157 heat flow

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Twelfth Annual Convention. June 1983

HEAT FLOW VARIATION ON WESTERN INDONESIAN BASINAL AREAS: IMPLICATION ON BASIN FORMATION AND HYDROCARBON POTENTIAL

B. Situmorang *, Siswoyo * M. Thainrin **, Barlian Yulianto *

ABSTRACT

Average heat flow values on several Tertiary Basins of Western Indonesia. i.e. tlie North Sumatra Basin, South Sumatra Basin, Northwest Java Basin and the Northeast Java Basiq fall in a I-ange of I .05 5 0.25 - 2.58 5 0.21 p Cal/cm-. s. except on the Central Siimatra, Basin where heat flow value is about 3.27 & 0.93 1-1 Cal/cni- s. I t appears that discrepancies in geological and geophysical cliaracte- ristics between each region. are also demonstrated by variation in heat flow values as exemplified by the Sumatra and Java basinal areas.

In general, less variabilitj. of heat flow is observed in Java Basinal Areas than those observed in Suniatra basinal areas. The lo\\rst variability occurs in tlie South Stiiiiatra Basin. \diereas the largest one take p1;icc.s i n tlic Central Sumatra Basin. This variability probably i-etlccts variiition in the aniuiint of extension wliicli has occurred in c:ic11 basin. wliicli in tu in bas played a n important role i n basin hydrocarbon potential.

1 . Introduction Initial lieat flow nieasurciiiciits in Western lniloiiesiiiii

basinal areah, was carried out by L.cinipas arid I'ertamiiia i i i 1977 i i i the Cciitr:il SiiiiiLiii.a Uasiii. Since then, subse- qiieiit n i c a s u i c i i i c n i s I I ; I V C bcen tlonc i i i tlic Soutli Sui i ia t ra Basin (1979) . tlic' NoItI i Suiii;iti-a kisiii (1980). tlic North- west and Noi-tlicast J a v ~ i 13asins ( I()SI ). Ka1ini:ir i tan Basinal Areas (1981) and t h e Niiiiina l3asiii (1081). These activities represent tlic first suif:icc ineiisui-eiiieiits 01' beat flow in tlic Tcrti;ii-y basins 01' Indoncsia

'The rcstrlt o f tliosc iIic:isiirI:iiicIits. undoubtedly, has been a valuablc coritiibiitioii to o u r understuncling on lieat disti-ibiitioii i n hach-arc b i i s i i i s 01' Western Inclonesi;i, wliich (as M C all hn ( iw) are tlic n i a in Iiydrocarbon poducing basins i n tlic c o i i n i i y . Not only ~isel'ul f o r studies i n n ia tura- tion of Iiydrocai h i s , tliose Ileal Ilow values are also valtiable foi- ot l ie r purposes. such a s basic studies on geo- thermal energy and iiiineralisation.

There arc not many I'iiblisliecl s t tidies o n tiic rclat ionship between lieat flow vai-iatioris ant1 tectonic I'catur flow nieasurenients i n the Bering and Caribbean Seas 1962; Gcrad et ~ l . , lO(17) wcrc the first nieasurenients made in back-arc settings, which gave the heat flow values cha- racteristics of a back-arc basin as normal o r subnornial, However, observation by Yasui and Watanabc (1065) in

' + i-baiig Geologi, Litban;! I l', I'crtamina, Jakari;~ * Div. I \pl~)r; ir icin~l.sploitation, OGI'IX "1 .c in ip~" . .lakarta

tlic Japan Sea - another back-arc basin - indicated that heat flow values are higli. It was in this region - the Japan Arc - t l i i i t scientists in the seventies recognized the importance of heat flow variation with respect to island arc tectonism (McKen/,ie & Sclater. 1068; Suginiura & Uyeda, 1073).

The heat flow variations across an arc-trench system are \\.ell exhibited by tlie Japan Arc, where the island arc coincides with a proiiiinent discontinuity in the pattern of lieat flow. Typical oceanic values (1 .O - 1.3 p Cal/cm&) occiii- on tlie oceanside of tlie arc (with sliglitly lower values in trenches), wliereas beliind the arc the lieat Ilow value is I .o,- 3.3 p CaL/cm? s with a niean value of I .56 p Gal/ cni- sec (Vacquier et al., 1067).

