An investigation of the relationship between the geology of indonesian sedimentary basins and heat...

Tectonophysics, 121 (1985) 45-62

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

AN INVESTIGATION OF THE RELATIONSHIP BETWEEN THE GEOLOGY OF INDONESIAN SEDIMENTARY BASINS AND HEAT FLOW DENSITY

MOCHAMAD THAMRIN

Department of Geological Evaluation and Development Pertamrna, Jakarta (Indonesia)

(Received February 6. 1985; revised April 23. 1985)

ABSTRACT

Thamrin, M., 1985. An investigation of the relationship between the geology of Indonesian sedimentary

basins and heat flow density. In: A.E. Beck (Editor), Terrestrial Heat Flow and Thermal Regimes.

Tectonophysics, 121: 45-62. ’

Geothermal data. collected from 929 wells drilled by oil companies in 20 Tertiary basins of Indonesia,

have been related to the geology and tectonics of the area. It is found that the thermal conductivity

increases with the depth of burial and degree of compaction in both transgressive and regressive

sediments but decreases with increases in impurities and heterogeneous grain sizes. The temperature

gradient is controlled mainly by the depth and temperature of a heat source beneath a basin. Very high

heat-flow densities ( > 125 mW mA2) found in central Sumatra, South Sumatra, Salawati Basin and

Bintuni Basin may be caused by relatively shallow magmatic diapirism.

INTRODUCTION

As part of a study of the role of temperature and depth of burial of hydrocarbon

source rocks in the process of oil generation, Pertamina instituted a program of

determining temperature gradients and thermal properties in numerous holes drilled

in oil fields of the sedimentary basins of Indonesia. These data have been related to

the local and regional geology and it is the purpose of this paper to report our

observations and conclusions.

In section I the regional and local geology is discussed, in section II we describe

the methods used for determining thermal conductivity and temperature gradients,

in section III we derive terrestrial heat flow density values, and draw conclusions

relating heat flow densities to the geology of the area of interest in section IV.

I. REGIONAL GEOLOGY

Indonesia is an area where three major lithospheric plates, the Eurasian, Pacific,

and Indian-Australian, as well as many smaller plates, interact in a complex

0040-1951/85,‘$03.30 0 1985 Elsevier Science Publishers B.V.

manner. The presently active subduction zones and strike-slip faults have been

identified by Carvalho et al. (1980), Eubank and Maki (1981) and Katili (1975); in

the regional geological setting eastern Indonesia is more complex than western

Indonesia.

Sedimentary basins

The Indonesian sedimentary basins of Tertiary age (Fig. 3) have been classified

on the basis of their tectonic position relative to the postulated Cainozoic plate

boundaries (Robertson Research, 1983).

Circum Sunda basins, that is the basins related to the Sunda Arc and the major

subduction zone marked by the present-day Java Trench, are subdivided as follows:

back-arc basins covering North. Central, and South Sumatra, Sunda, Northwest and

Northeast Java and North Lombok; fore-arc basins, between the outer non-volcanic

arc and the inner volcanic arc, consisting of Sibolga, Bengkulu and South Java; East

and West Natuna basins related to secondary spreading centres in the South China

Sea and subduction zones around its perimeter; Barito, Asem-Asem. Kutai, Tara-

kan, Lariang, and South Makasar basins which are related to rifting of Sulawesi

from Kalimantan along a spreading centre in the Makasar straits.

Zrian Jqva basins are Permo-Carboniferous to Quaternary in age and related to

the complex interactions of the Pacific, the Philippine Sea, the Caroline Plates and

the Banda Arc subduction zone.

The Tertiary basins of west Indonesia were formed on the flanks of the Sunda

Craton and were filled for the most part by sediment derived from its erosion

starting mostly in the Eocene and Oligocene. Carbonate rocks were locally formed as

reef growth. reefal beds and fossiliferous limestone. Pyroclastic sediments were also

poured into the basins and formed thin tuff and volcanic sandstone intercallated and

interbedded with other kinds of sediments. Most sediments were deposited intermit-

tently in transgressive-regressive series. This process in some parts of west In-

donesia, but especially in Sumatra and Java, was caused by local subsidence where

block faulting was active intermittently. Because of this, different thicknesses of

sediments and magmatic intrusions occur in different areas (Fig. 1).

From the pre-Tertiary there is mostly metamorphic basement consisting of

metasediment, orthometamorphic and igneous rocks. Tertiary intrusion of granite

and basalto-andesite of Oligocene, Middle Miocene. Plio-Pleistocene and recent ages

sometimes act as the economic basement; this activity occurred mainly in the

back-arc basin as the complement to the volcanic arc in Sumatra and Java.

