Advances in Orllaai¢ Geechemistry 1967 Org. Geochem. Vol. 13, Nos I-3, pp. 411--421, 1988 Printed in Great Britain. All rights reserved

0146-6380/88 $3.00 + 0.00 Copyright © 1988 Pergamon Pren pic

Source rocks of Western Newfoundland

FAYE J. WEAVER and STEPHEN A. MACKO* Department of Earth Sciences, Memorial University, St John's, Newfoundland, Canada AIB 3X5

Abstract--Studies on the organic geochemistry of natural oil seeps, although not always associated with major economic deposits, offer opportunity to compare chemical characteristics of an oil to a sedimentary depositional environment. One such seep occurs in the geologically well-documented Parsons Pond region of Western Newfoundland. A number of potential sources for the oil, which has been observed for 170 years, have been proposed including shales from the Shallow Bay and Green Point Formations of the Cow Head Group, the Table Head Group and similar sediments from the Curling Group, all which represent deposition along a continental margin in the Cambro-Ordovician.

Total organic carbon in the shales ranges from quite low (0.6%) to moderate (5.2%). Gas chroma- tography-mass spectrometry shows evidence that suggests weathering. Odd predominance prior to C2, is seen in these oils and is characteristic of most Ordovician oils. The oils contain a slight even predominance in the C~+ alkanes. Pristane and phytane were both observed to be present yielding a ratio of 1.6. Isotopic compositions from the Green Point Formation extracts are closest to that of the oils (-28.5~). Similar variations in isotope values from one fraction to another are observed in both the oils and Green Point extracts. Nitrogen isotope studies show a large range in the extracts with the best comparison between the Broom Point (0.5%0) and seep oils (0.6%0). The composition of the organic matter from the Broom Point and Martin Point Members of the Cow Head Group reflect the hydrocarbon compositions of the oil.

The effect of thermal maturation eastward across the Parsons Pond region is observed in increased bitumen contents and heavier carbon isotope compositions.

Key words: Cambro-Ordovician, carbon isotopes, nitrogen isotopes, oil/source correlation, bitumen, pristane/phytane

INTRODUCTION

Natural oil seeps present an excellent opportunity for comparison of chemical characteristics of oils with associated sediments which may have generated that oil. One such seep occurs within the Cambro- Ordovician sediments of Western Newfoundland, the Cow Head Group. The sedimentary sequence has been interpreted geologically for both depositional and tectonic environments, and underlies an area of about 600 square Kilometers forming the coastal lowlands of central west Newfoundland (Fig. 1). It represents an extensive deep water apron deposited along the rise of the early Paleozoic continental margin of North America. It has since been trans- ported and deformed during at least two major subsequent orogenies (James and Stevens, 1986). Natural oil seeps in the area have been noted since 1812. The first drilling was in 1867, with the most recent and last well being drilled in 1965. At least 27 known wells existed in the area with a total maximum cumulative production of 6000 barrels.

The geochemistry of petroleum source rocks and their associated oils have become an important tool in exploration and the assessment of basin potential (Grizzle et al., 1979; Tissot and WeRe, 1984; Philp, 1986). Various geochemical methods have been con-

*Author to whom correspondunce should be addressed.

trasted and compared to ascertain their viability as source rock indicators for natural seeps. Parameters used in this study for source--oil correlation include: (1) stable carbon isotope compositions on crude oils, extracts and chemical fractions of the oils and ex- tracts; (2) gas chromatography-mass spectrometry on the saturate fraction comparing distribution pat- terns (isoprenoids, n-alkanes); and O) stable nitrogen isotope compositions on the kerogen and asphaltenes from the oils. This paper is an attempt to combine some of the geochemical procedures commonly used in oil-source correlations to assess the most fikely source of the seeps.

GEOLOGICAL SETTING

Natural seeps occurring along the western margin of the province of Newfoundland in Eastern Canada are found within the geological region known as the Humber Arm Allochthon (Williams, 1975). The re- gion contains Cambro-Ordovician sediments of allo- chthonous (transported) rock composed of conglom- erates and the interbedded black shale of the Cow Head Group which can be subdivided into two major formations. First, a predominantly coarse grained litholngy proximal to the original depositional shore- line, the Shallow Bay Formation; and a second finer grained lithology distal to the shoreline along the continental rise, the Green Point Formation (James

411

412 FAYEJ. WEAVER and S1ZPtlL~A. MAcKO

Cow

Broom Pt.

