The Geology of the Central and South Sumatra Basins

© IPA, 2006 - 3rd Annual Convention Proceedings, 1974

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PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATION Third Annual Convention, June 1974. THE GEOLOGY

OF THE CENTRAL

AND

77

SOUTH SUMATRA BASINS

G.L. DE COSTER *)

BASIN DESCRIPTION The Central and South Sumatra basins are important oil producing areas on the island of Sumatra in the Republic of Indonesia (Figs. 1 and 2). They are Tertiary structural and depositional basins composed of a Tertiary sedimentary section lying o n an u n c o n f o r m i t y surface of pre-Tertiary metamorphic and igneous rocks. These are two of the three basins located on Sumatra ( t h e third is the North Sumatra basin) aligned northwest-southeast between the Barisan Mountains to the southwest and the Malacca and Karimata Straits and the Java Sea to the northeast and east. T h e rocks exposed in the basins consist almost wholly of Tertiary strata though several uplifted blocks i n the basins (including the Tigapuluh and Duabelas mountains) do expose pre-Tertiary rock at the surface. The rocks exposed in the Barisan Mountains are composed of Paleozoic and Mesozoic metamorphic and igneous rocks and of young Tertiary to Recent volcanics. Analysis of the Central and South.Sumatra basins show that they had very similar and related histories a n d c o u l d be considered as one large basin with many troughs and grabens. The North Sumatra basin, on the other hand, appears to have been separated from the Central Sumatra area throughout most of its h i s t o r y by the Asahan arch and can be treated as a separate basin. For this report the Central and South Sumatra areas will be discussed and described as separate basins. The Central and South Sumatra basins (see Fig. 2) are asymmetric basins bounded on the southwest by faults and uplifted exposures of pre-Tertiary rocks along the m o u n t a i n front of the Barisan Mountains;. on the northeast by the sedimentary or depositional boundaries of the Sunda shelf (site- of the ancestral Sunda landmass); to the south and east by the Lampung high and by an arch that parallels the

east coast of Sumatra; and to the north and northwest by the Asahan arch and the outcrops of pre-Tertiary rocks northwest of Pekanbaru. Both The Asahan and Lampung arches were positive elements throughout much of Tertiary time, separating the Central and South Sumatra basins from the adjoining North Sumatra and Sunda basins, respectively.The two arches were covered by shaUow marine seas only during the .Early and early Middle Miocene time to form temporary connections to the neighboring basins. The northeastern and eastern boundaries of the basins along the Sunda shelf and the Lampung high are difficult to define precisely, but are usually placed where the sedimentary section is less than 1500 feet (460 meters) thick and composed of Plio-Pleistocene and Younger strata lying o n Lower Tertiary or pre-Tertiary rocks. The boundary between the central and south basins is also indefinite due to l a c k of major structural features separating them. This factor is a n o t h e r argument for considering the two areas t o be one basin rather than two. The boundary between the two basins is usually drawn as a northeast-southwest band through the northern part of the Tigapuluh Mountains, joining the axis of a broad arch extending southwest from t h e Sunda landmass to a istructurally complex area in the m o u n t a i n front *) P.T. Stanvac Indonesia Jakarta, Indonesia. The writer wishes to acknowledge his indebtedness to the many geologists and geophysicists presently and fomaerly associated with P.T. Stanvac Indonesia for much of the geologic information add interpretation incorporated in this paper. Information was drawn freely from discussions with colleaguesin P.T.S.I. and from the f'des in the Exploration DepartmenL In addition, he is grateful to .personnel in the exploration and geology • departments of Pertamina and"of other eomp/mies operating in Indonesia for their contributions in general discussions about the regional geology of the Sumatra area.

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Schematic strotigraphic section for South Sumatra basin~ showing generalized lithologies.