The basins studied in Irian Jaya-Salawati and Bintuni overlie Paleozoic metamor-

phic basement, and contain unmetamorphosed Permo-Carboniferous and Mesozoic

deposits in which platform and reefal limestone are more prominent compared to

the other basins studied in western Indonesia.

The nomenclature for a given stratigraphic sequence sometimes differs between

companies: to standardize the names of stratigraphic sequences of the basins in this

3N3303W

3N3303

3N330E

. . . . . . . . . . . . . . *:. . . . . . . . . .:. OEI . . . . . . . . .:. . . . . . . . -:. . . . . . *:.

study, the epochs of the Tertiary will be used, at least when discussing thermal

conductivity.

Geological structures of most interest in the study area are found in western

Indonesia. especially Sumatra and Java (Fig. 2).

Good outcrops are found at Kedungbiru, Central Java; Wai Tebu, Lampung; and

Wai Insu Muara Dua in the Garba Mountains of South Sumatra (Thamrin. 1964,

1973. 1978). These indicate steep extension faults, dipping 50”-80” with a N-S

trend. The absence of sediments in most of these areas indicates that they emerged

in, or before. the early Tertiary. In the Eocene and Oligocene some Pre-Tertiary

faults appear to have been reactivated to form horst and graben blocks: some of the

latter were filled with sediments and accompanied by the intrusion and extrusion of

basalto-andesite (old andesite) and rhyolitic material which formed as dikes and sills

(Eubank and Maki, 1981; Thamrin, 1964, 1973, 1978). In the Middle Miocene there

was more tectonic activity and some parts of the so-called back-arc basin were

disturbed and the character of the sediments changed.

Orogenic activity in the Plio-Pleistocene caused most parts of western Indonesia

to be regionally uplifted, and much folding and faulting occurred, causing the

general structure of the sediments to trend NW-SE, i.e. in the present Sumatra

trend. This activity caused rejuvenation of some previous faults which appear to

have had two different styles of movement. In some parts younger faults, such as the

Sumatra strike-slip (the Semangko fault), were developed and in other parts sec-

ondary N-S faults were developed. The active part underneath the back-arc basins

might have stimulated the magmatic activity raising magmas up to shallow positions.

Il. DETERMINATION OF THERMAL CONDUCTIVITY AND TEMPERATURE GRADIENT

Thertnul conductiuities

A total of 2465 conventional core specimens were collected from 929 wells drilled

in several oil and gas fields in 20 Tertiary basins. Most of the core specimens can be

classified petrologically into shale, sandstone, limestone. igneous and metamorphic

rocks. In Table 1 these rock types have been grouped into stratigraphic units by their

relative age. The thermal conductivity was measured on a commercially available

transient heat flow apparatus. The results are summarised in Table 2, with the

results grouped according to rock type and age. Arithmetic mean values are given

together with the standard deviation and the number of specimens in the statistical

sample. The measured values of thermal conductivity for a given rock type of given

age showed considerable variations. For example, 42 specimens of Pleistocene shale

in western Indonesian basins gave a value of 1.58 + 0.14 W m-’ Km’. with the

lowest value being 1.09 and the highest 1.93.

It can be seen from Table 2 that there appears to be a systematic, but not smooth,

increase in the thermal conductivity of the elastic rocks with age; a relationship

tivity

.. ‘)

TABLE 2

Thermal conductivity of rock types in Western and Eastern Indonesia vs. age

Relative age

(cpoch~)

Pleistocene

Plmcene

Miocene

MIddIe

tocene

Thermal conductivity (mW m ’ Km’) Notes **

shale sandstone limestone volcanic rocks

(1) (2) (3) (1) (2) (3) (1) (2) (3) (1) (2) (3)

7.58 0.14 42 2.28 0.13 X W

1.63 0.14 120 2.35 0.13 51 2.52 0.34 10 W

1.5x 0.22 14 2.29 0.27 14 E

1.71 0.16 1X7 2.60 0.17 31 - _

1.68 0.18 20 2.37 0.32 22 2.13 0.28 31

1.85 0.25 104 2.61 0.30 220 2.51 0.28 93

1.70 0.15 17 2.40 0.10 I2 2.42 0.28 74

1.X6 0.11 127 2.96 0.33 370 2.68 0.43 130 _ _

1.96 0.26 113 2.98 0.39 240 2.68 0.45 35 2.71 0.50 3 - _

2.03 0.20 8 3.15 0.63 13 2.52 0.43 14 1.75

2.71 0.50 6 2.37

0.28 23 W

0.41 56 W

1.02 I9 w

Pre-Tertiary basement rocks (1) (2) (3)

phyllite

quartzite

graywacke

dolomite Imst.

granite and

granodlorite

quartzite 4.33 0.25 3 E

phyllitic shale 2.62 0.89 13 E

limestone 2.40 0.27 6 E

2.76 0.04 4 W

4.84 0.74 20 w

3.77 0.18 20 w

3.70 0.14 14 w

4.71 0.82 36 W

* Columns: (1) mean value; (2) standard deviation; (3) number of specimens in sample

** W-west: E-east.

between age and thermal properties has been noted in other basins (Judge and Beck,

1973). In this area it is believed that the increase with age and depth is an effect of

greater compaction with increasing depth of burial. The fact that some increases

occur suddenly is interpreted as being due to sedimentary structures such as

cross-bedding and interfingering formed principally in transgressive-regressive series.

