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PROCEEDINGS INDONESIAN PETROLEUM ASSOCIATIW Fifteenth Annual Convention, October 1986
TERTIARY STRUCTURAL FEATURES RELATED TO EXTENSIONAL AND COMPRESSIVE TECTONICS IN THE PALEMBANG BASIN, SOUTH SUMATRA
ABSTRACT
Prior to Tertiary sedimentation, a rugged (Pre-Tertiary) surface of highs and depressions pre-set the general shape andconf guration of the Palembang basinal area.
Tertiary basinal history from the Eocene to the Lower Miocene is marked by a tensional phase with maximal rates of subsidence of faulted "block-areas" of 0.04 cm/year, during Late Oligocene to Early Miocene times. This phase in basinal history coincides with a standstill in subduction of the Indian oceanic plate in the south below Sundaland. Oblique oriented compression of the northward converging Indian Ocean plate were instead solely accomcr dated by an intervening NW-SE trending proto-Barisan by way of lateral movements.
The early Middle Miocene marked the beginning of compressive movements throughout the Paleinbang (back- arc) basin, obviously connected with renewed subduction of the oceanic plate. Basinal history from the Middle Mio- cene on is furthermore characterized by a regression of the sea. Interactive movements among "block-areas" resulted a.0. in positive structural inversion and tilting of blocks in opposite directions, which in turn induced typical fold- shapes and patterns within Neogene strata, i.e. a progressive change in direction of anticlinal axes and opposing vergences among anticlinal structures of adjacent "block-areas". Synde- positional deformation of Air Benakat and Muara Enim sands took place on tilted surfaces with unconsolidated Cumai shales/clays as a plastic substratum.
However, diastrophism in the Palembang Basin was main- ly confined to a north-south stretching relativeiy narrow zone. Besides containing the region's highest heatflow, this "fairway" is the site of most of the basin's hydrocarbon- bearing sedimentary formations.
The relations between vertical movements of blocks within the Palembang Basin, rates and directions of the con- verging Indian Ocean plate as proposed by Schwan (1980), and relative changes of sea level (Vail et al, 1977) with respect to paroxysmal stages of the Barisan orogen, are out- lined in this study.
I. INTRODUCTION Western Indonesia is characterized by deep sea trenches,
volcanic chains, sedimentary basins and cratonic con- * PERTAMINA, Jakarta.
tinental areas, and is situated at the convergence of the northward moving Indian Ocean plate sub-ducting obliquely beneath the Eurasian continental plate.
In this plate tectonic setting, the presently known and established oil (and gas) production is primarily confined to the back-arc basins of North Sumatra, Central Sumatra, South Sumatra, Sunda and the Northwest Java basinal areas (Plate 1).
The Palembang Basin is the southeastern part of the South Sumatra Basin. situated west and south of the town Palembang, and covers an area of roughly 125 lun by 150 km. Structurally, it is bound to the southwest by faults and uplifted exposures of Pre-Tertiary rocks of the Barisan Mountain Range and to the northeast by the Pre-Tertiary outcrops of the Sunda craton. The so-called Lampung High forms its southeast and eastern boundary. Compared to other Sumatra back-arc basins, the Palembang Basin is the most extensively explored basin since the turn of the century, 'but nevertheless it still offers promise and attrac- tion to petroleum exploration.
Three parallel WNW-ESE trending rows of anticlinal structures or anticlinoria characterize the Tertiary sedimen- tary strata within the basinal area of which the centrally situated Pendopo-Limau Anticlinorium is the main oil and gas producer, hence its recognition as the "Palembang Basin in sensu stricto" (Plate 2). In conjunction with the souther- ly situated Muara Enim Anticlinorium, the area is also known as the South Palembang Basin.
The following sludy, which is based on a 11983 thesis, will mainly concentrate upon this particular area by focuss- ing upon structural and rdated sedimentary phenomena throughout the successive formations in the stratigraphic column with the aim that identification and analysis of these will disclose lateral as well as vertical movements which have influenced the Palembang basinal area during its development. This in turn will lead to the recognition of compressive as well as tensional phases within the ba- sinal history. Coupled with observed surface and subsurface anomalies mostly structural in character, and to relate these to recognized tectonic phenomena in the region, the resulting conclusions synthesize processes and mecha- nisms which were responsible for the presently observed prominent structural features in the Palembang Basin, par- ticularly the aforementioned oil and gas producing I'endopo- Lirnau Anticlinorium .