111 this paper, we observe tlie significance of lieat flow variations in terms of its relatioilship with major geological and geophysical features. Our discussion will be limited to back-arc basins of Sumatra and Java, which unlike the Jap; in Sea, have never been developed into an oceanic environment (Carvalho ct al., 1980). The procedures of licat flow iiieasurenient have been published elsewhere (e.g. Thanirin et ~ l . , 108 I) and will not be discussed in this p;iper. Any conclusions derived are considered as preliminary, which have to be confirmed by fiirtlier geological and geophysical studies.

2. Heat flow distribution 7.1. Suniatra Basinal Areas. The heat flow distribution in Western Indonesian basinal

;iicas is shown in Figure 1 . In the North Sumatra Basin, tlic average o f 302 values measured from 1 13 onshore and oi'l'sliorc wells is 2.39 + 0.44 p Cal/cm2 s. Higher values ( > 3 p Cal/ciii?- s.) arc observed in the northern corner and approxirnately in the central part of the basin, i.e: the NSB-AI, NSB-C1, Jambobale-I, Peulalu-2, Alur Pineung-I, Alur Pineung-2, S. Peneung-Al , Besitang-2, Telaga Said Timur-1 , Darat Utara-1, Telaga A-I , Watnpu-2, and Wam- pu-5. The generally lower values ( < 2 p Cal/cmd s) occur along the east coast of North Sumatra. At the westernmost part of the basin, heat flow values of < 2 p Cal/cm2 s increases sharply at the ('enozoic volcanic front, the Barisan Mountains.

The results of heat flow measurements in the Central Su. niatra Basin have been published by Carvalho et al., (1980). The average of 170 values a t 162 locations is 3.27 -t 0.93 1-1 Cal/cm2s. Unlike the North Sumatra Basin, the central part o f the Central Sumatra Basin is occupied by lower values, between 2 and 3 p Cal/cm2 s. Towards the northeastern part of the basin, the values increase to over 4 p Cal/cm2 s,

© IPA, 2006 - 12th Annual Convention Proceedings, 1983

158

from where it decreases onto the Malaysian Shield; whereas at the southwestern part, the values are steeply increased at the volcanic arc.

In the South Sumatra Basin, the average value of 358 measurements is 2.58 2 0.21 p Cal/cm2 s. This value is higher than the average value of the North Sumatra Basin, but lower than the mean value of the Central Sumatra Basin. The lowest heat flow value is observed at Betung-5 well (1.67 p Cal/cm2s), whereas the highest one is recognized at Benakat Timur-24 well (3.38 p Cal/cm2s). As in the case of the Central Sumatra Basin, the central part of the South Sumatra Basin is cooler, with the values less than 2.5 p CaIlcm2s. Farther to the northeast and southwest, it rises to > 2.75 p Cal/cm&. The values steeply increase at the volcanic zone in the southwest,and are expected to decrease on the stable Sunda Shield in the northeast.

In those three basins, the heat flow pattern is considered to be parallel t o the volcanic chain of Sumatra, i.e. the Barisan Mountains. Hence, in general, the heat flow contours are trending NW-SE, parallel with most of the structures in the island.

2.2. Java Basinal Areas In the onshore portion of the Northwest Java Basin, the

mean of 32 values is 1.95 5 0.25 p Cal/cm2s. This value is lower than all mean values observed in the basins of Suma- tra. Heat flow higher than the average value exists in the southern rim of the Jatibarang sub-basin (2.1 8 - 2.39 p Cal/ cm2 s), in the Rengasdengklok High (2.26 p Cal/cmLs) and in the Tangerang High (2.73 p Cal/cm2s). In general, distribution of heat flow appears to be uniform.

A rather high average, 2.20 f 0.27 p Cal/cm&, has been obtained from heat flow measuremens in 108 onshore and 22 offshore wells in the Northeast Java Basin. Two main trends of the heat flow contours can be distinguished in this basin, the NE-SW trend in the offshore area and an approximately W-E trend onshore. Heat flow values > 2.50 p Cal/cmLs are confined to the offshore area, with an eastern limit corresponding to Bawean Island.

