Diagenesis And Reservoir Formation Mechanism Of The Fourth Member Of The Sinian Dengying Formation In Northern Sichuan

Based on the observation of the field profiles,drilling cores and thin slices in the northern part of the Sichuan Basin,combining the paleogeographic background,the evolutionary history of regional geology,geophysical data and geochemical data,the characteristics of sedimentation,diagenesis and reservoir in the fourth member of Sinian Dengying Formation are discussed,and the comprehensive influence of sedimentation,tectonism and diagenesis on the formation of reservoir is clarified.The study area in Dengying fourth stage is in the shallow water carbonate platformbasin environment of the regression phase,which can be divided into 3 subfacies,such as restricted platform,platform edge and slope,and 9 microfacies.There are three main types of rocks:algal dolomite,granular dolomite and crystalline dolomite.The types of reservoir space can be summarized into 3 categories and 10 subcategories.The 3 categories are:pore reservoir space,cave reservoir space and crack reservoir space.The 10 subcategories are such as residual intergranular pore,lattice pore,intergranular pore,intragranular dissolution pore,karst and fracture.The diagenesis in the study area mainly includes dolomitization,hydrothermal dissolution,karstification and rupture which have positive effects on the reservoir,and cementation and filling,silicification,compaction and pressure dissolution which have destructive effects on the reservoir.The study area have 7 stage fluid fillings,including the first stage supergene fluid,the second stage brine water fluid,the third stage hydrothermal fluid,the fourth stage brine water fluid,the fifth stage hydrocarbon fluid,the sixth stage siliceous fluid,the seventh stage calcite fluid.And 8 generation minerals were developed in the pore space of the study area,including(1)the early dolomite(2)the early age two stages of quartz(3)hydrothermal dolomite(4)fluorite,celestite and other MVT minerals(5)the late age multi-stages dolomite(6)asphalt(7)the late age quartz and(8)the late age calcite metasomatism.Based on the matching of mineral generations,fluid stages,diagenetic stages and tectonic burial history,the diagenetic evolution models of different types of reservoir rocks are analyzed.The study shows that the mineral growth generations and the multi-stage fluid dissolution and transformation reflected by them are the key to controlling high-quality reservoirs.After the sedimentary period,the microbial carbonate rock and its early dissolution played a role of foundation in reservoir development.During the medium-deep burial period,the early rock is transformed into a characteristic high-quality reservoir by multi-stage fluids dissolution modification,including organic acid fluid,magnesium-rich(hydrothermal)fluid,and silicon-rich(hydrothermal)fluid.And in the final uplifting and folding period,effective micro-cracks are formed and further improve the porosity and permeability of reservoir.