© IPA, 2006 - 15th Annual Convention Proceedings, 1986
11. GENERAL STRATIGRAPHY
The foundations of the stratigraphic succession within the Tertiary sediments of the Palembang Basin were do- cumented by Tobler in 1906. Subsequent investigations in the mid-twenties marked the recognition of an uncon- formity between the Tertiary sediments and underlying formations of Pre-Tertiary ages.
Since then, discussions and reviews on the stratigraphy of the Tertiary sedimentary formations were mainly docu- mented in oil company reports (Pustowka, 1939; Thiadens, 1940; Dufour, 1941 ; van der Schilden, 1941 ; Martin, 1952; and others) apaf from Musper's 1937 and Westerveld's 1941 official publications.
A more detailed classification was produced later by Spruyt in 1956 on the basis of a substantial amount of data obtained through intensified drilling and other oilexplorati- on activities in the area. Spruyt's classification also had litho, bio and "time" stratigraphies clearly distinguished from one another.
More recent publications on the geology of the South Sumatra basinal area such as De Coster's 1974 study, had stratigraphy and sedimentation discussed along modern plate tectonic views of basin development.
At present, stratigraphic nomenclatures principally based upon work by Spruyt's 1956, Shell's 1978, and De Coster's 1974 are most in use among geologists working in the Pa- lembang Basin, although differences pertaining to local stratigraphic names still exist with more than one oil company operating in the same bas i~al area. Plate 3 illus- trates these stratigraphic nomenclatures.
Blow's 1969' biostratigraphic ronation had been increa- singly applied to the area in determining "time-lines" within the Tertiayy column with, lately, the complen~entary role of palynology.
The lithostratigraphic subdivision of the Tertiary in rhe Palembang Basin is principally based upon a group of sedi- ments deposited in non-marine, paralic, shallow marine and deep marine environments (Telisa Group), and another group of sediments characterized by a regressive environ- ment (Paleinbang Group). Yle whole sedimentary sequence is thus generally regarded as a megacycle consisting of a lower transgressive facies, composed mostly of coarse clastic material, and an upper regressive facies, which also con- sists mostly of coarse clastic material, with the plane of symmetry through a bathyal shale series that apparently represents the so-called intra-Miocene diastrophism.
A brief outline of the various rock-formations which make up the stratigraphy of the Palembang Basin is hereby given :
Pre-Tertiary Basement
Exposures of Pre-Tertiary rocks are found along the front of the Barisan Mountains (Gumai-Garba Mountains) which form the southwestern boundary of the Palem- bang Basin while to the northeast the Pre-Tertiary outcrops of the Sunda craton are only to be found on the islands of Bangka and Singkep. Within the limits of the Palembang Basin proper, the Pre-Tertiary crops cut only at Bukit Pendopo, west of the town Prabumulih and at Biikit Batu,
east of Palembang. A variety of lithologies characterize the Pre-Tertiary basement within the basinal area such as: andesite, andesite breccia, porphy rite, phyllite, quartzite, limestone, granite and granodiorite.
Lahat Formation (LAF)
Wells in the Palembang Basin (i.e. Pendopo-Limau Anticlinorium) show that this formation generally consists of : a) tuffs, agglomerates, claystones, tuff-breccia and andesite;
a variety of coarse, fragmented, angular volcanic materi- als overlain by a column consisting of :
b) shales with intercalations of tuff, siltstones, sandstones and some thin coal-layers. This formation is also called ax the Lemat Formation
(by SVPM/Stanvac geologists), and is subdivided into a Coarse Clastic Member and the Benakat Member.
