Bulletin of the Geological Society of Malaysia, Volume 37, July 1995, pp. 179 – 190
Idemitsu Oil Exploration (East Malaysia) Co. Ltd., Miri, Sarawak, Malaysia
Abstract: The Baram Delta oil and gas province covers a major part of the onshore and offshore areas of northern Sarawak, Malaysia and Brunei. Thick marine to deltaic sediments of Late Eocene to Pleistocene age were rapidly deposited in the basin resulting in widespread abnormal formation pressures. Overpressure zones are normally encountered within thick marine-deposited claystone intervals occurring below and/or just at the basal part of major oil and gas accumulations in the offshore areas of the Baram Delta Province. Due to drilling hazards, the onset of overpressuring is regarded as an arbitrary economic limit for oil exploration.
In onshore Sarawak, south of the Baram Delta Province are thick Late Eocene to Miocene (Cycle I-V) sediments deposited and uplifted after latest Miocene age. Abnormal pressure zones have been encountered in three different settings viz. a) in undercompacted claystone-predominant interval, b) in uplifted tight sandstone beds with hydrocarbons and c) in hydrodynamically pressured shallow sand intervals. A unique overpressure zone which occurs below stacks of hydrostatic and permeable coastal-deltaic sands has been observed.
The formation of these varied overpressure zones and the pressure regime in the area are studied in combination with the burial and structural development history of the southern margin of the Baram Delta Province. The implications to hydrocarbon expulsion, migration and entrapment are analyzed.