Abstract: In order to understand the formation and evolution of the normal pressure shale gas reservoirs in Wulong Syncline in Southwest Chonqing,China the major controlling factors of shale gas accumulation were studied through drilling data analysis,thin section observation,Ar-SEM,tectonic balanced section and burial history analyses,in combination with analysis of four dynamic preservation factors (sedimentation,burial evolution,hydrocarbon generation,and tectonic uplifting).Results show that there are 3 key elements controlling the formation and evolution of the typical normal pressure shale gas reservoirs in Wulong syncline.The first one is the favorable lithological association of the Wufeng and Longmaxi Formations.The quality shales of deep shelf facies are stable in lateral distribution and have high potential of hydrocarbon generation,while the overlying thick shale has good sealing capacity,thus is favorable for the preservation of shale gas.The second one is the favorable reservoir conditions.The organic nanopores provide high porosity for gas storage and the complex pore geometries improve the permeability of the shale reservoirs.In addition,the brittle minerals associated with the organic maters acted as skeleton and protected to some extent the organic nanopores from compaction during diagenesis.The third one is tectonic evolution which determines the reservoir scale.After the continuous deep burial and main period of hydrocarbon generation,the target reservoirs experienced reformation during tectonic uplift,which led to the partial gas migration and dispersion,and the formation of normal-pressure shale gas reservoir with a pressure coefficient in the range from 0.8 to 1.2.

Key words: accumulation factor, normal pressure reservoir, shale gas, Wufeng Formation, Longmaxi Formation, Wulong Syncline, Southeast Chongqing