Development constraints of marine source rocks in China.

The formation of source rocks is largely dependent on the nature of organisms from which kerogen is derived and the preservation conditions of organic matter,which are fundamentally dependent on a favourable combination of various elements in which organisms live and are subsequently buried.These elements include climate,oceanic currents,structure and environmental conditions.The formation of marine organic-rich source rocks is closely related to the oxygen content in the atmosphere,xerothermic climate,and the rapid rise of sea level caused by a rapid increase of temperatures and melting of glaciers,between a glacial age and a postglacial age.The formation and evolution of oceanic currents are considered as another important factor that controls the formation of marine organic-rich source rocks.The upwelling of waters rich in green-sulfur bacteria and elements,such as phosphorus,silicon and ferrous compounds,can greatly accelerate the productivity and burial rate of organic matter.A distribution model of the Cambrian upwelling in South China and Tarim Basin and a distribution model of anticyclone oceanic currents during the formation of the lime-mud mound source rock on the mesa-marginal slope of the Upper Ordovician Lianglitage Formation in Tarim Basin are proposed.These studies show that high-quality source rocks can develop only in rift valleys in passive continental margin,intracratonic rift valleys,intracratonic sag basins and sag basins at the cratonic margin.In addition,a favorable environment for the development of organic-rich source rocks also includes undercompensation basins,evaporite lagoons,mesa-marginal slopes(lime-mud mound) and semi-blocked or blocked undercompensation bays.Moreover,a low input of inorganic matter and a low depositional rate are of great advantage to the formation of organic-rich source rocks,and the slope seabed landform favors the formation of source rocks.