Tabte PT.

Parsons Pond

St. Paul s Inlet

Western Brook ~ ' ~ P o n d

pt. Green

Pt. .j j s S SS

s s S

s s S S j

I I | ! t I ! ! I ! I t I I I

g

oB

NEWFOUNDLAND

Inset Scal.e :

0 25k in

Island of Newfoundtand Scats: 0 50 km

* Crude oil. sampUng sites (Standpipes}

LID

Fig. 1. Location map of study

and Stevens, 1986; Fig. 2). These sequences have been interpreted as on-lap, base of the slope deposits, on a steep continental rise prism which evolved in re- sponse to unstable shelf conditions as a result of the evolution of the Paleozoic North American margin (James et aL, 1987). Black shales occur most notably in the distal deposits representing anaerobic to dys- aerobic deep water, hemipelagic sedimentation con- ditions. The occurrence of diverse graptolite fauna in the shales has been used as a tool in selection of various stratigraphic intervals and sampling zones. The graptolite rich units represent low oxidation depositional environments" with excellent organic matter preservation (Williams and Stevens, 1987). This study emphasizes work on 40 samples collected from individual members within the Shallow Bay and Green Point Formations. The Cow Head Group is structurally underlain by syndepositional shales, lime- stones and dolomites of the Table Head Formation.

The destruction of the ancient continental margin began toward the end of the Ordovician during the Taconic Orogeny. This period records the greatest

area, Western Newfoundland.

plate activity in the Appalachians at a time when the ancient Iapetus Ocean was closing as the African continent collided with the North American con- tinent (Schenk, 1978). The result was extensive short- ening across the continental margin as it was com- pacted and the crust was emplaced as a complete set of tectonic slices pushed from the southeast to the northwest (Stevens, 1970; Cawood and Williams, 1986; Fig. 3). Deep water sequences were transported westward across shallower sequences and general metamorphism was recorded in eastern-most slices due to the subduction of the shelf under the crust of the ancient Iapetus Ocean (Williams, 1979). A later, more pervasive tectonic shortening, the Acadia Orog- eny in the Devonian, caused the remobilization of the Long Range Mountains on top of the already de- formed allochthon and resulted in westerly inclined steep folds increasing in abundance eastward across the ailochthon. Thermal maturation studies using paleontology indicate that the sediments in the area are within or near the oil window (Nowlan and Barnes, 1987b). Shales from the Green Point For-

Source rocks of Western Newfoundland 413

Mainland LLAND. Sandstone

LLANV.

ARENIG

(3

TREMAD.

0 TREMP.

FRANC.

-. DRES. %

1 Black Cove Format ion 3 Table Point Format ion

4 Factory Cove Member 6 Tucker Cove Member

8 St. Paul's Member 10 Mart in Point Member

Lower Head Sandstone

St. George Group

Q

0

10 ¢l Q '1-

0 0

2 Table Cove Format ion

5 Steering Is land Member 7 Downe's Cove Member

9 Broom Point Member

Fig. 2. Simplified stratigraphic column of the geological groups found in Western Newfoundland (after James and Stevens, 1986).

n s

Cow Head Group B Table Heed Group [ °

I Shallow Bay Formation D St. George Group [ 2 k i n

r ~ Green Point Formation I ~ Mainland Sandstone o =kin

B Lower Head Sandstone

Fig. 3. Stylized cross section showing the structure of the Humber Arm Allochthon (after Cawood and Williams, 1986).

414 F^~ J. WEAVr~ and SrEpt~ A. MACKO

Table I. Chemical composition of Black Shales indicating stratigraphic and geographic locations

