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Geologic Evolution and Petroleum System of the Thailand Andaman Sea Basins
The Thailand Andaman Sea has regained exploratory interest due to recent gas discoveries offshore Myanmar and juxtaposition to the petrolific North Sumatra basin to the south. There have been only twelve exploratory wells drilled within the Thailand sector including two gas discoveries. Two back-arc basins, the Mergui and North Sumatra basin northern extension, exist within the Thailand Andaman Sea with sedimentary thicknesses exceeding 10,500 meters. Miocene reefs and Oligocene to Miocene fluvio-deltaics and turbidites constitute the major exploratory plays.
A shallow sag basin formed between the Burman and Shan plates during the Eocene in response to the northward drift of the Indian Plate. With the northward drag of the Burman Plate, the North Sumatra, Mergui, and Martaban rift basins open during the early Oligocene. Marine incursions from the south advanced throughout the Late Oligocene and large deltaic complexes formed in the Mergui Basin. Tectonic quiescence, a sea-level maxima, and an explosion of reef growth characterized the Early Miocene. During the Middle Miocene, the continued northward movement of the Indian and Burman Plates caused the modern Andaman Basin to open. As compressional movements intensified during the Late Miocene and Pliocene, the Barisan Mountains rose on Sumatra and the Andaman basins deepened.
Hydrocarbons primarily move laterally from mature source areas updip along unconformities and/or via permeable carrier beds while faults tend to block and redirect hydrocarbon migration. The source strata are generally lean, but this deficiency is compensated by substantial volumes of mature, fine-grained rocks in the deep basin grabens. The hydrocarbon accumulations are gas with significant amounts of associated condensate due, in part, to the low explusion efficiency of the source strata. Once temperatures reach the point that oil cracks to gas, the explusion efficiency changes and the gas, carrying with it condensates, will migrate from the source rocks.
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