3. Variation of heat flow values The mean heat flow value for each basin is depicted in

Fig. 2. The values in the back-arc basins of Sumatra range from 2.39 5 0.44 p Cal/cm2 s in the North Sumatra Basin to 3.27 f 0.93 p Cal/cm2s in the Central Sumatra Basin, whereas in back-arc basins of Java the values fall in a range of 1.95 2 0.25 - 2.20 _+_ 0.27 p Cal/cmLs. All values are higher than the worlds's mean heat flow, which is 1.5 5 10%. p Cal/cm2s (Sugimura & Uyeda, 1973). I t also appears that the average values of Sumatra basinal areas are higher than the mean heat flow of about 2.2 p Cal/cm2s observed in all back-arc basins of the Western Pacific region (Watanabe e t al., 1977).

Variation of heat flow values as presented in Fig. 2 must be related to the difference in tectonic styles between the Sumatra and Java Basinal Areas. Katili (1973) has pointed out that lateral variations and discrepancies in the geological and geophysical features occur along the Sumatra-Java Arc, which is attributed to the differential northward movement of the Indian-Australian Plate.

The following are the contrasting geological and geophysical phenomena between Sumatra and Java as identified by Katili (1 973):

Java Sumatra 1. Submarine trench maximum depth: maximum depth:

+ 5 k m - + 7 km. 2. Arc-trench gap non volcanic outer submarine ridge

arc 3. Present volcanic low to intermediate high

-

activity 4. lgnimbrites abundant small occurrence 5 . Lateral variation not clear very pronounced

in K 2 0 content of andesites

6. Deep earthquakcs not present present, maximum depth: 700 km

To that list, wc can add the following features:, 7. Fault types transcurrent d i p d i p

(Tjfd, 1967) 8. Turbidite dcposits unusual abundant

9. Heat flow va luc~ 2.39-3.27 pCal /cm2s 1.95-2.20pCal/cm2s (Martodjo, 1982)

(this paper)

The boundary between the Sumatra tectonic domain and the Java tectonic domain has been placed in the Sunda Strait (e.g. Ranncft, 1972; Budiarto, 1976). Based on detailed studies on the evolution of the Bogor zone, Marto- djojo (personal communication, April 1983) is of the opi; nion that part of West Java approximately west of 106°30 E longitude possibly belongs to the Sumatra tectonic domain. This view seems to be supported by the variation of heat flow values in the Northwest Java Basin, where an anomalous value o f 2.73 p Cal/cm-'s is observed at Tange- rang-] well. tlscwhcrc in the basin. heat flow values ;dre generally < 2 p Cal/cm7s except in the Southern Jatibarang Basin. The relatively high value in this area is attributed to the occurrence of the Jatibarang Volcanics (Thanirin et at , 1981).

Relatively low variability is observed in the Northwest and Northeast lava Basins, with the lowest one occurring in the South Suniatra Basin, wlrich ciitt be ascribed to the thick pile o f sediments deposited in the basin. The North- and Central Sumatra Basins exhibit ;I relatively high variability, with the largest one taking place in the Central Su- matra Basin.

The variability o f heat llow seems to be influenced also by ground-water circulation aloi ig exteiisional faults (Shearer & Keiter, 108 1 ). I t implies that the intensity of extension is less i n thc Noi Iliwcst Java. Northeast Java and the South Sumatra Basins, compared to those in the North and Central Sumatra B;isiiis. This intensity of extension should be visible in thc tectonic history of each basin.

4. Basin fonnation and hydrocarbon potential A serics o f s t ruc tuu l closs-sections across the basins

have been constructed, and is presented as Figs. 3 ,4 ,5 ,6 and

159

7. A sharp contrast between the Central Sumatra Basin (Figure 4) and other basins can be recognized in heat flow profrle and in basement depth. Significantly shallower basement depth is evident in the Central Sumatra Basin. The high heat flow values are probably an indication of the high temperature mantle underneath the basin. The cross-sections also display the block-faulted nature of the basins.

An extensional phase starting during early Tertiary and lasting until the lower Miocene, has been developed in all basins, giving way to rapid, faultcontrolled subsidence. It is recognized as a Paleogene extensional phase in the Central Sumatra Basin (Eubank & Makki, 1981), and has been described in some detail in the Makassar Basin (Situmorang, 1982a). Studies by Pulunggono (1982) in the South Suma- tra Basin, and subsidence data from the Northeast Java Basin seems to support the occurrence of rapid subsidence, mainly during Early Oligocene-Early Miocene times (Figs. 8,9). In the North Sumatra Basin, Cameron & Djunudin (1981) indicate that active block faulting occurred even earlier, i.e. during Mesozoic times.