84 Lemat Formation and Benakat Member The term " L e m a t " has been applied in the past to tuffaceous, coarse clastic, continental sedimentary rocks present in the South Sumatra basin that are now interpreted to comprise several distinct units that are lithologically similar but of differing ages. These units included 1) the extensive deposits in the basin of pre-Talang Akar tufts and coarse clastics (the " y o u n g " Lemat), 2) the "old" Lemat in the Lemat wells, and 3) the tufts present in the Laru wells and in outcrop near Lahat. The use of the name Lemat is now restricted to the " y o u n g " Lemat; the term "Kikim Tufts" is used for the "old" Lemat and for the occurrences in the Gumai Mountains and the Laru wells. The coarse clastic member of the Lemat is composed of sandstones, clays, rock fragments, breccias, "granite wash", occasional thin coal beds and tufts, all deposited in a continental environment. The Benakat Member of the Lemat (which is not to be confused with the Air Benakat Formation) occurs in central portions of the basin and is composed of grey-brown shales with some beds of tuffaceous shale, siltstone and sandstone, and occasional thin coal beds, carbonate stringers and glauconite. It was deposited in a fresh to brackish environment and lies conformably on the coarse clastic lower member of the Lemat, locally termed the granite wash member, in the center of troughs. In the proximal portions of the depositional troughs fhe Benakat may grade into the coarser elastic facies of the Lemaf. The relationship, however, is not too clear due to abrupt .thinning and truncation of the Lemat onto the flanks of the Pendopo anticlinorium. The Lemat is normally bounded at its base and its top by unconformities that coincide with seismic horizons. In distal parts of the basin the contact of the Lemat with the Talang Akar is interpreted to be paraconformable. The thickness of the formation is highly variable, ranging from a feather edge to more than 2500 feet (about 760 meters) in south Sumatra, and to as much as 3500 feet (about 1070 meters) (as defined by seismic data) in one fault depression in the central part of the basin. These maximum thicknesses of the unit have not been penetrated by the drill. The Lemat is

probably Paleocene-Early Oligocene and the Benakat Member, Late Eocene-Early Oligocene in age, as determined by spore-pollen and K-Ar age dating of some samples of shale and tuff. Kelesa Formation The Kelesa Formation occurs in the Central Sumatra basin and is composed of conglomerates, coarse quartz sands, variegated shales, coals and tuffaceous material which were deposited in a continental environment. It has a localized distribution in the basin area and was deposited as the initial sedimentary fill in troughs and grabens on the basin shelf. The unit is a sequence whose base is the u n c o n f o r m i t y with the pre-Tertiary that provides a good seismic reflection, and whose top is unconformable with the overlying Lakat Formation. The top is a distinct log pick and seismic reflector, commonly described as an u n c o n f o r m i t y from dipmeter data and the evident change in depositional environment. However, in local areas, continuous deposition undoubtedly t o o k place in a transitional environment. This continental facies is interpreted to grade northwest to the Tapanuli trough into the claystones, sandstones and shales of the pre-Sihapas Formation. The Kelesa attains a maximum thickness of about 4000 feet ( I 2 2 0 meters) in local areas in the southern part of the basin. It has been dated as Oligocene to Early Miocene on palynological evidence. The unit is therefore younger than its depositional cycle equivalent, the Lemat Formation, in the South Sumatra basin. Pre-Sihapas F o r m a t i o n The name Pre-Sihapas is applied by industry to the occurrence of the Kelesa Formation in the northern half of the Central Sumatra basin wh~re it is composed of variegated continental claystone and sandstone, and of massive dark brown lacustrine shale. It grades eastward and southward into the continental, coarse clastics of the Kelesa. The Pre-Sihapas is probably Oligocene but the basal sandstones in the deep portions of the trough may be as old as Upper Eocene. Talang Akar Formation This unit occurs in the South Sumatra basin,

85 overlying the Lemat Formation or the preTertiary section and underlying the Telisa Formation or the Basal Telisa Limestone Member. The Tal~ng Akar is here considered as a sequence which locally onlaps the underlying Lemat or pre-Tertiary and is essentially conformable with overlying strata of the Telisa Formation. Often distinct seismic reflections mark the upper and lower bounding surfaces of the Talang Akar and where the formation is quite thin, the upper seismic horizon is often masked by the reflection from the top of the Basal Telisa Limestone Member. Formerly the Talang Akar was considered to be essentially a sandstone unit with minor amounts of shale, but in this paper it is considered as a time-rock sequence that incorporates important facies and lithic changes within its boundaries. The Talang Akar is composed of delta plain sandstones, siltstones and shales that grade basinward into marginal marine sandstones and shales," and from there farther troughward into marine shales. The contact with the Lemat is unconformable in intermediate and rim parts of the basin and probably paraconformable in trough areas. The contact with the overlying Telisa and Basal Telisa Limestone is conformable over most of the basin, though likely to be disconformable on the basin rim, and is usually placed at the base of the " K " fimestone which is a widespread marker bed. This Talang Akar-Telisa contact is difficult to pick in wells in trough areas because the lithologies of the two formations are generally similar; there, seismic and paleontology information is used to assist in the correlations. The changes in thickness of the formation over the basin occur in a m o r e regallar pattern than it does in the Lemat Formation. The thickness of the Talang Akar varies from a feather edge around emergent highs and basin margins to as much as 1500 to 2000 feet (about 460 to 610 meters) in some of the trough areas in the basin. The thicknesses of the Talang Akar and the Lernat usually vary correspondently in the basin and where one is thick the other is likely to be thick as well. The age of the Talang Akar sequence is Upper Ofigocene and Lower Miocene and probably includes the N.3 (P.22), N.4 and part of the N.5 planktonic foraminiferal zones in the

