PETROGRAPEY AND TENTATIVE DEPOSITIONAL

ENVIRONMENT OF TEE CABALLOS FORMATION,

UPPER MAGDALENA VALLEY, COLOMBIA,

BASED ON DITCH CUTTINGS AND WLIRIEILXNE LOGS

FROM SEWN WELLS

JOHNMZLVIN

DRAFT REPORT

W O K By: J O H N r n L V N SEONAID MACDONALD

STUDY OF CABALLOS FORMATION FOR LASMO COLOMBIA

CONTENTS

1. mTRoDu~Lox

2. PETROGWEW RESULTS

3. PETROGRAFHiC DATA

1. PBOTOM[CROGWHS

5. OBSERVATIONS ON DIAGEhTSIS AND POROSITY EVOLUTION

OBSERVATIONS ON DEPOSITIONAL ENViRONMENT FROM PETROGRAPHY

DEPOSITION EKWRONMENT OF THE CABALLOS

APPENDIX

VISUAL ESTIMATES OF C U " G LiTEfOLOGY

PETROGRAPHIC DESCRIPTiIONS

PHOTOMICROG W H S

h

4

INTRODUCTION

This study arose from an original request ffam h s m o Oil (Colombia) Ltd to examine thin

sections of ditch cuttings from seven welIs in the Upper Magdalena valley in Colombia (see

Figurel). The objectives of the study were to attempt to assess the diagenetic history, and

evolution and controls on porosity, within the mid-Cretaceous Caballos Formation, a major

reservoir target in the area. In addition, and with the aid of a number of wireline logs

supplied by Lasmo, it was requested that an attempt be made to derive a model for the

deposiiional environment of the Cabailos Formation. Success in the petrographic part of the

study (diagenesis, porosity evolution) was limited for the reasons outlined below,

althaughmodelling of the depositional environment was rather more successful, and is

discussed in a later section of the report. A total of seventy (70) tlun sections of ditch

cuttings and a number of wireline logs were examined from the following wells: Yavi-1,

Alpujana-1, Suarez-1, Villanica-1, Chicoral-1, Apicala-1 and Estratigafico- I.

rq

Limitations of the Data

A number of factors conspired to limit success in this study. Although a total of seventy thin

sections were examined, the quality and amount of material from well to wet1 vaned greatly.

Thus Suarez-I is represented by 27 thin sections of wet ditch cuttings covering a total drilled

interval of 1040 feet, whereas Estratigrafico-1 only had 3 samples provided, and Chicord-1

had none at all, Furthemore, some wells provided wet cuttings and others had been washed

and the presence of contaminant drilling mud in many of the secibns at times proved

problematical. Other problems related to sample contamination and drilling history of given

wells included the presence (in phases an abundance) of suspecteTwel1 bore caklags and a

variety of lost circulation material CCM). Variability in sample quality is tabulated in Table

1. -

A

FIGURE 1

.-

SKETCH WAP OF ‘CIPPER MAGDALENA VALLEY SHOWWG RELATIVE IWLL LOCATIONS AND DATA QUALITY

c C)[ICORALl

2 Lots o cs

e AP t CALA- t o LOCS

t I cs

c

ALFUJARRA-1 SLOCS

a cs

II

VILLIUCRICA-~ . ILOC

5 cs

PETROGWPBIC RESULTS

Each thin section was examined, and a number of lithic types were identified as being

representative of the cuttings in each case. Consideration of the location of the nearest

casing shoe in each well provided a fair degree of confidence that most of the material

examined derived from the Caballos Formation (although, as noted previously, some samples

did have quite a large amount of caved material, based an the size and shape of the fragments

available on the thin section). The proportion of each lithic type in each thin section was

noted by visual estimate, and tabulated in Tables 2 - 7. A detailed witten description of each

sample, with an accompanying photomicrograph is presented in Appendix 1 .

Several major lithic types represent the drilled intervals in this study. They are:

A

Dark organic silty mudstone

Pale brown (?smectitic) silty mudstone

Quartz arenite

Chert-rich (sub) Lithareriite

Chert

Limestone

Monocrystalline quartz

Each lithic type is described below, and illustrated in Plates 1-7.

Dark organic siltv mudstone Plates la, 1 bl

This is a very dark brown, clay rich rock, which contains abundant dispersed silt-sized ga ins

of quartz. In places, the silt-sized material is found as regular laminations up to lmm thick;

elsewhere this dark mudrock contains scattered foraminifera and other microfossil debris.

TABLE 1

WELLNAME No. LOGS

VARZABJLLTY AND QUALITY OF DATA USED IN TEE STUDY

DEPTH DEPTH No.CTGS i ” t V A L INTERVAL, COlwMENTS SAMPLES OVER PER

CABALLOS SAMPLE

Suarer- 1 3 27

Yavi-l 3 16

- Apicala- 1 11

Alpujarra- t 5 8

Villarrica- 1 1 5

E s I I ~ & ~ o - I 1 3

Chicord-1 2 1

m WELL

1090 30’ (2 gaps] Ab& driH mud

50 (1x350’ 1050’ gap)

--II___pp-

Mixed wetldry SaUlpt€S 595’ 20’ (Sgaps)

Includes two 730’ 100’ (CIx30’) basement samples

460’ 100’ (71x60’)

150’ 100’ (+1x50’) Abundant LCM

- - -

c Pale brown í? smectitic) silty mudstone (Piate 2'1

A

This rock is characterised by its pale brown colour and the abundance of dispersed silt-sized

quartz m a t e d that occurs within the clayey matrix. Glauconite pellets are present. Cuttings

fragments of this material commonly exhibit sinuous cracks, indicating the desiccation of a

water sensitive clay component: hence the tentative interpretation that the clay fraction is

smectite in composition

Quartz arenite (Plate 3a, 3b)

This cuttings type is quite distinctive. It comprises a fine to medium grained sandstone with

well sorted, quartz grains of monocrystalline quartz. Rare grains of glauconite have been

observed in this lithic type. The grains generally occur as a tightly interlocking mosaic of

quartz, very commonly dispiaying quartz overgrowths with "dust rims". Grain contacts are

straight or sutured, although these phenomena are thought to be related more to the

interpenetration of quartz overgrowths rather than compaction. Visible porosity i s extremely

rare in this lithic type, and at best is only seen as the faintest pinprick of blue dye epoxy resin.

Chert-rich (Sub) litharenite Plate 4a, 4b)

This is the dominant sandstone lithrc type of these rocks. It is a very fine to medium grained,

poorly to moderately sorted sandstone with subequant grains of monocrystalline quartz and

varying amounts of very finely microcrystalline chert which is pale brown in plane polarised

light. In most examples there are traces of recrystallised illitdmica representing original

detrital clay. Glauconite pellets are present. Quartz overgrowths and cornpactional effects

have reduced visible porosity in the cuttings effectively to zero. - Chert Plate 51

P-

This Lithic type occurs as discrete cutting fragments compose entirely of chert. This is

present as either of two t ipes : very fmely microcrystalline quartz, or coarsely

microcrystalline quartz. No porosity is associated with this lithic type.

I

A

Limestone (Plate 6 )

Limestones are present in 4 of the wells. They comprise two types of limestone, bioclastic

limestones and grainstones. The bioclastic limestones show shallow marine microfossils and she11 fragments ( e g bivalves, gastropods), most of which have been altered and replaced by

ferroan calcite and dolomite. Micrite and micrite envelopes are common. The grainstones

also show replacement and alteration to ferruan calcite and dolomite, Very little internal

structure is evident in any of the limestones. Very little porosity is seen, The limestone

“cuttings” are in most cases very large, relative to the sample, and are generally considered to

be caved.

Monocrvstalline quartz Plate 7)

Although not present in every well, cuttings represented by this particular lithic type are most

distinctive. They comprise very angular pieces of monocrystaiIine quartz, rarely associated

with cuttings composed of two interlocking quartz crystals (i.e. derived from a very coarsely

polycrystalline source). The character of this material suggests that where it occurs in great

abundance, the dril1 bit has probably penetrated a conglomeratic rock rich in pebbles or even

cobbles of vein quartz.

h

Figures 2 to 7 illustrate the relative proportions of each lithic type in each sample in graphical

format, and a number of interesting observations can be made.

.-

In Suarez-1 and Yavi-I wells respectively (Figures 2, 31, each demonstmtes a

dramatic disappearance of chert above a point roughly mid-way through the sampled

interval of the Caballos Formation. Furthermore in the Y a w l well t h i s appears to

coincide with a similar toss of sandstone lithic types, and tmcuttings above that level

are dominated by mudrock lithic pes. Suarez-1, however, not only continues to

yield significant sandstone cuttings, there is even a suggestion that they increase in

abundance towards the top of the Caballos.

I Apicaia-1 we11 (9 samples) (Figure 4) shows a very consistent, and essentially high,

mudrack content in cuttings throughout the sample interval.

d

e Aipujarra-1 (8 samples), Villarrica-1 (5 samples) and Estratigraficei (3 samples)

(‘Figures 5 , 6, 7) are the only three wells to exhibit the monocqstalline quartz lithic

type (see plate 7). Further more, where this lithic type occurs, it does so in great

abundance, although only at very specific horizons. This may have some significance

regarding depositional environment, or palamgeography of the area in general during

mid-Cretaceous times.

The possible significance of these observations is discussed in a concluding section of this

report.

4

PLATES 1 - 7

DARK ORGANIC SLTY MUDSTONE

DESCRIIPIION FOR PLATE l a

An example of the very dark brown clay rich silty mudstone in plane polarised light

Magnification 2.5. Note the dispersed silt sized grains of quartz, in places showing

lamination (upper right.) A more fossiliferous silty mudstone is seen (botion left to right)

showing a possible brachiopod fragment, brown in colour and showing the foliated nature of

its centre layer. Interesting is a similar shell fragment (above) also showing the foliated

nature of its inner shell but replaced by ferroan calcite. Such shell fragments and

tnicrofossiis in these silty mudstones are common. No porosity is seen.

Plate la taken from Apicala-1

DESCRIPTXON FOR PLATE l b

A general view of different dark organic silty mudstones is plane plarised light.

Magnification 2.5. Note the piece in the middle of the photomicrograph showing

characteristic lamination, and siIt sized quartz grains. Darker pieces are evident thrúught the

drilling mud that binds them. No porosity is seen,

Plate 1 b taken from Apicala- 1

.- c

.

PALE BROWN SILTY MITDSTONE

DESCRPTION FOR PLATE 2

An example (upper left) of pale brown silty mudstone in plane polarised light Magnification

2.5. Note the charactenstic pale brown c o h ~ , the abundance of dispersed silt sized quartz

materials and the sinuos cracks. This piece is bound by a brown drilling mud containing dark

silty mudstones [top right}, sublitharenites (top) and limestones (bottom). Little porosity is

seen.

Plate taken from Suarez-l

F.

QUARTZ ARENITE

DESCRIPTIONS FUR PLATE 3a

An example of quartz arenite taken in plane poiarised light. Magnification 5. The exampies

shows fine to medium well sorted monocystall ine grains exhibiting a tightly interlocking

mosaic of quartz. Note the 2 grains G3, G4. They are very well rounded and show classic

dust rims. Little porosity is seen.

Plate Sa taken fiom Villarrica-1

.-

DESCRIPTION FOR PLATE 3b

The same exampIe as above except is crossed polarised light. Magnification 5, showing that

monocrystalline quartz grains dominate. Little porosity is seen.

Plate 3 b taken from Villamca-1

4

.

4

.- CHERT-RICH (SUB) LITHARENITE

DESCRIPTION FOR PLATE 4a

An example of litiiarenite in plane polarised light, magnification 5. grains of very fine to

medium quartz can be seen, gain contacts are straight or satured. Like grains of chert are

seen as pale brown in colour. Note the surrounding matrix, possible drilling mud. Little

porosity is seen.

Plate 4a taken from Villarrica- 1

PETROGRAPHIC DESCRIPTION FOR PLATE 4b

The same example as above except in crossed polarised light, magnification 5 . Note the day

traces, yellow is crossed polarised light (e.g. F7); this is possible illite/mica. Lirtle porosity

is seen.

Plate 4b taken from Villarrica-1

CHERT

DESCRIPTION FOR PLATE 5

An example of a rock fragment composed entirely of chert taken in crossed polarised light,

magnification 2.5. Finely microcrystaIline equant crystals only a few microns across and

with no visible relict texture are dominant. This fimiy crystalline masiac of quartz exhibits

pin p i n t extinction. No porosity is seen.

Plate 5 taken from Suarez-1

h

a

LIMESTONE

DESCrCIPTION FOR PLATE 6

An example of bioclastic limestone taken in plane polarked light, magnification 2.5.

Ooidpeloids are evident, showing no original internal structure. They have been altered and replaced by ferroan calcite and are undergoing dolomitisation from the inside out. a

yeliow/green cement is around the ooidípeloids and infilling. The cutting is surrounded by a

brown drilling mud containing much smaller pieces of silty mudstones. The size of the

limestone compared to the mudstones suggests that it is probabIy caved. Little porosity is

seen.

Plate 6 taken from Yavi-1

i

MONOCRYSTALLINE QUARTZ

DESCRIPTION FOR PLATE 7

This example exhibiting single grains of fine to coarse monocrystailine quartz was taken in

plane polorised light, magnification 2,5.

Note the distinct angularity of the quartz cuttrng fiagtnents.

Plate 7 taken from Aipujarra- I .

Observations on diagenesis and wrosity evolution in the Caballos Formation

Although always worthy of some attempt, diagenetic studies of ditch cuttings samples of

sandstones generally yield little conclusive information. This is due to a number of factors:

(j) t h e very small nature of the sample @e. t h e ditch cuttings pet se severely limits

appropriate observation); the fact that cuttings samples represent a range of depths drilled

through any p e n formation ( in this study, this could be as much as 100 feet) severely

restricts the accuracy with which any observed diagenetic phenomena can be specifically

assigned to the formation; uncertainty is compounded by the presence of contaminant

materials, especially caved material. Not with standing the above, some limited comments

can be made on the post-depositional history of the CabalIos Formation sandstones

encountered in this study The quartz areaites (Plate 3a, 3b) ate characteñsed by grains that

are tightly interlocking, as a result of wholesale development of quartz avergrowths on the

original detrital quartz grains. It is most likely that this overgrowth, rather than compactional

effects, has resulted in the extreme porosity reduction observed all sarnpIes encountered in

the study. Quartz overgrowths are also abundant in the chert rich (sub) litharenites (Plate 4a,

4b), and similarly have severely occluded any original porosity present. In some examples

of these chert-rich rocks, there is abundant selective replacement of the chert grains by

carbonat e.

c

Visible porosity in both the sandstone lithic types encountered in the Caballos Formation

samples is, to all intents and purposes almost non-existent. It could be argued that this is a

function of seIective identification of cemented sandstone cuttings Le. that more porous

sandstones do occur, but they are disaggregated by the drill bit into their component grains.

