Characteristics And Grading Evaluation Of Reservoir Space Of Tight Sandstone Of The Hetaoyuan Formation In The Biyang Sag

The Biyang Sag is an oil-bearing depression containing enriched tight oil and gas in the eastern Nanxiang Basin.However,less research has been done on the tight sandstone reservoirs in this area.In this article,the sandstone samples of the 3rd member of the Hetaoyuan Formation including Wells Bi-212?Bi-216?Bi-246?Cheng-1 and An-84 were selected for physical property anlysis,microscopic examination,X-ray diffraction(XRD),low-temperature nitrogen adsorption(LTNA),mercury intrusion capillary porosimetry(MICP),nuclear magnetic resonance(NMR)and other experimental analysis,to carry out detail characterization of reservoir spaces of the tight sandstone in the Biyang Sag and establish the grading evaluation criteria of the reservoir.Through the observation of casting thin sections as well as field emission-scanning electron microscope,the reservoir space of tight sandstone in the 3rd member of the Hetaoyuan Formation in the Biyang Sag is divided into three types,which are intergranular pores,intragranular pores and fractures.Among them,intergranular pores and intragranular pores are the main reservoir space.According to the mineral composition,the intergranular pores are further divided into quartz and feldspar intergranular pores,quartz and feldspar grain edge pores and carbonate minerals intergranular pores.The intragranular pores are divided into quartz and feldspar dissolved pores,carbonate minerals intragranular pores and clay mineral intercrystalline pores.The results of multiple experiments indicate that the number of pores with diameter larger than 1?m in the tight sandstone reservoirs is small,which mainly distribute in the intergranular pores of minerals such as quartz and feldspar.The size of intragranular pores is usually distributed from 50 nm to 200 nm,which has less contribution to pore volume on tight sandstone reservoir.Scanning electron microscopy,thin section microscopy and the homogenization temperature of fluid inclusions were used to determine the types of reservoir diagenesis and establish the diagenetic evolution sequence.The diagenesis process and porosity change process were revealed by inversion stripping method.The results show that the study area has experienced strong compaction and carbonate mineral cementation,and the continuous compaction is the main factor causing the densification of sandstone reservoirs in this area.Using high pressure mercury injection technology combined with its fractal theory,the pore throat structure characteristics of tight reservoirs were discussed and the reservoirs were classified.The results show that a division method for micropore pores(less than 100 nm),transitional pores(100 nm?1.0 ?m),mesopores(1.0?m?3.0 ?m)and macropores(greater than3.0 ?m)within tight sandstone reservoir in the study area was established.According to the volume percentage of various types of pores in the reservoir,the tight sandstone reservoirs were divided into I,II,III and IV categories.Further according to the correlation between different microscopic pore throat parameters and reservoir physical properties,the tight sandstone reservoirs were divided into four grades: the grade I is high quality reservoir,the grade II is relatively high quality reservoirs in tight sandstone reservoirs.The optimum reservoir is selected by the standard of division,which is aimed to provid guidance for the order of tight oil and gas development.