Banner and Blow P and N Zone system of planktonic foraminiferal zonation. There is a paucity of age dating information for the Talang Akar - few wells have been analyzed for planktonic foraminifera and furthermore most of the wells in the basin were drilled in areas that correspond to delta plain and shelf areas of deposition of the Talang Akar where planktonic. fauna are scarce or absent. No Middle Oligocene rocks have yet been encountered in the basin, although they might occur in the centers of troughs where sedimentation would be expected to be most continuous. Late Oligocene fauna have been identified in beds correlated as the trough facies of the Talang Akar sequence in outcrops in the Air Cawang Kikim section On the southwestern edge of the]~asin and in the Wahalo and Bingintelok wells[2) Samples from the Talang Akar from certain wells in the basin were analyzed as occurring in the Early Miocene Florschuetzia levipoli palynological zone; these samples came from areas where the Talang Akar was deposited in a delta plain environment and where sedimentation probably did not start until Early Miocene. A n o t h e r , informally named, faunal zone, the Black Globi zone, occurs in the trough facies of the upper Talang Akar. In this zone the Globigerinid fauna are dark brown to black, in contrast to the buff and white color of similar fauna in overlying beds, and are associated with pyrite. This suggests a euxenic environment of deposition for the upper Talang Akar in the distal parts of the basin. Lakat Formation This formation in the Central Sumatra basin is c.omposed of relatively clean quartz sandstones and thin interbedded shales deposited in an inner neritic to shoreface environment. The lower contact with the Kelesa was described previously; the upper contact is an electric log and lithology pick. The unit is a poor seismic reflector in the Kampar area anff can only be mapped where, it is tied to well control. The thickness varies with basin position but may be as much as 1100 feet (336 meters) or more in (2) CawangKikim03° 50S, 103 ° 23'E Wahalo-1 02° 59 25"S, 103° 39' 47" Bingintelok-I 02 ° 35'

38"S, 103° 06' 58"