However the evidence seems to suggest that this is not a widespread phenomenon, since any

disaggregated detrital quartz grains would be expected to exhibit a fair degree of the

rounding they underwent while being originally transported and - deposited as grains. The

only monocrystalline cuttings types observed are the monocrystalline quartz fragments

described earlier: these are charactensect, not by being in any sense rounded, but rather as

being extremely angular, (Plate 7 ) indicating that they have been derived by the drill bit

penetrating a tight, quartzose lithology such as vein quartz, rather than a porous quartzose

sandstone.

4

A

Observations on depositionai environment as suggested from ditch cuttings

Conclusive identification of depositional environment is indisputably, best achieved by

examination of cores taken from the reservoir, wherever possible. Attempts to achieve any

significant conclusions from ditch cuttings can only be made at a general level, at best. The

major limitations associated with such an approach are the same as those outlined above,

regarding the determination of diagenetic history etc.

There are two major sandstone types identified from ditch cuttings from the Caballos

Formation, viz. quartz arenite and chert-rich (sub) litharenite. Both of these rock types are

mineralogically very mature i.e. they comprise an extremely high proportion of silica. In

addition glauconite has been observed in several examples, particulariy of the quartz arenite.

Both rock types are texturally very mai-ure also, behg generally fine grained, welt sorted and

with equant, sub-rounded to rounded grains {to the exient that original depositional texture

can be determined). The maturity of the two sandstone tjpes, and the noted occurrence of

glauconite in several samples allows a classical interpretation that the rucks were deposited

in a marine high energy environment. Although this was probably a shallow marine

(?shelfal) setting, no further differenhation of depositional environment can be elucidated

without recourse to further information, such as wireline logs.

F.

Demsitional environment of the Caballos Formation, as interpreted from wireline logs

A number of wireline lugs have been supplied for this study. The number and quality varies

from well to well (see Table I), In particular, no logs were available from Apicala-l well.

The approach to assessing depositional environment most utilised the gamma ray and sonic

logs, with additional useful information from the density log. By paying particular attention

to changing trends on the gamma ray log a number of paiaeoenvironrnents can be tentatively

inferred in each of the wells for which data was supplied. .A

A Aipuiarra-1 well ( 5 logs)

The base Caballoshop Basement in this well occurs at 3894ft. From 3894ft up IO 3794ft the

gamma ray log exhibits an erratic (serrate) pattern, with on average, moderate API values

(40-100 H I units). This represents alternating shales or mudstones, and sandstones that are

either quite argillaceous or feldspathic (or both). It is difficult to assign a depositional

environment to th is interval, but it is not unreasonable to consider a continental (fluvial)

environment of moderate energy/sinuosity, considenng the presence of underlying basement,

and the possibility of feldspathic (arkosic) sandstones.

Between 3794ft and 3694ft (100ft) the g m m a ray i s dominated by a very distinctive "barrel

shaped" pattern of low MI values (20-30 API units), punctuated sharply by local occurrences

of high to very high M I values (100-220 APZ units). This type of gamma ray log pattern is

very commonly associated with rocks assigned from core studies to a braided stream fluvial

environment. The loft thick mudstone with the high gamma values at 3748-2758ft is very

distinctive, and probably represents the dramatic abandonment of the underlying braided

fluvial system. It is interesting to speculate on the possibility that such a dramatic

abandonment may be related to a significant change in base level w i h n the fluvial system

possibly having regional stratigraphic significance.

The interval between 3694feet and 3608feet commences wth a gradual "shaling upwards" to

3682A, followed by an erratichighly serrate pattern in the gamma ray log, which is

dominated by quite high API values (av 180 APE units) The basal "shaling upwards" unit

represents the Fadual abandonment of the second braided fluvial system, and the serrate

pattern that overlies it, with its predominately high APT values, most likely represents the

subsequent establishment of floodbasin environments. Between 3608ft and 3592ft there is a

single but highly distinctive "barrel shapedq gamma ray log trace, with very low APT values

(20 API units) and very sharp lower and upper búundaries. This is interpreted as a simple,

but highly significant channel sandstone, of a (probably) braided channel environment. If

this is the case, it implies a basinward shift in base level, and so could have major sequence

strati graphic imp1 i cati ons.

- -c

-

Above this distinctive channel sandstone, between 3542 and 3470ft, is a complex and

generally senate gamma ray log trace. There is insufficient evidence to sensibly assign a

depositionaf environment to this interval although the implied argillaceous nature of its basal

section may suggest the onset of flooding (? marine) over the area.

Between 3470ft and 3370% the gamma ray log trace is remarkably uniform with sustained

high API values (160 API units). This would suggest thar the lithology represented is a

mudstone, probably of marine origin (considering the thickness). A b v e this mudstone, the

interval between 3370ft and 3266ft is charactensed by a gradual "cleaning upwards" of the

gamma ray log trace, indicating an upward increase in proportion of sandstones in the

system: such upward - cleaning and coarsening trends over these interval thickness are

typically assigned to deltaic progradation.

From 3266ft to the top Caballos Formation al 3170ft, the gamma T ~ L Y shows an erratic, but

increasing smoothed profile with high APT values (1 20 N I units) except in the very highest 7

feet which is a low API (40 API units) sandstone. This upward-fining section in the upper

Caballos is thought to represent cessation of progradahon and the onset of major transgression.

-

Overall, the interval of the upper Caballos Formation in this well between 3470ft and 3 170fi

(300ft) can be interpreted a5 a lowermost marineiprodelta shale passing up into prograding

sands of the upper prodelta, delta fiont and delta top, which in turn are overlain by thin sands

and muds of a transgressive marine systems tract.

Yavi-1 well (3 logs)

-4

En this well, the upper CabaIlos Formation, from its top at 906sadown to 9700ft bears a

remarkable similarity to the upper Caballos described above in the Alpujarra-1 well. Thus

from 97OOft IO 9570fi there occurs a section displaying a highly erratic gamma ray log trace:

this is of indeterminate depositional environment. It is overlain between 9570ft and 943%

by a very uniform interval of high gamma ray activities representing prodelta mudstones.

-

rc. Between 9300ft and 9433ft is an interval of increasingly clean and thick sandstones,

representing the "classical" prograding delta sequence. This in turn is overlain by an

increasingly high gamma trace, indicating the probable onset of increasing Tnanne conditions.

All of the above pattern in the Yavi-1 well gamma ray log correlates well with the upper

Caballos Formation in the Alpujami-1 well. Below 9700ft, however it is e?rtremely difficult

to make any correlation between the wells. Furthermore it is noteworthy that below that

depth (970Oft) in Yavi-1 there appears to be an increase in the number of thin coals inferred

from the density log, and the mount of chert in the cuttings as observed in the present study.

In addition, there seems to be a significant shift in character of the entire %ireline log suite

below that depth. The possible significance of this will be addressed later: it is suficient to

reiterate at this point the correlateability and simiIarity of depositional environment (namely

prograding delta) of the upper Caballos in both Alpujarra-1 well and Yavi-1 well.

h Villarrica-1 well I1 log)

The only gamma ray log trace available for this we11 is distinctive and unique within the data

set supplied for this study. The fornation can be subdivided into two principal units. The

lower unit, from TD at 2912ft to 2745fi is charactensed by a stacked series of blocky, or

"barrel shaped" (20-30 APT units) intervals, punctuated sharply by high value gamma spikes

(30-120 APi units). This feature is interpreted as a series of stacked braided stream

sandstones separated by low stage or abandonment mudstones. At 2745ft there is a highly

distinctive, extremely high value gamma peak (247 MI units). This represents shale kith

unique characteristics, and is thought to be of significant stratigraphic significance to the

Caballos Formation in this area.

Overlying the "high gamma shale" (between 274% to 2448ft) the gamma ray log is highly

serrate, and the lowest values (15-25 API units) occur in the iower part of the interval

between 2745fi and 2650ft. These are interpreted as channel sandstones. In contrast to the

lower section of the Caballos in this well, described above, these sands pass gradationallv

into high gamma mudstones Le. the log trace shows classical "fining - upwards"

characteristics typical of relatively high sinuosity fluvial systems.

--i

-

.

- The uppermost part of the Caballos Fornation in this welI is charactericed by a highly

senate, but generaIly high value gamma log, and coals are described at 260oft on the

geological composite log: thus it is likely that the upper Caballos between 2650ft and 2448ft is dominated by shales, fine sandstones and occasional coals of a fluvial floodbasin

(overbank deposits).

in summary, the Caballos Formation at Villamca-1 weil comprises a lower series of stacked

braided stream channel sandstones With subordinate shales, overlain by a very different series

of sandstones, shales and coal more representative of a high sinousity (meandering) fluvial

system. T h s suggests a significant shift in base level at that time in this part of the basin, and

the very pronounced "high gamma shale" at 2745ft which separates the two fluvial systems

undoubtedly is a reflection of that sudden change in basin configuration.

Estratimafico - 1 well ( I log)

From TD at 7360 ft to 7290 A the gamma ray log in this well is characterised by a double

barrel shaped, very low gamma trace, which is interpreted as the stacking of two major intervals of sandstone deposited by low sinuosity (braided) streams. These are separated by a

high gamma spike (? 120 NI units) indicating an intervening interval of about 8 A of

mudstone representing abrupt abandonment of the lower braided system.

Between 7290 ft and 7210 ft the gamma log trace is characterised by an erratidserrate

character, but within which a number of upward cleaningí? corsening intervals can be

identified, each of about 20-25 feet in thickness. These intervals occur above a lowermost

thick mudstone interpreted from a high gamma trace between 7290 fl and 7265 ft. That

mudstone represents the final abandonment of the basal braided stream systems described

above and might well be indicative of extensive lacustrine deposits. The overlying upward

ckaning/coarsening "cycles" are interpreted as small stacked aEvial fan deposits. This

interpretation is to some extent substantiated by the abundance of highiy angular

monoctystalline quartz in the cuttings sample between 7200 and 7300 ft (see Figure 7),

suggesting penetration by the drill bit of coarse-grained quartz - boulder - rich conglomerate.

It is also noteworthy that this interval includes the highest part of the underlying braided

stream deposits, interpreted above.

-

I

3

k Abandonment of these inferred aliuvial fans was abruptiy initiated at 7210 R with the

deposition of an argillaceous sequence represented by moderate to high gamma ray log trace

which persists to 7185 ft.

From 7185 A to the top Caballos Formation at 6950 ft, the gamma ray log trace is higiily

serrate, but within which can be identified three major upward - fining units. These OCCUT

between 7185 ft and 7100 ft (85 A), 7100 ft and 7022 ft (78 fi) and 7022 ft and 6950 fl(72

ft). In each case the upward fining unit commences with a sharp base, followed by a gradual

(or stepped) increase in gamma values until a fully argillaceous (high gamma) situation is

achieved, Argillaceous deposits then prevail in each case for a considerable intend (about

40 feet), before being intempted by the abrupt base of the superceding fining unit. Thus at

least 50% of each of these upward - fining units is composed of fine - grained material

(mudstone), based on analysis of the ganuna ray log traces, Each of those fining units is

interpreted as representative of deposition in a muddy, highly sinuous fluvial system.

A

In Estratigmficcl - 1 well, therefore, the Caballos Fornation is represented in its lower part by

a regressive alluvial sequence of braided stream channel sandstones (with low

stageiabandonment mudstones) overlain by stacked , coarse - grained alluvial €an deposits.

Following deposition of a 25 ft thrck mudstone, the upper part of the Caballos was laid down

within a highly sinuous (meandering), and generally fine - grained, fluvial environment.

Suarez-1 well (3 logs)

The gamma ray log trace in this well is moderately serrate throughout, with a very subtie and

gradual increase in average value (ca 45 MI units to ca 70 API units) from bottom to top of

the formation. There are no recorded coals, although a number of low density spikes on the

density log may well indicate the regular occurrence-of coals throughout the formation,

although not in abundance. No extremely high (or low) gamma s p x s are noted in this well.

- Low gamma sandstones (av 30 API units) appear to be most common below ca 7400A, and

examination of the log for trends suggests a number of upward-

coarsening trends throughout the logged interval of the Caballos. These occur in

approximately equal abundance, and have an average gross thickness of about 20ft. The

upward-fining

I

4 1

A Low gamma sandstones {av 30 API units) appear to be most common below ca 7400% and

examination of the log for trends suggests a number of upward-fining & upward-

coarsening trends throughout the logged interval of the Caballos. These occur in

approximately equal abundance, and have an average gross thickness of about 20ft. The

upward-fining units are interpreted as channel sandstone deposits, but it is unclear as to

whether they represent fluvial, delta distributary or tidal channels The upward coarsening

units could represent offshore shallow rnanne bars, but it cannot be discounted that some of

them at least may be representative of alluvium in a proximal overbank environment.

Examination of the cutting dab (see Figures 2-7) shows that below 7550ft in th is well there is

a dramatic, and persistant, appearance of chert in the cuttings. This is similar to the sudden

appearance of chert below 9700ft in Yavi-1 well. However, it is not possible to make an

effective lug correlation beween the two welis, and so the significance of the presence (or

absence) of the chert with regard to depositional environment is unclear, and could of course

be non-existent. Indeed it is feasible that such petrographic variations are more related to

tectonic, rather than depositional, events. (It should be noted that both Suarez-1 and Yavi-1

wells occur on the Same thrust sheet along strike from each other, and on a separate thrust

sheet from the other wells.

h

Chicoral-1 well (2 logs)

The gamma ray log character for this well i s very similar in i ts general appearance to the

previously discussed well at Suarez-l. Thus, it shows a subdued erratic trace (amplitude of

the log does not exceed ca 45 APT units) which has quite a high average value (ca 105 API

units) and shows a subtle upward increasing trend. Individual fluctuations in the log chow

both upward-fining and upward-coarsening trends, which are on average 15-20 A thick. Thus similar ambiguities to Suarez-1 prevail when attempting to interpret depositional

environment from the logs alone: also, this well had no representative ditch cuttings supplied

for this study. All that can be said is that the Caballos Formation at Chicoral-1 appears to

have similar characteristics to Suarez-2, and couíd represent a coastal to shallow marine

depositiona1 environment.

