Shanxi Shale Sedimentary Facies And Reservoirs Characteristics In Daning-jixian Area Of East Margin Of Ordos Basin

This paper takes the Shanxi Formation shale in the Daningji County area of the eastern margin of the Ordos Basin as the main research object.Field outcrop detection and core observation as the basic part,combined with a series of subsequent analysis and test data,under the sedimentary background,the sedimentary facies and plane distribution characteristics of the study area are clarified;establish shale division scheme and describe various lithofacies characteristics;the corresponding reservoir characteristics of various lithofacies are clearly defined,and the advantages and disadvantages of shale lithofacies are divided in this study by combining with high-quality shale evaluation schemes at home and abroad,so as to identify favorable shale and provide a certain theoretical basis for future exploration and development.The study area is located in the south of the western Shanxi flexion belt,adjacent to the Yishan slope on the left and weibei uplift on the south,with typical monocline characteristics and extending westward.The strata in the east are older and the strata in the west are newer,with a total area of 2×10³km².The Shanxi Formation in the Daningji County area of the eastern margin of the Ordos Basin belongs to the transitional facies of marine and continental facies as a whole,and belongs to the subfacies of tidal flat and lagoon.Can be further divided into the following microfacies types:mud flat,sand flats,swamps,tidal waterways.Among them,the mud flat and sand flat microfacies are the main types of microfacies developed.Based on the comprehensive analysis of parameters such as shale thickness,the mud flat microfacies is identified as the optimal shale sedimentary microfacies,followed by the sand flat.The shale can be divided into 8 different lithofacies by using the"TOC+mineral comp-osition"classification scheme,organic-poor clay-siliceous shale（I₁）,organ-ic-rich clay-siliceous shale（I₂）,organic-rich calcium-containing clay-siliceous shale（I₃）,organic-poor siliceous-clay shale（II₁）,organic-rich siliceous-clay shale（II₂）,organic-rich calcium-containing siliceous-clay shale（II₃）,organic-rich calcium-containing siliceous-containing clay shale（II₄）,organic-rich clay mixed shale（III）.The organic matter content in the study area is relatively high,ranging from 0.86%to 15.97%,with an average of 3.93%.Ro is between 2.02%and2.61%,with an average of 2.29%.It is in the high-mature-over-mature stage,mainly developing Type II₂and type III kerogens.The porosity is between 0.99%and 4.58%,with an average value of 2.37%,the permeability distribution is between 0.0001m D and 0.2983m D,with an average value of 0.1123m D;the BET specific surface area is between 5.72m²/g and17.51m²/g,with an average value of 10.38m²/g,the total pore volume of BJH is between0.0024ml/g～0.0438ml/g,with an average of 0.0277ml/g;The pores are mainly organic pores,intergranular pores and intragranular pores.Referring to the evaluation criteria of favorable shale lithofacies at home and abroad,combined with the experimental analysis data of this paper,the total organic carbon content（TOC）,thermal evolution degree（Ro）,physical properties,pore volume and specific surface area of shale were selected as the evaluation parameters of favorable lithofacies,and the parameters of 8 different shale lithofacies are listed,compared and optimized.Finally,it is concluded that organic-rich calcium-containing clayey siliceous shale（I₃）and organic-rich calcium-containing siliceous clay shale（II₃）are favorable shale lithofacies and the potential favorable exploration areas in the study area are predicted,which provides a theoretical basis for future shale gas exploration.