Continental % % location Locality Unit" CaCO3 Bitumen TOC

g

o el

- - Parsons i Pond (PP) 2 GP 35.3 0.05 1.3

3 GP 40.7 0.02 I. 1 5 GP 7.8 0.06 2.4 6 GP 11.2 0.17 4.2 7 SB 37.6 0.06 1.0

Cow Head (CH) 8 SI 23.1 0.17 2.2

9 FC 19.6 0.09 1.2 10 FC 25.3 0.34 2.9 11 FC 21.3 0.18 1.4

St Paul's Inlet (SP) 13 SP 94.2 0.05

15 SP 76.3 0.11 1.2 16 SP 20.6 0.26 2.7

Broom Point (BP) 17 BP 17.0 0.22 2.3

18 BP 18.2 0.25 5.2 Western Brook (WB) 19 SP 21.1 0.16

20 SP 8.6 0.11 2.0 Martin Point (MP) 21 SP 67.3 0.26 1.8

22 SP 16.3 0.02 2.5 23 BP 14.9 0.14 1.1

Green Point (GP) 24 MP 8.4 0.21 2.8

25 BP 25.5 0.10 3.5 26 BP 45.2 0.38 1.7

Lobster -~ Cove (LC) 29 GP 44.5 0.05 1.0

30 GP 19.3 0.01 0.6 ?~ 31 SB 19.3 0.01 1.3

"GP: Green Point Fmb; SP: St. Paul's Mbr; BP: SB: Shallow Bay Fmb; SI: Steering Isle Members.

Broom Point Mbr; MP: Martin Point Mbr. Mbr; FC: Factory Cove Mbr. blndistinct

marion, although immature to marginally mature on the basis of Rock Eval data from surface exposed samples, could be prime source beds where buried sufficiently to generate hydrocarbons (Macauley et al., 1987). Fluoresence studies indicate thermal alter- ation indices (TAI) of 2.5-3.0 for the Cow Head Group (Hogan, 1986), this being equivalent to the oil window (Staplin, 1969). Conodant alteration indices (CAD of 1.5 for the Cow Head Group lie within the oil window of CAI values of I-2 (Nowland and Barnes, 1987a, b).

Sample collection was based on two factors: (l) the organic matter content preserved in local strati- graphic units, and (2) the average grain size. The finer grain size increases the potential for organic pre- servation (Demaison and Moore, 1980). The most organic rich units are those that are grey to black resulting from reducing conditions at the time of deposition (Williams and Stevens, 1987). Selection was mostly of black shales with the exception being those shales with secondary limestone nodular features.

M E T H O D S

Shale samples were obtained from a variety of localities to include ages from Upper Cambrian to Middle Ordovician within the Cow Head Group. A

limited number of samples were selected from the original 40. Those samples showing high total or- ganic carbon (TOC) values and increased bitumen contents with the least amount of biodegradation were chosen (Table 1). Collections were, for the most part, outcrop samples. The effects of biodegradation on organic matter can present a problem with analy- sis of outcrop samples (Forsberg and Bjorzy, 1981). Shales were collected from less exposed, more mas- sive units about 0.5 m below surface to minimize weathering. Samples wre powdered using an agate ball and mill to a sieve size of 74/~m. Overgrinding was avoided to prevent pyrolysis and oxidation (Peters, 1986).

All shales were soxhlet extracted for isolation and quantification of bitumens. Thirty gram samples were continually extracted for 48 h in extracted thimbles using two 100 ml volumes of toluene:methanol (3:1). This azeotropic mixture was chosen on the basis of a comparative examination for this study as well as on results of previous work (Rohrback, 1979). Ex- tracts were then filtered through prewashed glass fiber filters (GFC, Whatman), to remove fine clays, and were then rotary evaporated at 25°C.

Bitumens and oils were precipated prior to chro- matography to remove asphaltenes using a pen- tane:bitumen ratio of 25:1 (vol). Concentrated ex- tracts were separated on a mixed bed liquid

Source rocks of Western Newfoundland 415

chromatographic column (40 cm × 6 mm i.d.), packed with silica and alumina gel which had been activated at 180°C for 4 h. Volumes were calculated for a maximum of 30 g of alumina and 40 g of silica per 0.5 g of hydrocarbon (Hirsch et al., 1972). Separ- ations were accomplished at a flow rate of 50 ml per hour. Fractions were eluted in three stages (Durand et al., 1970). Saturates were removed from the col- umn using 11 ml of hexane. Aromatics were eluted with 45 ml of hexane:benzene (2:1), and finally resins were obtained by flushing the columns wit~l 9 ml each of benzene:methanol (1:1) and anhydrous ether.

