IPA 89-11.10

PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Eighteenth Annual Convention, October 1989

STRUCTURAL DEVELOPMENT OF HYDROCARBON TRAPS IN THE CEPU OIL FIELD NORTHEAST JAVA, INDONESIA

N. Soeparyono * P.G. Lennox * *

ABSTRACT

Reinterpretation of 18 local and 7 regional seismic lines in northeast Java, numerous exploration wells and their integration with newly me'asured stratigraphic sec- tions has enabled a new structural model to be developed for the Cepu oil fields. The generally shallow water hey-clastic sequence developed in a rifting back-arc basin with many northeast-southwest oriented basement faults.

Deformation in the early Middle Miocene caused reactivation of the basement faults in the Nglobo- emanggi area with wrenching and the initial develop- ment of flower structures. This deformation caused areally restricted erosion of the main reservoir rocks in this area. Later Pliocene deformation accelerated the development of the flower structures in the Nglobo- Semanggi area which were reflected at the surface as a series of en echelon, hydrocarbon-bearing anticlines. The Tambakromo-Kawengan area underwent minor north over south thrusting along east-west oriented lis- tric reverse faults with detachment at shallow depths and the development of hydrocarbon-bearing folds exist in the subsurface north of the Tambakromo-Kawengan structure. Such folds would be related to imbricate blind thrusts parallel to the Tambakromo- Kawengan thrust.

The wrench structures and detached compressional structures forming hydrocarbon-bearing folds in the Cepu Oil Fields are probably a result of the transference of stresses due to oblique subduction at the Java Trench during the Neogene to Pleistocene.

INTRODUCTION

Reinterpretation of twenty-five seismic lines, Bou- guer anomaly gravity maps and surface mapping have enabled new tectonic models to be proposed LO explain the formation of the Cepu Oil Fields.

*

** Sesksi Diklat Eksplorasi. PPT-Migas, JI. Sarogo I , Cepu, Central Java, Indonesia Department Of Applied Geology, University Of New South Wales, PO Box I . Kensington. Australia, 2033

The Neogene-Pliocene sequence was deposited in a back-arc basin oriented northeast-southwest. This basin was deformed twice with the development of a regional unconformity and two areally restricted unconformities (Sabardi, 1988), open folding and fault reactivation. This reactivation caused the cover sequence to develop en echelon folds. flower structures and detachments within incompetent units within the sequence (Soeparyono 1988).

Previous models for the tectonic development of nor- theast Java have been based on the Moody and Hill( 1956) wrench faulting model (Situmorang el af., 1976), the intrusion of diapiric shale (Soetarso and Suyitno, 1976) and the effects of detached compressional structures (Lowell. 1979). This paper details ihe evidence for some oil fields being formed in en echelon folds over flower structures reflecting wrench movement on palaeofaults and other oil fields being developed in asymmetric faul- ted-folds n i t h listric faults soling in detachment surfaces at depth reflecting compression. This compression may represent transmission of the stresses generated in the subduction zone (Froidevaux et al., 1988) once the pre- sent plate disposition had become established in the Ter- tiary (Katili and Reinemund, 1984).

GEOLOGY

The Cepu Oil Fields are located in northeast Java and are being exploited by the Indonesian government through PT hligas and more recently Pertamina (Fig. 1). The twenty-setfen oil fields were discovered from 1894 on\\.ards by Bataafsche Petroleum Maatschappij (BMP) oil company and Nederlandsctie Koloniale Petroleum Maatschappij (NKPM). Only five fields are still in pro- duction (Soetarltri ef a/., 1973). Over 150 MMbbls of oil hnve been extracted from the quartz sandstone and cal- carenite reservoir rocks over the two stratigraphic inter- vals within the Upper Miocene and Pliocene epochs.

Remapping of some structures plus five measured sec- tions have enabled refinement of the stratigraphic column constructed by Samuel & Gultom (1984) as

IPA, 2006 - 18th Annual Convention Proceedings, 1989

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shown in Fig. 2. This study has shown that the Early to Middle Miocene regional unconformity at the base of the Ngrayong Member is infact areally restricted and loca- ted at the top of this member. The Pliocene unconfor- mity between the Selorejo Member and Lidah Forma- tion has been relocated above the Mundu Mombar (Sa- bardi, 1988).