The extensional phase was followed by compression, which resulted in the uplifting of the previously down- thrown blocks along the fault set that was formed during extension (eg. South Sumatra Basin) (Pulunggono, 1982). However, in the Central Sumatra Basin, compression is related to a dextral wrench (Eubank & Makki, 1981). Subsidence during the compressional phase is not signifi- cant, as shown by the gentle slope of the curves since the Middle Miocene times (Figs. 8,9).

The two-stage pattern of the subsidence curve suggests that in general, the formation of the basin can be explained by the lithospheric stretching model of McKenzie (1978). The main phase of extension which was completed in Early Miocene times probably corresponds to the rifting stage with subsidence due to crustal thinning and thermal ex- pansion. Since Middle Miocene, decay of a thermal anomaly created during the rifting stage produced the so-called thermally controlled subsidence. Normally, flat lying and unfaulted sediments will be deposited at this time, but as already mentioned, compressional stresses are active in most basins (e.g. South Sumatra Basin). Such a case has also been observed by Sclater et al., (1980) in the intra- Carpathian Basins.

The relatively high heat flow in the basins in probably related to a deep-seated phenomenon following crustal extension, which produced the subsidence as presented in Figs. 8 and 9. Hence, the variation of heat flow values is determined by the amount of extension with a different value for each basin. In other words, heat flow variation is related to variation in the thickness of the crust. At present, we do not have data on crustal thickness for each basin, from which the amount of extension can be determined through comparison with unstretched crust of similar basement geology. The above discussion implies that configuration of each basin will be mostly controlled by variation in the amount of extension which in turn will result in the variation of crustal thickness.

The maturation of hydrocarbons in the basins has undoubtedly benefited from the high heat flow. The organic-

bearing sediments which filled the basin since it was formed in the early Tertiary are subjected to a sufficiently high temperature, resulting in generation of petroleum hydrocarbons. An example of hydrocarbon potential evaluation, using the relationship between the amount of subsidence produced by the stretching mechanism and heat flow has been presented by Sitimorang (1982b, in press), for the Makassar Basin. The five back-arc basins discussed above, have long been known as prolific hydrocarbon-bearing basins.

5. Conclusions Observation on heat flow distribution in back-arc basins

of Java and Sumatra, reveal the following topics All basins are characterised by heat flow values which

are higher than the world’s mean heat flow. Heat flow values in Sumatra basinal areas are higher than the mean heat flow of all back-arc basins in the Western Pacific region.

Variation of heat flow is due to the difference in tectonic styles as exemplified by Sumatra and Java, with its boundary trending approximately N-S possibly in the vicinity of Tangerang-1 well.

The variation of heat flow values is probably determined by the amount of extension, which results in the variation of crustal thickness. The latter is thought to be responsible for setting up the basin configuration. In this respect, it is necessary to have data on crustal thickness, which can be obtained by geophysical studies, e.g. deep seismic reflection profiling.

Acknowledgements The authors would like to express their gratitude to the

managements of the Oil and Gas Technology Development Center ”LEMIGAS” and Pertamina for permission to pu- blish this paper.

REFERENCES

Budiarto, R., 1976: Sunda Strait, a dividing line between Tertiary structural patterns in Sumatra and Java Islands, Geology Indonesia, 3 (l;, 11-20.

Cameron, N.R. and Djunudin, A., 1981: Pretertiary sedi- mentation in Northern Sumatra, Laporan Symposium Direktorat Sumber Daya Mineral, 1 (3b), 121-129.

Carvalho, H. da Silva, Purwoko, Siswoyo, Thamrin, M. and Vacquier, V., 1980: Terrestrial heat flow in the Tertiary Basin of Central Sumatra, Tectonophysics, 69, 163- 188.

Eubank, R.T. and Makki, C.A., 1981: Structural geology of the Central Sumatra back-arc basin, Proc. Xth. Ann. Convention, Indonesian Petrol. Assoc., 153-196.

