Differential Enrichment Of Organic Matter In The Third Member Of Shahejie Formation In Dongpu Depression Controls Geological Conditions And Selection Of Favorable Areas Controlling Factors And Patterns

The Dongpu sag is a typical secondary sag in the Bohai Bay Basin,located at the southeastern end of the Bohai Bay Basin,and its structural location belongs to the Linqing Depression.The Dongpu sag has experienced more than 50 years of exploration.Salt rocks are developed in the northern part of the sag,and freshwater deposits are mainly in the southern part.Existing exploration shows that the organic matter abundance of source rocks in the north and south of the Dongpu Sag differs significantly.The average organic carbon content of source rocks in the north is about1.0%,and that in the south is 0.49%.The reason for the difference in organic matter abundance has not been revealed.Taking the source rock of the third member of Shahejie Formation in Dongpu sag as an example,this thesis analyzes the geological factors that cause the differential enrichment of organic matter.In the study,through the observation and statistics of the coring data of exploration wells in the north and south of Dongpu Sag,it is clear that the source rocks in the north of Dongpu Sag are mostly black,dark gray mudstone and shale,and the source rocks in the south are mostly dark gray mudstone,mainly shale.Through statistical analysis of total organic carbon and hydrocarbon generation potential,it is found that the abundance of organic matter in the northern part of the third member of the Shahejie Formation is higher than that in the southern part.The organic matter in the northern part is mainly type II kerogen,and the southern part is mainly type III kerogen Mainly,the type of kerogen in the northern regions is better than the southern ones.The sedimentary paleoenvironment of the third member of Shahejie Formation was analyzed by the ratio method of major and trace elements such as Sr/Ba,B/Ga,Mo,U,V/V+Ni,Ni/Al and P/Al.Brackish water—salt water,reducing environment,the southern area is freshwater,reducing environment,the paleosalinity,reducibility and paleoproductivity in the northern area are higher than those in the southern area,so the depositional environment of organic matter in the northern area is better than that in the southern area.In order to deeply explore the reasons for the enrichment of organic matter in the northern region,the parent material sources of organic matter in the northern region and the influence of the saltwater environment on the enrichment of organic matter were analyzed.Taking the sample from Well Wen 248 as an example,through the observation of microscopic components and the detection of biomarker compounds,it was concluded that the source of parent material in the northern region was mainly algae,and the TOC was positively correlated with the suspected algae,indicating that these algae were rich in organic matter in the north.Sets provide an important material basis.Analysis of the relationship between TOC and paleosalinity and the number of algal fossils shows that: TOC first increases and then decreases with the increase of salinity.The paleosalinity(Sp)value range of good source rock(TOC>1%)samples is 6.49‰～11.80‰,This is because the brackish water-saline water environment in the north affects the quantity and preservation environment of algae,an important source of parent material,which affects the changes of TOC and provides an important material basis for the enrichment of organic matter.