Formation and evolution of effective reservoir petrofacies in tight sandstone reservoirs of Shaximiao Formation in Jinqiu Gas Field of Central Sichuan
Effective reservoirs are one of the key points of tight sandstone oil and gas exploration and develop-ment,and the mechanism and evolution process of effective reservoir space formation under the overall low per-meability to tight condition are the key to understanding effective reservoirs.Taking the second member of the Shaximiao Formation in the Jinqiu Gas Field in central Sichuan as the research object,the characteristics and differences of reservoir petrology and diagenesis were analyzed by comprehensive use of rock mineralogy,elec-tron microscopy(SEM),carbon and oxygen isotope analysis methods,and the formation and evolution process of effective reservoir petrofacies was explored.In view of the characteristics of strong heterogeneity of the reser-voir in the second member of the Shaximiao Formation,three sandstone petrofacies were divided according to the differences in petrological structure,diagenesis degree and process,and pore characteristics.Among them,permeable reservoir sandstone has medium compaction,rich cementation types but low total amount,strong dissolution and good porosity.The porous permeability of ductile lithic-rich sandstone and tightly carbonate-ce-mented sandstone is poor,the former is strongly compacted and leads to rock densification,and the latter is dense due to a large amount of calcite crystalline cementation in the pores.The composition and structure of the original sediments control the diagenesis of the rocks in the reservoir,and different types of petrofacies have un-dergone different diagenetic evolution.The ductile lithic-rich sandstone and tightly carbonate-cemented sand-stone have become dense in the early diagenetic stage,constituting interlayers of various scales in the reservoir,while the fluid activity and fluid-rock interaction occurred in the buried evolution of the permeable reservoir sand-stone,and a large number of primary pores were retained in the early diagenetic stage,and the fluid action was active in the later stage,forming an effective reservoir petrofacies with relatively developed pore space and good physical properties in the tight reservoir,this rock phase is mostly distributed in the middle to lower parts of sin-gle sand bodies in distributary channels.
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