In the area of the Pendopo-Limau Anticlinorium, LAF shows a variety of thicknesses which range from ca. 200 m to more than 760 m. Here, it occurs as separate and isolated deposits unconformably overlying Pre-Tertiary basement. A Paleocene - Lower Oligocene (post Cretaceous/pre- N3) age is inferred for the LAF. A Late Eocene - Early Oli- gocene age is assigned to the Benakat Member as determi- ned by spore, pollen and K-Ar age dating (De Coster, 1974).
The LAF depositional environment was initially con- tinental. Subsequent deposition took place in fresh to brackish water environments.
Talang Akar Formation (TAF)
Where the LAF is absent in certain parts of the area under study, the Talang Akar Formation was deposited directly onto the Pre-Tertiary basement. Fluvial-deltaic sediments typify the lower part of TAF while paralic and marine conditions prevail during deposition of the upper part of TAF. Thus the lower part generally consists of coarse to very coarse-grained sandstones intercalated with (thin) layers of shales and some coal seams. The upper part of the formation generally consists of a succession of sand- stones and shales (with some coals), gradually becoming more marine in character. Instead of containing coal seams, the marine shales and sandstones may then become cal- careous.
The Talang Akar Formation is generally subdivided into: A Gritsand Member (TAFIGRM) and a Transitional Member (TAFITRM) (Spruyt, 1956).
Within the area under study, the TAF/GRM shows a variation of thicknesses: 550 m in the Karangan field, 225m in the Tanjung Tiga field. Obviously, the TAF/GRM de- velopment is closely related to so-called "basement highs" such as the Limau East-Q section, Tanjung Menang, Betung and Labi areas. Laterally, the coarse to very coarse grained character of the sands grades rapidly into medium to fine- grained sands.
A maximum rhckness of ca. 300 m has been measured for the TAFlTRM in the area. Widely distributed in com- parison to the TAFIGRM, this Transitional Member may stdl be encountered by wells in distal and more central parts of the basin.
Isopachs of the Talang Akar Formation in the area show that the TAF Depocentres of the order of 1000 m and 800 m, are situated on present structurally uplifted areas i.e. the Pendopo - Limau High and the Lembak High (Plate 4 and 15).
An N-3 age is inferred for the non-fossiliferous TAF/ GRM while a lower Miocene (Aquitanian) age is assigned to the TAF/TRM.
Baturaja Limestone Formation (BRF):
I t is developed as thick coral reefs on structural highs within the Palembang Basin i.e. the Musi, Pagardewa, Prabu- menang, Betung and East Limau-Q section fields. The BRF is very marly in the eastern part of the Limau Anticlinorium (Talang Jimar, Tanjung Tiga, Tanjung Miring). Laterally, in the deeper parts of the basin, a more clayey and marly facies is encountered. In the central and deepest parts of the basin, the BRF interfingers with the shales of the over- lying Gumai Formation.
Thicknesses of the BRF in the Limau Anticlinorium ge- nerally range from 60 m to 75 m but where developed as reefs, it may attain a thickness of more than 100 m. The formation thickens from the East Limau field to the west (Limau Niru field) where it is usually encountered as hard, compact and layered limestones. Marl and also sandy layers are found as intercalations. Maximum thick- nesses of over 200 m are encountered in the Benuang, Raja and Betun areas to the north and northwest of the Limau Anticlinorium. Other depocentres are seen at Ogan, Musi and Pagardewa - Prabumenang.
Age determinations using Blow's planktonic zonation attest to a Lower Miocene (N5 - N8?) age.
Gumai Shale Formation (GUF):
The GUF was deposited during the time of maximum marine transgression into the basinal area. The lithology consists of a thick sequence of Globigerina bearing clayey and marly shales.
Within the Limau Anticlinorium, observed GUF thick- nesses vary from 150 m to 500 m, but a remarkable thick- ness of ca. 2500 m is seen in the Lematang Depression which is situated directly to the south of the Limau Anti- clinorium. The Gumai Shale Formation is widely distri- buted in the Palembang and Jambi basinal areas.
An N9 - N12 (Blow's planktonic zonation) age is general- ly assigned to the GUF in the Palembang Basin.
Air Benakat Formation (ABF):
This formation mainly consists of clays and claystones with an increasing number of sandstone layers higher up in the sequence. Glauconite and macro-forams also abound in the ABF.
