Geochemical Characteristics Of Deep Hydrocarbon Source Rocks In Xujiaweizi Fault Depression Of Songliao Basin

In the recent years,the deep reservoir exploration and development is one of the key areas of oil and gas exploration in China.With the further study in exploration of deep reservoirs,the deep oil and gas exploration has also made significant achievements in the Songliao Basin.Evaluation of hydrocarbon source rock is the key point for oil and gas exploration in a basin.The literature is limited to describe the distribution of hydrocarbon source rocks in the Xujiaweizi fault depression.So in this paper,the key is to describe the hydrocarbon source rock distribution and geochemical characteristics.This paper is based on the Xujiaweizi fault depression in the Songliao Basin.The hydrocarbon sources were evaluated through rock pyrolysis,total organic carbon?TOC?,whole rock maceral and total organic carbon?TOC?recovery analysis technology.In order to systematically study the geochemical characteristics of hydrocarbon source rocks,the molecular geochemistry of the source rock of the xujiaweizi fault depression is described in detail.It provides reference for exploration and exploitation of oil and gas resources in the Xujiaweizi area of the Songliao Basin.The analysis and test of the rock pyrolysis,total organic carbon?TOC?and the whole rock group showed that the organic matter abundances in three sets of hydrocarbon source rocks were all medium-good in the Yingcheng Shahezi and Huoshiling group.And the shahezi group was superior to the yingcheng group and the firestone group.Types of organic matter was analyzed by maceral microscopically,the test results is given priority to with?type kerogen and mixed with type II in a few samples.The hydrocarbon source rock maturity is determined mainly by vitrinite reflectivity.The reflectivity of the organic vitrinite is greater than 1.8%,and the highest is 2.8%,with the average of 2.17%in the yingcheng formation.The Ro in shahezi group is greater than 2.0%,and the highest is 3.7%,with the average of 2.38%.And in yingcheng group,The Ro suggested that the hydrocarbon source rocks of the study area were in the high mature stage.The distribution of normal alkanes in saturated hydrocarbons of hydrocarbon source rocks shows that the three sets of hydrocarbon source rock peaks are weak pre-peak.The ratio of nC₂₁+nC₂₂/nC₂₈+nC₂₉ is high.Its value is above 1.0.The carbon advantage index?CPI?ranges is from 0.75 to 1.31.And its value tends to 1.This suggests that the hydrocarbon source rock evolution degree is higher in the region.In the study area,the Pr/Ph value of the source rock samples is 0.08⁰.6,and the relationship between Pr/nC₁₇ and Ph/nC₁₈ is used to determine the reduction of the sedimentary environment,and the organic matter types were mainly"mixed".The tricyclic terpene?TT?series have basically the same distribution characteristics,in the study area which are in the normal distribution of C₂₃TT as the main peak.And C₂₄tetracycline(C₂₄Te)has development.The distribution characteristics of C₂₁ far higher than C₁₉9 are shown in the samples of study area,which are in line with the distribution characteristics of the lake facies source rocks.That is to say,C₁₉ and C₂₀ were lower than C₂₁ or C₂₃.The tetracyclic terpene(C₂₄)abundance is high,slightly lower than C₂₆ tricyclic terpene,suggesting that the source rocks in the region are deposited in the deep water environment with a strong reducibility.For the pentacyclic triterpenes,C30hopane is the main peak,and the compound abundance of C₃₁ is low.Namely,the pattern distribution is decreased with the increase of carbon number.The ratio of C₃₁22S/?22S+22R?tends to balance.The ratio of gammacerane/C₃₀ hopane is low.Which show that the hydrocarbon source rock maturity is higher and sedimentary water salinity is low in the study area.In addition,the triangular diagram of the regular sterane C₂₇-C₂₈-C₂₉ indicates that the source rocks are mainly mixed with the original input.The relative content of rearranged sterane is high.Moreover,C₂₉-20S/?20S+20R?,rearranged sterane C₂₉/regular sterane C₂₉,rearranged sterane C₂₇/regular sterane C₂₇and rearranged sterane C₂₉/regular sterane C₂₉ show a certain positive correlation.In addition,regular sterane C₂₇/regular sterane C₂₉ ratio tends to balance.All of these suggest that the hydrocarbon source rocks are mature.The content of aromatic hydrocarbons is dominated by the phenanthrene series and chrysene compounds in the three sets of hydrocarbon source rocks.The ratio of TMN/TeMN,MPI2,F1,and F2 in aromatics parameters indicate that the hydrocarbon source rocks in the study area is high maturity and have reached the stage of high-over mature evolutionary stage.At the same time,the relationship between the series DBT/PHEN series and Pr/Ph show that the sedimentary environment of the source rock in the Xujiaweizi fault depression is the deep lake-semi-deep lake facies.According to the methyl benzene and thiophene two distribution index?MDBI?,hydrocarbon source rock maturity is the highest in huoshiling group,shahezi group and yingchengzu group.The aromatic series compounds of fluoranthene,pyrene and anthracene have not only the special properties of higher plant input,but also have the identification of the maturity of source rocks.It is possible to make a better distinction between the different layers of hydrocarbon source rocks in the study area by using the relationship diagram of polycyclic aromatic hydrocarbons?pah?,retene/phenanthrene,fluoranthene/phenanthrene,pyrene/phenanthrene,anthracene/phenanthrene.In addition,the geochemical characteristics of high-over mature source rocks or crude oil could be distinguished and discussed by using these polycyclic aromatic hydrocarbons,which cannot be explained by using saturated hydrocarbons.