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20 OTHMAN Au MAHMUD H o TJIA AND MoHo toRus IsMAIL. rendah sesar utama dan unsur unsur lai n pada batuan dasar yang boleh membentuk struktur perangkapan stratigrafi di. dalam batuan ber klas Oligosen M iosen di atasnya Lokasi ke luasan dan punca yang menghasilkan anomali graviti. juga dikenal pasti Beberapa siri struktur tinggi dan lipatan yang bersekutu dengan tujahan batuan dasar merupakan. sasaran yang baik untuk penumpukan hidrokarbon Secara umumnya data graviti yang diperolehi baru baru ini. menyediakan panduan untuk mengenal pasti kawasan bagi aktiviti jelajahan dimasa hadapan di bahagian Utara tengah. daratan Sarawak, INTRODUCTION reconnaissance fieldwork and review of the well data and. subsurface sequences of the nearby offshore Ba1ingian. The Tinjar Province is located in the north central part basin where some of the major oil fields in Sarawak are. of onshore Sarawak Figure 1 It covers the area of located. approximately 25 000 square kilometres The Tinjar Based on the preliminary investigation a total of. Province was part of the total Sarawak onshore concession nearly 40 leads have been identified fro m SAR data The. area granted to Anglo Saxon Petroleum Company later leads include folds domal structures and fault dependen t. renamed Sarawak Shell Berhad SSB in 1909 Later Shell closures associated with strike slip faults and normal faults. relinquished the Tinjar Province and the entire Sarawak Mohd ldrus Ismail et al 1997 These structures were. onshore area in stages between 1957 to 1981 later reconfirmed by ground truthing works. During the concession period exploration activities. carried out by Shell were concentrated to the north eastern. part of Sarawak onshore where discovery of the Miri field. CORRELATION WITH THE BALINGIAN, was made in 1910 and to the southwest of Bintulu The PROVINCE. north central part of the Sarawak onshore or the Tinjar. The Balingia n Province located to the northwest of. Province was not thoroughly explored, the Tinjar Province is one of the petroliferous areas in. Minimal exploration activities during the early days. Sarawak It contributes about 40 and 18 of Sarawak, resulted in lack of data for the Tinjar Province Seismic. coverage is limited to the coastal area with only single to. six folds CDP stacked seism ic data The old seismic data. were acquired between 1948 to 1975 Figure 2 Only 12. wells were drilled in the coastal belt which tested 3 SOUTH. structures namely BuJak Setap Subis and Selungun Land CHI A. gravity and magnetic data acquired between 1928 to 1965. have limited coverage on account of difficult accessibility. PREVIOUS WELL RESULTS,PROVINCE 1, The wells were drilled in the very old days between.
1914 to 1956 Figure 3 and were tested with very old. technology In most cases the wells were selected based. on oil seepages and surface geological expression without SARA WA K. using the advantages of seismic data It is most probable SOKMS. that the wells were not drilled at the optimal location. Figure I Location map of the Tinjar Province,However some of the wells encountered oil and gas. shows in the Cycles I and II Miocene clastics The Suai N. 3 well drilled in 1953 was production tested and flowed. about 30 barrels of oil per day The presence of hydrocarbon 1. in these wells and the result of Suai 3 well indicate that. a working hydrocarbon system exists in this area,PRELIMINARY INVESTIGATIONS Tin jar. Since relinquished by Shell in 1976 this area has been. left untouched It is generally assumed that this area has. shallow basement and this idea discouraged explorationists. to look back into this area However in late 1996 a. Petroleum Management Unit Petronas Research, Scientific Services PMU PRSS team began a study on Figure 2 Coverage map of the land seismic survey by Sarawah. the hydrocarbon potential of the Tinjar Province The Shell Berhad over Sarawak onshore 1948 1975 In the area of. study covers interpretation and review of Synthetic Tinjar Province 2 D seismic coverage is limited to the coastal. Aperture Radar SAR and the existing geological data areas. Geological Society of Mala ysia Annual Geological Conference 2001. INTERPRETATION OF NEWLY ACQUIRED AEROGRAVITY DATA OF THE TINJAR PROVINCE ONSHORE SARAWAK 21. total oil reserves and production respectively Among the AIRBORNE GRAVITY AND MAGNETIC. major oil fields discovered and currently in production are DATA ACQUISITION. Temana Bayan Dl8 and D35 These fields range in size. from 