Observed thicknesses of the Air Benakat formation in the Palembang Basin vary from 100 m to 1100 m. ABF was deposited during the early stages of the regressive cycle of deposition and the formation is widely distributed in the Palembang and Jambi Basins. N l 1 IN12 to N16 age (Blow's planktonic zonation) is assigned to the ABF.
Muara Enim Formation (MEF):
The MEF consists of claystones and shales with several sandstone layers and some coal beds. Deposited in shallow marine to parglic and non-marine: environments. The ob- served thicknesses vary from 450 m to 750 m, and an Upper Miocene to Lower Pliocene age is generally assigned to the Muara Enim formation.
Kasai Tuff Formation (KAF):
The lithology of the KAF comprises pumice tuffs, tuffaceous sandstones and tuffaceous claystones. This continental deposit characterized the end of the regressive phase. The inferred age for this formation is Upper Pliocene to Lower Quaternary.
III. OBSERVED STRUCTURAL AND SEDIMENTARY PHENOMENA
Folds and faults are the most prominent structural fea- tures in the Palembang Basin. Previous investigations con- cluded that the Plio-Pleistocene tectonic phase (or Plio- Pleistocene orogeny) is responsible for these WNW-ESE trending folded stluctures with accompanying faults.
In the area of t l ~ c Limau Anticlinorium, normal faults are encountered in al~llost all anticlinal structures, generally trending SW-NE and dissecting the Air Benakat and Muara Enim Fornlations at the surface. At deeper horizons, ver- tical displacements of 7-0-40 in are detected within these generally shallow faults. Investigations also revealed the existence of another set of faults trending in the same direction but which, could be traced into far deepel hori- zons. These basement rooted faults which also offset most of the overlying Tertiary formations, were apparently already established structural features at the s tq t . of" tl>e basinal history. In the area under study, these faults are known as the B XI, F1, L2 and L3 (Lembak-Talang Jimar) faults (Plate 5).
Geophysical (gravity and seismic) work also revealed the existence of a WNW-ENE trending basement-rooted major fault or fault zone, known as the Lematang Fault, which bounds the Pendopo-Limau Anticlinorium to the south and forms the northern edge of the Lematang Depression. Vertical throws may reach ca. 1000 metres such as south of the Benakat Field. Eastward, the observed vertical displace- ment lessens rapidly until it becomes a flexure which sub- sequently dies out in the area to the southeast of t l e Lin~au East Field.
The important role these basement rooted faults or fault zones had during subsequent Tertiary basin evctlution is shown by modern investigations. 12-fold CDP seismic co- verage and inlproved processing techniques, together with a substantial amount of other subsurface data, highlight the block-faulted character of the %-Tertiary basement, which leads to the recognition of basement block fault tectonics in the area which apparently created the basic framework at the end of Cretaceous times especially in the case of the Pa- lembang Basin. Thus, the Tertiary deposits infill a rugged paleotopography with a number of those basement-rooted faults undergoing rejuvenation well into Tertiary times (Plate 6).
Only after 1974 there was an attempt by De Coster to classify existing inajor faults and to relate these to tectonic events of different ages. Nevertheless, the rather regional scope of this study did not lend itself t o a more detailed analysis and explanation of the structural phenomena and anomalies within the several block areas which basically make up the Palembang Basin.
Synsedimentary faulting and phases of maximum intensity in subsidence
Well to well correlations across the SW-NE trending ma- jor faults B XI, L2 and L3 (East Lirnau-Q section), Betung, Limau Middle & West and East Tanjung Miring show ob- vious discrepancies in thicknesses between the sedimentary columns of the same age in the downt hrown and upthrown blocks.
Vertical throws along these basement-rooted faults are seen to increase from the GUF (Gumai Formation) down- wards to the TAF (Talang Akar Formation). Comparison of thicknesses between the correlated layers throughout the sedimentary section of interest reveals that particular section within the column which shows the time-interval or phase of maximum subsidence.
Table 1 shows that subsidence at a maximum rate took place at an earlier time in the southeastern part of the Lirnau Anticlinorium than in the northwestern part. Fur- thermore, the observed durations of phases of diastrophism
shows that subsidence movements of the faulted block areas in the western part of the basin were much slower.