The limestone appears to have a fairly constant conductivity regardless of age.

Ninety-eight specimens of volcanic material from a single stratigraphic unit (Jati

Barang volcanics) and consisting of tuff, volcanic sand and volcanic breccia, gave

mean values of 1.75 & 0.28, 2.37 _t 0.41, and 3.64 + 1.02 W rn-’ K-i respectively.

The pre-Tertiary rocks cannot be grouped by epoch; the values given in Table 2

are for the specimens grouped without regard to age.

Temperature gradients

Because of the absence of any other data, the temperature gradient for each drill

hole was calculated from an ambient mean annual surface temperature of 26.7”C as

used by the Indonesian Petroleum Association (Aadland and Phoa, 1981) and the

bottom-hole temperature obtained after correction according to a Schlumberger

Nomograph, which is equivalent to the method used by Dowdle and Cobb (1975)

and Ferth and Whichman (1977).

III. CALCULATION OF HEAT FLOW DENSITIES

Since the temperature gradient is essentially being averaged over a considerable

depth of borehole, the heat flow density value must be calculated using a weighted

harmonic mean conductivity (A,,,,,, ) for the whole stratigraphic column of N

formations, where each of the formations with a mean conductivity X,,, as given in

Table 3, is weighted according to its thickness (d,) in the column. This is equivalent

to the calculation of the series type conductivity (Beck and Beck, 1965; Beck, 1976)

given by eqn. (1).

The values of heat flow density are classified according to the system of Anderson

et al. (1978), namely, the heat flow density is low if H < 40 mW me2, normal if

40 < H-c 85 mW mp2, high if 85 < H < 125 mW me2 and very high if H > 125 mW

m 2. If the thermal conductivity of the stratigraphic column through which the well

bore passes is similar for all heat flow density sites, then a similar classification can

be used for temperature gradients.

The results for heat flow density values are given in Table 4 and Fig. 4.

From Table 4 it can be seen that most of the basins have normal to high heat flow

densities with three of the back arc basins having high to very high values and two of

the fore arc basins having very low values.

Figures 3 and 4 demonstrate that in spite of the rather crude method of

estimating temperature gradients from a single mean annual surface temperature

and a single temperature at considerable depth, when several holes are available in a

given region the heat flow density values obtained are reasonably reliable. Because

of the much larger statistical errors associated with single sites, no attempt is made

to discuss differences in heat flow densities between individual wells.

tivity

I 3 I 80

N = 113

0 160 240 320mWm’~

N. SUMATRA C. SUMATRA S. SUMATRA

N = 16 30

SIBOLGA 8 BENGKULU

c , 240 320 mWme2

i 58 50

30 1 2:

80 1w 240 320 mwm-’ 30 160 240 320mwm~

SUNDA 8 NW. JAWA

N=56 30

0 30 Id0 240 3ZOmWm~f

l- 160 240 32OmWm*~

N=69 I N-47

N=26 30

0 a0 160 P40 320 mwm.’

80 160 240 320mWm’t

E 8 NE. JAWA SEA

*0 160 t40 3ZOmWm~’

BARITO, KUTAI 8 TARAKAN E 8 W NATUNA SEA SALAWATI 8 BINTUNI

Fig. 4. Heat flow density histograms for several of the basins studied.

tivity

4 5 94

0 8 15

9 6 1 14

10 8 9

IV. DISCUSSION

In this section we try to relate the statistical picture given by Fig. 4 to lithology,

geology and tectonics.

First we note from Table 2 that the thermal conductivity of some rock types

depends on age. and therefore depth. We attribute the changes to compaction; in

particular we note that shale increases by 28% and sandstone by 38%. Grouping rock

types by their ages or stratigraphic units may therefore be very important in detailed

petroleum engineering studies. From Table 4 and Fig. 5 we note that heat flow

densities appear to decrease regularly from high (to very high) values in the back-arc

basins to low in the fore-arc basins. The highest values of heat flow densities and

temperature gradients are found in the central Sumatra and the south Sumatra

basins. Since the north Sumatra basin was developed in the same tectonic framework

as central and south Sumatra, presumably its significantly different heat flow density

value is related to its submergence later than the central and southern parts (Fig. 2).

