Petrographic characteristics of lower Longmaxi Formation shale in South-east Chongqing and the formation mechanism of high quality reservoir

ZHAO Difei; GUO Yinghai; MAO Xiaoxiao; ZHANG Jingxia; BAI Wanbei; LI Gang

Abstract：Shale samples from lower Longmaxi Formation in Qijiang,Sanquan,Shizhu and Fuling in South-east Chongqing were selected for the study.Based on the analysis of the X-ray diffraction,TOC,brittleness test,high pressure mercury injection and low temperature nitrogen adsorption,the petrographic characteristics including mineral composition,organic characteristics,pore characteristics and brittleness of shale reservoir were studied.The perspect of reservoir petrology was evaluated and the influences of petrological characteristics on the formation mechaism of high-quality reservoirs were discussed.The results showed that mineral compositions of Longmaxi Formation shale reservoir can be divided into brittle minerals of quartz and clay minerals of illite.The content of brittle minerals at the bottom was significantly higher.The average value of TOC content is above 2% and the organic matter is over mature.The type of pores are complex and various types of pore were identified including organic nano pores,flake pores in clay mineral,dissolution pores in matrix mineral,and intergranular pores in framboidal pyrite.The main specific surface area and reservoir space were contributed by the nanoscale pores and the morpholohy characteristics of pores are sophisticated with various shapes such as parallel-plate size etc.The mechanics brittleness is good and the brittleness index is favorable for microfractures in reservoir and artificial fracturing.The lower Longmaxi Formation reservoir is high quality reservoir with special petrological characteristics which is formed in deep shelf and has experienced the late diagenesis stage,and therefore have suitable mineral compositions with developed nanopores,microfractures and good mechanics brittleness.Shale reservoir material compositions are the basis for the formation of high quality reservoir.The study is significance to explore the formation mechanism of physical characteristics and its control factors from the angle of the petrological characteristics in reservoir research.