Cenozoic lacustrine basins of South-east Asia, their tectonic setting, depositional environment and hydrocarbon potential

Summary Cenozoic strike-slip tectonism, which caused local crustal extension in South-east Asia, generated many short-lived but deeply subsiding basins. Under tropical conditions and assisted by intensive karstic weathering, the basins formed ideal sites for lakes during early basinal history in Oligocene to Miocene times. In several basins in northern Thailand, a shallow-lacustrine assemblage of mudstone and coal contains thin, alginite-rich oil shales of moderate grade which formed under oxic or anoxic conditions during the initial flooding of the lake, which was accompanied by high organic productivity. In the Mae Sot Basin, a carbonate-rich lacustrine assemblage contains thick, alginite-rich oil shales of high grade which formed under anoxic conditions in deeper, probably stratified lakes where the organic material was well preserved. The kerogen in both assemblages is Type I, hydrogen-rich. Under conditions of rapid sedimentation and abnormally high heat flow associated with active tectonism, the shallow-lacustrine oil shales have generated hydrocarbons which are exploited in several basins.