Post-sedimentary tectonic evolution, cap rock condition and hydrocarbon information of Carboniferous-Permian in Ejin Banner and its vicinities, western Inner Mongolia: a study of Carboniferous-Permian petroleum geological conditions (part 3)

Based on an analysis of Mesozoic-Cenozoic sedimentary evolvement and a study of structural reformation after Carboniferous-Permian and Mesozoic magmatism, the authors hold that Cretaceous was a flourishing period of Yingen-Ejin Benner basin evolution: there existed the combination of lacustrine facies-swamp facies sandstone and mudstone characterized by large thickness and stable horizontal spread, and Cretaceous mudstone consitituted a good regional cap of the Carboniferous-Permian petroleum system. Although multistage tectonic reworking took place after Carboniferous-Permian, tectonic stress was dominated by extruding and uplifting. In general, the extrusion and uplift didn't affect the Carboniferous-Permian petroleum system except for local areas where structural dynamometamorphism occurred. There happened four stages of magmatism, in which the Early Cretaceous magmatism was strong. Affected by this magmatism, Carboniferous-Permian strata locally experienced thermal metamorphism, and hydrocarbon source rocks reached the mature stage, but the effect range was limited. There were four sedimentary cycles during Carboniferous-Permian; as a result, there existed mud shale of large thickness and stable horizontal spread especially in Amushan and Maihanhada Formations, and the mud shale was not only a good source rock but also a good cap rock. There was a long-term uplift-erosion from Late Variscan to Indo-Chinese epoch in Carboniferous-Permian, but there existed stable spread of Carboniferous-Permian strata in Ejin Banner-Wutaohai area based on geophysical data interpretation, with the residual thickness being generally from 1000 to 2000m and the thickness of local areas being over 3000m. Information of a series of Carboniferous-Permian hydrocarbon source rocks related hydrocarbon shows indicate that there existed processes of hydrocarbon generation, migration and accumulation in Carboniferous-Permian period, which suggests good hydrocarbon prospects.