Foster, T.D., 1962: Heat Flow measurements in the North- east Pacific and in the Bering Sea, J. Geophys. Res., 67,

Gerard, R., Langseth, Jr. M.G. and Ewing, M., 1962 : Thermal gradient measurements in the water and bottom sediment of the Western Atlantic, J. Geophys. R e s , 67,

Katii, J.A., 1973: On fitting certain geological and geophy-

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sical features of the Indonesian Island Arc to the new global tectonics, In: P.J. Coleman (Ed.), 7he Western Pacific, Island Arcs, Marginal Seas, Geochemistry, Univ. Western Australia Press, 287-305.

Martodjojo, S., 1982: Evolusi Cekungan Bogor, Jawa Barat, Disertasi Doktor, Institut Teknologi Bandung, 41 2 pp.

McKenzie, D.P., 1978: Some remarks on the development of sedimentary basins,Earth Planet. Sci. Lett., 40, 25-32.

McKenzie, D.P. and Sclater, J.G., 1968: Heat flow inside the island arcs of the Northwestern Pacific, J. Geophys. Res., 73,3173-3179.

Pulunggono, A., 1982: Sistem sesar utama dan pembentuk- an Cekungan Palembang, Disertasi Doktor, Institut Tek- nologi Bandung, 239 pp.

Ranneft, T.S.M., 1972: The effects of continental drift on the petroleum geology of Western Indonesia, J. Austr. Petrol. Expl. Assoc., 12 (2), 55-63.

Sclater, J.G., Royden, L., Horvath, F., Burchfiel, B.C., Seniken, S., and Stegena, L., 1980: The formation of the Intra-Carpathian Basins as determined from subsidence data, Earth Planet. Sci. Lett., 5 I , 139-1 62.

Shearer, C. and Reiter, M., 1981: Terrestrial heat flow in Arizona, J. Geophys. Re&, 86,6249-6260.

Situmorang, B., l982a: The formation and evolution of the Makassar Basin, Indonesia, Ph. D. Thesis, University of London, 3 13pp.

Situmorang, B., 1982b: The formation of the Makassar Basin as determined from subsidence curves, Proc. XIth Ann. Convention, Indonesian Petrol Assoc., 83-1 07.

Situmorang, B., in press: Formation, evolution and hydrocarbon prospect of the Makassar Basin. Indonesia, Proc. IIIrd Circum Pacific Energy and Mineral Resources Conference, Hawai, August 23-27, 1982.

Sugimura, A. and Uyeda, S., 1973: Island arcs, Japan and its environs, in : Developments in Geotectonics, Vol. 3, Elsevier, Netherlands, 247 pp.

Thamrin, M., Prayitno and Siswoyo, 1981: Heat flow study in the oil basinal areas in Indonesia, Joint ASCOPE / CCOP Workshop on heat flow, Jakarta, October 19-23, 1981.

Tjia, H.D., 1967 : Volcanic lineaments in the Indonesian island arcs, Bull. Volcan, 3 1,85-96.

Vacquier, V., Uyeda, S., Yasui, M., Sclater, J., Corry, C. and Watanabe, T., 1967: Heat flow measurements in the Northwestern Pacific, Bull. Earthquake Res. Inst., 44,

Watanabe, T., Langseth, M.G. and Anderson, R.N., 1977: Heat flow in back-arc basins of the Western Pacific, In: M. Talwani and W.C. Pitman I11 (Eds.), Island arcs, deep sea trenches and back-arc basins, Am. Geophys. Union, Maurice Ewing Series, 1 , 137-1 61.

Yasui, M. and Watanabe, T., 1965: Terrestrial heat flow in the Japan Sea, 1, Bull. Earthquake Res. Inst., 43, 549- 563.

1 5 19-1 535.

FIGURE CAPTIONS

Figure 1 Heat flow distribution, Western Indonesian Back-

Figure 2 Mean heat flow values, Western Indonesian Back-

Figure 3 Schematic structural cross section and heat flow





Figure 8 Uncorrected basement subsidence curve, South

Figure 9 Uncorrected basement subsidence curve, North-

Arc Basins.

Arc Basins.

profile, North Sumatra Basin.

profile, Central Sumatra Basin.

profile, South Sumatra Basin.

profile, Northwest Java Basin.

profile, Northeast Java Basin.

Sumatra Basin (Pulunggono, 1982).

east Java Basin.

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157 heat flow

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