86 certain wells and areas. No paleontologic basin and is composed of calcareous shales information is available for the formation - it interbedded with thin, glauconitic silts and is usually interpreted to be Lower Miocene on • sands, representing the transitional facies from the basis of stratigraphic position. inner to outer neritic marine deposition. The electric log pick is placed at the top of the Si Hapas Group uppermost, relatively thick glauconitic sand below the massive Telisa shale. A widespread The Si Hapas Group is an equivalent term and continuous seismic reflector occurs at the used for the section that includes the Lakat and the Tualang Formations. The lower Si Hapas is top of this unit which is also a sequence the equivalent of the Lakat, and the Upper Si boundary defining the top of the basal wedge Hapas, of the Tualang. The age span for the Si of sedimentation of the Tertiary depositional cycle in the Central Sumatra basin. The Hapas Group is given as N.4 to N.8 in the P and formation is normally about 200 to 400 feet N Zone system of planktonic zonation. (61 to 122 meters) thick in the southern half of the basin and occasionally reaches 500 to 600 Basal Telisa Limestone feet (153 to 184 meters) in some of the wells. T h e Tualang is interpreted to be the This member was deposited in intermediate and shelfal portions of the South Sumatra approximate time-rock equivalent of the Basal basin, on and around platforms and highs. It is Tetisa. Limestone of the South Sumatra basin both a rock unit (member) and a sequence, in and to be of Early Miocene age. It is assigned contact at its base with tlie Talang Akar this age principally on the basis of stratigraphic sequence or with pre-Tertiary rocks and position and the rare occurrence of Spirocoinciding at its top with a strong seismic clypeus in the unit. reflection. The Basal Telisa Limestone is composed of Telisa Formation platform or bank limestone capped in restricted localities by further buildups of detrital, reefal The Telisa Formation is the most widespread and bank limestones. In the distal portions of occurring of tl3e Tertiary units, being deposited the basin the equivalent unit consists of shales during the time of maximum marine transgreswith thin limestone beds that are often mapped sion into the two basins. The Telisa is as a part of t h e undifferentiated Telisa characteristically a fossiliferous, marine shale Formation. The thickness of the lower bank containing occasional thin beds of glauconitic unit is fairly constant over the shelf area, limestone. On the basin rims and shelfal areas it averaging about 200 to 250 feet (about 60 to occurs in a shallow marine facies with siltstones 75 meters) although the thickness variations are and fine-grained sandstones as well as limegreater where the unit lies on the pre-Tertiary stones present with the shale. Sandstones on with more irregular topographic relief to be the basin shelf that heretofore were correlated filled. An additional 200 to 400 feet (about 60 as Talang Akar because they-were sandstones to 120 meters) of limestone are present in those occurring beneath Telisa shales, are now shown areas where the buildups of the upper unit to be sandstones developed within the Telisa. occur, The outcrop of the Baturadja Formation The Telisa lies on Talang Akar in the South in the Garba Mountains is reported to be about Sumatra basin and on the Tualang in the 1700 feet (about 520 meters) thick. The Central Sumatra basin, and on pre-Tertiary member is very fossiliferous and has been dated rocks on most of the basin rim~. It is overlain as Early Miocene. The fauna contained in the by the Lower Palembang (in South Sumatra) Basal Telisa Limestone probably correspond and the Binio ( in Central Sumatra ) Formations mostly to the N. 6 faunal zones but may extend at a contact that is very difficult to select locally into the N.5 and the N~7 zones. consistently over the entire basin areas. The top of the Telisa as picked on lithologic characteristics is diachronous, the characteristic Telisa Tualang Formation lithology persisting until later in time in basin The Tualang occurs in the Central Sumatra centers than on basin edges. Now with-better

87 seismic and regional information available, a more time-constant contact can be Selected, based in part on the occurrence of a seismic reflection within the lower part of the Lower Palembang and the Binio, at the top of or in a Section of sandstones interbedded with shales. Where lithic and seismic data are available, the top of the Telisa is placed at the base of this lower sandstone section in the Lower Palembang or Binio, above the massive shale section of the Telisa. The Telisa on the stratigraphic charts is shown as a sequence (or time-rock unit) with time constant top and base. The thickness of the Telisa varies greatly with basin position and probably is as much as 6000 to 9000 feet (about 1800 to 2700 meters) thick in trough areas. The Telisa can be dated with planktonic fauna. U s i n g the new sequence tops for the Telisa, its zonal ages extend from N.7 or possibly N.6 at the base to as high as into N. 11 at the top. In distal parts of the basin where it lies on the Talang Akar, the Telisa probably extends down as far as into the No.5 zone. The top of the formation may extend into the N.12 or higher zone but those zones in like manner have not been recognized in well samples perhaps because the marine environment has shallowed .to such an extent that few planktonics occur. In the Central Sumatra basin where some paleontologic studies have been made, the age of the Telisa, as well as can be interpreted from the limited data, probably occurs over the time span bracketed by faunal zones extending from approximately N.7 up to N.12 or N.13. It has been reported that the environment of deposition reached its maximum depths during the Globigerinoides bisphericus (G. sicanus) (lower N.8) zone in the South Sumatra basin and in the Globorotalia fohsi fohsi (N. 10-11) zone in the southern half of the Central Sumatra basin. A minor hiatus occurs in the uppermost Telisa and the lower Lower Palembang and lower Binio on the basin edges which is detectable as a disconformity in some of the basin edge wells but is not discernable in wells away from the basin edge nor on the seismic sections. This hiatus may be related to the Intra-Miocene diastrophism and is discussed in further detail in the section on structural and depositional history of the basins.