The central and south Sumatra basins suffered high intensity tectonic movement

which formed deep steep faults: there were at least three tectonic episodes in the

Oligocene, Middle Miocene and Plio-Pleistocene. All of them produced folding and

faulting, and were accompanied by magmatic intrusion and extrusion, which could

indicate magmatic diapirism. The shallow depths of magmatic diapirs, such as those

in central Sumatra (Carvalho et al., 1980; Eubank and Mnki, 1981) would obviously

have a significant effect on the heat flow density of the basin. Carvalho et al. (1980)

indicate how to estimate the depth of magmatic diapirism in central Sumatra.

In the south Sumatra basin, diapirism occurred westward of Lematang Basin

between the Garba Mountains and the Barisam Mountains. Therefore, the very high

heat flow densities and thermal gradients of some locations in the area may be

explained as being due to magmatic diapirism; this view is supported by the

existence of solfatars on the west rim of the basin. about 20 km from Muaradua.

SUMMARY AND CONCLUSION

The results of this study in 20 sedimentary basins of Indonesia indicate that the

thermal conductivity of sedimentary rocks is affected by textural composition and

compaction due to burial. Therefore. grouping the rock types by age may reveal very

high heat flow densities and temperature gradients which might indicate the ex-

istence of shallow magmatic diapirism.

As can be seen from Fig. 5, the Tertiary basins of Indonesia fall into the category

of normal to hot basins except those that are near the active subduction zone.

ACKNOWLEDGEMENTS

The author gratefully acknowledges the management of Pertamina for permission

to present this paper at a Heat Flow symposium of the IASPEI Regional Assembly

in Hyderabad, India. I thank Ir. AK. Soejoso and Drs. Luki Witoelar, Division

Head of Litbang EP, for approving a heat flow study in the working program, and

Professors A.E. Beck and S. Uyeda who provided constructive criticism and com-

REFERENCES

Aadland, A.J. and Phoa, R.S.K., 1981. Geothermal Gradient Map of Indonesia. Indonesian Petroleum

Association. Jakarta.

Beck. A.E., 19’76. An improved method of computing the thermal conducti~,ity of ~uid-filled sedimentary

rocks. Geophysics. 41: 133-144.

Beck, J.M. and Beck, A.E., 1965. Computing thermal conductivities of rocks from chips and conventinal

specimens. J. Geophys. Res.. 70: 5227-5259.

Carvalho, H. da S., Purwoko, Siswoyo, Thamrin, M. and Vacquier, V., 1980. Terrestrial Heat Row in the

Tertiary basin of central Sumatra. Tectonophysics. 69: 163-188.

Dowdle. W.L. and Cobb, W.M.. 1975. Static formation temperature from well logs-an empirical

method. J. Pet. Technol, 27: 1326-1330.

Eubank, L.R.T. and Maki. C.. 1981. Structural geology of the central Sumatra back-arc basin. IPA Proc.

Ferth, W.H. and Whichman, P.A., 1977. How to determine static BHT from well log data. World Oil.

Jan.: 105-106.

Judge, AS. and Beck, A.E., 1973. Analysis of heat flow data in several boreholes in a sedimentary basin.

Can. J. Earth Sci.. 10: 1494-1507.

Katili. J.A.. 1975. Volcanism and plate tectonics m the Indonesian Island arc. Tectonophysics, 26:

165-188.

Robertson Research, 1983. Petroleum Geochemistry of Indonesian Basins. Robertson Research lnterna-

tionai Ltd., Llandudno, U.K., Vol. 1, pp. 6-8.

Thamrin, M., 1964. Preliminary report on the geology of the north central part of South Serayu Mts.

Major thesis for Sarjana degree ITB Bandung (unpublished).

Thamrin, M.. 1973. Preliminary report on the geology of Tanjung Kemala, Lampong. Exploration report

for Pertamina.

Thamrin, M.. 1978. Preliminary report on the geology of Garba Mountains. Muara Dua, South Sumatra.

R&D report, Pertamina (unpublished).

Thamrin, M.. Siswoyo, Sandjojo, S.. Prayitno, and Siregar, I.. 1980. Heat flow in the Tertiary basin of

South Sumatra, Indonesia. Proc. 16th Session CCOP, Bandung, pp. 250-269.

Thamrin, M.. Siswoyo and Pray&no, 1981a. Heat flow in the Tertiary basin of North Sumatra. Indonesia.

Proc. 17th Session CCOP, Bangkok, pp. 394-408.

Thamrin. M., Siswoyo and Prayitno, 1981b. Heat flow in the Tertiary basin of Northwest Java, Indonesia.

Proc. 18th Session CCOP, Seoul.

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