Study On The Characteristics Of Biomarker Compounds Of Source Rocks In Southwestern Part Of Chang 7 In Ordos Basin

The Ordos Basin Yanchang Formation has a large oil enrichment.The oil in Yanchang Formation at Ordos Basin is mainly from source rocks of Chang 7 Formation.According to lithology,the source rocks of Chang 7 Formation can be divided into two types,one of them is shale,and the other is mudstone.Both kinds of source rocks have good hydrocarbon generating ability.There are some differences in the characteristics of the two types of source rocks,such as the characteristics of rock and electricity,the potential of hydrocarbon generation,the range of hydrocarbon supply and the characteristics of biomarker compounds.The study of these differences will help to reveal the development mechanism and distribution of high-quality source rocks,and it is significant to the study of the origin of oil and the characteristics of accumulation.This paper takes the shale and mudstone in Chang 7 Formation of different area as the object of the research,and on the basis of extensive investigation of domestic and foreign literature and the collection of relevant geological data in the study area,samples of shale and mudstone in different blocks are collected.First,logging identification standards of oil shale and dark mudstone are set up through lithological and electrical differences,clarifying their distributions.Then the two kinds of source rocks are evaluated and distinguished from 3 aspects:types of organic matter,abundance,maturity.At last,the dissertation selects the distinct biomarkers,identifies the characteristics of biomarkers between source rocks and crude oil,and works on the oil source correlation in typical blocks,combined with the detailed GC-MS analysis of saturated hydrocarbons and aromatic hydrocarbons of oil shale and dark mudstones in different layers and blocks of Chang 7.What can be seen from sample analyses,data management,the geological and geochemical comprehensive researches can be show as follows:(1)Based on the differences of well logging response characteristics of the shale and mudstone in Chang 7 Formation in the study area,the interpretation standard of shale and mudstone identification by natural gamma and resistivity has been established,and the horizontal distribution of the shale and mudstone in Chang 7 Formation in the study area has been clearly defined.(2)The evaluation results of hydrocarbon generation characteristics of shale and mudstone indicated that both types of the source rocks have high abundance of organic matter,meanwhile the type of organic matter are mainly sapropel-type,and both source rocks are at the stage of maturity as well as reach the standards of the effective source rocks.However,there are certain differences in hydrocarbon generation characteristics between shale and mudstone.In comparison,the organic content of shale in Chang 7 Formation is higher,organic matter type was better,the hydrocarbon generating potential is greater,and it is considered as the best quality source rock of Yanchang group.(3)There is a certain difference in the characteristics of the shale sterane and terpene in the different regions on the plane,and vertically,the characteristics of sterane and terpene of shale and mudstone in the same region are basically similar,indicating that the the biomarker characteristics of the shale and mudstone of Chang 7 Formation in the study area are mainly controlled by the sedimentary environment and the source of the parent material.(4)According to the parameter optimization of various biomarker compounds,it is found that the distinction of carbon number predominance of N-alkanes and relative content of C27-C28-C29 sterane in the source rocks at different area between shale and mud stone is small,and the effect of comparison analysis is not very ideal.On the other hand,the ratio of pristane to phytane,the biochemical indexes of rearranged hopanes,the relative content of pregnene and triterpan,the content of dibenzothiophene,dibenzofuran,fluorene and their honologues can be well used to distinguish the sedimentary environment of high quality source rocks as well as indicate the source of high-quality source rocks.(5)The characteristics of biomarker compounds between oil and their adjacent source rocks in different block are similar,which are characterized by typical near source accumulation.