Relative rates of subsidence
The relative rates of subsidence of the faulted block areas during the phase of l la xi mum subsidence are ob- tained by measuring the thickness of that section in the downthrown block which correlates with the abovemen- tioned particular phase of maximum subsidence move-\ ment. This is then expressed in years by taking 1.5 X lo6 years as the equivalent of N4 time interval. (The above figure is taken from Billman's Cenozoic Zonation and Correlation Charts which depicts among others planktonic foraminifera zonations versus absolute radiometric time- scales).
Table 2 shows the obvious role of the faults at Niru and at Limau East-P section.
Actual rate of subsidence
The actual rate of subsidence of faulted blocks is de- termined by the position of the downward moving block with respect to a fixed position throughout a certain time- period.
In this .connection, sedimentary formation surfaces or bottom planes may be regarded as the position of the down- ward moving block with the datum plane or sea-level being the only fixed position during the process of sedimentation
TABLE 1
(See also plates 7 , 8 , 9 , 10, 1 1 and 12).
TABLE 2
Field :
Time interval or phase of maximum sub- sidence of down- going faulted block:
Betung
(N5 in parti- cular) N4-N7
East Tanjung Miring
cmlyear
0.04
Limau East Q-sec.
cm/year
0.05 0.03
- -
East Tanjung Miring
N3/N4- lower N4
N3-N4
Niru
N4-pre N5
Limau East P-sec.
cmlyear
0.07
Limau W + M
cm/year
0.03 0.04
Structure/ Field
Relative rate of movement of subsiding fault block
-- -
Limau West + Middle
N4-N8 N4-N5 N4-pre NS
Limau East Q- section
N4-pre N5 N3 /N4-pre N5
Betung
cm/year
0.01
Niru
cm/year
0.07
in a basin. With diastrophism generally occuring during N4 time
which is equivalent to the time of Talang Akar Formation deposition, i.e. TAFITRM, the thicknesses of the sedimen- tary section which lies between the base of TAFITRM (or top TAFIGRM) and the base of the Baturaja Formation (BRF) is divided by 1.5 million years to obtain actual rates of subsidence.
The study shows the following:
Niru : 0.02 cm/year Limau W + M : 0.04 cmlyear
0.03 cmlyear Limau East-Q section : 0.01 cmlyear
0.02 cmlyear East Tanjung Miring : 0.01 crnlyear
Note: North-South trending basement rooted faults are also observed in the Palembang Basin. These are the Lembak, Kikirn and Lagan-Lenggaran faults.
Fold Characteristics
Although the three main anticlinoria (rows of anticlinal structures) in the Palembang Basin, namely the Muara Enim, PendopolLimau and North Palembang Anticlinoria, are trending parallel with respect to each other, the follow- ing phenomena and anomalies are observed :
The six asymmetric anticlinal structures which form the Muara Enim Anticlinoriurn have very steep to overturn- ed north slopes. This may also be due to thrusting wl~ich has affected most of the anticlinal structures. Structures situated further south are synlmetrically shaped. 'Ihe Pendopo-Limau Anticlinorium comprises two rows of WNW-ESE trending, parallel anticlinal structures. Surface and subsurface horizon maps show that these fold structures within the Neogene sedimentary forma- tions are sloping more steeply to the south. About 40 km further northwest, the so-called Central Palembang anticlinal structures (Mangunjaya, Babat, Kukui, Suban Burung and Keban) possess steeper dipping north slopes. The northernmost situated North Palembang Anticlinori- urn, ca 35 km northeast of the Central Palembang area, on the contrary shows steeper south slopes at Taniiang, Bentayan and Grisik (Plate 13). These phenomena of opposing vergences among anticlinal structures of adjacent areas is also seen in the southeast part of the Palembang Basin. The Meraksa, Kepayang and Riangbandung structures here show inclinations opposed to those observed within the neighbouring Muara Enim Anticlinorium to the west.
shifting direction of longitudinal axes within those structur- es. Examinatin of horizon maps.of successive layers within the Talang Akar Formation (TAF), contour maps on top of the Baturaja Formation (BRF) and surface geological maps with outcropping ABF and MEF, reveals that the aforementioned shifting was a gradual development from an original SW-NE trend within TAF into an E-W trend at top BRF and lastly into the established WNW-ESE trend at the surface.