90 to 150 million stock tank barrels MMSTB of As a follow up to the preliminary study in late 1998. estimated recoverable oil Most of the oil were found in a total of 4653 km of airborne gravity and magnetic data. Cycles I II and III consisting of late Oligocene to early with a grid spacing of 5 km X 5 km have been acquired. Miocene clastics The hydrocarbons are reservoired in the Figure 5 The main objectives of the geophysical survey. coastal plain and nearshore sandstones which are made up was to further investigate the hydrocarbon prospectivity. of channel stacked channel crevasse splay mouth bar of the Tinjar Province and to assess sediment thickness. and shore face sands The traping mechanisms are complex depth to the basement the areal extent of the onshore. and are intimately associated with the deeply seated fault basins as well as possible structures which cou ld be. basement movements associated with hydrocarbon accumulations The acquired. Figure 4 shows that the oil fields in the Balingian data were later merged with data from existing airborne. Province are distributed in a NW SE trending fairway It surveys and marine data over the adjacent Balingian basin. is believed that this fairway extends onto the onshore part Together the data form fairly good coverage of gravity. of the Tinjar Province including the area of the identified and magnetic conditions over the north central part area. leads structures from SAR images This NW SE trending of Sarawak. fairway is possibly related to the trend of the paleocoastal The modelling and interpretation of gravity data were. zone during Cycles I and II in the late Oligocene to early carried out jointly with SSB SSPC in the Shell office in. Miocene Surface geological data gathered from the field Miri using LCTSIES gravity magnetic and seismic. work carried out in the interior area to the south east of. Bintulu Town and in the vicinity of Bintulu indicate the F131 1. environment of deposition was Coastal to Coastal Plain 4 E11 S Clastics. ES I Rebab, This is similar as the environment of the hydrocarbon EB. bearing reservoirs of the major oil fields in Balingian Sulak Selap. Well data from the producing oil fields in the Balingian I. Province show that the sediments in the Cycles I and II. were deposited in the Near Shore to Coastal Plain, environment Abundance of coal and lignite indicates I.
good source rock potential for this area The offshore. Balingian and the onshore Tinjar were subjected to the. same series of deformations and tectonic events during. late Oligocene to late Miocene Hence the structural. patterns and types of hydrocarbon closure in the Balingian ldt ntificd lcnd i. SOKMS from SAR ta, are very probably good analogues for the structures of the. onshore Tinjar Province Figure 4 The Balingian oil fields trending in a NW SE belt that. is believed to extend into the onshore part of the Tinjar Province. The belt could be associated with the paleocoastline during Cycles. TO ft OB J RESULTS I and II times,1 B Seta J r1 1914 1438 CYI Oil Gas Shows. 2 B Seta J r2 1915 1922 CYVJI Gas Shows,3 B Seta J r3 1951 Oil Shows. 11 623 CY 11111111,4 Subis 1 1939 1000 CYVJI Suspended. due to war,5 Subis 2 1953 10 708 CYVJI Gas Shows,6 Suai 1 1952 3833 CYII Ruorescence.
7 Suai 2 1953 3567 CYII Ruorescence,8 Suai 3 1953 1286 CYII Tested 30 BOPD. between 840 890,9 Suai 4 1955 2232 CYII Gas Shows,10 Suai 5 1955 5443 CYVJI Gas Shows. 11 Suai 6 1955 1979 CYI 11 LST Residual Oil,12 Selungun 1 1956 7374 CYI Ruorescence. Figure 3 Table showing results of wells drilled in the Tinjar Figure 5 Airborne gravity and magnetic data acquired over the. Province Most of the wells are very old and were drilled based on Tinjar Province in 1998 A total of 4653 km of gravity and. surface geological data without using the advantages of seismic magnetic data have been acquired with a grid spacing of 5 km X. data 5 kin,June 2 3 2001 Pangkor Island Malaysia, 22 O THMAN Au M AHMUD H o TJI A AND MoHo loRus IsMAIL. modelling software from FUGRO LCT Inc However in the Cycles 1 II III structures Some of the hydrocarbon. magnetic data that turn out to be very comp lex is not migrated quite a great distance of up to 100 km into the. included in this paper higher areas where it was trapped in f ields li ke Bentara. Balai and J4 The possibility of the hydrocarbon migratin g. RESULTS AND DISCUSSION toward the onshore area and being trap ped in structures o r. the Tinjar Province cannot be rul ed out,Bouguer Gravity Map Gravity Profile.