The productive Mio-Pliocene formations are located above pre-Tertiary pelagic sediments, melange, serpen- tinite, acid volcanics, schists and phyllites which do not outcrop in this area. The Ngimbang Formation which does not outcrop consists of basal coarse sandstone, shale and minor coal seams which changes upsection into predominantly micrograined limestone with minor mads and sandstone (Fig. 2). This formation represents deposition in a fluviatile to shallow marine environment during transgression upon a pre-Tertiary basement. The Kujung Formation consists of clay, shale or marl intercalated with sandy calcarenites representing deposi- tion in a deep water environment. The Ngimbang and Kujung Formations thicken to the east reflecting the presence of a deeper basin in this area at this time (see Fig. 4). Middle Miocene Orogeny caused development of a basement high near Tuban with resultant weathering of the Tuban Formation and some parts of t h e K u j u n s Formation.

Stratigraphy

A single well, surface exposures and seismic traverses have enabled development of models of the lateral and vertical configuration of the three economic formations in the Cepu Oil Fields. Proposals to rehabilitate the Cepu Oil Fields in the mid 1970s resulted in the shooting of 18 new seismic lines (Cu 2-20) of 181 km total length (Fig. 3). These lines supplement a number of regional seismic sections which can be tied to suitable wells both east and west of the Cepu Oil Fields. These seismic lines were divided vertically using five seismic reflectors which have been defined in stratigraphic terms using ties to wells. These seismic reflectors correspond to the middle part of the Wonocolo Member and the top of the Ledok Member, Ngrayong Member, Prupuh Member and basement rocks.

The Tuban Formarion

The Tuban Formation consists of fine clastic sediments which are thicker in the northern and eastern parts of the basin and of uniform thickness in the western part of the basin (Fig. 2). The Tuban Formation is subdivided into the Tawun Member (lower) and Ngrayong Member (up- per). The Tawun Member consists of grey carbona- ceous shale and calcarenite. The calcarenite is rich in or- bitoidal foraminifera suggestive of a shallow water envi- ronment. The Ngrayong Member is a poorly cemented quarrz sandstone (and represents the main reservoir

(75%) of the Cepu Oil Field). Seismic records indicate the presence of local deltaic facies within the Ngrayong Member suggesting southeastern progradation (Fig. 4). A deeper starved basin on which slope deposits accumu- lated was located in the southwestern part of the study area. There is also evidence of a Middle Miocene tecto- nic uplift over a restricted area because of erosion of the Ngrayong Member. Downlapping seismic facies corres- pond to erosional truncation on the upper part of the Ngrayong Member in this area.

Kawenangan Formation

The Kawengan Formation consists of four confor- mable members and it outcrops extensively over the Cepu Oil Fields (Figs 2,4). It is composed dominantly of mark interbedded with thin sandy calcarenite at its base, becoming well bedded calcarenite towards its top.

The Kawengan Formation lies in part unconformably upon the Tuban Formation. Its basal member consists of well bedded, trough cross bedded calcarenite indicative of a high energy, shallow platform type environment (Figs 2,4). The basal member is overlain by thickly bed- ded marl which is in turn overlain by glauconitic calcare- nite. The lithology, cross-bedding and biogenic structu- res are indicative of shallow marine conditions. The uppermost member of the Kawengan Formation consists of monotonous unstratified marl with abundant small foraminifera1 fossils indicative of a bathyal environ - ment.

Lidah Formation

The extensively outcropping Lidah Formation con- sists predominantly of clay or marl (Fig. 2). The basal member, the Selorejo Member, has a very restricted distribution in the Cepu Oil Fields. This may reflect eit- her subsequent erosion or wedging out of facies in this direction or restricted sediment supply to this area. It was deposited under shallow marine conditions. The fol- lowing Tambakromo Member consists largely of blue clay with some sub-millimetre thick streaks of very fine grained quartz sandstone and was deposited under shal- low marine conditions. The Tambakromo Member is distinguished from the overlying calcareous Turi Mem- ber because the Turi Member consists of intensively weathered marl. The Turi Member is everywhere uncon- formably overlain by recent alluvial deposits.