Lower Palembang (South Sumatra) and Binio Formation (Central Sumatra) These units which are usually considered to be equivalents in the two basins, were deposited during the early stages of the regressive cycle of deposition. They are composed of shales with glauconitic sandstones and occasional limestones, deposited in a neritic environment at the base grading to a shallow marine environment at the top. The base of the unit and its contact with the Telisa were discussed above and is an approximate time constant surface. The upper contact with the Middle Palembang and the Korintji is essentially a lithologic contact, based in South Sumatra on the occurrence of coals, in the Middle Palembang. The Pangadang coals in South Sumatra and the coals in the middle Korintji usually give a strong seismic reflection which is mappable over broad areas. This would suggest that these are fairly synchronous surfaces that can be used in correlations; however, the occurrence of the coals vary in different parts of the basin so that an interpreter cannot be certain that the same coal beds are being mapped on the opposite sides of the larger anticlines which expose Middle Palembang or Korintji strata. There is no diagnostic age dating information available for the two formations. The units have been interpreted in most reports to be mostly Late Miocene in age, though their age range may extend into slightly older and slightly younger stages. The thickness of the units vary considerably with basin position and on how the contacts are selected; thicknesses usually range up to 3300-5000 feet (1000 to 1500 meters). Middle Palembang and Korintji Formation These units were deposited in shallow marine-brackish (at the base), paludal, delta plain and non-marine environments and are composed of sandstones, mudstones and coal beds. They are rock units identified and correlated by lithologic criteria. The lower boundary of the Middle Palembang in the southern part of the basin is usually marked by coal beds, and in the Jambi trough area by equivalent strata where the top contact is picked at the highest glauconitic sandstone of the Lower Palembang. The number of beds and thicknesses of coal decrease from south to

88 north in the South Sumatra basin, l-n South Sumatra strong seismic reflections come from the coal beds as described in paragraphs above. In Central Sumatra the contact with the Binio Formation is picked by lithic and log correlations with the coals (that are locally good seismic reflectors) falling within the Korintji. The upper contact with the overlying Upper Palembang or Nilo probably varies considerably with the basin position, usually being disconformable or unconformable, and is picked at the base of the lowest thick tuffaceous beds in the overlying Upper Palembang or Nilo. The thickness of the Middle Palembang and Korintji vary with basin position and how the contacts are picked; the maximum thickness may reach 1500 to 2500 feet (450 to 750 meters). There are no faunal data from these units that are usable for age dating. The formations are usually interpreted to be uppermost Miocene to Pliocene in age on the basis of stratigraphic position. Upper Palembang and Nilo Formation These units were deposited during the Plio-Pleistocene orogeny and are mostly erosion products derived from the uplifted Barisan and Tigapuluh Mountains and from the uplifted folds being formed in the basins during the orogeny. The formations are composed of tuffaceous sands, clays and gravels, and occasional thin lentils of coal, with great variations of thickness and composition. The basal contact is usually placed at the base of the lowest thick tuffaceous layer. The units occur in the synclines formed during the orGgeny and are absent from the anticlinal folds. A Plio-Pleistocene age is usually assigned to the units based o n their association with the orogeny of that age.

STRUCTURAL GEOLOGY SUMMARY The structural features present in the two basins axe the result of orogenic activity that occurrea in at least three separate episodes the mid-Mesozoic orogeny, the Late Cretaceous-Early Tertiary tectonism and the PlioPleistocene orogeny. The earliest of the major episodes was the mid-Mesozoic orogeny when

