Abstract

The Malacca Strait PSC is a mature region of the Central Sumatra Basin which has yielded prolific oil and minor gas discoveries. The principal, and probably the only source rock for Malacca PSC oil is the Palaeogene, lacustrine Brown Shale Member of the Pematang Group, which is known to be mature in the Bengkalis Graben, although significant petroleum potential also occurs within coals of the overlying Sihapas Group. A multi-1D modeling exercise using Platte-River's BasinMod™ for Windows™ 3.1 was performed on two small sub-basins proximal to discoveries up to 20 km from the Bengkalis Graben, to determine their maturity, and the probability of their having sourced these fields. Possible volumes of petroleum expelled, and migration directions were determined to assist in deciding the most prospective areas for future drilling.

The study involved a comprehensive review of thermal and geochemical data to provide a firm basis for modeling. A number of wells and pseudowells were modeled in each sub-basin, using a variable gradient method to match measured temperatures. The method of matching vitrinite reflectance results was downgraded as they were significantly depressed in wells drilled in the region, and thus a poor guide to the level of organic maturation.

The modeling indicates that the discovery farthest from the known kitchen (Bengkalis Graben) is likely to be sourced by long distance migration (some 25 km), as the local sub-basin (Rangsang Trough) is immature for expulsion from both source rocks. The other sub-basin (Padang Trough) by contrast is highly mature at present day due to a very high geothermal gradient, and could have expelled almost 1 billion barrels of oil from the Brown Shale. There is some evidence that the heating event responsible for petroleum expulsion is extremely recent in the Central Sumatra Basin.