Palaeogeography

The palaeogeographic map shown in Figure 4 was derived from a much larger fence diagram covering a 20 x 50 km area centered on Cepu which was constrwted using data from seven oil wells and a twenty-five line seis- mic grid (Fig. 3). This map aetails the lateral and vertical lithological variations in the Tuban and Kawengan For-

14 1

mations. A landmass lay to the northwest of Cepu (Sun- daland of Katili, 1974j: The palaeoshoreline probably was oriented northeast-southwest and lay to the north of Cepu. Shallow marine deposition in the northwest gra- des to open marine basinal deposits in the southeast. The local deltaic facies within the Ngrayong Member and the presence of a starved basin in which slope deposits accu- mulated in the southwest suggest localized shallowing parallel to the palaeoshoreline. In the northeast, carbo- nate facies were being deposited suggesting reworking of a carbonate shelf at this time. The presence of thick, shallow, offshore carbonate deposits northeast of Cepu may be due to shells being derived from reefs north and northeast of Cepu. In this respect, reefal environments have been present throughout northeast Java not only within the Oligocene Kujung Formation but also in the Neogene-Pleistocene Paciran Formation in the Tuban area.

There were coarser sediments in the north which grade south into finer sediments. Alternation between sandstone, calcarenite and sandy marl are predominan- tly developed in the northern part of the area and repre- sent cyclic and reciprocal sedimentary processes bet- ween the clastic detritus from a hinterland and the rewor- ked carbonate detritus from reef atolls or patch reefs. The bulk of the clastic detritus is quartz, consistent with a granite hinterland northof Java island at this time (Katili, 1974).

The calcarenite to the north is surrounded by marl in the south. Thus these facies changes are consistent with a reef framework grading down slope into beds of calcare- nite and grading to marl in the deeper parts of the basin. The thick shale and marl in the southern part of the area indicates continuing stability of the shelf during Neogene deposition.

TECTONIC SETTING

The present day tectonic features in east Java reflect its development as a convergent zone between the Eura- sian plate and Indian Ocean-Australian Plate (Katili, 1974, Hamilton 1975, Lowell, 1979. Katili and Reine- mund, 1984). There has been a rotation of the magmatic arc from being oriented northeast-southwest in the Late Mesozoic to east-west in the Tertiary in northeastern Java (Katili and Reinemund.1984). Given this magmatic arc rotation the Cepu area would have changed from a Mesozoic fore-arc setting to a Tertiary back-arc setting. The pre-Tertiary basement lithologies are indicative of an accretionary complex whilst the northeast-southwest oriented basement faulting and horsts and grabens arc consistent with rifting in a fore-arc setting suckas those developed elsewhere adjacent to subduction zones (Beck, 1983, Jarrard, 1986). The plate vectors indicate that convergence was not always at right angles, even at

the present time, which would aid sinistral strike-slip movement on any northeast trending faults in this nor- theast Java basin (Lowell 1979).

The northeast Java basin is considered to consist of four blocks differentiated on the basis of fold orienta- tion, faulting and outcrop distribution (Fig. 5a). Block l consists of an area covered by the oldest surface sedi- mentary rocks reflecting the presence of an elevated basement in this area. Block 2 in the western section of the oil fields exhibits en echelon, east-west trending folds with Neogene rocks in their cores (Fig. 5c). It is separa- ted from Block 3 by a major sinistral fault (Fig. Sb & c). The evidence for this major sinistral strike-slip fault includes the differences in the nature of hydrocarbons either side of this line, the differences in the trend of folds and the presence of abundant similarly trending faults in the basement. Block 3 in the eastern half of the oil fields has mainly northwest-southeast trending folds (Fig. 5c). Block 4 to the south of the other blocks exhibits a lower basement. contains extensive alluvial deposits and has east-west trending, en echelon folds (Fig. Sc). The development of the oil fields structures involved a pre-Late Oligocene basement configuration in which Block 1 basement \vas higher than basement in any other block (Fig. 5a). Shear couple movement, possibly due to oblique subduction, resulted in reactivation of the deep-seated northeast-southwest trending palaeofault o n the eastern edge of Block 2. This was accompanied by conjugate, sinistral, strike slip movement and formation of flower structures and en echelon folds in Block 2 as described below. In Block 3 northwest-southeast tren- ding reverse faults and folds were initiated as described in the section below on the Tambakromo Mawengan structures.

