Analysis Of Favorable Hydrocarbon-forming Facies Belts Of The Permian Shanxi Formation In The Southern Ordos Basin

The Ordos Basin is located in the western part of North China,covering the five provinces of Shaanxi,Gansu,Ningxia,Meng and Jin.The tectonic position is in the transition zone between the eastern tectonic domain and the western tectonic domain.It is the second largest sedimentary basin in China.The study area is located in the southern part of the Ordos Basin,in the transition zone between the stable block of the Ordos Basin and the Qinling orogenic belt.The structural location is in the Weibei uplift belt.In recent years,as a key exploration target horizon,the oil and gas exploration of the Permian in the Ordos Basin has gradually attracted people's attention.A large number of studies have been carried out on the Permian sedimentary facies and source rocks,but the combination of the two is further discussed to further discuss the facies that are conducive to the deposition of source rocks.Therefore,this paper mainly studies the favorable hydrocarbon-forming facies belt of the Permian Shanxi Formation in the southern Ordos Basin,and combines the geochemical indicators such as organic matter abundance,organic matter type and organic matter maturity of the source rock in the study area with the sedimentary facies belt of the study area.Conducive to hydrocarbon source rock into hydrocarbon phase zone.Through the outcrop observation and measured geological section,using sedimentary,stratigraphic,geochemical and other theories,the Permian Shanxi group in the Liangliang Erdaogou area in the southeastern part of the Ordos Basin and the Hanshui River in the southern part of the Ordos Basin the study.On the basis of two measured sections of Pingliang Erdaogou and Hancheng Lishui River,the standard of stratigraphic division was defined and the stratigraphic division scheme was formulated.By studying petrological features,sedimentary structures and paleontological signs,the sedimentary facies markers are determined,and the sedimentary facies of the study area are analyzed.Combined with the development characteristics and sedimentary tectonic evolution of the basin,the sedimentary facies belt distribution of the Permian Shanxi Formation is established.Geochemical data of source rocks in the statistical study area,and the distribution characteristics of the source rocks of the Permian Shanxi Formation in the southern part of the basin.Starting from the abundance of organic matter,the type of organic matter and the maturity of organic matter,the corresponding evaluation of source rocks in the study area was made.Finally,the sedimentary facies of the study area are combined with the characteristics of the source rocks to predict the formation of hydrocarbon zones.The study area is divided into two sedimentary systems,two sedimentary facies,three sedimentary subfacies,and eight sedimentary microfacies.The overall sedimentary appearance is the basin subsidence center in Fuxian,Yichuan and Yanchang areas,and the shallow lake sediments are developed.The surrounding areas are mainly delta deposits.The source rocks of the Shanxi Formation in the study area are mainly dark mudstone,carbonaceous mudstone and coal rock.The main developmental facies of the source rocks are the delta plain peat swamp and the shallow lake swamp.The dark mudstone organic matter abundance(TOC)in the delta plain peat swamp is between 0.43% and 4.47%,and the kerogen type is mainly type III,organic matter.Maturity(Ro)is between 2.04% and 3.74%,which is in a stage of high maturity-over-maturation.The TOC content of coal rock source rocks is between 76% and 84%,and the organic matter type is type III.The organic matter abundance(TOC)of dark mudstone in Binshui Lake swamp is between 0.24% and 2.7%,the kerogen type is mainly II and III,and the organic matter maturity(Ro)is between 1.94% and 3.47%.-Over mature stage.In the horizontal direction,the source rock in the study area is broadly covered,and the dark mudstone has the characteristics of thicker northeastern thin section.The coal rock source rock gradually thickens from southwest to northeast.In the longitudinal direction,the dark mudstone source rocks of the Permian Shanxi Formation in the southern Ordos Basin are mainly developed in the Shan 1 section,while the coal rock source rocks are exposed in the 2nd strata.Finally,through the comprehensive evaluation of the characteristics of different source-bearing source rocks,it is considered that the favorable hydrocarbonforming phase of the Permian Shanxi Formation in the southern Ordos Basin is a delta plain peat swamp.