Study on Anoxic Environments of Permian in the Middle and Lower Yangtze Region

Anoxic environments developed best in Permian,especially in Gufeng and Dalong Stages in the Middle and Lower Yangtze region. Special complex of anoxic sediments of "black shale-chert- limestone "was formed in this kind of environments. According to palaeoecology and sedimentary petrology,anoxic sediments can be divided into two kinds, dysaerobic and anaerobic sediments. Anaerobic sediments are characterized by black and dark-grey, thin-beded or laminated,organicrich, and phosphoric-rich chert, shale and mud-bearing limestone, which are devoid of benthic fossils and biodisturbance,but bear plankton fossils, e. g. radiolaria and Ammonoids. Dysaerobic sediments are characterized by black-grey and dark-grey, thin or middle-beded,organic-rich and phosphoric-rich shale, mud-bearing limestone and chert, which are devoid of epifaunal fossils, but bear infauna,plankton fossils and biodisturbance. The distribution of anoxic sediments of Permian in the study area,is closely related to the framework of lithofaces and palaeogeography. Anoxic sediments of Gufeng Formation are distributed in the north and south of Lower Yangtze region,and those of Dalong Formation in the north of Lower and Middle Yangtze region,both of which are closely related to deeper slope and basin. There are two main origins of anoxic environments of Permian in the region. One is the euxinic orgin. Under the palaeogeographic conditions of low paleolatitude of Permian,high temperature of marine water,low energy of marine water and subaqueous relief of "twofold platform and basin"result in the euxinic condition of deeper water layers. The other orgin is the influence of upwelling.The active upwelling coming from the northward ocean promotes high biologic productivity,and in turn the recycling of dead organic matter in the water column creates a very high oxygen demand which can trigger anoxic conditions in deeper water layers under the upwelling. The distribution of anoxic and aerobic environments is determined by the depth of marine water. The sedimentary model of anoxic environments of Permian in the region is established on the basis of comprehensive analysis. The depth of sea floor of anoxic environments is analyzed from the viewpoint of palaeogeography, petrology,and comparative study of modern anoxic environments. It is estimated that the sea floor of anoxic envirionments of Permian in the study area is not very deep. The depth of dysaerobic environments is less than 300m,and anaerobic environments less than carbonate compensation depth(CCD). Abundant organic matter in the anoxic sediments may result in.the formation of potential hydrocarbon source beds. According to the difference of various anoxic sediments, We classified three kinds of potential hydrocarbon source beds, and estimated the distribution of those beds in the study area. Research shows that there are excellent conditions of the formation of oil and gas in Permian of the Lower and Middle Yangtze region.