\

-

.r?

h Vil iarrica- 1, Aipujma- 1 and Estratigrafico- 1 wells exhibited petrographic similarity in

having distinct stratigraphic levels of abundance of highly angular cuttings of

monocrystalline quartz. n o s e intervals correlate with the inferred occurence of braided

stream sandstone and (at Estratigrafm-1) alluvial fan sediments [on wireline logs): it is not udikely that such environments would contain coarse grained, conglomeratic

sediment, and any pebbles or cobbles of yein quartz would disintegate under the drill bit

to yield just such anguiar quartz fragments. Thus the distribution of those cutting types is

thought to be of a rea1 depositional significance.

Chicoral-l and Suarez-1 wells exhibit very similar gamma ray log traces throu& the

Caballos Formation. Those traces, however, lack distinctive character, and interpretations

regarding depositional environment of the Caballos in those wells is very tentative: it may

well have been a shallow marine setting Lwth tidal channels and bars.

h Petrographically, Apicala- 1 well is dominated by cuttings of a mudrock type (no logs were This is consistant with regional knowledge suggesting that this well is available).

relatively distalIy situated with respect to the Caballos basin.

--.

h

DISCUSSION

im

From the preceeding discussion of the petrography of the ditch cuttings, and the wireline log

character of the Caballos Formation in the seven subject wells, a number of observations can

be made, which are summariced beiow:

A limited number of lithologies are identified which charactense the Caballos Formation

from the subject wells, which include mudstone (two types), chert, monocrystalline quartz,

quartz arenite, chert litharenite and limestone (probably caved).

Suarez-1 and Yavi-1 wells have a petrographic similarity in that chert, while abundant in

the lower half of each well, dramatically disappears in the upper half of each well.

However well log correlation cannot be effected between these wells, suggesting a

dissimilarity of depositional environment between them. The correlatability of the chert

occurence between these wells is probably not related to depositional processes; it is

possible that it represents a tectonic correlation, and it is not impossible that it is purely

coincidental.

Yavi-1 and AIpujarra-1 weils in the south of the study area have highly comelatable

gamma ray log traces in the upper Caballos, and analysis of trends suggests a prograding

deltaic environment of deposiiion,

The lower Cabdlos in Alpujarra-1 appears from similar analysis to comprise a number of

differing fluvial environments, including distinctive braided stream deposits in the lower

part.

Ln Villarrica-l well, analysis of ?he character of the gamma?ay log suggests a lower

braided stream sandstone interval below an upper meandering stream interval. The two

are separated by a highly distinctive gamma ray peak, which may have stratigraphic

significance as an indicator of base level shift ("flooding surface") in the fluvial

environment. Very similar facies associations are identified at Esiratigrafico - I well.

A

COPU’CLUSIONS

Petrographic and wireline log information suggest that in that part of the basin represented by

the seven study wells, the Caballos Formation is represented by a complex, vaned

depositional facies rnosiac. Furthermore evolution of the depositional milieu through time

allow infomal subdivision of the Caballos into an upper and a lower member in some of the

wells, based solely on the information derived from examination of ditch cuttings and

wireline logs from the seven subject wells. In particular the lower Caballos Formation is

cleaTIy identified in the three wells on the East of the study area (Figwe sa), namely

Alpujarra - I, Villarrica-1 and Ectratigrafico-1, Tn each of these wells, the lower Caballos is

identified on logs as an alluvial succession, more varied in Alpujarra - I than the others, but

in all cases dominated by braided stream sandstones (with some influence from alluvial fans

possible at Estratigrafico - I).

h

It seems highly significant that is was only in these three wells, and from appropriately

relevant intervals in each, that the cuttings data yielded very high quantities of very angular

monocrystalline quartz. As discussed, this is consistent with a highly quartzose, very coarse - grained (congiorneratic) deposit, such as braided stream alluvium andor alluvial fans. Such

reservoirs would be desirable given appropriate trap and sed, since they would be laterally

continuous and could have good reservoir properties (an abundance of LChd material from

the sample from 7300 - 7380 A at Estatigratico - 1 implies considerable permeability in such

deposits in that location). Potential intra-reservoir problems could ame however relating to

IateraIly continuous shales which could lead to significant production problems if associated

with particularly highly prmeable zones. The identification of the lower Caballos braided

fluvial deposits as being apparently restricted to the eastern side of the study atea may

present a problem whose resolution is beyond t h e scope of the available data in the present

study. -

*

The upper Cabaílos presents a very different and such more variable paleoenvironmenial

picture (Figure 8b). Thus in the southern pan of the study area, at Yavi - 1 and Alpujarra - 1

weils, these rocks indicate deposition in a deltaic setting,

f

FIGURE Sa

A P I C A U - l

- -

SKETCH hihP SIiOWING INTERPRETED PALEOENVIRONM ENTS IN STUDY AREA DURING LOWER CABALLOS TIMES

8

Ct~IC0RAL-I

CA-I

FIGURE Sb

SKETCli MAP SNOWING TNTERPRETED PhLEOENVLRONhlENTS IN STUDY AREA DURING UPPER CABALLOS TIMES

f

Further north, at Villarrica - 1 and Estratigrafico - 1, the upper Caballos Formation appears

to be dominated by hi@ sinuosity fluvial, or possibly delta top distributary channel

environments. The wells at Chicoral - 1 and Suarez - 1, however, give rise to an entirely

different interpretation, and in those locations the upper Caballos is more likely represented

by shallow rnarine?nearshorei? tida[ channels and bars. Ditch cuttings from Apicala - 1 (no

log available) suggest that the Caballos Formation at that location is represented by offshore

muds, in a distal setting.

m

From the ditch cuttings, the reservoir quality of the two main sandstone lithic types (quartz

arenite and chert litharenite) appears to be extremely poor, due to extensive cementation by

quartz overgrowth on the original sediment, which is highly mature, both texturally and

meneralogically. However, it is possible (as argued above) that coarser - grained, ? less

mature deposits of the lower Caballos alluvial suite might provide more attractive targets

(based on pnmary depositional criteria). There is some evidence for this in the abundance of

LCM material. identified at the bottom of Estratigrafico - 1 well. .--.

3

Tiit>lc 1ii

PETROGRAPH iC DATA: VISUAL ESTIMATES OF CU'lI'ING LITHOLOGY (WELL: S IJAR E 7, I )

T:ililc 1 Ii

PETROCFWPlflC DATA: VISUAL ESTIMATES O F CUTTING LITl1C)LOGY (Continued) (WELL: SUAREZ-1)

I

21)

t 0

3

3

3

15

3

3

2

5

3

PETRUGRAPRTC DESCRIPTIONS FOR SUAREZ-1

1. DEPTH 6690 Si, 4 different rock types were present in the thin section. The rock cuttings are bound by brown drilling mud containing quartz grains. Sublitharenites form 45% of the thin section and show quartz arenites of single monocrystalline grains with rare polycrystalline grains and cherts showiny niicrocrystailine quartz with no visible relict texture. Some alteration to ferroan calcite is seen. Sandy siltsrone forms 20% of the sample The siltstones are brown and show la~iiination. Organic siltstone form 15% of the sample. These siltstones are dark browidblack in colour and show calcareous cement in small amounts. One large piece shows sniaIi brown forininifera. Bioclastic limestone forms 20% of the sample and shows abuiidant shell fracments ceiiiented by a calcareous cement. Some fragments show a micrits envelope and some alteration to ferroan calcite. Very little porosity is seen in any of the rock types.

2. DEPTH 7020 Si. 3 different rock types are present in thc thin section. The rock cuttings are bound by brown drilling m u d containing quartz grains. Sublitharenites form 50% of the thin section and show quartz arenites of single monocrystalline grains with rare polycrystalline grains and cherts showin= iiiÍcrocrystalline quartz with no visible relict texture. Some alteration to ferroan calcite is seen. Quartz arenite sandsíone forms 5% of the sample showing sincle crystals with sutuwd contacts and signs of compaction. Sandy siltstone forms 45% of the sample. The siltstones are brown and laminated. Abundant quartz is seen. Some pieces show alteration to a ferroan calcite cement, Very little porosity is seen in any of the rock types.

h

3. DEPTH 7080 Si. 4 different rock types can be seen in this thin section. The rock cuttings are bound by brown driIling mud containins quartz p i n s . Sublitharenites form 35% of the thin section and show quartz arenites of single monocrystatline ya ins with rare polycrystalline grains and cherts sliowing niicrocrystdline quartz with no visible relict texture. Some alteration to ferroan caicite is seen. Quartz arenite fornis 25% of the sample. The grains are nionocrystalline. Sutuied con tms are evident between some grains Sandy siltstone form 35% of the sample. The siltstones are brown and show lamination. Aiterated to calcite cement is present in minor amounts, Bioclastic limestone forms 5% of the sample and shows replaced bioclasts to ferroan calcite and neomorphic spar. Very little porosity is seen in any ofthe rock types.

x

4. DEPTH 7110 Si. 4 different rock types are present in the thin section. The rock cuttings are bound by brown drillinS mud containing quartz grains. Subiitliarenites form 25% of the t h in section and show quartz arenites of sii-tgle rrianocrystalline grains with rare polycrystalline grains and cherts stiowiny microcrystallii-ic q u a m with no visible relict texture. Quartz arenite forms 15% of the sample showins fine grained nioderarely ~vvell sorted monocqstalline grains. Sandy siltstone fornis 35% of the sample. The siltstones are b r w and show lamination. Organic siltstone forms 25% of the saiiiple. The siltstones are dark brown in colour and show calcareous cement in m a i l amounts. There is also the presence of radial white and light brown crystal foriiis showing aliiiost black birefri iigence. Very little porosity is seen in any of the rock types.

Some aheration to ferroan cak i te i s seen

PETROGRAPHIC DESCRIPTIONS FOR SUAREZ-i (Coiitiiiiied)

5 . DEPTH 7140 Si 3 difterent rock types rare present in the cutting. The rock cuttings are bound by brown drilling mud. Sublitlmenites foriii 40% of the thin section and show quartz arenites of single moiiocrystalline grains with rare polycrystalline grains and cherts showing mic.rocrystalline qiiartz with no visible relict texture. Some alteration co ferroan calcite is seen. Quartz arenite fornis 70% of the saiiiple. The sample is medium grained subrounded to subangular and moderately sorted. The sample shows sutured grains. Elongated white radiating crystals are present i v i t h dark birefrincence. Sandy siltstones form 20% of the sample. They are brown and sliow hiination. Siltstones containing less amounts of quartz form 20% of the sample, they also are brown in colorir and show lamination. Colourless radiating crysrals are again found within the siltstones. Very little porosity is seen in any of the rock types.

6 . DEPTH 7170 Si 4 DiffeI-ent rock types are present in the thin section. The rock types are bound by brown drilling mud. Sublitharenites form 45% of the th in section and show quartz arenites of single iiionocrystalline grains with rai-e polycrystafline grains and cherts showing microcrystalline quartz with no visible relict texture. S o i m alteration to ferroan calcite is seen. Quarrz arenite forms 1O0h of the sample, showing fine subrounded grains with sutured contacts. Sandy siltstoms form 35% of the sample. They are brown in colour and show lamination. White elongated crystals with a radial structure are found within the siltstones. There are also sonic dark salmon pink crystal shapes showing dark birefringence. Organic siltstones formi IO% of the sample. They are dark browrúblack in caiour and also show lamination. Very lirtle porosity is seen in any of these samples.

7. DEPTH 7200 Si 5 Different rock types are present in rhs thin section and are bound by brown drilling mud. Sublitharenites h i - t r i 35% of the th in section and show quartz arenites of single monocrystalline g a i n s witti rare polycrystalline ga ins and cherts showing microcrystalline qiiartz with no visible reIict texture. Some alteration to ferroan calcite is seen. Quartz arenite forms I O % of the sample. showi iig fine subrounded grains with sutured contacts. Sandy siltstones foriii 25% of the sample. They are brown in colour and show lamination White eionpted crystals with a radial structure are found within the siltstones. There are also some dark salmon pink crystal shapes showing dark birefringence. Organic siltsrones form I O % of t h e sample. They are dark browdblack in colour and also show laniination. i 'ery little porosity i s seen in any of these samples. Bioclactic limestone forms 70% of the sample and shaws abundant shell fragments cemented by a calcareous cement Soiiie fragments show a micrite envelope and alteration 10 ferroan calcite. Very little porasiry is seen in any of the rock types.

8. DEPTH 7290 Si There are 4 differenr rock types in rhis th in section. These rock types are bound by a brown drilling mud. Sublitharenites form 70Y0 of the thin section and show quartz arenites of single nionocqmlline grains with rar.olycr)rstaIIine grains and cherts showing microcrystalline q u a m Lvitli ni3 visible relict texture. Some alteration to ferroan calcite is seen. Quartz arenite foriiis 20% of the sample. Sandy silstone forms 50% of the sample. The pieces are brouri i n coloui- and show lamination. Ferroan calcite is found in the lamination. Very dark brown i-adiatiny crystal forms with a dark birefrinsence are also found Dioclastic liinestone forills IO% of. the sample and shows abundant shell fragments composed of calcite spar after dissolution of the original shell. The shapes of the original shells are mit i ra ined by a dark micrite slíell. Very little porosity is found in any of these rock types.

PETROGFUPHI C DESCRTPTI ON'S FOR S UAREZ- 1 (Co II t iii lied)

9. DEPTH 7330 Si There are 3 differmt rock types in this thin section. These rock types are bound by a brown drilling mud. Sublitharenites form 20% of the thin section and show quartz arenites of single monocrytatiine srains with rare polycrystalline grains and cherts showing microcrystalline quam with no visible relict texture. Some alteration to ferroan calcite is seen. Quartz arenire fortiis 30% of the sample and shows fine wrell sorted and subrounded grains. Sandy siltstone forms 50% of the sample. The pieces are brown i n colour and show lamination. Ferroan calcite is found in the lamination. Dolomite or calcite rhonibs are also seen. Very dark brown radiating crystal forms with a dark birefringence are found. Very little porosity is found in any of these rock types.

IO. DEPTH 7380 Si There are 3 difterenr rock types in this rhin section, these rock types are bound by a brown drilling iriud. Sublitliareiites form 30% of the thin section and show quartz arenites of single monocrystalliiie p i n s w i t h rare polycrystalline grains and cherts showing microcrystalline quartz with iio visible relict texture. Soirie alteration to ferroan calcite is seen. Quartz arenite forms 20% of the saiiiple and shows fine grains, well sorted and subrounded. Sandy siltstone h i i s 50% of the sample. The pieces are brown in colour and show lamination. Ferroan calcite is found within the siltstones. Colourless radiating crystals with a dark birefringence are found. Very little porosity is found in arty of the rock types.