Previous Studies

Martins (1951 ) axial-surface. thrust-reverse faulting model for the Kawengan Anticline was the first to explain this structures form. The present study results are consistent with the reverse fault elements of Martins (1951) cross section, although this study shows that at depth the faults sole out on a subhorizontal detachment within the Tuban Formation. Lemigas and Beicip (1969) considered that dog-legs in the generally east- west trending oil producing anticlines nere due to shea- ring caused by re-activation of northeast-southwest tren- ding palaeofaults during the Pleistocene deformation. This may p:titly explain the fold pattern.

The two major fold trends identified in the Cepu area were ascribed to the Neogene and Pelistocene orogenies (Soetantri et al., 1973). Soetantri et a/ . . (1973) proposed that DI resulted in the north\vest-southeast trending folds and rare east-west trending folds; while D2 resul- ted in east-west trending folds only (Figs 1 & 5 ) . The two

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structures which are the focus of this study, the Kawengan-Tambakromo structure and Nglobo- Senianggi structure, were formed durifig D1 and DI(?)- D2 respectively according to this model. This folding model for the Kawengan structure contrasts with the diapiric shale model proposed by Soetarso & Suyitno (1976) for this structure. The diapiric shale was derived from the Wonocolo Member and Upper Tuban Forma- tion.

Other models for northeast Java have emphasized the impor- tance of faults in controlling the development of folds (Situmorang et al., 1976). East-southeast to west- northwest oriented 2nd order dextral faults develop east- west oriented drag folds and these are deformed by 3rd order sinistral, northeast- southwest oriented faults.

Nglobo-Sernanggi

Well developed flower structures can be identified from two of the five north-south seismic lines across the crestal region of the Nglobo-Semanggi structure (Fig. 6, Harding, 1985). These are positive flower structures because the upward spreading faults have reverse sepa- ration on most of their elements. These structures reflect a convergent wrench zone where there is strike-slip motion on the fault system (Harding and Lowell, 1979). The isochron map of the Prupuh Member indicates dominantly northeast-southwest trending reverse faults (Fig. 7d), while the overlying sequence shows only nor- mal faulting at approximately right angles to the fold axes (Fig. 7a-c). This basal reverse fault pattern is consis- tent with the flower structures while the normal fault pat- tern in the cover sequence reflects accommodation in the stretched hinge zone of the anticlines.

There is a significant diffcrcnce between thc thickncss of the Tuban Formation over the crest of these flowcr structures compared with the limbs (Figs 6 8: 8). This differential thickness cannot be accounted for using displacements on faults within the flower structure; and is visible on both sections normal and parallel to the east- west trending fold axes (figs 6 & 8). The Tuban Forma- tion thickens over the crest of folds as a probable result of its original deposition in a graben at this locality between the northeast-southwest trending normal faults in t h e pre-Tuban basement. These normal Paults have been reactivated during transpression in the Neogene to their present listric, reverse form. The development of anti- forms and synforms in the overlying Tuban Formation reflects the amount and type of strike- slip displacement on these faults and the sense and degree of overlap of these faults. If the faults are curving or have dog- legs there will develop either pop-ups or pull apart basins (Aydin and Nur, 1985). The basement reverse faults cha- nge strike from northeast-southwest to eastnortheast- westsouthwest between the Nglobo and Semanggi Antic-

lines and sinistral movement on these faults would result in flower structures with pop-ups (Figs 6,9).

There is some suggestion in one seismic sectior. of detachment within the Tuban Formation beneath the Ngloho anticline (Fig. 8b). Further processing of the other seismic sections may clarify whether such detac- hments are more commonly developed in the Tuban For- mation reflects the ability of the pre-Tuban faulting to accommodate the imposed Neogene transpression. If these basement faults do not substantially reactivate as strike-slip faults during Neogene transpression, then the Tuban Formation will detach to accommodate deforma- tion. In the case of the Nglobo anticline it owes its exis- tence to development of a flower structure in the base- ment and lower Tuban Formation and possibly some deep-level detachment within the Tuban Formation (Fig. 8b). This detachment within the Tuban Formation was probably triggered by the thicker section of Tuban Formation in this former graben and the presence of the bounding reactivated faults. This detachment within the

.Tuban Formation soles at about 4 km unlike that dedu- ced from the Tamhakronio-Kawengan structure which soles at 1.5 km.

