鄂尔多斯盆地东部二叠系山西组页岩特征与分类评价OA

The Permian Shanxi Formation shale in eastern Ordos Basin is the product of continental sedimentation,and has great potential in shale gas exploration and development.However,the shale characteristics in this area are not understood systematically,there is no available classified evaluation system,and the favorable exploration targets are not defined clearly.To address these core issues,this paper systematically investigates the petrological and geochemical characteristics of the shale by means of experimental tests such as core observation,X-ray diffraction(XRD),total organic carbon(TOC)analysis,rock pyrolysis,and scanning electron microscope(SEM).On this basis,a classification system is constructed.In addition,the characteristics of reservoir spaces and pore structures are analyzed,and the lithological types and controlling factors of quality reservoirs are determined.The following results are obtained.First,the minerals in the Shanxi Formation shale are dominantly felsic(dominated by quartz)and clayey,with average TOC of 1.25%,organic vitrinite reflectance(Rₒ)ranging from 2.07%to 3.13%,and average rock pyrolysis(S1+S2)of 0.31 mg/g,and the kerogen is mainly type III,indicating the Shanxi Formation shale is overmature quality source rock.Second,based on mineral compositions and TOC characteristics,the Shanxi Formation shale is classified into three types,i.e.,organic-lean silty shale,organic-rich clayey shale,and organic-lean clayey mudstone(shale).And its reservoir spaces are mostly acted by intergranular(intercrystalline)pores,intragranular(intracrystalline)pores and organic pores,with nano-scale medium and large pores being the predominant pores in organic-lean silty shale,while relatively limited development of reservoir spaces in the two types of clayey shale.Third,geological evaluation demonstrates that the organic-lean silty shale is the core lithology of quality reservoir for Shanxi Formation shale gas due to its high porosity and developed medium to large pore throats.In conclusion,the shale classification system and quality reservoir evaluation criteria established in this paper provide a key geological basis for the selection of favorable explorations areas of Shanxi Formation shale gas in eastern Ordos Basin,and are of great significance for promoting the shale gas exploration and development in this area.