Stable isotope analyses of organic carbon and nitrogen on the whole oils, extracts, fractions and acidified whole rock powders were completed using a modified Dumas technique (Macko et al., 1984). Isotopic compositions were determined on a Micro- mass 903E (V.G. Ltd) ratio mass spectrometer with a standard error for &lsN of better than +0.2%0 based on replicate samples (Macko et al., 1982). Samples run in duplicate for 13C had a mean average deviation of 0.05%0. The carbon and nitrogen isotope data are presented in 6 notation relative to the Chicago PDB carbonate (6~3C) and to atmos- pheric nitrogen (&~SN). Total organic carbon con- tent is measured in the form of COs on a calibrated manometer.

Gas chromatography was performed using a Hewlett-Packard 5792GC equipped with a 10m 0.2 mm methyl silicon fused silica capillary column (Ultrex 1, Hewlett-Packard), coupled to a Hewlett- Packard 5970A mass selective detector. Programmed conditions for the separation were: initial tem- perature (70°C) for 3 rain, 5°C per rain rate, final temperature of 270°C.

RESULTS AND DISCUSSION

Whole rock powder TOC range from quite low (<1%) to moderate (>5%) with the majority be- tween 2 and 4%. TOC trends are observed with a decrease in age and the location of the sample with relation to the origin depositional environment (Table 1). Those samples that are Tremadocian in age have a larger percentage of extractable material and higher TOC than younger samples of Arenigian age. This could be interpreted as a response to the initiation of the Taconic Orogeny and eventual clos- ing of the ancient Iapetus Ocean with subsequent destruction of the continental margin. Geographi- cally the samples show an increase in hydrocarbon yield from the nearshore to offshore indicating greater preservation of organic matter in deeper continental rise sediments or preference of fine sedi- ments to preserve organic matter (Demaison and Moore, 1980). The exception to the TOC values is the Lobster Cove locality which has undergone higher levels of thermal alteration and has been interpreted as a floundering melange block (James and Stevens, 1986). The Green Point samples could be excellent

source rocks on the basis of extractable organic content relative to TOC being greater than 10% (Oudin, 1970). All samples lie above the minimum organic carbon content of 0.5% by weight proposed as the minimum required for petroleum source rocks (Tissot and Welte, 1984).

Whole rock compositions were used to select the suite of samples for further analysis. Extracted bitu- mens show an increase in non-hydrocarbons (NHC; asphaltenes and resins) relative to hydrocarbons (HC; saturates and aromatics) when compared to the oils. Such an increase in the percent of NHC has been previously observed in a comparative study (Tissot and Pelet, 1971) and has been interpreted to be the result of retention of the heaviest most polar fraction during migration (Pelet et al., 1986). Cow Head extracts contained an average of 37% NHC as com- pared to 8% for the seep oil. The amount of saturate from the source to the oil increases from an average of 49-74% (Table 2).

Gas chromatography-mass spectrometry shows normal alkane distributions which exhibit an odd predominance in the nClf-nC21 chains (Fig. 4). This is characteristic of marine algae with inheritance from algae and bacterial HC (Powell and McKirdy, 1972; Zumberge, 1983; Macauley et al., 1987). The pattern of odd carbon dominance has been observed in Ordovician oils (Palmer, 1984; Reed et al., 1986; Longman and Palmer 1987); however a decrease in n- alkanes after ?/CI9, is also common in Ordovician oils, and is not evident in the Green Point samples (Fowler and Douglas, 1984). The smooth profiles after nCm9 are possibly a result of dilution of the original organic saturate material by n-alkanes ran- domly generated from kerogen (Macauley et al., 1987). Acyclic isoprenoids of oils have been described as being very low in concentration in many but not all Ordovician oils (Fowler and Douglas, 1984; Jack- son et al., 1984; Reed et ai., 1986). Observable concentrations of pristane and phytane (Fig. 4) in these extracts and oils give ratios ranging from 0.2 to 2.2, with the minimum values occurring in the most mature samples which have the greatest bitumen contents. Pristane and phytane values from samples of the Green Point average 1.6, and compare most favorably to the seep oils which average 1.8. All samples show similar normalized alkane distributions (Fig. 5), with prominant maxima at nC,7 and nCl9 in the oils and the extracts. The tendancy for the distributions to have highest concentrations near the low molecular weight carbon chains is distinctive of marine Paleozoic organic material (Maximov et aL, 1973).