In areas of transpressional tectonics i t is likely that ste- pover between subparallel overlapping faults will result in the development of flower structures as an accommo- dation mechanism during strike-slip movement (Aydin and Nur. 1985). A model for the development of the ste- povers in the Nglobo-Semanggi area is shown in Fig. 9

The eight seismic lines and the basal isochron map indicate the north-northeast trending basement faults causing the Nglobo-Semanggi anticlines must have slip- ped in a sinktral sense during the Pleistocene deforma- tion. This would be consistent, using the strain ellipse method of Wilcox ef a/., (1973), with a major sinistral fault trending northeast. This fault forms the boundary between the eastern and western oil fields (Fig. 5 ) . The Nglobo-Semanggi structure is but one of the east-west oriented. open. upright folds in the block west of the regional fault. Folds to the east of the regional fault are generally northwest-southeast oriented, open, upright faulted structures

Tarnbakrorno-Kawengan

Using the ten seismic sections it is possible to deter- mine that there is a fundamental difference between the shallow and deep structures in the Tambakromo- Kawengan area.

There is a half-flower deep structure affecting the Pru- puh Member and the lower part of the Tawun Member but not the overlying cover sequence (Fig. 10). The major reverse fault within the half flower structure has a

143

vertical displacement of 160 m which I S much less than the 550 m throw calculated for the major reverse faults in the Nglobo-Semanggi area. The Prupuh Member shows abundant generally nor1 heast-southwest trending faul- ting, probably as a reflection of basement faults of a simi- lar trend which were reactivated during the Neogene orogeny (Fig. l lc) . In contrast to the faulting which dominates the deep structures, the shallow structures occur as both faults and folds.

Whereas the open Kadewan and Tambi anticlines affect the top of the Ledok Member and Middle part of the Wonocolo Member, the gently plunging Tamba- kromo anticline involves the top of the Ngrayong Mem- ber. Thus the Tambakromo anticline formed during both Neogene and Pleistocene orogenies whereas the Kade- wan and Tambi anticlines were affected only by the Pleis- tocene deformation. The Kawengan anticline differs from the other folds in this eastern block in that it is an open asymmetrical fold with a southern more steeply- dipping limb dissected by a reverse fault which soles into a detachment within the Tuban Formation.

The isochron map of the top of the Ledok Member shows the northwest-southeast trending culmination of the Tambakromo anticline and a single normal fault at right angles to the fold axis (Fig. 1 la). The isochron maps for the middle of the Wonocolo Member and top of the Ngrayong Member show a major reverse fault in the nor- theast corner of the map (Fig. 1 Ib & c). Since the nor- theast-southwest trending normal fault identified at the top of the Ledok Member isochron map occurs on these two other isochron maps and in all cases terminates against the northwest-southeast trending reverse fault, this implies that the reverse fault formed after the normal fault. The northwest-southeast trending reverse fault identified from the middle of the Wonocolo Member and top of Ngrayong Member isochron maps does not out- crop the surface. The isochron map for the top of the Prupuh Member shows many northeast-southwest tren- ding normal and reverse faults (Fig. 1 Id). This fault pat- tern is similar to the comparable isochron map in the Nglobo-Semanggi area (Fig. 8d). This reflects the perva- sive northeast-southwest trending basement faulting gen- erated in the Cretaceous fore-arc and re-activated during defor- mations in the Neogene and Pleistocene when the northeast Java basin was in a back-arc setting. The absence of the northwest- southeast trending reverse fault in the Prupuh Member, which was recogni- zed in the Wonocolo and Ngrayong Members indicates this reverse fault is detached within the Tawun Member.

The deep structures were formed during the Middle Miocene deformation whereas the shallow structures, which are important for oil field formation, formed during the Middle Miocene and Early Pleistocene defor- mations. Thus some deep seated faults were formed and

other reactivated during the Middle Miocene deforma- tion whilst rare half flower structures and folds in the cover sequence were formed during Early Pleistocene deformation (Fig. 10) . The restricted angular unconfor- mity upon the Ngrayong Member supports such a two phase deformation model.