This phenomenon (or anomaly) is particularly seen at East Lirnau P section and Niru fields (Plate 14; see also Plate 2).
IV DISCUSSION AND CONCLUSIONS
The observations described in the previous chapter show that anonlalous phenomena concerning sedimenta- tion (i.e. optimum rates of deposition), and TAF depocentres (now on top of present uplifted block areas) are principally encountered within the interval of TAF deposition. Those anomalies of structural character such as shifting anticlinal axes from an original SW-NE trend to a WNW- ESE trend and opposing vergences of groups or rows of anticlinal structures within very short distances, are mainly if not wholly detected in the ABF and MEF. Another phe- nomenon, in turn anomalous to the abovementioned, is the GUF attaining remarkable thicknesses in areas which pre- sently are known us depressions or lows (Plate 15)
Within an anticlinorium, fold structures are generaily encountered as an alignment of two or more parallel rows - of anticlines with interestingly, all steeper flanks consistent- ly verging to the same direction. Cross-sections also show that folding and vergence is only obvious in the Air Benakat (ABF) and Muara Eni~n (M EF) Formations (Plate 16).
In conjunction with the structure niap constructed on top of "basement", a pattern of faulted block areas, with major WNW-ESE and N-S trending deep-seated baselllent rooted faults as elements which separate those areas, is evident. In this way, the Palembang Basin (in sensu .stricto) colnprises the Baturaja-Ogan Block, the Muara Enam- Lematang Block, the Pendopo-Liniau Block and the Kukui- Babat Block with the Lematang Fault, the Benuang Fault, the Kikim Fault - Benakat Culley, the Lembak Fault and the Prabumenang Fualt as the intervening elements (Plate 17).
It may also be deduced from the anomalous position of the GUF depocentre, with regard to the Baturaja and Ta- lang Akar Formations, that a marked tectonic event took place within early Middle Miocene time.
Seismic sections across the Lematang Depression and the Pendopo-Limau Anticlinorium, together with observed vergences, the present uplifted position of TAF depocen- tres and fault characteristics only corroborate the above mentioned early Middle Miocene tectonic event which marks the beginning of compressive movements througli-
Shift in direction of longitudinal axes out the Palembang Basin (Plate 18). A consequence of the compressive movements is the
Horizon contour maps of the successive layers within so-called positive structural inversion of the Pendopo-Limau the Tertiary formations at some anticlinal structures in the Block area, which is schematically depicted in Plate 19. Limau Anticlinorium show the phenomenon of a gradually Interactive movements among the separate blocks within
the basin brought about a tilt of these blocks which ex- plains the presently observed pattern of alternating opposing vergences of groups of anticlinal structures within very short distances.
Moreover, the development of the fold structures within the ABF and MEF are not to be disassociated from the role exerted by the underlying Gumai Formation (GUF) which at the time of ABF and MEF deposition had tilted surfaces comprising unconsolidated clays, claystones and shales which behaved as a plastic substratum and which only en- hance lateral and compressive movements towards the di- rection of tilt. Thus, ABF and MEF sand layers were in a way synsedimentarily folded.
Relation to regional tectonics
The foregoing discussion only shows that movements in a vertical and lateral sense i.e. extensional and compressive tectonics, have strongly influenced the basinal history during the Tertiary. Nevertheless, fault characteristics within the Lematang Fault, Kikim Fault-Benakat G d e y and the northernmost Lalang Fault indicate that compressive move- ments were prevdng at the end of Pre-Tertiary times. Only since the Paleocene (?)/Eocene have tensional move- ments influenced the basinal area, with the uppermost Oligocene- lowermost Miocene marking the period of op- timum subsidence of the faulted block areas.
