Reservoir Evaluation And Accumulation Of Tight Gas In Shanxi Formation Of Qinshui Basin

Coal measures strata are widely developed in Carboniferous-Permian in Qinshui Basin,which has abundant natural gas resources.Now,the natural gas exploration in Qinshui Basin is mainly focused on coalbed methane,and a large-scale coalbed methane field with up to 100 billion cubic meters of coalbed methane has been established in the south of Qinshui Basin.The rise of tight gas research stems from the discovery of good gas logging display in several sets of sand layers in the Upper Paleozoic during the exploration and exploitation of several coalbed methane blocks in recent years,which has attracted the attention of domestic experts and scholars.However,at present,the pore structure characteristics and reservoir physical property control factors of sandstone reservoirs in Qinshui Basin are not identified,and the diagenetic evolution process,tight gas reservoir characteristics,accumulation period and accumulation process of tight gas are not clear,which limits the exploration of tight gas in this area.Therefore,based on X-ray diffraction,casting thin section,scanning electron microscope,high-pressure mercury injection,organic geochemistry,gas adsorption,noble gas measurement and fluid inclusion and other experimental analysis and testing methods,in combination with core,well logging,gas logging and regional geological data,this paper carries out detailed reservoir evaluation and tight gas accumulation characteristics analysis on Shanxi Formation sandstone reservoir,defines the pore structure characteristics,the main control factors of reservoir physical properties and the diagenetic evolution process of sandstone reservoir,quantitatively evaluates the hydrocarbon generation capacity of source rocks and identifies the accumulation process of tight gas.The main innovative achievements of this paper are as follows:1.The sandstone reservoir space of Shanxi Formation is mainly composed of dissolution pores,intergranular pores and microfractures,and the primary pores are poorly developed.The reservoir mainly develops nanoscale pore system.The pore diameter is mainly between 40 nm and600 nm.The pores of the reservoir are mainly small pores?<0.1 um?,followed by medium pores?0.1¹ um?,with a few large pores?>1 um?locally developed.2.The reservoir has small throat diameter,complicated pore structure,high displacement pressure,poor pore-throat connectivity and low porosity and permeability,which is a typical tight reservoir.Dissolution can improve the physical properties of reservoir and form dessert areas locally.Reservoir physical properties are mainly controlled by pore structure and mineral composition.Throat diameter is the direct factor controlling reservoir physical properties,while sedimentation is the fundamental factor controlling reservoir physical properties.3.The calculation models of free gas and adsorbed gas in source rocks are established,and the storage capacity of source rocks for natural gas is quantitatively evaluated.Mudstone has smaller hydrocarbon generation capacity and larger storage capacity,while coal has larger hydrocarbon generation capacity and smaller storage capacity.The gas expulsion amount of the two are 0.6×10¹² m³ and 11.96×10¹² m³,respectively.The calculated tight gas resource of Shanxi Formation is 0.38×10¹² m³.4.There are two stages of accumulation of tight gas in Shanxi Formation,and the corresponding accumulation times are Middle-Late Triassic and Late Jurassic-Early Cretaceous respectively.According to the diagenetic history of the reservoir and the gas injection history,the first gas accumulation stage occurred before the densification of the reservoir,and the second gas accumulation stage occurred after the reservoir is densified.