11. DEPTH 7410 Si There are 3 difierent rock types in this thin section. These rock types are bound by a brown drilling mud. Sublitharenites form 30% of the thin section and show quartz arenites of single nionocrystallirie grains wit t i rare polycrystalline grains and cherts showing microcrystalline quartz with no visible relict texture. Some alteration to ferroan calcite is seen. Quartz arenitc: form 30% of the sample and shows medium grained polycrystalline grains. The yraiiis are ~iiodei-ately sorted subrounded and show oversrowths. Sandy siltstones fortiis 50% of the sample. The pieces are brown in colour and show lamination. Ferroan calcite is found. ColourIess radiating crystal forms with a dark birefringence are found V e r y little porosity is found in any ofthese rock types.

c

12 DEPTH 7440 Si There are 5 different rock types in this t h i n section These rock types are bound by a brown drilling miid Sublitharenites form 30% of the th in section and chow quartz arenites of single monocqstallitie grains with rare polycrystalline grains and cherts showing microcrystalline quarrz wi th 110 visible relict texture Some alteration to ferroan calcite is seen. Quartz arenite forins 10% of the caniplc and shows medium grained monocrystalline and poIycqsralline grains. The grains are moderately sorted and subrounded. Sandy cilrstone forms 30% of the sample. Tlie pieces are brown in colour and show lamination, Brown radiating crystal forms are present. Organic siltstone forms 20% of the sample The pieces are dark brownhhck in coIour and show lamination. Bioclastic limestone forins 10% of the sample and shows shellfragments composed of a calcite spar i n a cherty matris. V e r y little porosity is found in any of these rock types.

A

b

PETROGRA PHTC DESC Ri PTI O K S FOR SU.4 R EZ- 1 IC0 I I t i r i ired)

13. DEPTH 7470 Si There are 5 different rock types in rhis thin section. These rock types are bound by a brown drilling rn~id . Sciblitliarenites form 25% of the Ehin section and show quartz arenites of single monocrystalline grains wi tI i rare po'ycrysralline grains and cherts showing microcrystalline quartz with no visible relict texture. Some alteration to ferroan caicite is seen. Quartz arenite f o r m 10% of the sample and shows medium grained monocrystalline and polycrystalline p i n s . The g a i n s are moderately sorted and subrounded, Sandy siltstone forms 25% of the sample. The pieces are brown in colour and show lamination, colourlecs radiating c-stals with a dark birefringence are found. Rare grains of anhydrite are present, Orsank siltstone fornis 30% of the sample. The pieces are dark brownMack in colour and show lamination. Bioclastic limestone forms 10% of the sample and shows shell fragments composed of a neomorphic spar i n a cherty matrix A bivalve shell is found sliowin~ pr-esenatjon of its internal structure. Very little porosity is found in any oftlie rock types.

14. DEPTH 7500 Si. There are 5 different rock types in this thin section. These rock types are bound by a brown drilling mud SriblitIiarenitec form 20% of the thin section and show quartz arenites of single monocrystaltine grains with rare polycrystalline grains and cherts showing microcrystalline quartz with no visible reIict texture. Some alteration to ferroan calcite is seen. Quartz arenite fariiis IOYb of the sample and shows medium monocrystalline and polycrystalline gi-ains. The grains are nioderateiy sorted and subrounded to subangular. Sandy siltstone f o r m 30% of ttie sampIe. The pieces are brown in colour and show lamination. Ferroan calcite cement is found in the lamination. Calcareous shell fragments are also found Colourless radiaring crystat forms are found showins dark birefringence. More orsanic siltstone forms 30% of the sample. The pieces are dark brown/black in colour and show lamination Ferroan calcite cement is found. Bioclastic limestone forins 10% of the sample and shows shell fragments composed of micrite cement and neomorphic spar bíicrite eiiveIopes are stiown around the shell fragments. One of the micrite envelopes is broken indicating cotupaction. Some calcite cement is also present. Very little porosity is seen i n any ofthe rock types.

15. DEPTH 7530 Si. There are 7 rock types in this thin section. Both ai-e bound by a brown drilling mud. Argillaceous sandstone foriiis 70 % of the sample and shows fine to medium grains, subrounded, moderately well sorted in a niafrix composed of ferrroan calcite cement with some quartz cenisnt. Saiidy siltstone forms 30% of the sample. The pieces are brown in colour and show lamination. Ferroan calcite cement is found in small quantities. Very little porosity is found in either rock type.

PETROGRAPHTC DE SCRiPT IONS FOR S U AREZ- 1 (Con t i n 11 ed)

16. DEPTH 7560 Si There are 4 different rock types in this thin section. These rock types are bound by a brown drilIiny mud. Sublittiñrenites form 30% of the thin section and show quartz arenites of siiigle riionociystalliiie grains with rare polycrystalline grains and cherts showing microcrystalline quartz n i t h no visible relict texture Some alteration to ferroan calcite is seen. Minor amounts of medium polyci-ystalline g a i n s are also found. Rare chlorite is found Kare glauconitic grains are found. Quartz arenite sandstone forms 20% of the sample and shows fine to medium grains w i t h sutured contacts. Overgrowths are present. Conipaction is so high this sandstone type could be described as metamorphic grade. Sandy siltstone fornis 10% of the sampie and is brown in colour with no lamination. Ferroan calcite rhoinbs are present within the siltstone Atered radiating crystal forms are found, possibly originally gypsum. Chert forms 40% of the sample showing uniform microcrystalline quartz w i r h no visible relict texture. The grains are surrourided by a carbonate matrix and by a fine riin of clay, possibly illite/smectite. Rare mica is found. Very little porosity i s found in any of these saiiipies.

17 DEPTH 7590 Si There are 4 dif’terent rock types in this thin section. These rock types are bound by a brown drillinc niud. Sublithareiiites form 30% of the thin section and show quartz arenites of single monocrystalline grains with rare polycrystalline grains and cherts showing microcrystalline quartz \ v d i no visible relict texture. Some alteration to ferroan calcite is seen. Minor amounts of medium polycrystalline grains are also found. Rare chlorite is found. Rare glauconitic grains are found. Quartz arenite sandstone forms 20% of the sample and sliows fine to medium grains with sutured contacts. Overgowtlis are present. Conipaction is 50 high this sandstone type could be described as metamorphic grade. Sandy siltstone f o r m 10% of the sample and is brown in colour with no lamination. Ferroan calcite I-hornbs are present within the siltstone. Aitered radiatins crystal form are found, possibly or-iginally gypsum. Chert forms 40% of the sample showing uniform microcrystalline qiiai-tz with no visible relict texture. The grains are surrounded by a carbonate matrix and by a fine rim of clay, possibly illirdsmectite. Rare mica is fotind. Ver,! l i t t le porosity is found i n any of These samples.

18. DEPTH 7620 Si There are 4 dittereiit rock types in this thin section. These rock types are bound by a brown drillins niiid Sublitliarenites form 30% of the thin section and show quartz arenites of single nionocrystalline g a i n s with rare polycrystalline grains and cherts showing microcrystalline quartz u ith no visible relict teutiire Some alteration to ferroan calcite i s seen. Minor aiiioiiiitc of medium polycrystalline p i n s are ais0 found Rare glauconitic grains are found. Quartz arenite sandstone forms 10 % of the sample and shows fine to medium grains with sutured contacts. This sandstone type is metamorphic grade. Sandy siltstone form 30% ofthe sample. It is brown in coIour fine grained and shows common laniination Mediumgrained pieces show no lamination. Aitered radiating crystals are found, possibly originally gypsur i lChert forms 50% of the sample showiny uniform niicrocystalline qiiartz icirli no visible relict texture. The grains are surrounded by a brown calcite matrix and by a fine rim of clay, possibly ilIite/smectite. Rare mica is found. Very little porosity is found in any of these samples.

PETROGRAPHIC DESCRIPTIOXS FOR SUAREZ-I ( C ~ i i t i i i i i ~ d )

19 DEPTH 7650. There are 6 difereiit rock types in this th in section. The rock types are bound by a brown drilling mud. Sirbtitharenites form 25% of the thin section and show quartz arenites of single riionocystaltine grains with rare polycrystalline grains and cherts showing microcrystalline quartz with no visible relict t e m r e . Some alteration to ferroan calcite is seen. Quartz arenite sandstone forms 5% of rhe sample and shows fine to medium grains ~ i t l i sutured contacts. The sandstone type is metamorphic crade. Sandy siltstone forms 30% of the sample and is brown in colour showing lamination and some alternation to ferroan calcite. Altered radiating crystals are found. Organic siltstone forms 10% of the sample and is dark browiiiblnck in colour Slight lamination is evident. Chert forms 3 O 0 h of the sriniple sbv ing aiiiorphous microcrystalline quartz with no visible reIicr texture, The grains are surrounded by a carbonate/cIay matrix. Mica is present. Bioclactic limestone forms 5% and shows a micrite matrix. No micrite envelopes are found. Limesrone composed of peioidc showing some internal structure form 5% of the sample. L'ey tittle porosity is seen in any of the rock types.

They show a dark birefringence possible originaly gypsum

20. DEPTH 7680. There are 6 difierent rock types in this rhin section. The rock types are bound by a brown drilliiig i~ iud . Siiblitliai-eniteb form 25% of the thin section and show quartz arenires of sin$e tiioiiocrystalline grains ~vitli rare polycrystallirie grains and cherts showing niic~-ocrystalliiie quai-tz with no visible relict texture. Some alteration to ferroan calcite is seen Quartz arenite sandstone forms 5% ofthe sampie and shows fine t o medium grains with sutured contacts The sandstone type is metamorphic grade. Sandy siltstone forms 20% of the sample and is brown in colour showing lamination and some alternation t o ferroaii calcite. Altered radiating crystals are found. Organic siltstone forms 10% of the saniple and is dark browdblack in colour. Slight lamination is evident. Chert forms 30% of the saniyie showing amorphous microcrystalline quartz with no visible relict texture. The grains are surrounded by a carbonate/ctay matrix. Mica is present. Bioclast limestone fornis 55% and shows a micrite matrix. No rnicrite envelopes are found. Limestoi-ic coiiiposed of pelokls showing some internal structure forms 5% of the sample. Very litt le poimity is seen in any of the rock types.

They show a dark birefringence possible originaly gypsum.

.-

21. DEPTH 7710, There are 5 different rock types in this thin section. These rock types are bound by a brown drilling mud. Sublithareriites fort-n 35% of the thin section and show quartz arenites o f singie rnonocq stalliiie grains with rare polycrystalline grains and cherts showing niicracrystatline quartz with no visible relict texture. Some alteration t o ferroan calcite is seen. Quartz arenite sandstone fori~is 5% of the sample and shows fine to itiediiiiii grains wirli stitirred contacts. The sandstone type is metamorphic grade. Sandy siltstone foriiis -3 5° í~of the sample and is btokvn in colour showing lamination and some altcrnatioii to ikrrom calcite-Aitered radiating crystals are found. They show a dark birefringence possible originally gypsum.Organic siltstone forms 10% ofthe saiiiple and is dark brownibIack in colour. Sligbt lamination is evident. Limestone/grainstone forms IO% of the sample and shows ooiddpeloids in a ferroan caIcite cement. Bioclast limestone Ycrrns 5% of the sample and shows completely altered shsll fi-agiiieiits to caicite. Very little porosity is seen in any of the rock types.

PETROGRAPHIC DESCRIPTIOKS I;OR S1JAREZ-1 (Coli tiriiied) h

.c

rcc

22. DEPTH 7740. There are 5 different rock types in this thin section. The rock types are bound by a brown drilling mud, Siiblitliai-enites form 30% of the thin section and show quartz arenires of single nionociystalline grains with rare polycrystalline grains and cherts showing microcrystalline qiiarrz with no visible relict texture. Some alteration to ferroan calcite is seen. Qiirinz arenite sandstone fornis 15% of the sample and shows fine to medium grains with sutiired contacts. The sandstone type is metamorphic grade. Sandy siltstone forms 20% of the sample and is brown in cotour showing laniination and some alternation to ferroan calcite. Aliered radiating crystals are found. They show a dark bi refringeiice possible originaiy gypsum. Organic siltstone form 15% ofthe sample arid i s clark broivnhiack in calour. Sliyht lamination is evident. Bioclasts linit'ctone forms IO% of the sample showing shell fragments altered to a calcite spar surroundcd by a micrite matrix. Ccittinc of'micritt cement are also found void of shell fragments. A bivalve fragment is found showing attraction to calcite with the preservation of its internal structiire. Limestandgainstone forms IO% of the sample and shows ooiddpeloids in a carbonate nitid matrix. A shell fragment is found within this matrix.

23. DEPTH 7770. There arc G different rock types i i i this thin section. The rock types are bound by a brown driIling mud. Strblirliarenites form 20% of the thin section and show quartz arenites of single monoci-ysralline grains ivith rare polycrysralline grains and cherts showing microcrystalline qiiat-tz with no visible relict tekmre. Some alteration to ferroan calcite is seen Quartz arenite saiidstone f o r m 5% of the sample and shows fine to inediuni grains with sutured contacts. The sandstone type is metamorphic g a d e . Sandy siltstone forins 20% of tlie sarnpie and is brown in colour showing Iarnination and some altei'natioii to ferroan calcite. Altered radiating crystals are found. Organic siltstone forms 5% of the sample and is dark brown/bIack in coloiir . Slight lamination is evident. Chert foi-rns 40% of the saiiiyle showiii; amorphous microcrystalline q u a m with no visible relict tesiure. Tliz g a i n s are surrounded by a carbonate/ciay matrix. hlica is present. Bioclast liinestone foriiis 10% of the sample showing shell fragments altered to a calcite spar surrounded by a niicritic matrix. Very little porosity is seen in any of the rock types.

They show a dark biretingence possible originally gypsum.

24. DEPTH 7800. There are 5 different rock types in this thin section. The rock types are bound by a brown drilfiiig mud. Sublitharenites fori11 20% of the thin section and show quartz arenites of sincle monocrystalliiie si-ains k b i t h rare polycrystalline grains and cherts showins microcrystalline quartz with no visible relict texture. Some alteration to ferroan calcite is seen. Qriartz arenite sandstone forms 5% of the sample and shows fine to medium grains witti sutured contacts. The sandstone type is metamorphic grade. Sandy siltstone for-itis 10% o f the s a d e and is brown i n colour showing lamination and soiiie alternation to ferroan calcite. Altered radiating crystals are found. Drcanjc siltstone forriis j?h of the saii iple and is d a - k browIiiblack in coIour . Slight lamination i s evident. Chert forms 6Ooh of the sariiple showins amorphous microcrystalline quartz with no visible relict testure. The grains are stirrounded by a carbonateíclay matrix. Mica Is present. Very l i tr!e porosity is seen in any of d i e rock types.