The Bouguer map in Figure 6 reflects the regional Figure 8 shows a profile of the Bouguer anomali e. gravity In general the offshore part shows low Bouguer which cuts across the exposed basement of the Tatau. anomali es whi ch correspond with the low density fill of High If the sed iment thickness is taken as zero at the. the Balingian and Acis depocentres The onshore part cropped out basement Tatau High then relatively there. shows moderate to high Bouguer ano mali es which is quite a substantial amou nt of sed iment toward the. correspond with the high density of the uplifted basement northeast of the profile of the Ti njar Prov in ce Th e. rocks difference in anomaly is in the range of20 to 30 mgals and. The Bouguer gravity data shows a small part of the thi s will give sed iment thickness of app rox imate ly between. Ba lin gian to extend onshore to the south of the Tinjar 2000 to 3000 m Ass uming that density contrast of 0 25. Province A coupl e of inversions in the offs hore part in glee between basement rock and the overlyin g clastics. the northeast are caused by the density contrast of the Thi s amo unt of sed iment as supported by gravity data. hi gh area with abundant carbonate buildups and platforms rules out the earli er assumption of a sha ll ow basement fo. i e E7 E8 E5 and Kalung the onshore area, The prominent West Balingian Line is well expressed F igure 9 shows a profile of the residual anomali es. by the Bouguer anomaly To the west of thi s lineament which is believed to be associated with the structures in. hi gh grav ity marks the Penian that runs subparallel to the the shallower levels above the ba ement Based on tha i. coast line is clearly seen on the Bouguer gravity map. In the onshore part and to the southwest of Bintulu. Town a maximum anomaly of 84 mgals is observed This. anomaly is probably caused by th e outcropping, metamorphic basement in the Tatau area This anomaly. the Tatau High exposes rocks of the economic basement. whi ch is the Rajang Group as confirmed in the fie ld The. Rajang Group of sedi me nts is strongly deformed partly. metamorphosed and consists of deep water distal turbidites. of interbedded of argillite and greywacke Wolfenden. 1960 Towards northeast in the Bintulu area this gravity. high forms a saddle but rises again in the Suai and Subis. area Gravity highs are generall y assoc iated with structural. hi ghs whi ch are potentiall y good hydrocarbon traps. However the Suai and Subis structures have been drilled Figure 6 Bouguer gravity map showing regiona l gravity anomal. of the area Prominent features such as Balingian basin We L. but onl y oil and gas shows were found The saddle area in. Balingian Line Pen ian High An au Nyalau Thrust and Tatau High. between two grav ity high s near Bintulu could allow. are well expressed by the Bouguer anomalies, hydrocarbon to migrate from the Balingian basin into. stratigraphi c pinch outs or other structures in the onshore. A sign ificant gravity low occ urs to the southeast of. the Tatau High This gravity low is evide nce of a small. onshore basin which could be prospective fo r hydrocarbon. exploration, Figure 7 shows the Bouguer grav ity map superimposed. by hydrocarbo n f ields of the Balingian Province Most of. the hydrocarbon f ields are situ ated within and at the edge. of the huge depocentre th at includes the Balingian Acis. and North Acis basins This deep depocentre is believed. to be the kitchen area fo r the hydrocarbon generation and. expulsion which fed the existing oil fie lds in the Balingian. Province The hydrocarbon migrated vertically along the Figure 7 Bouguer anoma ly map superi mposed wid1 the distribut ion. faults and laterally through carrier beds becomin g tr tpped of hydrocarbo n fields in the Balingian Province. Geological Socie v of Malaysia Annual Geological Conference 2001. INTERPRETATION OF NEWLY ACQU IRED AEROGRAVITY DATA OF THE T INJAR PROVINC E ONSHORE S ARAWAK 23. profile numerous of potential structures for hydrocarbon Gravity Modelling. entrapment could exist in the onshore Tinjar Province The basic aim of grav ity modelling in LCTSEIS is. Stratigraphy simply to match the calculated Bouguer grav ity of the. input model to that of the observed Bouguer gravity Four. The stratigraphic scheme fo r Sarawak basin used in profiles namely AA BB CC and DD were selected for. this study is based on previous work done by Ho 1978 modellin g and the results are shown in Figures 11 12 13. which subdivided Sarawak basin into 8 cycles Figure. 0 The base of each cycle is marked by the maximum, Profile AA was selected over existi ng marine seismic.
