CENTRAL SUMATRA BASIN

CENTRAL SUMATRA BASIN

Regional Tectonics

The Central Sumatra Basin is the largest tertiary sedimentation hydrocarbon producer in Indonesia. Judging from its tectonic position, the central Sumatra Basin is a hollow behind the arc.

This central Sumatra basin extends to the northwest-southeast, where its formation is influenced by the presence of the Indian-Australian plate subduction under the Asian plate. The boundary of the southwestern basin is the Barisan Mountains, which are composed of pre-Tertiary rocks, while to the Northeast are limited by Sundanese exposure. The southeast boundary of this basin is the Thirty Mountains which also separates the central Sumatra Basin from the southern Sumatra Basin. The boundary of the northwest basin is the Asahan Bow, which separates the central Sumatra Basin from the northern Sumatra Basin.

Map of Sumatra plate movement and other Southeast Asian regions at present

The process of subduction of the Indian-Australian plate produces a stretch of crust in the lower part of the basin and results in the emergence of hot convection upward and flanked by magma with the resulting magma products which are mainly acidic, the nature of magma inside and hipabisal. In addition, there is also a flow of heat from the mantle towards the top through fault lines. Overall, these are the things that cause high heat flow in the central Sumatra basin area (Eubank et al., 1981 in Wibowo, 1995).

Central Sumatra Basin location and its boundaries

The main controlling factor of regional geological structures in the central Sumatra basin is the existence of a Sumatran Fault formed in the limestone era. Sloping plate subduction from the southwestern direction of Sumatra results in strong dextral wrenching stress in the central Sumatra Basin (Wibowo, 1995). This is reflected in the steep field of fault that changes throughout the course of rock laying, the upward fault structure and the flower structure formed during tectonic inversion and structural reversals. In addition, the formation of the folding axis in the direction of the fault mode with sediment thickening occurs in the inverted part (Shaw et al., 1999).

The geological structure of the central Sumatra basin has a pattern similar to that of the South Sumatra basin, where the main pattern of structures developing in the form is the Northwest-Southeast and North-South structures (Eubank et al., 1981 in Wibowo, 1995). However, the North-South trending structure is far more dominant than the Northwest-Southeast structure.

The tectonic elements that make up the configuration of the Central Sumatra Basin are influenced by the pre-Tertiary High-Low morphology, the effect of High-Low structure and morphology on the basin configuration in the Central Sumatra Basin (Bengkalis Graben area), including depocenter distribution from graben and half graben. The Northwest-Southeast Basement lineage is very visible in this area and can be traced along the central Sumatra basin. This line has been formed and reactivated by the youngest tectonic movements (Plio-Pleistocene tectonism). However, this basement lineage can still be observed as a component that influences the formation of formations from the Paleogene basin in the central Sumatra Basin.

The tectonic history of the central Sumatra basin in general can be summarized into several stages, namely:

1. Basement consolidation in the Yura era, consisting of sutures that trended Northwest-Southeast.
2. The basement was exposed to magmatism and erosion during the late Yura and Cretaceous times.
3. Extensional tectonics during the Early and Middle Tertiary Tertiary (Paleogen) yielded a graben system with a North-South and Northwest-Southeast direction. The relation of this tectonic activity to the paleogeomorphology in the central Sumatra Basin is the occurrence of changes in depositional environment from terrestrial avalanches, swamps to lacustrine environments, and closed by fluvial-delta environmental conditions at the end of the rifting phase.
4. As long as the deposition takes place in the late Oligocene to the beginning of the early Miocene which deposits the main reservoir rock from the Sihapas group, Sumatra's tectonics are relatively quiet. Clastic sediments were deposited, mainly sourced from the Sunda land and from the Northeast covering the Malay Peninsula. The process of sediment accumulation from the northeastern direction of the island of Sumatra towards the basin, is accommodated by the presence of North-South trending structures. The sedimentation condition in the middle of the Tertiary is more influenced by global sea level (eustation) fluctuations that produce transgressive sedimentation episodes from the Sihapas group and the Telisa Formation, closed by regressive sedimentation episodes which produce the Petani Formation.
5. The end of the late Miocene volcanism increased and tectonism returned intensively with the compression regime lifting the mountain ranges in the southwest of the basin. Mountains These rows are a source of later basin fill. The direction of sedimentation in the late Miocene in the Sumatra Basin is running from the south to the north with control of the trending structures north of the south.
6. Compressive Plio-Pleistocene tectonism results in the inversion of the Basement structure forming upward and fold faults that traverse Northwest-Southeast. This Plio-Pleistocene tectonism also produces regional inconsistencies between the Minas formation and quarter alluvial deposits against the formations below.