the Paleozoic and Mesozoic strata were metamorphosed, faulted, and folded into large structural blocks or belts and intruded by granite batholiths. These belts of metamorphic rock are composed of strata of varying lithologies, of differing degrees of metamorphism and varying intensities of deformation. They are exposed in the Barisan Mountains, are postulated to extend into the basin subsurface, and together form the basic "structural grain" of Sumatra. The second significant tectonic event occ u r r e d probabl~, in Late Cretaceous and Early Tertiary time, when major tensional structures that include grabens and fault blocks were formed in the basins of Sumatra and in the adjoining Sunda basin as well. The general trend direction of these faults and grabens is N-S and NNW-SSE. These tensional features and the remnant structures from the mid-Mesozoic orogeny, coupled with rugged paleotopography developed by differential weathering of the pre-Tertiary rocks, comprise the "old" structural elements of the basin. These elements structured the pre-Tertiary unconformity surface which in turn controlled the deposition of the Lemat, Kelesa and Pre-Sihapas Formations. The other "old" features include the stable platforms in existence prior to Lemat and Kelesa deposition and which remained essentially unaffected by structural movements during the Late Tertiary orogeny. The most prominent structural features in the basins are the northwest trending folds and faults formed during the .Plio-Pleistocene orogeny. The convergence of the Indian Ocean plate against the Sumatra portion of the Southeast Asia plate is postulated to have been the cause of the final uplift of the Barisan Mountains, the development of the major right-lateral wrenching through the length of these mountains, and the formation of the associated fold-fault structures in the basin. In many instances the faulting appears to be partly controlled, their trends interrupted, terminated or offset, and some of their afignments influenced by the boundary faults of the mid-Mesozoic belts of metamorphosed rocks and by the N-S or NNW-SSE faults formed during the Late Cretaceous-Early Tertiary. All these Plio-Pleistocene features are referred to as the '~young" structures of the basins and are

89 easily the most dominant features of Sumatran geology. THE PRE-TERTIARY AND EARLY TERTIARY STRUCTURAL FEATURES The "old" structures in the basin have long been the subject for speculation and interpretation, especially by those exploring for hydrocarbon accumulations that might have been controlled by these older features. Good quality, deep-penetration seismic data and regional and isopach maps of time-rock units have been used in the current studies of these older features. The seismic sections and maps, and the isopach maps of the Lemat and the Kelesa show a considerable amount of topographic relief, some of it fault controlled, to have been present at the time these units were being deposited. Onlap onto the irregular surface that was cut into the pre-Tertiary rocks is common throughout at least the shelf and intermediate portions of the basin where we have data; it is presumed to be present in the deeper parts of the basin as well. These old features are now interpreted to be a combination of paleotopography formed by differential erosion of the pre-Tertiary rocks and of structural elements composed of fault blocks, grabens and stable platform areas formed in Late Cretaceous and Early Tertiary time. The remnants of the belts of Paleozoic and Mesozoic metamorphic and igneous rocks formed during the mid-Mesozoic orogeny also may have comprised part of the regional pre-Tertiary structuring on which the Lemat and Kelesa Formations were deposited. The Late Cretaceous-Early Tertiary tension structures are shown on Figure 6. The major troughs and fault blocks containing thick sections of Lemat, Kelesa and Pre-Sihapas strata include the following: The Benakat "gulley" flanked on the west by faults. This trough, opening to the south into the Lematang trough and to the north into the extension of the Jambi trough, may be a half graben. The Lematang trough, which was probably flanked on the north by a boundary fault separating major blocks of pre-Tertiary rocks.

The Jambi trough. Very little is known about the older Tertiary section in this trough and it is from inference, some seismic fault evidence, gravity data and occurrence of thick sections of Lemat at the northeast end and the west end (in the Barisan mountain-front outcrop) that a trough is postulated here during the time of Lemat deposition. The un-named, north-south trough associated with the Kampar high in central Sumatra. This trough is partially flanked by faults and extends northward onto the shallow basin shelf. The deep graben portion of the Tapanuli trough which received many thousands of feet of Oligocene Sediments. The western flank of the trough may have had intermittent connection to the open sea during the Oligocene. Other faults and smaller grabens of pre- or Early Tertiary age also are present in both basins as shown on Figure 6. In marked contrast to the deep grabens are the platform areas on which no sedimentation occurred until Basal Telisa Limestone and Tualang time. These platforms apparently subsided as stable blocks during the Middle and Late Tertiary and were little affected by the Plio-Pleistocene orogeny. Another broad regional high, partly cut by faults, extended northwest from Palembang toward the basin center. This high, whose surface is composed of hard, massive pre-Tertiary limestone, is flanked by thick sections of L e m a t and Talang Akar clastics and is onlapped b y t h e s e strata from the west and north. " The Tigapuluh and Duabelas Mountains on the other hand, probably were receiving sediments during the deposition of the entire Tertiary section and were not uplifted until Plio-Pleistocene time. Both mountains are rimmed by exposed outcrop of the section from Lemat up to Middle Palembang, 'sugg e s t i n g they were subsiding and receiving sediments during these times. True, the cores of the mountains may have been above wave base or been positive features but there is no way to document this as the Tertiary rock section in the m o u n t a i n areas has been stripped away by erosion.

