The Evolution Model of High-Quality Source Rocks in the Lower Part of the Third Member of Shahejie Formation in Bonan Sag, Jiyang Depression

Abstract A set of high quality source rock is preserved in the lower part of the third member of Shahejie Formation in Bonan Sag, Jiyang Depression, Bohai Bay Basin. In order to perfect its formation mechanism, evolution model of high grade source rocks in the study area is researched, based on detailed observation of the whole core, organic geochemical and microelement analysis, and combined with regional geological characteristics. Results show that this set of source rock deposits a great thickness approximately to 200m with high organic carbon content (0.71% to 9.32%) with the average of 3.1%, and the organic matter dominated by Type one and Type two is in the oil generation stage. Besides, it is a complete third-order sequence in the lower part of the third member of Shahejie Formation in Bonan Sag. By using trace element analysis of the region, factors such as paleosalinity, paleoclimate, the flux of terrigenous debris, biological production and redox conditions are studied. It is revealed that the spatial distribution of elements reflects obvious differences between Transgressive Systems Tract(TST) and Highstand Systems Tract (HST). Additionally, the ancient sedimentary environment is crucial to the abundance of organic matter. The whole interval formed in a warm, moist saline lake anoxic environment, the terrigenous clastic supply is the main controlling factors of excellent source rock, high biological yield is the premise of organic matter enrichment. Most of the shale oil and gas researches focus on marine shale, but with fewer attention to lacustrine basin, especially in Jiyang depression. The lower part of the third member of Shahejie Formation is not only the main source rocks in Jiyang Depression but also the most development strata of shale oil and gas. Hence, the study of the evolution model of source rock development will provide an important reference for the organic matter enrichment causes in continental salt lake.