Regional Stratigraphy

The process of sedimentation in the central Sumatra Basin begins at the beginning of tertiary (Paleogen), following the process of forming a half graben basin that has been going on since the Cretaceous period to the beginning of tertiary.

Basement configuration of the basin is composed of metasedimentary rocks in the form of greywacke, quartzite and argilit. This basic rock is thought to be Mesozoic. In some places, these metasedimentary rocks are intruded by granite (Koning & Darmono, 1984 in Wibowo, 1995).

Central Sumatra Regional Basin Stratigraphy

In general, the sedimentation process for filling this basin can be grouped as follows:

Rift (Pematang Siklis)

Overall, these basin fill sediments in the extensional tectonic phase (rift) are grouped as Pematang Group composed of claystone, carbonan flakes, fine sandstones and colorful siltstone. Weak seismic reflection and strong amplitude in seismic data provide indications of facies associated with the lacustrine environment.

Precipitation at the beginning of the rifting process takes the form of terrestrial clastics and lacustrine from the Lower Red Bed Formation and Brown Shale Formation. Upward towards the late rifting phase, the sedimentation completely changed into the lacustrine environment and deposited the Pematang Formation as Lacustrine Fill sediments.

a) Lower Red Bed Formation

Composed of claystone colored red - green, siltstone, gravel sandstones and a little conglomerate and breccias which are composed of pebble quartzite and filit. Deposition conditions are interpreted in the form of alluvial braid-plain seen from the number of muddy matrix in conglomerates and breccia

b) Brown Shale Formation

This formation contains quite a lot of organic material, characterized by dark brown to black colors. Composed of flakes with siltstone inserts, in some places there are intervals of sandstones, conglomerates and paleosols. The thickness of this formation reaches more than 530 m in the depocenter section.

This formation is interpreted deposited in deep lake environments with anoxic conditions seen from the absence of evidence of bioturbation. Intercalation of sandstone-conglomerate sandstones was deposited by the fluvial channel fill process. Intersecting the center of this formation, there are several paleosol horizons which are possible to form at the edges of the lake that surface (local horst), shown by rock core recordings in the Bukit Susah complex. Tectonically, this formation is deposited in a rapidly decreasing basin so that fluvial activity is not so dominant.

c) Coal Zone Formation

Laterally, this formation is in several equivalent places with the Brown Shale Formation. This formation is composed of alternating shale with coal and a little sandstone.

The depositional environment of this formation is interpreted as a shallow lake with control of a non-dominant fluvial process. Judging from the configuration of the basin, this formation is deposited in the shallow area in the active part of the graben away from the depocenter.

d) Lake Fill Formation

Composed by sandstones, conglomerates and shale. The rock composition is mainly in the form of dominant filitic clusters, vertically increasing the content of litoclase quartz and quartzite. Normal gradation sedimentary structures with several reverse gradations indicate a fluvial-deltaic depositional environment.

This formation is deposited progradation in the fluvial environment towards the delta in the lake environment. During the deposition of this formation, tectonic conditions began to calm down with decreasing basins that began to slow down (late rifting stage). The thickness of the formation reaches 600 m.

e) Fanglomerate Formation

Deposited along the descending portion of the cesare as a series of alluvial deposits. Composed of sandstones, conglomerates, few claystones are green to red. Both vertically and laterally, this formation can transition into the Lower Red Bed formation, Brown Shale, Coal Zone and Lake Fill.

In some areas such as in the Safe Sub-Basin, the last two formations (Lake Fill and Fanglomerat) are considered to be one unit equivalent to the Pematang Formation based on the nature and distribution of the seismic cross section.

Sag

Inconsistently above the Neutral sediment group deposited. This sedimentation phase begins with episodes of transgression represented by the Sihapas Group and reaches its peak in the Telisa Formation.

