Characteristics Of Organic-rich Fine-grained Sedimentary Rocks Of The Second Member Of Kongdian Formation In Cangdong Sag

Cangdong Sag is located in the southern part of Huanghua Depression in Bohaiwan Basin.Wide distribution and huge verticle thickness are main characteristics of the shale in the second member of Kongdian Formation?Ek₂?in Cangdong Sag,which makes it a very important field for the prospecting of shale oil.Under the guideline of sedimentology and geochemistry and based on previous research,this paper makes full use of regional geological data,core observation,thin section,analysis and testing data and logging data to analyze shale lithofacies,characteristics of source rocks,sedimentary environment and prediction of distribution of high-quality source rock in the second member of Kongdian Formation?Ek₂?in Cangdong Sag.According to core observation,thin section analysis,X-ray diffraction analysis and scanning electron microscope technology,based on the differences of mineral composition and sedimentary structure,fine-grained sedimentary rocks in the research area are divided into six types of lithofacies,which are laminated fine-grained felsic sedimentary rocks,massive fine-grained felsic sedimentary rocks,laminated fine-grained mix sedimentary rocks,massive fine-grained mix sedimentary rocks,laminated dolostone and massive dolostone.The organic matter characteristics of all kinds of lithofacies are studied.The results show that the laminated fine-grained felsic sedimentary rocks and the laminated fine-grained mix sedimentary rocks have the high hydrocarbon generation potential and are evaluated as high-quality source rocks.The hydrocarbon generation potential of dolostones are weaker in general.The hydrocarbon generation potential of the massive fine-grained felsic sedimentary rocks and the massive fine-grained mix sedimentary rocks are the weakest.The sedimentary environment of lakes is an important factor controlling the development of source rock.Based on the analysis of biomarkers,main mineral contents,trace element ratios and sporopollen fossils,the sedimentary environments of different lithofacies are analyzed and the favorable environment for the formation of high-quality source rocks are ensured.When the climate is warm and humid and the water have more nutritive substance with certain salinity,and the environment is reducing with water column stratification,the sedimentary environments are suitable for the aquatic organism and algae growth and beneficial to the preservation of organic matter.Taking the lithofacies assemblage as the research unit for predicting the development and distribution of high-quality source rocks,it is concluded that there are five major lithofacies assemblages in the basin with different characteristics.In the vertical direction,the source rocks in the 3rd and 2nd sub-member of the Ek2 Formation are well developed.In the plane,the development of high-quality source rocks in different periods is quite different.However,the high-quality source rocks are basically close to the center of the lake basin and the source rock development is poor to the edge of the basin.