They show a dark birefiiigence possible oi-iginaIly gypsum

PETROGRAPHTC 1) FISC K I PTIOXS FOR SUA REZ- 1 (Coii t i 11 lied) n

25. DEPTH 7830 Si There are 6 diferent rock types in this thin section. These rocks are bound by a brown drilliiig mud. Sublitliarmites forin 20% of the thin section and show quartz arenites of single nionocrystallins grains with rare polycrystalline p i n s and cherts showing microcvstalline quam with no visible relict testure. Some alteration to ferroan calcite is seen. Rare glauconitic yains are present. Quartz arenite fomis 5% of the sample and shows fine to niediuiii grains with sutured contacts. This sandstone type is rnetaiiioi-pliic grade. Sandy siltstone forms 10% of the sampIe and is brown i n cotour showing lamiiiatioii a i d sotile alteration 10 fertoan calcite. Altered radiating grains possibly oi-iginaliy gypsum are present. They show chert almost block birefringence. Orcanic siltstone forms S O X I of the sample and is dark brownhlack in coIour . Slight lamination is evident Chert fbrms 50% of the sample showing amorphous microcrystalline quartz with no visible relict texture. The grains are surrounded by a carbonate/cIay ma ti’ix. Mica is present. Bioclastic limestone forms 5% of the sample and shows altered shell fragments to calcite in a rnicritic matrix. Micrite envelopes surround t h e fragments. Limestones comprised of peloids showing some internal structure fornis 5% ofrhe sample. Very little porosity is seen in any of the above rock types.

h

26. DEPTH 7860 Si There are 6 different rock types in this thin section. These rocks are bound by a brown drilling niud. Subtitharenites form 30% of the thin section and show quartz arenites of single monocrystalline grains with rare polycrystalline Srajns and cherts showing niicrocrystalline quartz with 110 visible relict texture. Some alteration to ferroan calcite is seen. Quartz arenite Forms 5% of the sample and shows fine to medium grains with sutured comers. This sandstone type is metamorphic grade, Sandy siltstone fornis 10% of thc sample and is bi-own in colour showing lamination and sortie alteration to ferroriii calcire. Al tered radiating p i n s possibly originally gypsum are found. They slioiv chert aliiiost black birefringence Organic silstone forms 5% of the sample and is dark browdblack iii colour . Slight lamination is evident. Chert forms 50% of the sample showing amorphous microcrystalline quartz with no visible relict texture. The grains are surrounded by a carbonate/clay matrix. Mica is present. Bioclastic limestone foriris 5% of the sample and shows altered shelf fragments to calcite in a micritic marrix. hqicrite envelopes surround the fragments. Limestones comprised of peloids showins some internal structure forms 5% of the sample. Very little porosity is seen in any of the above rock types. A highly pleochroic red mineral with high relief is present in rare amounts.

.- PETROGUPH rC DESCRTYTIONS FOR SIJ.4 REZ- I ICotitÍiiiieti)

27. DEPTH 8010 Si There are 6 different rock types in this thin section. These rocks are bound by a brown drillins niud. Sublitharenites form 30% of the thin section and show quartz arenites of s i n ~ l e nionocrystdIine grains with rare polyc~-ystalline grains and cherts showing microcrystalline quartz with no visible relict testure. Some alteration to ferroan calcite is ceeii. Rarc chlorite is foiind. Quartz arenite form 5% of the sample and shows fine to riiediuni p i n s w i t h sutured contacts. This sandstone type is metamorphic grade. Sandy siltstone f o r m 10% of the sampIe and i s b r o w in colour showing lamination and soille alteration to ferroan calcite. Altered radiating grains possibly originally gypmini are found. They show alniost black birefringence. Orcanic siltstone forms 5% of the sample and is dark browdblack in colour . Slight lamination is evident. Chert foriiis 50% of the sample slioi.vi ng amorphous microcr)lstalline quartz with no visible relict texutiiit The grains are surrounded by a carbonate/clay matrix. blica is present. Bioclastic liinesto~ie foriiis 5?4 of the sanipie and shows altered shell fragments to calcite in a iiiicritic matrix. Micrite envelopes surround the fragments. Limestones comprised of peloids sliowiiig so~iie interrial structure forms 5% of the sample. .4 highly pleochroic red mineiA with high relief is present in rare amounts.

Very little porosity is seeti in a n y of the above rock types.

. T;tlilc 2

I'ETI~OGRAPI1lC DATA: VISUAL ESTIMAI'ES UF CUTTING LITIIOLOGY (WELIA: YAVI-I)

SUBLil'HAREN L'iE ("/.I

I o

10 I

QUARTZ AREN 1TE

('%I

-

2 0

-

2 0

1 o

2 0

IO

5

5

10

S 1 LTY MU D STONE

(%)

ORGANIC SILTY

M UD STON E

LIMESTONE ("/I REMARKS

-m t

3

PETROCRAAPHIC DESCRIPTIOYS FUR YAVI-I h

1. DEPTH 9050 Yi. There is one rock tr'pe in this thin section. Sandy siltstone forms 200% of the sample and consists of aheration of silt grade quartz and clay-organic matter. The clay material is likely to be illitdsr-riectite. The siltstones are light brown in colour. Similar siltstones are found. They art: slightly ddrker brown in colour and consist of graded silt passing up into clay material. These siltstones show alteration and replacement to ferroan calcite and doloiiiite. V e r y little porosity is seen in this rock types.

2. DEPTH 9100 Yi There are 2 different rock types in this thin section. Qiiartz arenite foms 20% of the sample and shows fin?, moderately well sorted subrounded grains showing some overgrowths and replacement of grains by clay. In the medium grained sandstone some intergranular primary porosity is seen. Sandy siltstones forms 80% of the sample and consist of alternarions of silt grade qiiai-tz and clay-organic matter. The clay material is likely to be illite/ciiiectite, rhe siltstoiies are light brown in colour. Similar siltstones are found with a slightly darker bi-own colour, consisting of graded silt passing up into clay nialerial. These siltstones diaw sortie alteraiion to ferroan calcite and doiomite. Rare mica flakes are found. V c i y little porosity is seen in either of the rock types.

3 . DEPTH 9150 Yi There are 2 diRerttiit rock types found in this thin section. Sandy siltstones form 70% of the sample and c~ns is t iiiostly of alterations of silt g a d e quartz and d a y organic inatter. The clay rnareiGl i s likely to be illiteísniectite. The siltstones are light brown in colour. Simiiar siltstones are found slightly darker brown in colour. They are silt grade but show no laniinrition. Alteration and replacement t o ferroan caIcite and doIomite. They are dark brown/black in colour, some show lainination. No quartz is found. Some alternation to ferroan calcite is seen. Veiy little porosity is seen in either of these rock types.

A

Organic siltstones form 3OYO of the sample.

4. DEPTH 9200 Yi There are 4 different rock types in this thin section Quartz arenite forms 70% of this thin section and shows fine moderately ~vell sorted subrounded Qrains showing rare over-growths. Medium grained qtiartz arenitts are also present. Sandy siltstones form 65% of the thin section the sample and consists OF alternations of silt grade quartz aiid clay-organic matter. The clay inaterial is likely to be iIlitdsniectite, the siitstones are light brown in colour. Siniilar siltstones are found wi th a slightly darker brown colour, consistiiig of graded silt passing Lip into cIay niaterial These siltstones show some alteration to ferroan calcite and dolomire Rare mica flakes are found Organic siltstones farm 10% of the t tiin sectirin. They are dark brownhlack in colour, some show lamination No quartz is found. Rare grainstone:liIiiestune forms 5% of the thin section is seen in a brown carbonate miid slirnving pellets altered [O calcite. Very little porosity is seen in any of the rock types. .12

PETROG RA PI I I C DESCRIPTIONS FOR YAVI- 1 [Con tiiiiied)

5 . DEPTH 9250 Y¡ There are 3 differen[ rock types i n this thin section. Siltstones form 60% of the thin section but have been conipletely replaced by a fen-oan caIcite cement which is itself undergoing replacement by dolomite. Dolornitised bivalves fragments are found within the sihstones. Rare siltstones with bioclasts showing their original composition are found. They are light brown in colour, of silt grade material and laminated. Quartz arenites form 10% of the thin section. They are fine to medium grained, subrounded and moderately will sorted. Some primas. intergranular porosity is found. Rare alteration of grains has taken place. possibly by iIIite/smectite. Limestone forms 30% of the thin section. The shell frl-iynients have been completely obliterated by ferroan calcite with some undersoitig fLrrthei- replacen-ient by dolomite. Micrite is common, some pieces showins no alteration and no fossil content, other pieces showing replacement by ferroan calcite and dolomite. Grainstones are evident, showing replacement by calcite and further replacement by dolomite. Ooid/peloids are evident, showing no original internal siructure. They have been replaced by ferroan calcite and are undergoing dolornitisation from the inside out. .4 yellow/green cement is around the ooid/peloids and infillin:.. Very little porosity is seen in either the siltstones or the limestones.

6. DEPTH 9300 Y ¡ There are 3 different rock types is this then section. Sublitharenites form 10% of the thin section and show qiiar~z arenites of single monocrystalline grains with rare polyciystalline grains and cheIi sliowins rnicrocrystalline quartz with no visable reiict texture Some alteration to feri-oan calite is seen. Sandy siltstone forms 50% of ihe thin section and show alteration to ferroan calcite and dolomite. In some cases the siltstones are have been completely rcplaced by quartz which is undergoing dolornitisation. The siltstones that show their original composition are light brown in colour of silt grade material and laminated are found within the siltstones. A cluster of giauconite pellers that have undergone coniposition. Clay is pore firling and forms an outer rim around the pellets. This clay is possibly iliite/smectite. Bioclastic limestone forms 3.0% of the thin section and shows shell fragitientc altered to ferroan calcite and undergoing dolomitisation in a rnicrire and calcite matris. Part rnicrite envelopes are found around some of the fiagnients. Gr-ainstone/lIinestone forms 10% of the thin section and shows alteration and replaced by calcite and dolomite. A hematite coating is seen around some of the examples. Little porosity is seen in any of the rock types.

A

7. DEPTH 9350 Y¡ There are 1 different rock types in this thin section. Sandy siltstone forms 70% of the sample and consists of alternations of silt grade quartz and clay- orsanic matter. The day material is IIkely to be illitdsmectite. Fossils are found within the siitstones. A gastropod is present where the wicinal aragonite walls hac inverted to calcite with the loss of internal detail, yet the original i n t e r n a n d external outiines have becn preserved. Other shell fragments show a sirnilar invertion t o calcite and dolomite. BiocIastic limestone forms 25% of the sriiiiple and shows she11 fi-a;ments altered and replaced by ferroan calcite and dolomite, In m n i e cases micrite cnvelopes are preserved. Grainstonellimestone forms 5% of the saniplc and shows pellets i n carbonate mud undergoin2 dolomitisation. Very little porosity is seen in either ofthe rock types.

f

PETROGRAPHIC DESCRIPTIONS FOR Y A W - i (Coritiriiied)

8. DEPTH 9700 Yi There are 4 different rock types in this thin section. The rock types are bound by a brown drilling niud. Sublitharenites form 10% of the thin section and show quartz arenites of single monocrystalline grains with rare polycrystalline grains and chert showing microcrystalline quartz with no vicable relict texture. Some alternation to ferroan calcite is seen. Quartz arenite sandstone forms 20% o€ the thin section and show fine to medium rnonocryctalIine grains with sutured contacts. Sandy siltstones forms 20% of the thin section sample and are brown in d o u r and show lamination consisting of alternates of silt grade q u m z in clay organic rnatter. the clay is possibly illitekmectite. Chert fornis 50% of the í l i in section and shows niicrocrystalline and niacrocrystalline quartz with no visibie relict testtire. The grains are surrounded by a carbonate/clay matrix. Some alteration to illite/srnectite is seen. Very littie porosity is seen in any of the samples.

1

9. DEPTH 9750 Yi There are 4 different rock types in this thin section. The rock types are bound by a brown drilling mud. Sublitliarenites forin IO% of the thin section and show quartz arenites of single monocqstalline grains with rare polycrystalline grains and chert showing microcrystalline q u a m \ k i t h no visable relict texture. Some alternation to ferroan calcite is seen. Quartz arenite sandstone forms 20% of the thin section and shows fine to medium monocrystalline grains with sutures contacts. Sandy siltstones foi-rns 20% of the th in section sample aiid are brown in colour and show lamination consisting of alternaíes of silt y a d e q~iartz in clay organic matter, the clay is possibly illitdsmectite. Chert forms 50% of the thin section and shows microcrystalline and macrocrystalline quartz with no visible relict texture. The grains are surrounded by a carbonatdclay matrix Some aheration to illite/smectite is seen. Very Iittle porosity is seen in any of the samplcs.

10. DEPTH 9800 Y¡ There are 4 different rock types in t h i s t h i n section. The rock types are bound by a brown drilling mud. Sublitlinrenites forin 5% of the thin section and show quartz arenites of single monocrysralline Srains with rare polycrystalline grains and chert showing microcrystalline q u a m with no visable re1 ict texture. Quartz arenite sandstone forms 5% of the thin section and shows tine to niediurn monocrystalline grains with sutured contacts. Sandy siltstones fornis 30% of the thin section sample and are brown in colour and show laiuinarion consisting of ahernares of silt grade quartz in clay organic matter, the clay is possibly illite/sniectire. Chert forms 60% of the thin section and shows microcrystalline and macrocrystalline quartz with no visible relict texture. The grains are surrounded by a carbonateiclay matrix. Some alteration to illite/smectite is seen. Mica is present. Very little porosity is seen in any of the samples.

11. DEPTH 9850 Y¡ There are 4 ditterent rock types-in this thin section. The rock types are bound by a brown drilling i i i ~ i d ~ Sublitharenites foi-iii l & of' the thin section and show quartz arenites of single monocrystalline =rains with rare polycrystalline grains and chert showing mic.rocrystalline quartz with no visable relict texture. Quartz arenite sandcrone forms 5% of the thin section and shows fine to medium monocrystalline grains with sutured contacts. Sandy siltstones forms 30% of the thin section and are brown in colour and show lamination consisting of alternatcs of silt grade quartz i n clay orpnic matter, the clay is possibly illite/sniectite. Chert forins GO?4 of the th in section and shows microcrystalline and riiacrocrystrill¡iie quartz with no visible relict texture. The grains are surrounded by a carbonare/clay niat riu. Sonie alteration to illite/srnectite is seen. Mica is present. Very lirtle porosity is seen i n acy oftlie samples.