90

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91 The finer details of the trends and locations of the old structural features can be reasonably well depicted by the isopach map of the interval between the top of the Talang Akar (or equivalent surface in Central Sumatra) and the pre-Tertiary unconformity (Figure 7). The isopach data on Figure 7 incorporates the effects of pre-Tertiary/Early Tertiary structures, of paleotopography on the pre-Tertiary surface, and of basin, tilting (or differential basin subsidence) during the deposition of the Lemat-Talang Akar/Kelesa-Lahat Formations. Despite the restriction of the effects of basin tilt, the map can be used to analyze the gross features of "old" structure and of relative paleotopography on the pre-Tertiary surface at the time of Lemat and Kel~sa deposition. On the map are seen broad platforms, arches, grabens, fault blocks, numerous isolated mounds and ridges, and many terraces and noses flanking or extending away from regional highs. Most of the faults and troughs are oriented nearly north-south; some are northeast-southwest. The isopach maps and the seismic sections indicate that many of the faults were active during Lemat and Talang Akar deposition. MIDDLE TERTIARY FEATURES

STRUCTURAL

Very little structural movement, other than basin subsidence, seems to have occurred after Lemat and Pre-Sihapas-Kelesa time and before the Plio-Pleistocene orogeny. Some of the "old" faults appear to have been active as late as during early Talang Akar and Lakat deposition but were inactive after that. Some seismic sections indicate some movement along the faults during Telisa deposition b u t these appear to have been of local and only minor significance. Middle Miocene tectonic events interpreted by many geologists from structural evidence in the Barisan Mountains apparently had some effect on the structures in the northern part of the Central Sumatra basin but had httle effect elsewhere in the two basins. There is evidence from some wells located on the northeast edge of the-basins that an hiatus occfirred o n the basin shelfs in the Middle Miocene, possibly a result of eustatic dropin sea level, but there is no indication of structural

movements break.

accompanying this

PLIO-PLEISTOCENE TURES

depositional

STRUCTURAL

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The most prominent structural features in the basins are those developed during the Plio-Pleistocene orogeny. These features, whose dominant trend is northwest-southeast, are clearly seen on surface geology m a p s and on structure maps of shallow horizons. Figure 8 is a compilation of the axes of Plio-Pleistocene anticlines and synclines and the traces of the major faults formed at the same time. Some of the Plio-Pleistocene features can be seen on structure maps of the Talang Akar and Tualang Formations though earlier events also affect the structuring on this horizon. Structure on the pre-Tertiary unconformity (Figure 9) is the resultant of several events that includes the pre-Tertiary structuring and paleotopography, and differential basin subsidence during the deposition of the Tertiary section, as well as the Plio-Pleistocene folding. This structure map i therefore least representative of pure Plio Pleistocene structuring. Regional analysis of the Sumatra basins highlights some of the significant Plio Pleistocene structural elements that include the following (Figure 8): 1. The Semangko wrench fault, extending the length of the island of Sumatra formed ir the Plio-Pleistocene orogeny as a result o~ the convergent collision of the northeast moving Indian Ocean plate against Sumatra. It is not known if this zone existed as a wrench fault in earlier Tertiary times or iv the Paleozoic or Mesozoic Eras. Evidence against its being present in the Mesozoic i~ that the fault seems to transect the major blocks of pre-Tertiary rocks in the Barisan Mountains that were formed during the mid-Mesoziaic. 2. The northwest trending folds w i t h i n the basin, aligned more nearly parallel to the trace of the Semangko fault than are the blocks of pre-Tertiary rocks in the Barisans. Many of the folds occur in the basins in sets that appear to have right lateral offset. About five sets of folds or anticlinoria are

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CENTRAL 8 SOUTH SUMATRA RELATIVE PALEOTOPOGRAPHY AND OIL - G A S FIELDS ISOPACH MAP OF LOWERWEDGE OF T E R T I A R Y

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RelatiVe paleotopogra•hy on pre-Tertiary unconformity, illustrated by the isopach map of lower wedge of Tertiary section (isopach map of top Talang Akar to pre-Tertiary in South, and top Lakat to preTertiary inCentral Sumatra basin).

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