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Abstract: The Far East Hydrocarbon Habitat--The Charge Perspective

Harry Doust

From a hydrocarbon exploration point of view, the Far East is typically the realm of Tertiary basins with youthful prospects. Considering the archipelagic nature of the area and the extensive marine environments associated with shallow seas, it is perhaps surprising that nearly all of the oil and gas in these basins is of terrestrial origin (typical for the basins are low-sulfur, light waxy oils with strong land-plant inprint, and a superabundance of gas). The reason for this can be sought in the late Mesozoic-early Tertiary history, when the current cycle of tectonic development commenced. At that time, much of what is now east and southeast Asia consisted of a large land area, comprised of microcontinental blocks welded together by fold belts. In the Paleogene, this continental area became subject to back-arc extension and collapse as a consequence of complex plate readjustments.

Subsidence took place in fault-bounded (graben) depressions of many orientations throughout the area, and widespread lacustrine environments were established, especially in the Oligocene. The middle to late Tertiary history of these basins was dictated by their proximity to the open ocean and by the extent of crustal subsidence, but follows a transgressive-regressive cycle that gives rise to the following groups of plays: (1) early Tertiary transgressive clastics, basically oil-prone, (2) Miocene carbonates of the maximum transgression, gas prone, and (3) late Tertiary regressive clastics, oil and gas prone.

The mechanisms of generation and migration in Far East Tertiary basins are poorly understood, and the type and volumes of hydrocarbons trapped often appear to be unpredictable. In general, however, there is convincing evidence that charge is one of the most dominant parameters in determining basin and play prospectivity and that source rock development is both local and strongly environment related. At present, three basic depositional realms are envisaged for source rock accumulation: (1) lacustrine

grabens ("deep lakes") that contain rich algal-SOM type I-II source rocks and are oil prone, (2) paralic coal and coaly shale swamps, developed in a variety of onshore deltaic environments; charge from these source rocks is very variable, and (3) coastal and marine, especially related to deltas, where terrestrial organic material is introduced, accumulates, and is bacterially degraded and preserved.

The distribution and types of hydrocarbons produced from these source rocks and their relations to hydrocarbon habitat are examined in this review.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994