PETROGR.4 I’H IC t) E SC R I PTI O K S FOR l’.4i.’i- I (Coii t i 11 lied)

12. DEPTH 9900 Yi There are 4 different rock typcs in this thin section. The rock types are bound by a brown drilling mud. S~iblitharenites form i0Yó of the thin section and show quartz arenites of single monocrysdline grains with rare polycrystalline grains and chert showing microcrystalline quartz with no visable relict texture Some alternation to ferrúan calcite is seen. Quartz arenite sandstone forms 10% of the thin section and shows fine to medium monocrystalline grains wiih sutured contacts. This sandstone type is metamorphic grade Rare mediuiii grade polycrystaili ne grains are found. Sandy siltstones form 60% of the thin section. They are light brown in colour and little lamination. However they are silt grade and show clay, possibly illitdsmectite. Chert forms 20% of the thin section and shows microcrystalline and macrocrystalline quartz with no visible relict textiire. The grains are surrounded by a carbonate clay matrix, possibly illitelsmectite. Mica i s present. Chalcedonic quartz is common. Very little porosity is seen in any of the rock types.

13. DEPTH 9950 Yi There are I different rock types in this thin section. These rock types are bound by a brown drilling mud. Siiblitiiarenites form 30% of the thin section and show quartz arenites of single iriorioiI-)israll¡ne graii is \bi t t i rare polycrystalline grains and chert showins microciyctalliiie quartz with 110 visable reIict tcsture. Sortie alternation to ferroan calcite is seen. Kare mica tlakes are fuiind Sandy siltstone forms 45% of the thin section. They are brown and show lamination consistins of alternates of silt grade quartz and clay organic matter. They clay is probabaly ilIitdsmectite. Orcanic siltstone forms 10% of the thin section and is dark brown/bIack in colorir. Slight lamination is evident. Bioclastic limestone she11 fi-rtgiiients form 15% of the t h i n section. A transverse section of a stick like bryozoan colony shows the zooscia within and a red grain coating, possibly hematite. This is surrounded by a yellow cement, perhaps micrite. Other shell fragments are convened to calcire. Very little porosity i s seen in any of these rock types.

14. DEPTH 10000 Yi There are 3 diffirent rock types in this th in section. These rock types are bound by a brown drilling mud. Sublithai-enites for-rti 155’0 of the thin section and show quartz arenites of si rigle rnorioci.ystalline grains wilh rare polycrystaliine grains and chert showitig microciystalline q u a m wiili no visable relict texture. Some alternation to ferroan calcite is seen. Rare niica flakes ate fourid.Saridy siltstone forins 35% of the ihin section. They are brown in colour arid show laiiiiriation consisting of aIternates of silt grade quartz and clay organic niattei-. They clay i s probabaly iIlite/simectite. Organic siltstone forms 15% of the thin section and i s dark browidblack i n colour. Slight lamination i s evidenf Bioclastic limestone f o r m WO of the th in section and shows shell fragments replaced by calcite Very little porosity i s seen iri any a f t l i e rock types -

h

PETROGRAPHIC DESCRJPTTOSS FOR \ ' A V I - I (Coritiri~ied)

IS. DEPTH 10050 k'i There are 1 diftt.rent rock types in this thin section. These rock types are bound by a brown drilling iiiiid. Siiblitliarenites form 15% of the thin section and show quartz arenites of single monocqmll ine grains with rare polycrystalline grains and chert showiny microcrysralline quai-rz with no visable relict texture. Some alternation to ferroan calcite is seen. Rare niica flakes are found. Sandy siltstones form 30% ofthe thin section. They are brown and show lamination consisting of alternates of silt grade qriartz and clay organic matter. They clay is probabaly illite/srriectite. Organic siltstone forms 25% of tlie th in section and i s dark browm'black in colour. Slight lamination is evident. Bioclastic Iiniestone 5% of the thin section and shows sheiI fragments replaced by calcite. Very little porosity is seen in any of the rock types.

16, DEPTH 10100 Y ¡ There are 4 different rock types in this thin section. These rock types are bound by a brown drilling iiiud. Siiblitharenites form 30% of the thin section and show qiiartz arenites of single monocrystalline grains with rare polycrystalline grains and chert showing microcrystalline qiiartz with no visable relict texture, Some alternation to ferrlian calcite is seen. Rare mica flakes are found. Sandy siltstone forms 40% of the thin section. They are brown and show lamination consisting of alternates of siIt grade quartz and clay organic niattsr-. They clay is probabaly illitdsmectite. Complete replacement by ferroan calcite has taken place i n many of the silstones. Organic siltstone forms 10Y0 of tlie thin section and i s dark browdblack in colour. Slight Iarnination is evident. Chei-t foi-ins 20% of the thin section and shows microcrystalline and macrocrystalline qiiartz i v i t l i no visible relict texture. The grains are surrounded by a carbonate clay matrix, possibly iItitdsniectire. Mica is present. Chalcedonic quartz is common. Very little porosity is seen in any of these rock types.

nfl:l.l.

T i I l I k 3

PETKOCRAPIlIC DATA: VISUAL ESTIMATES OF CUTTING LITHOLOGY (WELL: APICALA-1)

Qi JARI'X A I t l ~ N i ' r l ~

( X )

3(1

I o

20

2 0

I o

PETROGKAPH IC DESCRIPTIONS FOR X P r c A L . w

DEPTH 13325 Ai There are 5 different rock types in this thin section. These rock types are bound by a brown drilling iiiu3. Sublitharenites form 10% of the thin section and show quartz arenites of single rnoiiocryctalline grains with rare polycrystalline grains and cherts showing microcrystalline quartz wi th no visible relict testure. Some alteration to ferroan calcite is seen. Q u a m arenite forms 30% of the thin section and shows fine to medium grains with sutured contacts. Overgrowths are present. Sandy siltstones form 25% of the thin section. They are light brown in colour and show an alternation and parallelism of silt grade q u a m and clay organic matter. They clay is probably illitdsmectite. Fossi t s are present within the siltstones. A brachiopod fragment mid brown in colour is seen with its center layer preserved. The shape of the fragment suggests that it was pari of a ribbed shell. A brachiopod shell replaced by ferroan calcite shows the foliated nature of its inner shell. A small fragment of a grainstoneAimestone is seen undergoing alteration 10 ferroan calcite. Organic siltstones for 35% of the thin section. They are bIack in colour and show slight lamination some show shell fragments. Chert forms 5% of the thin section and shows uniform microcrystalline quartz with no visible relict texture. Very little porosity i s seen in any of these rock types.

2. DEPTH 13380 Ai Thert: are 5 different rock types i n this Thin section. These rock types are bound by a brown drilling mud. Sublitharenites form 5% of the thin section and show quartz arenites of single monocrystalline grains with rare polycrystalline grains and cherts showing microcrystalline quartz with no visible re1 ict testure. Some alteration to ferroan calcite is seen. Qttartz chert forms 5% of the thin section and shows uniform microcrystalline quartz with no visible testure. Sortie of the grains are surrounded by a fine rim of day, possibly itlite/sniectite. Sandy siltstone form 80% of the thin section. They are light brown in d o u r and 5how alternation and parallelism of silt grade quartz and clay organic matter. The clay is probably illite/smectite. Fossils are present within the siltstones and shell fragnients are C O M ~ I I O ~ . ,4 transverse section of an echinoderm spine is seen amongst other bioclastic fragments. A dolomite is present some of the sihstanes are darker brown in colour and consist of graded silt passing up into clay material. Organic siltstones form 594 of the thin section. They are dark browriblack in cotour and show slight lamination. Micrite forms 5% of the thin section. The cement is seen in fragments with tio fossil coritent. Very little porosity is seen in any of these rock types.

3. DEPTH 13400 Ai There are 3 different rock types in this thin section. Sublitharenites form 5% of the th in section and show qtiartz arenites of single inonocrystalline grains with rare polycrystalline grains and chei-ts showing microcvstalline quartz with no visible relict texture. Some alteration to ferroan calcite is seen. Quartz chert forms 5% of the thin section and shows uniform inicrocrystalline quartz with no visible texture. Some of the grains are surrounded by a fine rim 2 day, possibly illite/smectite. Sandy siltstones fortti 90% of the thin section. They are light h n in colour and show alrernations and parallelisni of silt grade quartz and clayiorganic matter. The clay is probably illite/smecrite. Fossil fragments are present within the siltstones and show replacement by ferroan calcite cement. Salmon pink and colourless crystals are abundant showing a radiating structure and replacement. They here possibly originally gypsum crystals that have been replaced by phosphate. 'LVhole siltstone fragments have been completely replaced by ferroan calcitddolonii te cement. Rare ooidsipeloids are seen. They are brown in colour ivi th a iron rich grain coating. Phosphate fragments are found within the th in section. Very little porosity is seen in any of the rock types.

L

PETROGRXPHiC DESCRIPTIONS FOR APIC4.4LA-I (Coritiriiied)

4. DEPTH 13100 A¡ There are 3 different rock types in this th in section. They are bound by a brown drilling mud. Sublithareiiites form 5% of the thin section and show quartz arenites of single monocrysralline grains with rare poIycrystaIIine grains and cherts showing microcrystalline quartz with no visible relict texture. Some alteration to ferroan calcite is seen. Quartz chert f o r m 5% of the thin section and shows uniform microcrystalline quartz with no visible texture. Soinc of the grains are surrounded by a fine rim of clay, possibly illite/smectite Sandy siltstones form 90% of the thin section. They are light brown in colour and shoii. alternations and parallelism of silt grade quartz and clay/organic matter. Fossil fragments are present within the siltstones and show replacemenr: by ferroan calcite cement. Salmon pink and colourless cqsrals ai-e abundant showing a radiating structure and repiacemene. They were possibly urigirial ly gypsu~ii crystals that tiave been replaced by phosphate. Whole siltstone fragments have been completely 1-eplaced by feri-om calcite/doIomite cement. Rare ooids/peloidc are seen. They are b rown in coiour with an iron rich grain coating Pliospliate fragments are present within the th in section. Very little porosity is seen in any of the rock types

The clay is probably illite/sniectite.

5 . DEPTH 13440 (wet) Ai There are 2 dift'erent rock types in this thin section. They are bound by a brown driIling niud. Sublithareiiites form 5% of the thin section and show quartz arenites of single nionocqstalline grains w i r h rare polycrystalline grains and cherts showing microcrystalline q u a m with no visible relict Texture. Some alteration to ferroan calcite is seen. Quartz chert forms 5% of the thin section and shows uniform microcrystalline quartz with no visible testure. Some of the grains are surrounded by a fine rim of clay, possibly illitelsmectite. Sandy siltstones form 90% of the thin section. They are tight brown in colour- and show alternations and pai-alleIism of silt grade quartz and clay!orcanic matter. Fossil fragments are present within the siltstones and SIIOLV replacement by ferroan calcite cement. Salmon pink and colouriess crystals are abundant showing a radiatins structure and replacement. They were possibly originally gypsum crystals that have been replaced by phosphate. Whole siltstone fragments tiave been completely replaced by ferroan caIcite/dolomite cement. Rare ooiddpeloids are seen. They are brown in colour with an iron rich grain coating. Phosphate fragments are present within the thin section. Very little porosity is 5een in any of the rock types.

The clay is probably illitdsiiiectite.

6. DEPTH 13JGO Ai There are 3 different rock types in this thin section. Sublitharenites for 10% of the Ihin secrion and show quartz arenites of single monocrystalline grains with rare poiycysralline grains and cl ie i ts showing ~ i i i c r o c ~ s t a l l i n e quartz with no visible relict texture Sandy siltstones form 70% of the thin section. They are Iiylit in colour and show a&ation and parallelism of silt grade quartz and clay/organic ntattei'. The clay is probably illiteísmectite. Fossil fragments ai-e present within the siltstones and show replacetnent by fenoan calcite cement. Colourless r-adiatinz crystals are coninion showing a radiating structure and rephcenient possibly by pliocpliate. Darker coarser' siltstones are present, They contain more quartz and show laiiiination ]<are ferr-oan calcite rhombs are present. Organic siltstones form 30% of the sample I'hey are dark brown/btack in colorir and show lamination Coarser organic siltstones are coimmon and s h o ~ gaded medium subangular monocrystalline quartz g a i n s . Rare fossil fragments are present showing no internal structure and alternatiun to fcrroan calcite cciticnt. Very little porosity is secn in any of

Some alteration to fen-oanralcite is seen

+I.,.-?. , * * l . .,..-._I

PETROGRAPHTC D ESCR I PTIOSS FOR A PTC.4L.A-1 (Co ti ti ti t i ed)

DEPTH 13480 Ai There are 5 ditkrent rock types in this thin section. Sublitharenites form 10% of the thin section and show quartz arenites of singIe monocIytalline grains with rare polycrystalline grains and cherts showins microcrystalline quartz with no visible relict texture. Some alteration to fet-i-oan caicite is seen. Quartz arenite forms 10% of the th in section and shows niedium grains with sutured contacts. Overgrowths are common. Chert tornis 5% of tlie t h i n sectioii and shows uniform microcrystalline quartz with no visible texture. Some of the grains are surrounded by a fine rim of clay, possibly illitekmectite. Sandy siltstone foriiis 70% of the thin section. The fragments are brown in colour, laminations show alternations and pai-allelism of silt grade quartz and clay organic matter, the clay is probably illite/sinectite. Fossil fragments are corninon within the siltstones. A transverse section through a single aragonitic gastropod fragment is seen. All traces of the oi-iginal wall structure has been obliterated during inversion to caIcite but the recognisabk outline has been preserved by internal and external sediment plus cement. Coarser sandy siltstone is common showing siltstone frazrnents with medium subarguiar quartz grains. Some siltstones have been completely replaced by ferroan calcire. Organic siltstones forni 10% of the thin section, they are dark browru'black in coloiir and show s:iglit Iaiiiinatioii. Very l i t t le porosity is seen in any of the rock types.

8. DEPTH 13500 Ai There are 5 difierent rock types in this thin section Sublitharenites form 10% of the thin section and show quartz arenites of single rnonocrystalline grains with rare polycrystalline grains and cliei-ts showins rnicrocrystaliine quartz with no visible relict texture. Quartz arenite foriris 20% of tlie thin section and shows fine to medium grains with sritured conracts. Overgt-owths are pi-ssent. Compaction is so high, this sandstone type coiild be described as nietamorphic grade. Chert forms 10% of the thin section find chows uniform microcrystalline chalcedonic quartz with no visable relict texture. The grains are surrounded by a finc rim of clay, possibly ilIite/smectite. Sandy siltstones form 50% of the thin section. The kagments are brown in colour. Lamination shows alternations and parallelisrii of silt grade quaitz and clay/organic matter. The clay is probably illite/sniectite. A brachiopod spine fragment shows alteration 10 ferroan calcite but stili showing a layered fibrous wall structure. Coarser sandy silt~tones are coinrnon. These siltstones show laminated medium subangular q u a m grains. Some siltstones show alteration to ferroan calcite. Organic siltstones form 1Oo/;i of the thin section. They are dark brownhlack in colour and chow slight IariiiIiarion. Vei-y little porosity is seen in any of these rock types.