Carbon isotope compositions of the kerogen range from -28.2 to -33.8%0, and are not unique with respect to other marine sourced oils (Softer, 1984). Isotope compositions of the oil (-28.5%0) most closely resemble those of the Martin Point and Broom Point Member shales ( -28 .6 ° ) (Table 3). Asphaitene values for the oil samples (-29.3%0)

416 FAYE J. WF.AV~R and STEPHEN A. MACKO

Table 2. Normalized fraction percents for shale extracts from various localities

% % % % Locality" Asphalt Resins Aromatics Saturates

PP 2 16.5 36.7 8.0 41.4 3 20.6 37.6 19.2 26.2 5 17.2 36.3 12.3 36.4 6 19.0 40.8 14.1 29.2 7 22. I 33.0 26.0 26.9

CH 8 5.8 21,7 27.4 45.0 9 4,3 63. I 6.9 25.7

10 9.1 19.5 23.1 48.2 II 8.6 19,3 20.6 51.6

SP 13 14.6 21.1 7.0 54.4 15 19.5 27.7 4.1 48.6 16 8.8 19.4 10.5 61.3

BP 17 13.3 35.7 9.7 41.3 18 7.2 22.5 7.8 62.9

WB 19 13.3 29.7 13.9 43.1 20 10.0 31.0 8.3 50.9

MP 21 8.4 32.6 8.3 51.4 22 29.6 25.5 11.7 33.1 23 12.3 39.7 12.6 36.8

GP 24 5.0 21.4 15.8 58.1 25 17.2 41.1 7.9 33.8 26 2.3 39.9 15.5 42.2

LC 29 11.2 61.4 12.5 15.9 30 10.4 30.8 22.2 37.6 31 38.1 30.1 10.5 21.9

"Refer to Table !.

compare closely to the asphaltenes of the Broom Point shales (-29.5Yee), Carbon isotope composi- tions within all the extracts and fractions show values in the more mature samples with a trend eastward across the allochthon (Fig. 6). This decrease in the amount of '3C may be an effect, as observed by other authors, of maturation in organic matter subjected to geologically rapid heating where either tectonic, met-

amorphic or igneous features have accelerated the maturation process (Simoneit et al., 1981; Clayton and Bostick, 1985). The original deposifional se- quence and tectonic imbricated wedges parallel the observed maturation trends in bitumen content and stable isotopes as well as CAI, TAI and Rock-Eval data previously mentioned. The most organic rich and finer grained samples from the edge of the

m ~

0

WHOLE ROCK EXTRACT Martin Point Member SATURATE FRACTION

P,

m

SEEP OtL i ~. SATURATE FRACTION

b.

! i

Fig. 4. Saturate fraction chromatogram of whole rock extract and Ordovician seep oil from Parsons Pond, Partial chromatograms show Ct7 to phytane separations.

Source rocks of Western Newfoundland 417

I - z r~ ,2 UJ O.

W z < ~ o <

till

_1 < • •

n-ALKANE DISTRI B UTIONS t

t. i. t "t

t. t

' " ' . t / %

~ - , ~ ? , \

),.,, ,,~.... X "

. . . . . . . /

V"

v '.....,......" " "~....~.

INCREASING NUMBER OF CARBONS " , .

Fig. 5. Normalized alkane distr ibution of the oil (heavy line) and extracts from Cow Head Shales.

continental margin in the east have been buried the deepest and transported farthest during tectonic destruction.

Carbon isotope values of asphaltenes and kerogens have been seen to correlate significantly, and can be useful as a parameter of source rock delineation (Macko and Quick, 1986; Fig. 7). Samples from this study that lie nearest to this linear correlation are the

most favourable choice for source rocks. Samples from the Green Point Formation, excluding the St Paul's Member extracts, lie nearest to the source rock correlation line, and the seep oil lies within the Green Point Formation zone. St Paul's Member shales have kerogen values (-30.5%0) which are more negative than all other samples. Shallow Bay Formation shales have asphaltene values which are more nega-

Table 3. Carbon and Nitrogen isotope values for kerogen and extract fraction for shale= from various localities

61~N 61~ 613C 613C 6"C 6"C Locadity" Kerogen Kerogen Saturate Aromatic Rains Asphalt

