Study On Characteristics Of Tight Sandstone Reservoir Of Shan-2 Member Of Shanxi Formation In The Eastern Ordos Basin

The Shanxi Formation reservoirs in the eastern Ordos Basin are mainly tight sandstone gas reservoirs.The tight reservoirs generally have many gas-bearing strata,wide reservoir distribution,low porosity,low permeability and complex microscopic pore structure.They are also characterized by strong heterogeneity,complex gas-water relationship in the reservoir,and many abnormal pressures.All of such characteristics make exploration difficult.In the early exploration process,coarse sandstone and medium sandstone reservoirs were always regarded as research and exploration targets,and some industrial productivity has been obtained.By contrast,fine sandstone and siltstone reservoirs with poorer physical properties and more complex microscopic pore structures were neglected.The Shanxi Group mainly contains No.4 and No.5 coal seams,which exert a great influence on the gas bearing property of the reservoir,so that the fine sandstone and siltstone in the vicinity of the coal seam also have certain exploration potential.Researches on fine sandstone and siltstone reservoirs have important significance for the late exploration in the study area.Based on theories of sedimentology and reservoir geology,this article makes full use of field outcrops,cores,well logging data,logging data,employing cast thin sheets,scanning electron microscopy,X-ray diffraction analysis,high pressure mercury intrusion,nuclear magnetic resonance,conventional physical property analysis,nitrogen adsorption,micro-CT and many other techniques to analyze the basic petrological characteristics,pore-permeability characteristics and micro-pore structure of fine sandstone and siltstone reservoirs in the study area,classification and evaluation of the study area.The main rock types of fine sandstone and siltstone reservoirs in the Shan-2member of the Shanxi Formation in the study area are lithic quartz sandstone,lithic sandstone,and a small amount of quartz sandstone.Both structural maturity and ingredient maturity are relatively high.The interstitial fillings are mainly composed of hydromica,kaolinite,iron dolomite and siliceous,and also contains a small amount of dolomite,siderite and pyrite.The pore types of the Shan-2 member of the Shanxi Formation in the study area are primary pores such as intergranular pores,intergranular dissolved pores,secondary pores such as intragranular dissolved pores,lithic pores,intercrystalline pores,mold pores and micro-cracks,etc.Intercrystalline pores are the main pore type,while secondary ones are micro-cracks and inter-granular pores.Using the micro-CT technique,this study establishes a three-dimensional model offine sandstone and siltstone in the Shan-2 member of the Shanxi Formation in the study area to analyze the spatial structure of the microscopic pore throat,and to study the distribution of pore throats in fine sandstone and siltstone through shape factor,pore throat coordination number and pore throat connectivity function.Using high pressure mercury and nitrogen adsorption reveals that the pore radius distribution of fine sandstone is in the range of 4-630 nm.The pore volume is mainly provided by mesopores and transition pores.The mesopores,transition pores and micropores provide most of the specific surface area.The pore radius distribution of siltstone ranges from 4to 100 nm.The pore volume is mainly provided by mesopores and transition pores.The transition pores and micropores provide most of the specific surface area.According to X-diffraction analysis,the study finds that quartz mineral content of the fine sandstone and siltstone reservoirs in the Shan-2 member of the Shanxi Formation is relatively high,which is proportional to the brittleness index.It can be seen from the overall brittleness index of the study area that the brittleness index is generally greater than 60% which is conducive to large-scale fracturing transformation.Through comprehensive analysis of physical properties of fine sandstone and siltstone reservoirs,reservoir pore structure parameters and reservoir fluid seepage parameters,and comparison of diagenetic characteristics of cement content,the main diagenetic facies in the study area are divided into four types.The relatively favorable diagenetic facies are the dissolution phase of kaolinite cementation and the microporous phase of illite+kaolinite crystal.Based on lithology analysis,the study also concludes 9 common sand-mud-coal basic combinations of the Shan-2 member of the Shanxi Formation in the study area,and the basic combinations were superposed to form a complete gas-bearing combination.The analysis shows that fine sandstone and siltstone near coal seams have good gas-bearing properties.According to the reservoir physical characteristics,microscopic pore structure characteristics,sedimentary facies type,diagenetic facies type,sand body structure type and sand body thickness,the study carries out a comprehensive evaluation of fine sandstone and siltstone reservoirs in the Shan-2 member of Shanxi Formation in the study area.Fine sandstone and siltstone reservoirs are divided into four categories.Class III(poorer)and Class IV(poor)are main areas,while Class I(good)and Class II(medium)are smaller,distributed in the northwest and south of the study area.