Reservoir Characteristics And Factors Controlling The Physical Properties Of The Sandstone In The Forth Member Of Xujiahe Formation In The Bazhong Area,Northeastern Sichuan Basin

The fourth member of Xujiahe Formation in Bazhong area,an important reservoir for gas exploration in the northeast of Sichuan Basin,is a typical tight sandstone reservoir.To clarify the main controlling factor of properties of this reservoir,methods such as pore cast,scanning electron microscope(SEM),cathode luminescence(CL),NESE Geo image analysis,property test,mercury intrusion pore measurement(MIP),X-ray diffraction(XRD)and fluid inclusions were used to analyse characteristics of diagenesis and its influence on reservoir properties and to explore the effect of feldspar dissolution and its associated cementation on pore structure of reservoir.The results show that the sedimentary environment of the fourth member of Xujiahe Formation in Bazhong area is dominated by underwater river channels.The rock types mainly include rock fragment feldspathic sandstone and feldspathic debris sandstone.In terms of physical properties,the sandstone in the fourth member of Xujiahe Formation in Bazhong area has poor physical properties,with an average porosity of 3.18% and an average permeability of 0.051 m D,indicating that it is a tight sandstone reservoir.In terms of pore characteristics,the pore types of the sandstone reservoirs in the fourth member of Xujiahe Formation in the Bazhong area mainly include primary intergranular pores,intragranular dissolved pores and intergranular micropores.There are few primary intergranular pores,and their edges are relatively flat and regular,with diameters ranging from 20 to 40 ?m,and clay minerals often develop on the surface of them.Most of the intragranular dissolved pores are formed by partial dissolution of feldspar and rock fragments,with irregular or harbor like edge.The diameter of these pores is 10?20 ?m.Intercrystalline micropores are formed during the precipitation of clay minerals and the growth of its crystal.They mainly exist between the crystals of clay minerals such as illite and chlorite.The diameter of these micropores is below 10?m.In terms of diagenesis characteristics,the compaction of the sandstone in the fourth member of Xujiahe Formation in Bazhong area is strong.Quantitative analysis shows that the porosity-reducing effect of compaction was significant,the average compaction rate is 66.9%,and the amount of porosity lost during the process of compaction is24.3%(The original porosity is 36.2%).In period of mesodiagenesis,the burial dissolution of particles lead to an increase in porosity of 3% and a rise in permeability,and most of these particles are feldspars.The cementation mainly includes calcareous cementation,clay mineral cementation and siliceous cementation.Calcareous cementation occurred in the early stage of diagenesis.The composition of calcareous cement is calcite,and the cementing method is porous cementation.X-ray diffraction shows that the average content of calcium is 2.48%.Clay mineral cements are widely distributed,and the cementing method is pore lining cementation.The composition of siliceous cement is quartz.The cementation method is mainly pore cementation.The products of dissolution,such as illites,chlorites and quartz,precipitated in pores which formed during the process of dissolution or pores(throats)few meters away form the location where dissolution occurred,reducing the radius of pores and throats,reduces the connectivity of pore throats,which makes the frequency distribution of radius of pores and throats of reservoirs finely shifted.The proportion of micropores in the reservoir increased and physical properties deteriorated.The porosity reductions caused by clay minerals precipitation and quartz cementation are 3%?4% and 4.2%respectively.And in this process,the radius of pores and throats corresponding to the peak frequency decreased,and the proportion of micropores in the reservoir increased.Compaction is the most important factor that leads to the densification of the sandstone in the fourth member of the Xujiahe Formation in Bazhong area.Dissolution is an important factor that controls the development of dissolved pores and leads to the improvement of reservoir physical properties,and cementation influences reservoir properties at the meter scaler.