(Siklis Sihapas - initial transgression)

The Sihapas group formed at the beginning of the transgression episode consists of the Menggala Formation, the Bangko Formation, the Bekasap Formation and the Duri Formation. This group is composed of fluvial-deltaic environmental clastic rocks until the shallow sea. Deposition of this group takes place in the early Miocene - middle Miocene.

a) Digala Formation

Composed by conglomerate sandstones with coarse grain sizes ranging from gravel to medium grain size. Laterally, these sandstones graded into medium to fine sandstones. The main composition of rock is the dominant quartz, with sedimentary structure through cross-bedding and erosive basal scour. Based on the constituent lithology it is estimated that it is deposited on the fluvial-channel braided stream environment.

This formation is distinguished by Lake Fill Formation from the upper bundling group based on the absence of oxygenated clay in the matrix (Wain et al., 1995). The thickness of this formation reaches 250 m, estimated to be in the early Miocene age.

b) Bangko Formation

This formation is composed of carbonan flakes with alternating medium-fine sandstones. Deposited in open ocean exposure environment. From the fossil planktonic foraminifera obtained at the age of N5 (Blow, 1963). The maximum thickness of the formation is approximately 100 m.

c) Bekasap Formation

This formation is composed of medium-coarse-sized massive sandstones with little shale intercalation, coal and limestone. Based on the lithology and fossil characteristics, this formation is deposited in brackish water and open sea environments. Fossils on shale show the age of N6 - N7. The thickness of the entire formation reaches 400 m.

d) Duri Formation

At the top in several places, this formation was equivalent to the Bekasap formation. Composed of fine-medium sandstones and flakes. Maximum thickness reaches 300 m. This formation is N6 - N8.

(Telisa Formation - final transgression)

The Telisa Formation representing sedimentary episodes at the peak of transgression is composed of flakes with a slight intercalation of fine sandstones at the bottom. In some places there are limestone lenses at the bottom of the formation. Upward, lithology turns into flakes characterizing deeper environmental conditions. Interpreted the formation environment of this formation in the form of Neritik environment - upper Bathyal.

Regionally, marine shale from this formation has the same age as the Sihapas group, so the contact of the Telisa Formation with below is the transition of different lithological facies in the stratigraphic position and location. The thickness of this formation reaches 550 m, from the analysis of fossils obtained age N6 - N11.

(Petani Formation - regression)

Composed of fossil-rich gray shale, a little carbonate with several layers of sandstone and siltstone. Vertically, the tuff content in rocks is increasing.

During the deposition of this unit, compression and volcanic tectonic activity became active (the initial removal of Bukit Barisan), resulting in abundant volcanic material. Global seawater conditions (eustation) fluctuate significantly with a decrease in sea level so that some local inconsistencies are formed in several places.

This formation is deposited in regressive episodes in harmony above the Telisa Formation. However, to the north-east locally this formation has contact not in harmony with the formation below. The maximum thickness of this formation reaches 1500 m, deposited in the middle Miocene - Pliocene.

INVERSION

At the end of the tertiary major tectonic activity occurs in the form of a peak from the elevation of Bukit Barisan which results in regional inconsistency in Plio-Pleistocene. This tectonic activity results in the inversion of the fault structure down to an upward fault. In this inversion tectonic phase Minas Formation is deposited which is composed of land deposits and alluvium in the Pleistocene age of conglomerates, sandstone, gravel, clay and alluvium - Resen.

Reference:

• Moulds, P.J., 1989, Development Of The Bengkalis Depression, Central Sumatra and Ins Subsequent Deformation – A Model for Other Sumatran Grabens, Proceedings Indonesian Petroleum Association – Eighteenth Annual Convention vol.1, Jakarta.
• Shaw, J.H., Hook, S.C. dan Sitohang E.P., 1999, Extensional Fault-Bend Folding and Synrift Deposition: An Example from the Central Sumatra Basin, Indonesia, AAPG Bulletin, V. 81, No. 3 - Online presentation.
• http://www.searchanddiscovery.net/documents/Indonesia
• Wain, A.S. dan Jackson, B.A., 1995, New Pematang Depocentres on The Kampar Uplift, Central Sumatra, Proceedings Indonesian Petroleum Association – Twenty Fourth Annual Convention vol.1, Jakarta.
• Wibowo, R.A., 1995, Pemodelan Termal Sub-Cekungan Aman Utara Sumatra Tengah, Bidang Studi Ilmu Kebumian – Program Pasca Sarjana Institut Teknologi Bandung, Unpublished.