transgressive unit and the top is represented the maximum data and extended to the onshore area where no seismic. regress ive unit The 8 cycles Cycle I to VIII are of ages data is avail able The seismic interpretation was extracted. ranging from early Oligocene to Holocene The lithology from the 1989 SSB Exploration Data Book that provided. of the 8 cycles comprises of sand stones siltstones and the initi al model and the constraint required for the. shales and sometime interbedded with shallow marine exercise Additional constraints were provided by the data. carbonates Underlying the 8 cycles are the pre and mid from offshore wells E3 1 X and Temana 55 and surface. 0 gocene age metamorphic basement rock known as, geological data gathered during ground truthing work. Belaga Formation of the Rajang Group, The gravity model s in Figures 11 to 14 suggest that. the basement in the onshore part has been extensively. uplifted as compared to the offshore Balingian However. in so me parts of the Tinjar Province substantial sediment. still exist and could reach as thick as 4000 to 5000 m This. thickness is s ufficient for hyd roc arbon generatio n. expulsion and entrapment The basement rock as shown in. the models are heavily faulted in verted forming horst. and graben structures which provide the basic traps fo r. Foram Cycle,TIME EPOCH,Ma Zones Boundaries,PLEISTOCENE N23 VIII. PLI OC ENt N20,E ARLY N19, Figure 8 Map and profile of the Bouguer anomaly in the Tinjar VI. Province The 20 to 40 mgals differe nce in Bouguer ano maly N17. between cropped out basement and the deepes t part of the basement. are eq ui valent to about 2000 m to 4000 m of sediments assu min g N16. a density contrast of 0 25 glee between basement rock an d the N15. overl ying sediments 10 v,15 MIOCENE 1 IV,OLIGOCEN LATE.
Fig ure 9 Map an d profi le of res idual gravity anomaly showing Figure 10 General stratigraphic scheme of Sarawak basin by Ho. high freq uency of anomaly in the onshore part which co uld be 1978. assoc iated with structures at the shallower levels. Ju ne 2 3 2001 Pangkor Island Malaysia, 24 O THMAN Au M AHMUD H o T JIA AND MoHo loRu s IsMAIL. Figure 11 Gravity model based on initial,seismic interpretation of regional lines. 75 1560 and 70155 The observed 0 and,calculated C gravity response show a. good match The model shows that active,MODEL FOR PROFILE A A. movements of the basement during middle,to late Miocene time caused the overlying.
younger sediments to be pushed up and c,formed the present oil fields in the offshore 2. Balingian Province The same structural X,traps could also be present in the onshore z. I counterpart m,Figure 12 Profile along B B show ing. simi lar pattern of basement structures in,the offshore and onshore counterpart The. model shows that sedi ment thickness could,reach as high as 5000 m in the onshore.