PP 2 -28.4 -34.2 -29.3 -31.2 -28.8 3 2.9 -27.8 -29.4 -31.3 -29.3 -29.8 5 -2.5 -29.4 -30.8 -31.1 -29.7 -30.3 6 -29.7 -30.8 -29.3 -26.4 -30.1 7 2.3 -29.4 -28.1 -32.8 -31.3 -28.6

CH 8 - I . 3 -28.7 -32.0 -33.9 -30.0 -30.2 9 0.1 -28.0 -32.1 -30.1 -30.1 -30.3

10 -28.2 -29.0 -30.4 -30.0 -30.5 I1 1.7 -30.3 -31.4 -34.1 -34.2 -30.8

SP 13 - i . 3 -33.8 -31.5 -30.6 -29.9 -32.0 15 - I . 1 -32.7 -34.0 -34.2 -33.2 -28.7 16 0,7 -32.8 -31.6 -33,1 -30.5 -32.3

BP 17 - I . 2 -28.2 -28.5 -31.9 -28.4 18 -2 .6 -28.5 -31.7 -30.5 -28.9 -28.6

WB 19 -3,1 -31.0 -30.7 -30.9 -32.1 -31.7 20 -2.1 -30.6 -29.7 -29.3 -30.9 -30.2

MP 21 -7.1 -30.7 -30.5 -31.2 -30.8 -29.9 22 -5.2 -30.0 -30.7 -29.7 -30.2 -30.1 23 -33.0 -30.7 -30.8 -30.9 -30.4

GP 24 - i . i -29.6 -31.3 -33.1 -30.4 -29.3 25 0.5 -28.7 -30.2 -28.5 -30.1 -29.7 26 - 5.2 - 29.4 - 29.9 - 29.9 - 29.4 - 29.5

LC 29 -3.5 -29.2 -28.1 -26.8 -28.9 -28.4 30 -6.3 -28.0 -27.6 -27.5 -26.9 -31.3 31 -6.8 -28.6 -27.3 -27.6 -27.3 -29.2

• Refer to Table I.

418 FAYE J. WEAV~ and S~PHI~N A. M^CKO

- 2 9

Fig. 6. Geological map of the study area indicating kerogen isotopic values.

five (-30.0960) than shales from the Green Point Formation. Isotope distribution curves of the vari- ation from one fraction to another of both oils and extracts relative to their increasing polarity can serve

as a tool for source rock delineation (Stahl, 1978; Galimov and Frik, 1986; Fuex, 1987). When the variations between fractions are similar in both the oil and extract than a correlation can be made, and source kerogen t3C estimated. This type of isotope curve relationship, when applied to the sample suite (Fig. 8), indicates that samples from the Martin Point and Broom Point Members are most similar to the oils. The majority of curves with the exception of the Green Point and Broom Point samples exhibit irregu- lar patterns indicative of extensive alteration or con- tamination (Galimov and Frik, 1986). The isotopic composition of the Broom Point whole rock kerogen (-28.7%0) compares well with the assumed kerogen values (-28.8%0) extrapolated from the isotopic curves of the individual fractions.

The use of nitrogen isotopes for geochemical ex- ploration and source rock evaluation is relatively new (Grizzle et al., 1979; Macko and Quick, 1986). Nitro- gen isotopes for the sample suite have quite a large range (10.0%0) from +2.9 to -7.1%0. Grizzle et al. (1979) observed a range of 10.3%o from +1.5 to + 13.3%0 from a representative suite of the world population of oils. Nitrogen isotopes from older oils of the Silurian-Permian are similar to younger oils from the Mesozoic; however, samples from the Cow Head Group show an average nitrogen isotope com- position lower than that reported by Grizzle et al. (1979). The nitrogen isotope composition of the asphaltene fraction of the extract is within approxi- mately 1.5%0 of the crude, and the isotopes of the crude oil reflect the nitrogen isotopes of the original

UJ

< Z

t/3 m

I n- 0 14-

UJ ~o

Ul eL 0

0 m

0

I

X Seep Oil • Green Point Formet i0n •

• St. Peui'e Member Q Broom Poln; Member

B, Green P o i n t ex¢ iud Jng

[] ::":;?:'7:°":'TI:r" J ' P''"' , , 0 •

B Steering ,s iena Member / • ~ ~ s

• ,,otory c o . Memb,, / e o / ] / ° / /

/ "" 0 • /f" ~

/ ° A /

\ l . , / / s / /

' !

