Evaluation Of Shale Gas Preservation Condition Of The Wufeng-Longmaxi Formations In The Southeastern Chongqing

This study was focused on the shale gas preservation conditions of the WufengLongmaxi Formations in the southeastern Chongqing.Through the analyses of core photographs,logging data and test data,the geological and geochemical characteristics as well as the reservoir characteristics of the shale were analyzed in detail,and the gas-bearing characteristics and its influence factors of the shales were studied.Based on those,we determined a comprehensive study and evaluation standard of shale gas preservation conditions through the analyses of formation pressure,fracture development,burial evolution and diffusion characteristics of current shale gas.The Wufeng-Longmaxi shales are rich in organic carbon and high in maturity.There are high content quartz and clay mineral with complex pore structure in the shales.The shale gas content varies in different areas,and it is high in the northwest but low in the southeast horizontally and high in the lower part and low in the upper part vertically.There is apparent overpressure in the target layer.The Early Paleozoic strata in the southeast Chongqing experienced a long burial period,and were characterized by early deep burial and late uplift with a high degree of diagenetic evolution.The analysis results indicate that the overpressure is mainly generated by the hydrocarbon generation of the target layer.The study area has experienced multiple and complex tectonic movements,leading to that the structural fractures are very developed.There are less structural fractures and mainly the low angle and bevel fractures in the west,while there are more structural fractures and mainly high angle cutting fractures in the southeast(most filled by calcite).Based on the shale thickness,overpressure,fracture development degree and diffusion coefficient,the preservation conditions of shale gas in the southeastern Chongqing were comprehensively evaluated.The preservation conditions are good in Wulong and Pengshui,medium in Nanchuan,Puzi,Huangjia and Qiangjiang,and poor in Maoba,Youyangdong and Pingyanggai.