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December 12, 2006

Geology Of South Sumatra Basin

Filed under: Geology of Indonesia — haris @ 7:20 am

The South Sumatra basin is located to the east of the Barisan Mountains and extends into the offshore areas to the northeast and is regarded as a foreland (back-arc) basin bounded by the Barisan Mountains to the southwest, and the Pre-Tertiary of the Sunda Shelf to the northeast (de Coster, 1974). The
South Sumatra basin was formed during east-west extension at the end of the Pre-Tertiary to the beginning of Tertiary times (Daly et al., 1987).

TECTONIC SETTING

The structural features present in the basin are the result of the three main tectonic events. They are Middle-Mesozoic orogeny, Late Cretaceous-Eocene tectonism and the Plio-Pleistocene orogeny. The first two events provided the basement configuration including the formation of half grabens, horsts, and fault block. The last event, the Plio-Pleistocene orogeny, resulted in formation of the present northwest-southeast structural features and the depression to the northeast (de Coster, 1974). Structurally the 21 block is located in the Palembang High. This high is located at the southern edge of a positive feature in the eastern margin of

South
Sumatra
Basin.

STRATIGRAPHY

The sediments of the

South
Sumatra
Basin comprise an economic basement of Pre-Tertiary rocks that is overlain unconformably by a thick Tertiary sequence. The first Tertiary sedimentation occurred during Early Oligocene and gave rise to the Lahat Formation consisting of mainly volcanic rocks, claystone and shale that was deposited locally in the graben areas. The Talang Akar Formation (Late Oligocene) overlies the basement where the Lahat Formation is missing. It is a transgresive sequence resulting from Late Oligocene to Middle Miocene subsidence. The later sedimentation during mid-Miocene to recent produced a regressive sequence (de Coster, 1974).

In most parts of the Palembang High, sediment deposition started in Early Miocene, except in local low areas where terrestrial to nearshore sediments of Talang Akar Formation (Oligocene) and terrestrial sediments of Lahat and Lemat Formation (Eo-Oligocene) had been deposited. In many places, the Talang Akar, the Lahat and the Lemat Formations cannot be distinguished from each other and hence are called Pre-Baturaja clastics. During Early Miocene, carbonate sediments of the Baturaja Formation were developed either directly above the Pre-Tertiary basement or above a thin Talang Akar Formation. Off the Palembang High, hundreds of feet thick of the Talang Akar Formation are present (Ferianto et al., 2005).

Approximately 17.5 MYBP the sea level dropped, and the Baturaja carbonates were exposed and subject to diagenetic processes that led to development of good secondary porosity. Afterward, the sea level rose again and the Telisa marine shales (Early-Mid. Miocene) were deposited. In around 15.5 MYBP the sea level gradually dropped again and nearshore to terrestrial sediments of Palembang Formation (Upper Miocene-Pliocene) were deposited (Ferianto et al., 2005).

Plio-Pleistocene orogeny (in 2.8 MYBP) created the current structures in the region. A NW-SE striking monocline dipping to SW is the predominant feature there. The detail Stratigraphy of South Sumatra Basin is described in other literature, such as De Coster (1974) (Ferianto et al., 2005).

Regional sediment sources were generally from the Sunda Plate to the north and
Palembang or Lampung High to the east (Sitompul et al., 1992). Maximum transgression in the middle Miocene deposited the marine Gumai Shale Formation seal across the region before uplift and compression resulted in deposition of shallow marine and continental sandstones and shales. Development of the

Barisan
Mountains, and possible volcanic islands to the south and southeast, further decreased and then cut off and overwhelmed marine influences and added new clastic and volcaniclastic sources from those directions. Erosion of northwest trending anticlines that were formed during compression resulted in local Plio-Pleistocene continental deposits within the intervening synclines. Continued volcanic activity has covered much of the surface of the

South
Sumatra
Basin (Bishop, 2000).

