PALEOGEOGRAPHY RECONSTRUCTION OF PEMATANG GROUP IN NORTH AMAN TROUGH, CENTRAL SUMATRA BASIN

• Main Author: Ferari Budhi Prasojo, Kirandra
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/53533
• Institution: Institut Teknologi Bandung

Central Sumatra Basin is a proven hydrocarbon prolific basin. Most of hydrocarbon accumulations are in post-rift interval of Sihapas Group, meanwhile the Pematang Group is still considered under-explored. Nevertheless, there are remaining risk and uncertainty of reservoir geometry and distribution in this interval. The previous study on Pematang Group were not utilizing 3D seismic with integration with well data to identify sandstone reservoir distribution in North Aman Trough. Reconstruction of paleogeography in Pematang Group with integration of well and seismic data as in this study will be useful to reduce risk and increase chance to find another hydrocarbon accumulation. Approximately 1,230 sq. km of 3D merged post-migrated seismic cube which were tied to 45 wells in North Aman Trough have been interpreted with tectonostratigraphic approach. Main structure on this area is a series of Northeast – Southwest normal fault that was developed during Eo-Oligocene. Those faults were inverted to thrust fault during Middle Miocene – recent. Paleogeography reconstruction were inferred from conventional core data, wireline log electro-facies, seismic geomorphology, isopach map as proxy of paleo-topography and rift tectonostratigraphy. Biostratigraphy data suggests gradual changes on Eocene-Oligocene paleoenvironment from terrestrial to marine transition. The first appearance of foraminifera and nannoplankton fossil were in early Miocene as suggestion to marine incursion. Core interpretation shows heterolithic structure of flaser and lenticular as indication of rapid changes of sediment flow controlled by rain intensity or climate. North Aman Trough can be classified into three areas based on its half-graben unit: North, Central and South. The evidence of variance in fault activity during rift development and filling could be inferred from variance of sediment thickness on hanging wall blocks. The variance of fault activity possibly associated with fault orientation in correspond to maximum principles stress orientation. According to seismic data there are 5 (five) tectonostratigraphy unit bounded by lateral termination on seismic reflector such as: onlap, downlap, toplap and truncation. Meanwhile based on rift tectonostratigraphy model there are 3 (three) phases: 1) Rift Initiation – Lower Red Bed (Pematang-1), 2) Rift Climax – Brown Shale (Pematang-2) and 3) Immediate Post rift – Upper Red Bed (Pematang 3-5). Paleogeography reconstruction on Rift Initiation suggests 4 (four) main depositional environments: 1) fluvial meandering, 2) alluvial fan and 3) alluvial plain. On the next phase of Rift Climax, there are 8 (eight) main depositional environments: 1) deep lacustrine, 2) shallow lacustrine, 3) marginal lacustrine, 4) nearshore lacustrine, 5) deltaic, 6) fluvial meandering, 7) alluvial plain dan 8) fan delta. Last, on the Immediate Post-rift there are 2 (two) main depositional environments: 1) fluvial braided and 2) alluvial plain.