Diagenesis And Densification Of Tight Sandstone Reservoirs In Transitional Facies Of The Longtan Formation In Huangqiao Area

The Longtan Formation of the Upper Permian in Huang area of the Lower Yangtze region develops a tight sandstone reservoir of delta front under the sedimentary background of marine-continental transitional facies.It coexists with organic-rich mudstones,the process of reservoir densification of which remains to be revealed.In this study,the process of reservoir densification and the indication of the fluid environment of the cement under the circumstance coexisting with organic-rich mudstones are investigated using a combination of core observation,thin section identification,fluid inclusion temperature and its salinity,fluorescence analysis,scanning electron microscopy,and X-ray diffraction.The main conclusions are as follows:(1)The rock types of tight sandstone in Longtan Formation are mainly quartz sandstone,with a single mineral composition,mainly quartz,followed by clay minerals,plagioclase,siderite,and a small amount of calcite.The general clay mineral content of tight sandstone near coal seam is slightly higher.(2)There are various types of diagenesis in the tight sandstone of the Longtan Formation,which mainly include compaction,cementation and metasomatism,dissolution and pressure dissolution,in which compaction,cementation and metasomatism play a destructive role in pore space,and dissolution is an important constructive diagenesis.(3)There are two cementation types in the tight sandstone,quartz overgrowth and siderite cement with the former earlier than the latter.The temperature and salinity formed by both are similar,but they show obvious transition from acidic fluids to alkaline fluids.The quartz overgrowth is related to the early carbon-rich acidic water,while the siderite may be related to the activity of anaerobic bacteria under the condition of shallow buried environment.The formation of two types of cements generally experienced the influence of hot fluid activity relating to the thermal event of Dongwu movement.However,the formation process of two types of cements did not involve the influence of oil and gas.The maturation of organic matter and the filling of oil and gas after the formation of siderite lead to the transition of tight sandstone fluids from alkaline to acid environment.(4)There are two cementation types in the tight sandstone,quartz overgrowth and siderite cement with the former earlier than the latter.The temperature and salinity formed by both are similar,but they show obvious transition from acidic fluids to alkaline fluids.The quartz overgrowth is related to the early carbon-rich acidic water,while the siderite may be related to the activity of anaerobic bacteria under the condition of shallow buried environment.The formation of two types of cements generally experienced the influence of hot fluid activity relating to the thermal event of Dongwu movement.However,the formation process of two types of cements did not involve the influence of oil and gas.The maturation of organic matter and the filling of oil and gas after the formation of siderite lead to the transition of tight sandstone fluids from alkaline to acid environment.Two kinds of sandstone densification processes including continuous densification and staged densification were identified.The continuous densification mainly occurs in sandstones with high debris content and small particle size.The compaction results in the continuous densification of the pores.The staged densification mainly occurs in the medium-grained quartz sandstones with high compositional maturity and structural maturity.Internally,there are two primary densification stages in staged densification,including quartz overgrowth pore densification(accompanied by dissolution of feldspar)in acid environment,and densification of siderite cementing intergranular and intergranular pores(accompanied by dissolution of quartz)in shallow buried alkaline fluids.