Fossil fr'agriients are present within the siltstone.

9. DEPTH 13200 (wet) A i There are 5 differenr rock types in this thin section. Sublitharenites form 100/0 of the rhiii section and show quartz arenites of single monocrystalline grains with rare polycIystalline grains and cherts showing microcrystalline quartz with no visible relict test& Quartz arenite form 30% of the thin section and shows fine [ O medium y - a i m with sutured contacts. Overgrowths are present. Compaction i s so high, this sandstone type could be described as metamorphic grade. Chert foriiis 1 O'?; o f the t h i n section and shows irniforrn microcrystalline chalcedonic quartz with no visibte relict resttii-e The grains are surrounded by a fine rim of clay. possibly illite/sniectite. The fragments are brown in coloiir. Laniinntioii show alterntions and parallclisni of silt grade q u a m and clay/organic matter. The clay i s probably illite/srnectite. Fossil fragnents are present within tlie siltstones. OrSanic siltstones foriiis I O % of the thin section. They are dark browrúblack in colour aad show slight lriiiiiiiation. Very little porosity is seen in any of these rock woes.

Sandy siltstones form 50% of the thin section

PETROGRAPHIC DESCRIPTIOSS FOR APlCAL.4-1 (Coritiniiedl

10. DEPTH 13520 Ai There are 5 different rock type; in this sample. They are bound by sonie drilling mud. Sublitharenites form 10% of the tliiil section and show q u a m arenites of single monocrystalline grains wi t ti rare polycrystalline grains and cherts showing microcrystalline quartz witli no visible relict texture. Some alteration to ferroan calcite is se en^ Quartz arenite forms 10% of the thin section and shows fine to medium grains with sutured contacts. Overgrowths are present. Compaction is so high, this sandsrone type could be described as metamorphic grade. Chert fornis io% of the thin section and shows uniform microcrystalline chalcedonic quartz with no visible relict tesrure. The grains are surrounded by a fine rim of clay, possibly illitdsniectite. Sandy siltstones form 65% of the thin section. Lainination show alterntions and paratlelisrn of silt grade qtartz and clay!organic matter. The clay is probably illite/cmecrite. Fossil fragiiisnts are present within the siltstones. A transverse section through two originally aragonitic gstropods is secn. The gastropods have been replaced by a cherty matrix. A niici.ire envelope partly surrounds the original wall structure and sorile clay, possibiy illiteisriiectite is also seen around the rim of the gastropods. The surrounding inatris is ferroan calcite. Organic siltstone forms 5% of the thin section. They are dark brown,black in colour- and show slight lamination and shell fragments. Very little porosity is seen in any of these I-ock types.

The fragments are brown in cotour.

11. DEPTH 13920 Ai There are 2 different rock types i n this thin section, Chert forms 'iQo/o of the thin section and shows uniform microcrystalline and inacrocrystalline chalcedonic quartz with no visibic relict testrire. The grains are often surrounded by a carbonate matrix and by a fine rim of clay, possibly illitdsniectite. Mica is present. Light brown sandy siltstones form 40% of the thin section. They show slicht or no lamination. Clay is common, possibly illire/smectire.Brown sandy siltstones form 30% of the thin section. Laminations consist of alterations of silt grade quartz and clay/organic matter. The clay is possibly illiteisrnect¡te. SiiialI shell fragments that have undergone replacement by ferroaii calcite are coi~man. Very little porosity is sten in either of the rack types.

m

Td)k 4

PETROGRAPHIC DATA: VISUAL ES'I'IMATES OF CUTTING IJTliOLOGY (WEiAI,: ALPUJARRA-1)

h

PETROGRAPEUC DESCRIPTIONS FOR ALPUJAFtRA - 1

2. DEPTH 3170-3200 There are 4 different rock types in this thin section. Sublitharenites

form 10% of the thin section. They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with no visible relict texture. Quartz arenite forms

10% of the thin section and shows fine to medium equant grains with saíured and straight

contacts. Light brown silty mudstones form 70% of the thin section and show an alternation

and parallelism of silt grade quartz and possible clay material. Very fine grained grey/green

slates are present. Organic silty mudstones form 10% of the thin section. They are very dark

brown in colour and show silt sized material in the form of laminations. Very little porosity

in seen in any of these rock types,

2. DEPTH 32W3300 There are 5 different rock types is this thin section. Sublitharenites

form 10% of the thin section. They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with no visible relict texture, Quartz arenite forms

10% of the fhrn section and shows fine to medium equant grains with sutured and straight

contacts. Dust rims are common. Some alteration to dolomite is present. Light brown silty

mudstones form 65% of the thin section and show an alternation and paralIe1ism of silt grade

quartz and possible clay material. Organic silty mudstones form 10% of the thin section.

They are very dark brawn in colour and show silt sized material in the form of laminations.

A single piece of orangejbrown organic material is seen, possible loss circulation material.

Bioclastic limestone forms 5% of the thin section and shows shell fragments replaced by

ferroan calcite and dolomite. Whole pieces of rnicrite containing no shell fragments are

found. Very little porosity is seen in any of these rock types.

-

3. DEPTH 3500-3600 There are 2 different rock type3 in this thin section. Sublitharenites

form 60% of the thin section. 'hey show monocrystalljne sandstone fragments and cherts

which exhibit microcrystalhe quartz with no visible relict texture. Small amounts of a

yellow cement are present in crossed polarisation. Possibly illiteimica. Silty mudstones form

40% of the thin section. Light brown and darker brown mudstones show an alternation and

parallelism of silt grade quartz and clay material. Very fine grained greyigreen slates are

_c

**

present. Alteration to dolomite is common witiUn the silty mudstones. Glauconite pellets are

common. Very little porosity is seen in either of the rock types.

4. DEPTH 3600-3700 There are 2 different rock types y this thin section. Sublithamites

form 60% of the thin section. They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with novisible relict texture. Small amounts of a

yellow< cement are present in crossed polarisation.. Possibly illiteimica. Silty mudstones

form 40% of the thin section. Light brown and darker brown mudstones show an atternation

and parallelism of silt gade quartz and clay material, Very fine grained grey/green slates are

present. Alteration to dolomite is common witkin the silty mudstones. Glauconite peIiets are

common. Very little porosity is seen in either of the rock types.

5 . DEPTH 37W3800 There are 5 different rock types in this thin section. Sublitharenites

from 10% of the thin section. They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with no visible reliect texture. Quartz arenite forms

10% of the thin section and shows fine to medium equant grains with sutured and straight

contacts. Dust rims are seen, Light brown sandy siltstones form 5% of the thin section and

show an alternation and parallelism of silt grade quartz and possible clay rnateriai. Organic

silty mudstones form 5% of the thin section. They are very dark brown in d o u r and show

silt sized material in the form of laminations. Monocrystalline quartz forms 70% of this thin

section and shows single grains of fine to coarse monocrystalline quartz. There is a distinct

angularity of the quam grains. Very little porosity is seen in any of the rack types.

h

6. DEPTH 3800.3900 There are 5 different rock types in this thin section. Sublitharenites

form 10% of t h e thin section. They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with no visibie relict texture. Quartz arenite forms

10% of the thin section and shows fine to medium ecpmt grains with sutured and straight

contacts. Duct rims are seen. Light brown sandy siltstones from% of the thin section and

show an alternation and parallelism of silt grade quartz and possible clay material. Organic

silty mudstones form 5% of the thin section. They are very dark brown in colow and show

silt sized material in the form of laminations. Monocrystalline quartz forms 70% of this thin

section and shows single grains of fine to coarse monocrystalline quartz. There i s a distinct

angularity of the quartz grains. Very little porosity is seen in any of the rock types.

m

1

section and chows single grains of fme to coarse monocrystalline quartz. There is a distinct

angulariv of the quartz grains. Very little porosiv is seen in any of the rock t y p s . rr-

7. DEPTH 7300-7400 There are 5 different rock types in this thin section. Sublitharenites

form 5% of the thin section. They show monocrystabe sandstone fragments and cherts

which exhibit microcrystalline quartz with no visible relict t eme . Quartz arenite forms 5%

of the thin section and shows fine to medium equant grains with sutured and straight

contacts. Silty mudstones form 80% of the thin section and show an alternation and parallelism of silt grade quartz and possible clay material. Very fine grained gre-green

slates are present. Organic silty mudstone form 5% of the thin section. They are very dark

brown in COIOW and show silt sized material in the form of laminations. Very little porosity

is seen in any of the rock types. This thin section appears to come from below the Caballos

Formati on.

8. DEPTH 74W7500 There are 5 different rock types in this thin secbon. Sublitharenites

form 5% of the thin section. They show monocrystalline sandstone frrigments and cherts

which exhibit microcrystalline quartz with no visible relict texture. Quartz arenite foms 5%

of the thin section and shows fine to medium equant grains with sutured and straight

contacts. Silty mudstones form 80% of the thin section and show an alternation and

parellelism of silt grade quartz and possible clay material. Very fine grained greyígreen

slates are present. Organic silty mudstone form 5% of the thin section. They are very dark

brown in colour and show silt sized material in the form of laminations. Very little porosity

is seen in any of the rock types. This thin section appears tu come from below the Caballos

Formation.

c

T;ible S

PETROGRAPHIC DATA: VISUAL ESTIMATES OF CUTTING LITHOLOGY (WELL: VILLA RRI C A- l)

3 0

70

IUiMAllKS

. .-m

.-

PETROGWsIC DESCRIPTIONS FOR VILLARRICA-I

1 . DEPTH 2440-2500 There are 4 different rock types in this thin section. Sublitharenites

form 30% of the thin section. They show monocrystalline sandstone ftagments and cherts

which exhibit microcrystalline quartz with no visible rellict texture. Quam arenite forms 5%

of the thin section and shows fine to medium equant grains with sutured and straight

contacts. Brown silty mudstones form 60% of the thin section and show an alternation and

parallelism of silt grade quartz and possibie clay material. Bivalve fragments are present

within the mudstones. Organic silty

mudstone forms 5% of the thin section. They are dark brown in colour and show silt sized

material in the form of lamination. Rare foraminifera are seen. Very little porosi@ is in any

ofthe rock types.

Very fine grained greyígreen slates are present.

2. DEPTH 2500-2600 There are 3 different rock types in this thin section. Subfitharenites

form 70% of the thin section. They show rnonocrystalIine sandstone fiagments and cherts

which exhibit microcrystalline quartz with no visible relict texture. Glauconite pellets are

common. Brown silty mudstone form 10% of the thin section and show an alternation and

parellelism of silt grade quartz and possible clay material. Chert forms 20% of t h e thin

section and shows uniform microcrystalline quartz with no visible relict texture. Very little

porosity is seen in any of these rock types.

cl

3. DEPTH 26W2700 There are 5 diflerent rock types in this thin section. Sublitharenites

form 35% of the thin section They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with no visible relict texture. Quartz arenite forms 5%

of the thin section and shows fine to medium equant grains with sutured and straight

contacts. Brown silty mudstones form 15% of the thiniection and show an alternation and

parallelism of silt grade quartz and possible clay material. VeFf ine grained greyfgreen

slatestone are present. Organic silty mudstone forms 5% of the thin section. They are dark

brown in colour and show silt sized material in the form of lamination. Chert forms 40% of

the thin section and shows uniform microcrystalline quartz with no visible relict texture.

Very little porosity is seen in any of these rock types.

p1

1 b

I’ETROCI1APIiIC DATA: VISUAL ESTIMATES OF CUTTING LII‘HOLOCY (WELL: ESTKATIG RAFl C 0 - 1 1

PETROGRAPEHC DESCRIPTIONS FOR ESTRATIGRAFKX&l

1 . DEPTH 7150-7200 There are 4 different rock types in t h i s thin section. Sublitharenites

form 70% of this thin section. They show monocrystalline sandstone fragments and cherts

which exhibit microcrystalline quartz with no visible relict texture. Quartz arenite forms 5%

of the thin section and shows fine to medium quartz grains with sutwed and straights

contracts. Brown silty mudstones form 20% of this thin section and show an alternative and

parallelism of silt grade quartz and clay material. Very dark brown, more organic mudstone

form 5% of the thin section and also show silt sized material in the form of laminations.

Very little porosity is seen.

2. DEPTFí 7200-7300 There are 3 different rock types in this thin section. Sublitharenites

form 10% of the thin section. They show monocrystaIline sandstone fragments and cherts

which exhibit microcrystalhe quartz with no visible relict texture. Quartz arenite foms

10% of the thin section and shows fine to medium equant grains with sutured and straight

contracts. monocrystalline quartz forms 80% of the thin section and shows single grain of

fine to coarse quartz grains. There is a distinct angularity of the grains. Very little porosity is

seen in any of the rock types,

-

3. DEPTH 7300-7380 There are 3 different lithologies in this thin section. Sublitharenites

form 5% of this thin section. They show monocrystalline sandstone fragments and chens

which exhibit microcrystalline quartz with no visible relict texture. Quartz arenites form 5%

of the thin section and show fine to medium equant grains with sutured and straight contracts.

90% Of this section is made up of an orangeíbrown organic maaterid. It is possible this is

IOW circulation material. Very M e porosity is seen. 7

h

4. DEPTH 2700-2800 There are 4 different rock types in this thin section. Sublitharenites

form 75% of this thin section. They show monucrystalline sandstone fragments and cherts

whcih exhibit micrcrystalline quartz with no visible relict texture. Silty mudstone form 14%

of this thin section. They are brown in colour and show an alternation and parallelism of silt

grade quartz and possibIe clay material. Glauconite pellets are common. Organic silty

mudstone form 5% of this thin section. They are dark brown in colour and show silt sized

material in the f u m of arninations. Chert forms 5% of this t h section and shows uniform

microcrystalline quartz with novisible relict textwe. Very little porosity is seen in any of

these rock types.

