Lithofacies Characteristics And Genesis Analysis Of Continental Organic-rich Shale

Lacustrine basins in China are small in scale,complex in structure and fast in phase transformation.There are many organic shale series in continental freshwater and saline lake basins.The purpose of this paper is to compare and analyze the lithofacies characteristics and genetic differences of organic-rich shales in saline and freshwater lake basins of continental facies,so as to provide basis for exploration and development of shale oil.This paper takes the 10th rhythm of Eq3~4 in Qianjiang Sag of Jianghan Basin and the third submember of Chang-7 Formation of Ordos Basin as the object of study.Based on a series of analytical means from macro to micro,the study selects mineral composition,sedimentary structure and organic matter abundance as criteria for lithofacies classification,and then analyses the types and characteristics of organic-rich shale facies.The 10th rhythm of Eq3~4 can be divided into four types:carbonaceous laminated glauberite bearing dolomitic mudstone facies?carbon-rich laminated argillaceous dolomite facies?carbon-rich laminated dolomitic mudstone?carbon-rich laminated calcareous mudstone.And the third submember of Chang-7 Formation of Ordos Basin can be divide into carbon-rich laminated shale facies and carbon-rich massive tuff facies.On the basis of lithofacies division,favorable lithofacies in different continental lake basins are identified by taking reservoir space,oil-bearing conditions and brittleness as evaluation indexes.The most favorable lithofacies of the 10th rhythm of Eq3~4 is carbon-rich laminated argillaceous dolomite facies,and the most favorable lithofacies of the the third submember of Chang-7 Formation is carbon-rich laminated shale facies.Otherwise,the lithofacies genesis is analyzed.The paleo-sedimentary environments such as paleoclimate,paleo-salinity,paleo-water depth and paleo-redox environment were analyzed by geochemical methods,such as major and trace elements analysis.The results show that Qianjiang depression has a tropical-subtropical arid climate during the rhythmic sedimentary period of the 10th rhythm of Eq3~4,with high salinity and strong reducibility.The overall climate of the third submember of Chang-7Formation in Ordos Basin is suitable,warm and humid.There is a terrestrial freshwater lake basin environment with strong reductivity and close to provenance,which is conducive to the development and enrichment of organic matter and creates favorable conditions for large-scale shale oil enrichment.Combining with lithofacies microscopic characteristics and element geochemical indices,it is considered that the quartz of the10th rhythm of Eq3~4 is mainly terrigenous clastic quartz,and the dolomite is the product of chemical precipitation in primary-paragenetic stage.In addition,the tuff in southwestern Ordos Basin is deposited from volcanic clastic material after long transportation,which has undergone a certain degree of alteration in this process.