Sedimentary Characteristics And The Formation Conditions Of Fine-grained Mixed Sedimentary Rocks In The Second Member Of Kongdian Formation In Cangdong Sag

Most of the fine-grained sedimentary rocks in lacustrine basin are mixed by two or more mineral components(carbonate minerals,clay minerals,felsic minerals,etc.).They are mixed by disorderly,orderly or overlayed.The diversity of the mineral composition and the complexity of micro grain layer reflect the "hybrid" characteristics of fine-grained sedimentary rock.Therefore,fine-grained sedimentary rocks of lacustrine basin actually should be generalized fine-grained mixed sedimentary rocks.Through detailed core description,thin section identification and X-ray diffraction analysis,this paper reveals the mineral composition,rock types,mixed styles and rock assemblage types of fine-grained mixed sedimentary rocks.And on this basis,the paper discusses the environmental characteristics,formation conditions and controlling factors of fine-grained mixed sedimentary rocks in the second member of Kongdian formation in Cangdong sag.Meanwhile this paper evaluates the source rock characteristics of fine-grained mixed sedimentary rocks.The research shows that the fine-grained mixed sedimentary rocks of the second member of Kongdian formation in Cangdong sag is characterized by high carbonate content,high felsic content,low clay content and analcite segment development.According to the main mineral components,the fine-grained mixed sedimentary rocks are divided into dolomitic(limy)fine-grained mixed sedimentary rocks,felsic fine-grained mixed sedimentary rocks and clay fine-grained mixed sedimentary rocks.And the two former types are highly developed meanwhile clay fine-grained mixed sedimentary rocks are rarely developed.The mixed stypes of fine-grained mixed sedimentary rocks in the second member of Kongdian formation in Cangdong sag are mainly components mixture and tructural mixture.Based on the micro mixed mineral components and combination relationship,components mixture can be divided into CM1,CM2 and tructural mixture can be divided into SM1,SM2,SM3,SM4.On a macro level,it can be recognized four vertical combinations,which are the felsic-dolomitic felsic interbed-dolomitic vertical combination,the felsic-dolomitic vertical combination,the felsic-dolomitic felsic interbed vertical combination and the dolomitic felsic interbed-dolomitic vertical combination.Through comprehensively analysis of element ratio method,it shows that the sedimentary environment in the second member of Kongdian formation period is warm and humid.At the same time,the hot and dry climate also occurs.In the warm and humid climate environment,terrigenous clast is supplied adequately.The water is oxygen-poor reducing environment and water salinity is low.In hot and dry climate conditions,carbonate mineral is rich and terrigenous input is weak.The water is partial oxidation environment and the water is salt during this period.The formation of fine-grained mixed sedimentary rocks is mainly affected by climate variation and the supply of terrigenous clastic.And it can form different mixed styles and vertical combinations of fine-grained mixed sedimentary rocks.In addition,by means of rock pyrolysis,TOC and vitrinite reflectance analysis,it shows that the difference between organic matter and organic matter maturity is small.The organic matter types are mainly type I kerogen and type II kerogen and they are in low-mature and mature stage.According to the organic matter abundance,felsic fine-grained mixed sedimentary rocks are regarded as beneficial generative hydrocarbon rock types.