The evolution of the Shiwandashan Basin in Guangxi and its response to oil and gas resources

The Shiwandashan Basin in southern Guangxi is tectonically located on the northwestern edge of the South China plate. The basin began to accept the sediments during the Late Palaeozoic when the South China Ocean opened once again posterior to the collage of the South China and Yangtze plates. During the Devonian to the Early Permian, this area appeared as a passive continental margin, and during the latest Late Permian, it was developed into a back-arc basin due to the westward subduction of the Pacific plate and further into a foreland basin during the Early Triassic to the Jurassic. The clastic rocks as the source rocks were formed during the deposition of the siliciclastic shelf on the passive continental margins, followed by calcarenite, algal limestones, reefal limestones and dolostones as reservoir rocks during the subsidence of the carbonate platforms, thus constituting an old source rock-new reservoir rock association. A sequence of clastic rocks as the source rocks was laid down in the foredeep basin during the early stage of the foreland basin, and constituted a new source rock-old reservoir rock association together with the early reservoir rocks. This kind of associations may serve as the sealing strata of the underlying strata. The westward progradation of the overthrusting faults permitted the sedimentary strata to overlap upslope upon the cratonic slopes and then the stratigraphic traps were formed. The flysch rapidly deposited-during the middle stages of the foreland basin and mollase during the late stages facilitated the rapid burial, maturation and preservation of the early source rocks. It can be seen that the Shiwandashan Basin provides a superb example of a representative foreland basin that is highly prospective for oil and gas resources.