ABSTRACT: Malay Basin natural gases: classification, distribution and geochemical controls

Hoesni, Mohammad Jamaal and Peter Abolins , PETRONAS Research & Scientific Services Sdn Bhd, 43000 Kajang, Selangor, Malaysia

The Malay Basin, located east of Peninsular Malaysia, contributes a third of Malaysia's gas production. Total gas reserves are estimated to be in excess of 30 TCF. This will probably increase in the near future as targeted reservoirs are becoming deeper. The main contributary factors leading to this abundant supply are the presence of a thick, often coaly, source sequence and a high geothermal gradient.

Several factors control the hydrocarbon composition of a gas. In the Malay Basin such factors include: biogenic vs thermogenic origin, source maturity (wet vs dry thermogenic), and source rock type (lacustrine vs coaly).

Superimposed on this composition is the input of large volumes of inorganic-derived carbon dioxide. Furthermore, post emplacement process, such as biodegradation, also influence the gas composition and isotopic signature. This paper discusses the role of each of the above factors and the resulting distribution of gases in the Malay Basin.

The gases can be genetically grouped based on isotopic and compositional data. Gases found in shallower reservoirs in the basin axis area have heavier methane and ethane isotopic values, indicative of high maturity. Compositional and isotopic differences of methane-ethane suggests these dry thermogenic gases were derived from a coaly percursor, and migrated vertically from deep matured sources together with inorganic CO₂. Gases in the older reservoirs (late Middle Miocene and older) are mainly oil-associated gases and cracked oil. They could either be derived from coaly or lacustrine shale sources, as recognized from their reservoir age.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia