Abstract: Petroleum Occurrences in S.E. Asia Tertiary Basins

Matthew Barrett

Approximately 20 billion barrels of oil has been produced to date from the Tertiary basins of Southeast Asia. In a broad sense the geology of these basins is remarkably uniform.

Southeast Asian basins are characterized by several geologic phases: (1) Eocene-Oligocene extension, clastic fill of half grabens; (2) Oligocene basin "swamp" fill and anoxic stagnation, development of oil prone terrestrial source facies in shallow marine settings; (3) late Oligocene-early Miocene marine transgression/regression, predominantly shale with carbonate facies development; (4) mid-late Miocene, initiation of compressive stress, regressive sandstone development; (5) late Miocene-Pliocene strong compression, folding, wrenching, fault reactivation and reversal.

The hydrocarbon systems of these basins are characterized by post mid-Miocene thermal maturity and hydrocarbon charge. Source beds occur in Eocene-Oligocene graben-fill lacustrine facies as well as the Oligocene anoxic stagnating basin facies. Hydrocarbons commonly occur in linear trends associated with late stage structural compression. Fields are concentrated within or adjacent to depocenters containing mature source rocks. Field sizes vary with the size of the structural closure and situation with respect to the hydrocarbon source area.

Field size distribution indicates the predominant field size to be less than 50 million barrels recoverable reserves. Yet a significant number of fields with greater than 100 million barrels recoverable reserves will occur in virtually every major basin. In comparison, gas production is small and is restricted to basin deeps which correspond to high levels of thermal maturity.

By analogy with the field size distributions of the mature Southeast Asian producing basins, field size distribution of the lesser explored areas can be inferred. The South China Sea is an example where incorrect perception of field size distribution has had a profound effect on perceived petroleum potential.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994