PETROLEUM SYSTEM

SOURCE ROCK Hydrocarbons in

South
Sumatra
Province are derived from both lacustrine source rocks of the Lahat Formation and terrestrial coal and coaly shale source rocks of the Talang Akar Formation. The lacustrine source rocks were deposited in a complex of half-grabens whereas the subsequent coal and coaly shales were deposited in and extended beyond the limits of the half-grabens. The Batu Raja Limestone and the Gumai Formation shales may also be mature and have generated hydrocarbons in local areas (Sarjono and Sardjito, 1989, Bishop, 2000).

The middle to late Eocene through early Oligocene Lahat or Lemat Formation includes the Benakat Shale with oil prone Type I and II kerogen and gas prone Type III kerogen. Organic material in the late Eocene through middle Miocene Talang Akar Formation contains oil- and gas-prone Type I, II and III kerogen. The general temperature gradient in
South Sumatra is 49o Ckm-1 (Hutchinson, 1996 in Bishop, 2000).

TRAP TYPES

Northwest to southeast trending anticlines were the first traps explored and remain the most important traps in the
South Sumatra basin. Drape, facies-change, and stratigraphic traps are also important and may be attractive future targets (Bishop, 2000).

RESERVOIR ROCK There are several formations in the

South
Sumatra
Basin which play as reservoir rocks, event basement rocks. They are basement rocks, Lahat Formation, Talang Akar Formation, Batu Raja Formation, Gumai Formation, Air Benakat Formation, and Muara Enim Formation (Bishop, 2000).

Uplifted areas and paleohighs of Mesozoic and Eocene fractured and weathered basement granite and quartzite are effective reservoirs, in ten fields in
South Sumatra (Sardjito et al., 1991; Bishop, 2000).

The Eocene to Oligocene Lahat Formation is composed of synrift deposits that are as much as 1,070 m thick. Although locally absent, this formation, is in most locations, more than 760 m thick. The formation was deposited in continental, lacustrine, and brackish lacustrine depositional settings (Hutchinson, 1996 in Bishop, 2000).

The Talang Akar Formation is as much as 610 m thick. It is a late synrift to post-rift formation that is thick where the underlying Lahat Formation is thickest. The Talang Akar Formation unconformably overlies the Lahat Formation. It onlaps the Lahat and the basement, extending farther outside of the depositional basins than the depositional limits of the Lahat Formation. This reservoir consists of quartzose sandstones, siltstones, and shales deposited in a delta plain setting that changed basinward, generally to the south and west, into marginal marine sandstones and shales (Hutchinson, 1996 in Bishop, 2000). Specific depositional environments that have been identified include open marine, nearshore, delta plain, delta, distributary channel, fluvial, and beach (Hutapea, 1981 in Bishop, 2000).

The Batu Raja formation consists of widespread platform carbonates, 20—75 m thick, with additional carbonate buildups and reefs, from 60—120 m thick. The Basal Telisa is shale and calcareous shale deposited in deeper water as the carbonates were being developed on the platforms and highs. Reservoir porosity ranges from 18—38% and reservoir permeability is as much as 1 Darcy. Porosity has been enhanced in the upper parts of the formation due to subaerial exposure late in the early Miocene, at approximately 17.5 Ma, and also because of only partially cemented fractures (Hartanto et al., 1991; Bishop, 2000).

The Oligocene to middle Miocene Gumai Formation, is composed of fossiliferous marine shales with thin, glauconitic limestones that represent a rapid, widespread maximum transgression. The thickness of the Gumai Formation varies and is as much as 2,700 m thick in basins. The formation thins at basin margins and across highs. The Gumai Formation is the regional seal for the Batu Raja Limestone in
South Sumatra but also contains some reservoir intervals (Hutchinson, 1996 in Bishop, 2000).

The middle Miocene Air Benakat Formation was deposited during the regression that ended deposition of the Gumai Shale. The Air Benakat Formation changes upward from deep marine to shallow marine conditions. Marine glauconitic clays decrease in frequency and marine sands increase. The formation ranges from 1,000—1,500 m thick. Coal beds mark the upper contact with the overlying Muara Enim Formation. The average porosity of the sandstone is 25% (Bishop, 2000).

The Late Miocene to Pliocene Muara Enim Formation, also known as the Middle Palembang Formation, was deposited as shallow marine to continental sands, muds, and coals. The formation thins to the north from a maximum of 750 m in the south. Uplift of the

Barisan
Mountains provided source terrains for clastics from the south and southwest during deposition of the Muara Enim Formation (Bishop, 2000).

SEAL ROCK

The Gumai Formation represents the maximum highstand transgression following development of Batu Raja carbonates. Shales of this regional formation seal carbonate reservoirs and locally seal a series of stacked sandstone reservoirs of the Talang Akar Formation (Hartanto et al., 1991). Hydrocarbons that are found above the regional seal either have migrated there due to faults that broke the seal during the compression phase or were generated by the Gumai Formation shales in local areas where this formation might be mature. Intraformational seals within the Talang Akar consist of shallow marine and overbank shales that are important seals that compartmentalize the sandstones (Bishop, 2000).

References:

Argakoesoemah R.M.I., Raharja M., Winardhi S., Tarigan R., Maksum T.F., Aimar A., 2005, Telisa Shallow Marine Sandstone As An Emerging Exploration Target In Palembang High, South Sumatra Basin, In proceedings Indonesian Petroleum Association, Thirtieth Annual Convention, Jakarta

Bishop M. G., 2000, South Sumatra Basin Province, Indonesia: The Lahat/Talang Akar-Cenozoic Total Petroleum System, U.S. Department of The Interior U.S. Geological Survey, http://geology.cr.usgs.gov/energy/WorldEnergy/OF99-50S/province.html

De Coster G. L., 1974, The Geology of the Central and

South
Sumatra
Basins
. In proceedings Indonesian Petroleum Association, Third Annual Convention,
Jakarta, pp. 77 – 110.

Dally M. C., Hooper B. G. D., and Smith D. G., 1987, Tertiary Plate Tectonics and Basin Evolution in
Indonesia
. In proceedings Indonesian Petroleum Association, Sixteenth Annual Convention,
Jakarta, pp. 399-428.

Feriyanto, Kamil F., Kusnandar Y., and Yanto Y, 2005, Successful Identification of Thin Carbonate On Paleo-Basement High: Special Case In Palembang High, South Sumatra Basin, In proceedings Indonesian Petroleum Association, Thirtieth Annual Convention, Jakarta.

Hartanto, Karsan, Widianto, Eko, and Safrizal, 1991, Hydrocarbon prospect related to the local unconformities of the Duang area,

South
Sumatra
Basin
: Proceedings Indonesian Petroleum Association Twentieth Annual Convention, October, 1991, p. 17-36.

Kamal A., 1999, Hydrocarbon Potential in the Pasemah Block, A Frontier Area in South Sumatra, In proceedings Indonesian Petroleum Association, Twenty Seventh Annual Convention, Jakarta, pp. 49-63.

Sarjono, S., and Sardjito, 1989, Hydrocarbon source rock identification in the
South Palembang sub-basin
: Proceedings Indonesian Petroleum Association Eighteenth Annual Convention, October, 1989, p. 427-467.

Sardjito, Fadianto, Eddy, Djumlati, and Hansen, S., 1991, Hydrocarbon prospect of the pre-Tertiary basement in Kuang area,
South Sumatra
: Proceedings Indonesian Petroleum Association Twentieth Annual Convention, October, 1991, p. 255-278.

Sitompul N., Rudiyanto, Wirawan A., and Zaim Y., 1992, Effects of Sea Level Drops During Late Early Miocene To The Reservoirs In
South
Palembang
Sub
Basin, South Sumatra,
Indonesia
, Proceedings Indonesian Petroleum Association Twenty First Annual Convention,

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3 Comments »

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