5 . DEPTH 2800-2900 There are 3 different rock types in this section. Silty mudstone

forms 10% of the thin section and shows an alternation and paralIelisrn of silt grade quartz

and possible clay material. Chert forms 5% of the thin section and shows uniform

microcrystaíline quartz with no visible relict texture. Monocrystalline quartz forms 85% of

this thin section and shows single gains of the fine to coarse quartz. There is a distinct

angularity of the grains, mica is abundant in t h i s thin section, and there are come very large

pieces indicating possible loss circulation material. Very little porosity is seen in any of the

rock s p e s .

b

COREX

A

PHOTOMICROGRAPHS FROM

c WELLS: SUAREZ-I YAVI-1 APICAL A- 1 ALPUJARRA-1 VILLARRICA-1 ESTRATIGRAFICO- 1

L

.- PLATE Sa

WELL: SUAREZ-1

DEPTH: 6690

PLANE POLARTSED LIGHT

.4 generai view of siIty mudstones bound by a brown quartz rich driIting mud. The brown

mudstone (EI, C14) shows an abundance of dispersed silt-sized quartz material. Sinuous

cracks are evident. The piece is a light browdorange colour in crossed palars indicating the

presence of clays. Beneath is a darker brown mudstone (F2, 112) also containing siIt grains of

quartz

PLATE 8b

WELL: SUAREZ-I

DEPTH: 6690

PLANE POLARTSED LIGHT

A general view showing alteration of the silty mudstones to ferroan calcite. illteration of a

shell fragment to ferroan calcite is seen (CiO, D12). A cutting of a sublitharenite (€31, B6)

shows a fine to Inediutn grained poorly sorted sandstone showingrn9nocrystalline quartz and

microcrystalline chert (pale brown in plane polarised iighr) Porosity of the sublitharenite

cutting has been largely occluded by quartz overgrow&- and compactional effects. -

h

PLATE 9a

WELL: SUAREZ- 1

DEPTH: 7020

PLAKE POLARTSED LTGHT

A genera1 view showing silty rnudstones (Es, DF, G4, F8) and sandstone cuttins (H10, CS)

bound by a brown drilling mild. Note how alteration and replacement of pore throats and

grains by a ferroan calcite cement in the sandstone cuttings has occluded any visible porosity.

h

PLATE 9b

WELL: SUAREZ-1

DEPTH: 7020

PLANE POLARTSED LIGHT - A general view showing siity mudstones and a sublitharenite cutt&(HIS). A possible shell

fragment (17) shows complete replacement by a ferroan calcite cement.

h

A

I

PLATE 10

WELL: SUAREZ-1

DEPTH: 7080

PLANE PULARISED LIGHT

A general view showing silty mudstones, some showing sinuous cracks and alleration by a

ferroan caicite cement, A quartz arenite

cutting ( C 2 ) shows fine equant well sorted grains. Rare amounts of organic material are found

within the quartz arenites. Porosity is occtiided by quadz over~rowíhs.

Sublitharenite cuttings are seen (€310, G4, 1.5).

4

PLATE I 1

WELL: SUAREZ-I

DEPTH: 7110

PLANE POLARTSED LIGHT

A general view of silty mudstones and a sandstone cutting bound by a brown quartz rich

dritling mud. Note ihe bes. dark brownhlack organic cuttings (Dio, H6), They have an

almost coal like appearance. A sandstone cutting, very fine grained, moderately well sorted

(G12) shows rare amounts of organic material and common glauconite pellets. No visible

porosity is seen within the sandstone cutting. White radiating crystal shapes (C6) are seen.

They are perhaps a product of thc drilling mud.

h

PLATE 12a

WELL: SUAREZ-1

DEPTH: 7140

PLANE POLARISED LIGHT

A general view of silty mudstones and sublitharenites bound by a brown drilling mud. The

sublitharenite cutting sliows fine to medium moderately sorted monocrystalIine grains and

chert. Note the glauconite pellet (F13). Porosity is occluded by quartz overgrowths and

possibly clay material.

PLATE 1%

WELL: SUAREZ-t

DEPTH: 71-10

- CROSSED POLARISED LIGHT

A crossed polarised view of plate 12a. Note the calour of the siltpmudstone cutting (B 10) in

crossed polarised indicating a clay (?smectite) content. Evidence of a clay material is seen

around the grains of this sublitliarenite cutting.

PLATE 13

WELL: SUAKEZ-I

DEPTH: 7170

PLANE POLARTSED L K E T

A general view ofsublitharenites, dark brown organic silty mudstone, and quart arenite (C 11)

cuttings, bound by a brown quart rich drilling mud, Note the characteristic 'dust rims' in the

quartz arenite cutting. Porosity is occluded by quartz overgrowths.

c

PLATE 14a

WELL: SUAREZ-1

DEPTH: 7200

PLANE POLARTSED LIGHT

A general view of silty mudstones and' sublitharenite cutting. They are bound by a quartz rich

brown drilling mud. Note the alteration of the subfitharenite cutting (C8) to ferroan calcite.

Very little porosity is seen.

PLATE I. 4 b

WELL: SUAREZ-I

DEPTH: 7200

CROSSED POLARTSED LIGHT

The same view as above except in crossed polarised light. -

-

PLATE 15

WELL: SUAREZ-I

DEPTH: 7240

PLANE POLARISED LIGHT

A general view showing silty mudstones, sublitharenites and shell fragments which show

replacement by a ferroan calcite cement. V e r y M e visible porosity is seen.

.-

a

PLATE 16

WELL: SUA4REZ- I

DEPTH: 7350

PLANE POLARBED LTGWT

A general view showing silty mudstones and very fine grained, moderately sorted

sublitharenites bound by a brown drilling mud. The radiating crystal shapes 0 3 ) are perhaps

a product of the drilling mud. Very little porosity is seen.

h

4

PLATE 17;i

WELL: SUAREZ-1

DEPTH: 7380

PLANE POLAIUSED LIGHT

A view of very fine to medium grained moderately sorted sublitharenites showing single

monocrystalline grains and microcrystalline chen (pale brown in plane polarised light). Some

alteration to ferroan calcite is seen. Quartz overgrowths and possible compactjonai effects

have reduced the visible porosity to zero.

PLATE 17b

WELL: SUAREZ-I

DEPTH: 7380

CROSSED POLARTSED EIGHT

A crossed polarised view of the above to show the possible t r a c e d recrystallised illitehica

representing original detrital clay. CI

PLATE 17c

WELL: SWAREZ-1

DEPTH: 7380

PLAKE POLARISED LIGHT

An alternative view of the same thin section as 17a 17b showing the abundance of silty

mudstones. Note the sinuous cracks (B9).

A

.

h

PLATE 18a

WELL SUAREZ-1

DEPTH: 7410

PLANE POLARISED LTGHT

A general view showing very fine to medium grained, poorIy sorted sublitharenites with

subsequent %rains of monocrystalline q u a m and varying amounts of chert (pale brown in plane

polarised light). Note the organic material present within the sublitharenites. Very little

porosity is seen,

PLATE 18b

WELL: SUAmZ-1

DEPTH: 7410

CROSSED POLARISED LTGHT

The same view as above except in crossed polarised light -

h

PLATE 19

WELL: SUAREZ-I

DEPTH: 7440

PLANE POLARISED LIGHT

A general view of cublitharenites and organic material bound by a quartz rich drilling mud.

Porosity is occluded in the sublitharenites by quartz overgrow2hs and possible compactional

effects. Note the alteration to a ferroan calcite cement surrounding the quartz arenite (E3).

PLATE 20

WELL: SUAREL1

DEPTH: 7470

PLANE POLARISED LIGHT

A general view of subtitharenites and organic siltstones bound by a quartz rich drilling mud.

Note the foraminifera (H6) and the shell fragments (G2, G3). Very little porosity is seen.

h

PLATE 21

WELL: SUAREZ-1

DEPTH: 7500

PLANE POLARISED LTGHT

A general view of very fine to medium grained subfitharenites, silty mudstones showing

lamination and sinuous cracks, and organic mudstones. These cuttings are bound by a quartz

rich brown drilling mud. Very little porosity is seen.

4

m

PLATE 22a

\%'ELL: SUXREZ-I

DEPTH: 7530

PLANE POLARTSED LIGHT

A view of a very fine to medium grained, poorly sorted sublitharenite with subsequent

subangular monocrystalline quartz grains and common chert (light brown in plane polarised

light). Quartz overgrowths and possible compactional effects have occluded all porosity

+-

PLATE 225

W'ELL: SUAMZ-1

DEPTH: 7530

PLANE POLARTSED LIGHT

An alternative view of the same thin section showing an organic silty c- mudstone cutting (H3)

exhibitins sinuous cracks

--..

PLATE 23a

WELL: SUAREZ-1

DEPTH: 7560

PLANE POLAFUSED LIGHT

A view of fine to medium grained, pooriy sorted sublitharenites with subsequent subangular

monocrystalline quartz grains and microcrystalline chert showing no visible relict texture.

Quartz overgrowths and possible compactional effects have occluded all porosity. These

sublitharenites are bound by a quartz rich drilling mud.

h

PLATE 23 b

WELL: SUhREZ-I

DEPTH: 7560

CROSSED POLARISED LIGHT

The same view as above except in crossed polarised light to sho-e microcrystalline quartz

(chert).

a

PLATE 24a

WELL: SUAREZ-1

DEPTH: 7590

PLANE POLARISED LIGHT

A general view of sublitharenite cuttings, and c h m cuttings (light brown in plane polarised

fight). Both cuttings, types are bound by a brown, q u a m rich drilling mud. Very little visible

porosity is associated with either currings type.

PLATE 24b

WELL: SUAREZ- 1

DEPTH: 7590

CROSSED POLARTSED LIGHT

The same view as above except in crossed polarised light. Notethe chert cuttin;s showing

microcrystalline quartz (Hi 7) with no visible relict [enure.

-

.

PLATE 2%

WELL: SUAFKEZ-1

DEPTH: 7620

CROSSED POLARJSED LIGHT

A crossed poíarised view of sublitharenites bound by a quartz rich drilling mud. Note the

angularity of the medium quartz grains and the grain coating clay (E4, E5), perhaps traces of

recrystallised illitelmica. Quartz overgrowths and compactional effects have occluded any

porosity connected with these cuttings types.

PLATE 25b

WELL: S1I:AREZ-1

DEPTH: 7620

- PLANE POLARXSED LIGHT

The same view as above except in plane polarised light showingthe chert as light brown in

d o u r and the sutured contacts (G4, H4) between quartz grains. P

PLATE 26a

WELL: SUAKEZ-1

DEPTH: 7650

PLANE POLARiSED LIGHT

A general view of sublitharenites and a chen cutting (Gi3) bound by a brown quartz rich

drillins mud. Very little porosity is seen in either rock type.

n

PLATE 26b

WELL: SUAREZ-1

DEPTH: 7650

- CROSSED POLARTSED LIGHT

The same view as above but in crossed polarised light. Note the micro/rnacrocrystalline quartz

showing no visible relict texture in the chert cutting. This piece shows pin point extinction on

rotation of the stage.

PLATE 27a

WELL: SUAREZ- 1

DEPTH: 7680

PLANE POLARTSED LTGHT

A general view showing sublitharenite and chert cuttings bound by a brown drilling mud.

Very little porosity is associated with either of rock type. Note the organic material (I381 (112,

J1 I).

h

PLATE 27b

WELL: SUAREZ-I

DEPTH: 7680

- CROSSED POLARTSED LTGHT

The same view as above but in crossed polariced light. Note thmnicrocrystalline texture of

the chert cutting.

PLATE 28a

WELL: SUAREZ-1

DEPTH: 7710

PLANE POLARTSED LIGHT

A general view showing silty mudstones @3), organic material (541, sublitharenites (Bi3) and

very fine grained sandstonesl(rnacrocrysralline chert) (E1 2, 19). Note the replacement of

quartz grains to calcite? dolomite? This cutting was probably originally a sublitharenite (I31 O).

You can see (A141 a similar alteration starting in this sublitharenite cutting. Note the

glauconite pellet (Al) . All of these cuttings are bound by a quanz rich drilling mud. Very

little porosity is associated with any of these rock types.

PLATE 28b

WELL: SUAREZ-I

D'EPTFI: 7710

CROSSED POLAFUSED LIGHT

The same view as above but i n crossed polarised light.

PLATE 29a

WELL: SUAREZ-1

DEPTH: 7740

PLANE POLARISED LIGHT

A general view showing subiitharenites bound by a quartz rich brown drilling mud. Note the

quartz overgrowths (F11) a possible contributor to loss of porosity. The chert fragments

@i2) within the sublitharenites are light brown in plane poiarised light. Very little porosity is

seen,

.-

PLATE 29b

WELL: SUAREZ-I

DEPTH: 7740

PLANE POLARISED LIGHT

An alternative view of the 5arne thin section showing silty mudstoss bound by a quartz rich

drilling mud. Note the dispersal of silt sized quartz and the sinuous cracks, indicaling the

desiccation of a wrater sensitive clay component.

-

h

PL-4TE 30a

WELL: SUAREZ-I

DEPTH: 7770

PLANE POLARTSED LIGHT

A view of a very fine to medium, poorly sorted sublitharenite. Note the anguianty of the

medium quartz grains. Chert is sliown light brown in plane polarised light. Note the organic

material (E2, E12) and the alteration to ferroan calcite (H4). Very little porosity js evident.

PLATE 30b

CVELL: SUAREZ-1

DEPTH: 7770

- CROSSED POLARISED LTGHT

1c

The same view as above. Note the different pieces of chedsandstone finely microcrystalline

h

{B), macrocrystalline {El l ) , and the very fine sandstone ( C l > cuttings

around the grains are evident possibly representing original detritial clay

Traces of illite/mica

PLATE 31

WELL: SUAREZ-1

DEPTH: 7800

PLANE POLARISED LIGHT

A general view showing chert (FB), quartz arenite (11 1) and sublitharenite cuttings (Dl 1, 19,

BS> bound in a quartz rich brown drilling mud. Organic material is present within some of the

sublitharenites and quartz arenites. Note the 'dust rims' in the quartz arenite. Porosity has

been occluded by quartz overgowths and compactional effects.

4

PLATE 32a

WELL: SUAREZ-1.

DEPTH: 7830

PLANE POLARlSED LIGHT

A general view of sublitharenites (CZ, G8, B14) chert (H13) and silty mudstones (B7). .a1 these rock types are bound by a brown drilling mud, Very little porosity is evident in any of

the rock types.

PLATE 32b

WELL: SUAREZ-1

DEPTH: 7830

- CROSSED POLAFUSED LIGHT

The same view as above escept in crossed polarised lisht. Note the evidence e.g. (ES) of

recrystalled illite/mica representing original detritial clay. Note the red, possible iron rich

coating at the base of the sublitharenite cutting (IG).

A

PLATE 32c

WELL: SUAREZ-1

DEPTH: 7830

PLANE POLARTSED LIGHT

An example within the same thin section of nicrocrystalline chert showing no visible relict

texture. Note the fossil fragments an alinost black colour i r i piane yolarised light. Both the

rock types Are bound by a quartz rich drilling mud. Little porosity is evident in either rock

types.

PLATE 326

WELL: SUAREZ-1

DEPTH: 7830

CROSSED POLARTSED LIGHT

The same view as above except in crossed polarised light showing the chert cutting exhibiting

pin point extinction.