J cALCULATEO CiW m,Tinjar Province,fr g Jiik 1 1i Y. Figure 13 Profile along C C,C ltCUt t TfD CRAVIlY, Geological Society of Malaysia Annual Geological Conference 2001. INTERPRETATION OF NEWLY ACQUIRED AEROGRAVITY DATA OF THE TINJAR PROVINCE ONSHORE S ARAWAK 25. hydrocarbon accumulation in the overlying clastics Tate that are associated with the closures The next step is to. 1992 reported that graben structures with splayed fault better delineate the closures by acquiring regional seismic. patterns have been recognised in the offshore Balingian coverage We propose that 500 km seismic lines are. Province These structures are parallel or sub parellel to required for the Tinjar Province Figure 16 This seismic. the Tatau structure onshore coverage has taken into account sediment thickness. It is proposed that the Luconia Platform rifted from potential basement associated structures depth to. the shelf of Vietnam South China followed by its basement and leads that were identified on SAR images. southward drift that has compressed the deep water deposits. of the Rajang accretionary prism Hutchison 1988 This CONCLUSIONS. rifting and drifting occurred during Eocene Ologocene. time The rising Rajang Group provided the clastics for From this study we conclude that. Cycles I II and III sediments in the Tinjar and Balingian 1 The Suai 3 well result which tested 30 barrels oil per. basin Continued tectonic rejuvenation resulted in the day BOPD shows that a vaJid hydrocarbon system. periodic uplift and erosion of the earlier deposited exist in the Tinjar Province. sediments Part of the Cycles II III deposited in the u Onshore Tinjar Province and offshore Balingian are. Tinjar Province has been eroded and redeposited as younger eq ui valent in terms of structural styles and. Cycles IV to VI in the Balingian Basin hydrocarbon closure and both have experienced the. Later a series of compressional events partially same tectonic events. inverted the basin and also resulted in reactivated the m Gravity data show the presence of substantial amounts. deep seated faults in the basement developing anticlines of sediment in the Tinjar Province which are. and other structural traps for the current oil fields in the sufficient for the generation expulsion and entrapment. Balingian Province These series of late Miocene to of hydrocarbon. Pliocene deformations also destroyed or modified existing iv Gravity data detect basement highs and lows which. structural traps causing remigration of hydrocarbons Some could provide potential traps for hydrocarbon in the. of the hydrocarbons have reached the surface as oil and overlying clastics. gas seepages On the positive side remigration probably v With its meagre seismic coverage limited number of. created other prospective accumulations wells drilled and old drilling testing methods applied. We consider the offshore Balingian Basin in terms of in the previous days the Tinjar Province is considered. structural styles trapping mechanisms generation and an underexplored area. migration of hydrocarbons as model for the Tinjar Province. Hence the latter s prospectivity is still attractive for further ACKNOWLEDGEMENT. exploration efforts, The clastic sediments of Cycles I and II are sufficiently We would like to thank Mr Wong Kah Luk and Mr. thick and possess structures of the right types for Graeme Smith of SSB SSPC for their invaluable. hydrocarbon generation expulsion and entrapment The contributions during the modelling and interpretation phase. uplifted basement provides potential structures for of the gravity data at the Shell office in Miri We also. hydrocarbon accumulation Figure 15 Seals could be thank PETRONAS for permission to present and publish. provided by the intraformational shales of Cycles I and II this paper. Figure 14 Profile along D D The Anau,Nyalau Thrust is well expressed in this. CAI CULATE D Gf AVIT i L,June 2 3 2001 Pangkor Island Malaysia.
26 OTHMAN Au MAHMUD H o TJIA AND MoHo loRus ISMAIL. REFERENCES, Ho K F 1978 Stratigraphic Framework For Oi l Exp loration In. Sarawak Geological Sociery of Malaysia Bulletin 10 1 14. HuTCHISON C S 1988 Stratigraphic tectonic model for eastern. Borneo Geological Society of Malaysia Bulletin 22 135. MOHD lDR US ISMAIL OTHMAN Au MAHMUD AWALLUDIN HARUNAND. H D THA 1997 Hydrocarbon Potential ofThe TinjarProvince. Onshore Sarawak Abstract Geological Society ofMalaysia. Petroleum Geology C01 erence 97 Programme Abstracts. of Papers p 13 14, TATE R B 1992 The Mulu Shear Zone a majo r structural. feature of NW Borneo Geological Society of Mala ysic. Figure 15 Bouguer anomaly in 3 D showing basement highs in Bulletin 31 51 65. the onshore Tinjar Province The associated structures in the WoLFENDEN E B 1960 The Geology and Mineral Resources of. overlying clastics are good potential traps for hydrocarbon the Lower Rajang Valley and adjoining Areas Sarawak. acc umul ations Geological Survey Depa rtment British territories in Borneo. Memoir 11 167 p,TOTAL lillll m, Figure 16 Proposed 2 D seismic programme in the Tinjar Province. A total of 500 km of 2 D lines have been proposed in the interest. area The lines have been identified based on sediment thickness. potential prospects and structures, Geological Society of Malaysia Annual Geological COJiference 2001.

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