An attempt !:as been made to relate these movements to regional tectonics by correlating the events observed within the Palembang Basin (i.e. vertical movements of block areas during the period of optimum subsidence), with
Schwan's 1980 study concerning the genetic relation bet- ween orogenetic paroxysms and changes in seafloor directi- onlplate motion, and Vail et al's 1977 research concerning the relation between sea-level changes and phases of up- lifts within orogens. In this connection, Schwan's data concerning the Indian ocean-floor and the Sunda (Sumatra, Java) orogen were used since the Palembang Basin is only ca. 100 km from the Sumatra (or Barisan) orogen. Table 3 summarizes the correlation. Also, it is possible to correlate known ocean-floor rates of movement (Uyeda, 1978; Kanamori, 1978; Karig et al., 1979)' and changes in direction of ocean floor motions (Johnson et al., 1974) with the phase of maximum subsi- dence of block areas in the Palembang Basin (Plate 20).
Hydrocarbon prospects
At present, oil and gas production in the Palembang Basin (i.e. the Pendopo-Limau Anticlinorium), is obtained from the TAF and BRF. In the Muara Enim Anticlinorium however, production is known from the regressive sands of the younger formations.
It is generally acknowledged that the South Sumatra backarc basin opened during the Paleogene (Paleocene?) as rifts with a north-south trend resulting from extension. Sediments originating from adjacent uplifted basement blocks, were initially deposited in terrestrial environments. A marine transgression followed in the early Miocene with the deposition of sediments which form the main reselvoir horizons. The observed tectonic event in early Mid-Mio- cene times, which also marks the beginning of compressive
T A B L E 3
1 = correlated events.
1
Changes in seafloor direction/plate motions and related orogenetic paroxysms.
Schwan (1980)
5 Ma
10 Ma
20 18Ma Ma 1 27 Ma
Sea-level changes and related phases of uplift within orogens.
Vait et al. (1977)
- + 6 Ma
10 Ma
22,s Ma
30 Ma
Phase of optimum subsidence of block areas in the Palembang Basin.
This study
Uplift of 1 Pendopo-Limau Block
24.5 Ma i.4) } 26 Ma (N3 28.5 Ma
Age
Pliocene
Miocene ( Late 1
Miocene (Middle)
Miocene (Early) Oligocene
movements in the Palembang Basin, was also responsible for the forming of structural traps by folding and faulting.
With regard to the Palembang Basin in sensu stricto, the main reservoir horizons and/or most prolific structures are seemingly confined within the relatively narrow area which are bound by the north-south trending Kikim Fault- Benakat Gulley in the west and the Lembak Fault in the east.
Thinning of the underlying crust with the uprise of mantle material beneath, has also been contemplated for the area (Pulunggono and Cameron, 1984; Barber, 1985). Certainly, a regime of high heatflow mark this parti- cular north-south trending fairway in the basinal area. On this basis, further oil and gas prospects may be anticipated in this area, if not in the northern as well as in the southern extensions of this zone of tectonic mobility (i.e. the areas presently known as the Corridor Block and the Lematang Block).
Conclusions
a) N-S and WNW-ESE trending major faults were already important structural elements at the end of Pre-Tertiary times in the region. A rugged Pre-Tertiary surface of highs and depressions thus pre-set the general shape and configuration of the Palembang Basinal area prior to Tertiary sedimentation.
b) Subsequent tensional movements during Paleocene (?)/ Eocene to Early Miocene times, for the greater part east-west oriented, enhanced block faulting with con- sequent subsidence of faulted block areas along existing faults and dong newly induced SW-NE trending syn- sedimentary normal faults. Optimum rates of subsidence of the faulted blocks were measured in uppermost Oligocene - most early Miocene times. This phase is also marked by deposition of sediments in a continental- limnic, deltaic-fluvial, littoral and marine to deep-marine environments.
c) The early Middle Miocene marks the beginning of com- pressive movements in the basinal area, obviously con- nected with renewed subduction of the oceanic plate further south and southwest. This compressive phase in basinal history witnesses the uplift of the Pendopo- Limau Block and the development of the Lematang Depression. Interaction among block areas caused tilting of blocks in opposing directions along a WNW-ESE trend with consequent folding and structuring of Air Benakat - and Muara Enim sandlayers on a tilted substratum of plastic GUF clays and shales which were responsible for the presently observed anomalous vergences in the re- ~ognised anticlinoria in the Palembang Basin and readily explains the peculiar gradual shift in the directions of longitudinal axes throughout the sedimentary column as seen in the Limau area.
