Abstract The geological history of the Central Sumatra Basin, situated between the Bila River to the north and the Kampar River to the south, is discussed. The structural style in Central Sumatra is produced by a tensional regime strongly overprinted by a dextral wrench component. Compression does not appear to have been a dominant force; most compression observed in the Tertiary section can be ascribed to that component of wrench tectonics. Factors that have contributed to the prolific oil production of the Central Sumatra Basin include presence of graben sequences with thick organic-rich lacustrine shales, an overlying marine sag sequence with excellent reservoirs, development of early structures and high basin heat flows. The shallow Pre-Tertiary basement and thin sediments of the Central Sumatra Basin are characterized by a very high geothermal gradient, with an average gradient of 6.1°C/100 m (3.38°F/100′). Distribution of the oilfields in the Central Sumatra Basin is largely fault controlled. The oil has migrated vertically out of the Eocene-Oligocene age Pematang Group shales and laterally up the flanks of the graben, generally in an easterly direction, filling younger, Miocene age Sihapas Formation reservoirs. Hydrocarbon accumulations are also present within the rift-fill sequence. Migration distance is generally short, ranging up to 20 km. Models are presented to explain the structural evolution and depositional history of the Pematang graben. Oil, and most of the gas, in the Central Sumatra Basin has been generated from organic-rich shales of lacustrine origin within the Palaeogene Pematang Group. During Palaeogene time, large freshwater lake systems developed within structurally controlled rift graben. Palynology indicates freshwater conditions prevailed, however, occurrences of slightly saline conditions suggest minor climatic changes, chemical stratification, or occasional marine incursions. Tropical climatic conditions without an annual lake turnover favoured lacustrine anoxia and deposition of non-marine hydrocarbon source beds. Climatic changes strongly influence the stratigraphic position of oil-prone source material in the Pematang Brown Shale Formation. Differences in oils, however, appear to reflect the different depositional and environmental histories of the individual lake systems. Geochemical analyses are used in the identification of source beds, interpretation of their depositional environments, oil to source rock correlations and determining the maturation history of the source rock. Systematic variations in organic matter reflect different depositional environments and largely determine the composition of the hydrocarbons generated.
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