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.