CA. .ON ,SOTO~,E vALUeS F o . KenooeNs ~' Ind io t lnot Members

Fig. 7. Graphical correlation of carbon isotope values for kerogen and asphaltenes. Dashed line indicates significant correlation between kero~n and extracted bitumen (ah~ Maeko and Quick, 1986).

Source rocks of Western Newfoundland 419

Location

Parson's Pond

Cow Head

Friction* Carbon Isotope Values -3.3 -3.2 -3.1 -3.0 -2.9 -2.8 -2.7

I St. Pauls 2

inlet 3 4 5

1 Broom 2 Point 3

4 5

1 Western 2

Brook 3 Pond 4

5

1 2

Martin 3 Point 4

5

1 2

Green 3 Point 4

5

t 2

Lobster 3 Cove 4

5

1 2

Seep 011 3 4 5

% T y p i c a l oH curve (after Stahl, 1978) star ~l extrapolated karoglm value

2 o/~ 3 1 , /

I) Karogen 2) Asphaltene$ 3) NOS 4) Aromatlc 5) Saturate

Fig. 8. Carbon isotope curves show increasing polarity of petroleum fractions.

source organic material (Grizzle et ai., 1979; Macko and Quick, 1986). Nitrogen isotopes for the Parsons Pond seep oils analysed by McRae (1986) show asphaltenes averaging 0.6%0. This compares very favorably with the kerogens from the Broom Point samples which are 0.5%0.

CONCLUSIONS

Source rock determination for the oil seeps in Western Newfoundland is based on the following parameters: (1) hydrocarbons chemistry: total yield, pristane/phytane, saturate distributions and OEP, and (2) isotopes: asphaltene/kerogen relationships, carbon isotopes of kerogen, isotope relationship curves and nitrogen isotope values.

Total hydrocarbon yield averages about 0.2% and is at a maximum in the samples taken from the deepest original depositional environment of the

Broom Point and Martin Point samples of the Green Point Formation. The total hydrocarbon yield is at a minimum at the most eastern edge of the Humber Arm Allochthon adjacent to the Long Range Moun- tains averaging 0.03%. This reflects the thermal alteration recorded in TAI, CAI, Rock-Eval analysis, increases in bitumen content and stable isotope com- positions. Pristine to phytane ratios are most similar between the seep oil (1.8) and Green Point extracts (1.6) and show a trend to a lower value of 0.2 eastward towards the Long Range Mountains. Satur- ate distributions exhibit an odd predominance of nCls-nC21 paraffins and a slight even predominance after nC2s.

Carbon isotope analysis of asphaltenes and ker- ogen show a similar relationship between the oils and Broom Point and Martin Point shales. Estimated kerogen source values for the oil of -28.8960 reflect normal marine oils and most closely resemble those

420 F ^ ~ J. WV.AV~ and STEPtmN A. MACKO

Fig. 9. Isotopic trends observed in all fractions and kerogens. Arrow indicates increasing m3C content towards

the Long Range Mountains.

of the Broom Point kerogen (-28.7%Yee), asphaltene values of both average -29.4%o. Isotopes from all fractions, kerogens and extracts show a trend to a more enriched 613C values nearer the Long Range Mountains (Fig. 9). This increase in t3C is likely a result of thermal maturation and loss of the lighter nC, and overprints the original isotopic values mak- ing correlations based on single isotope measure- ments difficult. Isotopic curves alleviate this problem and present a strong correlation from fraction to fraction observed in both the Broom Point Member and seep oil. Nitrogen isotopes of the oil (0.6Yoe) compare favorably to the Green Point Formation (0.5~.).

The sequence that is most likely the source due to a comparison of all the data is the Broom Point Member shale. The effect of thermal alteration on the entire allochthon is observed in increased bitumen contents and isotopic trends.

AcknowledgementsmAcknowledgement is made to the do- nors of the Petroleum Research Fund, administered by the American Chemical Society (grant 14805-AC2) and NSERC (grant A2644) for partial support. F.J.W. acknowledges Memorial University for assistance in the form of a gradu- ate __r~__rch fellowship. Thanks also to Dr. S. H. Williams for his guidance in the field collection of samples.

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