CONCLUSIONS

Anticlines are the dominant structural trap in the Cepu Oil Fields. The western and eastern oil fields differ markedly in their structural style (Fig. 12). The Nglobo- Semanggi oil field would be a mainly basement-involved structure whereas the Tambakromo-Kawengan oil field would be a detached structure (Lowell 1979). Wrench faulting appears to be a minor factor in the development of the Tambakromo-Kawengan structure whereas it was crucial in the formation of the Nglobo-Semanggi structu- re. Nglobo-Semanggi oil fields are characterized by the presence of a flower structure at approximately 5-6 km depth and, at the surface, by en echelon fold patterns (Fig. 12a). There is some suggestion of detachment wit- hin the Tuban Formation at about 3 km beneath the Nglobo Anticline. Further processing would be required to verify how often such detachments occur in this area. The Kawengan oil field is mainly characterized by asym- metrical folding formed above a listric, detached, reverse fault at 1.5-2 km depth. There is a possibility that blind thrusts north of the present Tambakromo- Kawengan structure caused further anticlinal closures containing hydrocarbons.

The Cepu Oil Fields contain both wrench and compres- sional structures which formed hydrocarbon traps. These structures probably reflect transpressional stres- ses associated with oblique subduction in the Java trench during the Neogene to Pleistocene.

ACKNOWLEDGMENTS

The authors are grateful to Associate Professor Evans for his comments on numerous seismic sections during the course of this study. Dr M. Johnstone, Esso, is than- ked for reviewing the manuscript. N. Soeparyono was in receipt of an Australian Development Training Award, Australian Internal Development Assistance Bureau during this study. This paper is published with the kind permission of PPT-MIGAS. Mrs M. Kadar drafted the figures and Mrs M. Clark and Miss K. Jones typed the manuscript.

REFERENCES

Aydin, A. and Nur, A. 1985. The types and role of stepovers in strike-slip tectonics. In: Strike slip deformation, basin formation and sedimentation (Edited by Biddle, K.T. and Christie-Blick, N.) SEPM 3 1 , 3 5 4 3 .

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Beck, M.E. 1983. On the mechanism of tectonic trans- port in zones of oblique subduction. Tectonophy- sics 93, 1-1 1.

Froidevaux. C. , Uyeda. S. and Uyeshima, hf. 198s. Island arc tectonics. Tectonophysics 148, 1-9.

Hamilton, W. 1975. Subduction in the Indonesian region. Southeasi Asia Petroleum Exploration Society Proceedings 2, 37-40.

Harding, T.P. 1985. Seismic characteristics and iden- tification of negative flower structures, positive flower structures and positive structural inver- sion. Bulletin of the American Association of Petroleum Geologists 582-600.

Harding, T.P. and Lowell, L.D. 1979. Structural styles. their plate tectonic habitab and hydrocarbon traps in petroleum provinces. Bulletin of the Ameri- can Association of Petroleum Geologists 101 6- 1058.

Jarrard, R.D. 1986. Terrane motion by strike-slip faulting of forearc slivers. Geology 14, 780-783.

Katili, J .A. 1974. Geological environment of the Indo- nesian mineral deposits, a plate tectonic approach. Publikasi Teknik . Seri Geoiogi Eko- nomi 7,16 pp, Geological Survey of Indonesia.

Katili, J.A. and Reinemund. J.A. 1984. Southeast Asia: Tectonic framework, earth resources and regional geological programs. International Union of Geological Sciences 13, 68p.

Koesoemadinata, R.D. 1969. Outline of geolo;

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(contour Interval 50 m e t r e s )

L, H Low a n d hlgh a r e a in s u r f a c e Undeflned Et o b s e r v e d r e v e r s e fault

FIGURE 11 - Isochron maps of the Tambakromo-Kawengan structure at different stratigraphic levels; a) top of Ledok Member, b) Middle Wonocolo Member, c) top of Ngrayong Member and d) top of Prupuh Member. The legend in d) is applicable for all these isochron maps.

156

T U B A N FM.

\ 0 I

SURFACE / TUBA N FM.

2 3 4

km

g--- I I KUJUNG B NGIMBANG FM.

FIGURE 12 - Model showing the different structural styles in the NgIobo-Semanggi and Tambakromo-Kawengan oil fields. The former structure was formed during wrenching and reactivation of basement faults whereas the latter was formed during compressional tectonics with .detachment in the cover sequence.

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