d) Tectonic development and mobility were seemingly res- tricted within a relatively narrow, north-south trending zone in the Palembang Basin. This particular fairway within the basinal area which contains the bulk of sedi-
ments deposited and is furthermore prone to the region's highest heatflow, may have its extensions furtner north and further south, thus opening up further possibilities for hydrocarbon exploration.
e) With structuring of ABF and MEF sediments already established within Late Middle Miocene times, the so- called Plio-Pleistocene tectonic phase for the area may be regarded solely as the final (or paroxysmal) stage of a diastrophism which had started earlier.
Selected References
Barber, A.J., 1985, "The Relationship between the Tectonic Evolution of Southeast Asia and Hydrocarbon Occcur- rences"; Tectonostratigraphic Terranes, Pacific South- west Quadrant.
Bemmelen, R.W. van., 1949, "The Geology of Indonesia"; Gov. Printing Office, The Hague, 1949.
Bilman, H.G., 1973, "Indo-Pacific Area Cenozoic Zonation and Correlation", Compiled by Harold G. Billman : Oil Company internal report (unpublished).
Blow, W.H., 1969, "Late Middle Eocene to Recent Plank- tonic Foraminifera1 Biostratigraphy"; Proc. I st Internat. Conf. Planktonic Minofossils, Vol. 1, 1969, pp 199-422.
De Coster, G.L., 1974, "The Geology of the Central and South Sumatra Basins"; Proc. 3rd Ann. Conv. Indo- nesian Petroleum Association, June 1974., Jakarta.
Johnson, B.D., C. Mc. A. Powell and J.J. Veevers, 1976 "Spreading History of the Eastern Indian Oceah and Greater India's Northward Flight from Antarctica and Australia"; Geol. Soc. of Amerika Bulletin, Vol. 87, Doc. NO. 61 104, pp 1560 - 1 566.
Musper, K.A.F.R., 1937, "Geologische Kaart van Sumatra"; Toelichting bij blad 16 (Lahat), 1:200.000 : Dienst Mijnbouw Ned Indie, 1 10 pages.
Pullunggono, A., 1983, "Sistem Sesar Utama dan Pemben- tukan Cekungan Palembang"; Dissertasi Doktor, Insti- tut Teknologi Bandung (ITB) Bandung, pp. 239 (unpubl.)
Pulunggono, A. and Cameron, N.R., 1984, "Sumatran Microplates, their Characteristics and their Role in the Evolution of the Central and South Sumatra Basins"; Proc. 13th Ann. Conv. Indonesian Petroleum Associa- tion, May 1984, Jakarta.
Pustowka, A.E., 1939, "Die Tiefentektonlk von Aufgescho- benen Antiklinalen"; Shell Indonesia Report (unpubl.).
Schwan, W., 1980, "Geodynamic Peaks in Alpinotype Oro- genies and Changes in Ocean Floor Spreading During Late Jurassic - Late Tertiary Time"; Bull AAPG, Vol. 64, No. 3, pp 359-373.
Spruy t , J.M., 1956, "Onderverdeling en Nomenclatuur der Tertiaire Sedimenten van het Palembang-Jambi bekken"; Shell EP ReportIPertamina Report (unpubl.)
Uyeda, S. and H. Kanamori., 1979, "Back-arc Opening and the Mode of Subduction"; Journal of Geophysical Research, 1979.
Vail, P.R., R.M. Mitchum Jr. and S. Thompson III., 1977, "Global Cycles of Relative changes of Sea Level"; The AAPG Memoir no. 26,1977.
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Spruy t (1956) De Coster (1974) Shell Team( 1978)
:tigraphic subdivision FORMATIONS FORMATIONS AGE - rternary sediments QUARTERNARY
KASAITUFF FM(UAF) UPPER PALEMBANG KASAl FM
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A Pulunggono,l982,1986
STRATI GRAPHIC NOMENCLATURE PRESENTLY IN USE IN THE
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