Study On Reservoir Characteristics Of The Eighth Member,Shihezi Formation In Linxing Area,Ordos Basin,NW China

The eighth member of Shihezi formation in the Linxing area,Ordos Basin,is one of the main production zones for natural gas exploration.The carefiul study of its reservoir characteristics is of great significance for the exploration and development of natural gas.In this paper,based on the previous studies,taking modem stratigraphy,sedimentology,reservoir geology,logging geology and geochemistry as a theoretical guide.Through the observation and analysis of more than 200 core samples from 32 wells in the study area,combined with logging,gas testing,and production data,a detailed study of the physical characteristics of the reservoir in the eighth member of Shihezi formation in the study area was conducted.Using Integral Prediction Error Filter Analysis(INPEFA)technique,the sedimentary cycle of the eighth member of Shihezi formation in Linxing area was analyzed and divided into three mid-term cycles.On the basis of the three mid-term cycles,the sandstone reservoir is divided into 5 sand groups.among them,the ? sand group developed best with an average thickness of 11.69m.,followed by the ? sand group,with an average thickness of 9.21m,the ? sand group and the V sand group are generally developed,the ? sand group is the worst,,with an average thickness of only 2.82m.The lithology of the reservoirs in the eighth member of Shihezi formation of Linxing area is mainly lithic sandstone and feldspar lithic sandstone.The main components of the flller are kaolinite,ferrocalcite,hydromica etc.Coarse middle sand and middle fine sand accounts for the vast majority of grain size.The separation degree of rock debris particles is medium to poor with normal roundness of particles,and the shape of the particle is mainly the sub prismatic to sub round.The texture maturity of sandstone is lower.The contact relationship between particles is mainly point-line contact and line contact,most of the pore type cementation.The reservoir space mainly consists of primary pores,secondary pores and fractures,and secondary pores are the main ones.The eighth member of Shihezi formation in the Linxing area are mainly consists of continental facies.The sedimentary environment is a weak oxidizing environment of freshwater-brackish water,and the paleoclimate is warm and humid with a slow deposition rate.The diagenetic process in the eighth member of Shihezi formation of the study area can be divided into four stages:Late Carboniferous period to mid-late Permian quasi-symbiotic-early diagenetic A stage and B stage,during the compaction action resulting in sandstone porosity decreases sharply;Late Triassic to Middle Jurassic was diagenetic stage A,during which cementation dominated and sandstone porosity continued to decrease.After the Middle Jurassic,it was a meta-diagenetic stage B,mainly for ferruginous calcite and ferrocalcite precipitation,sandstone porosity is still further reduced.A well logging interpretation model for porosity and permeability was established for different facies(fine sandstone,medium sandstone,and coarse sandstone).Multiple regression analysis showed that there was a good eorrelation between porosity and density,acoustic,deep lateral resistivity and the shale content,and the correlation coefficient was above 0.9.Permeability has a good correlation with porosity,density and natural gamma ray,and the correlation coefficient is about 0.8.A detailed study was conducted on the plane distribution characteristics of the reservoir physical properties of the five sets of sand groups in the study area.On the whole,the porosity of the ?,?,and ? sand groups was relatively good,with an average value of around 9%.The distribution characteristics of "lower in the north and high in the south,,appears on the plane.The sandstone pores in the northern part of the study area were most developed(porosity is about 12%),and the porosity of the sandstone near the Zijinshan rock in the central part is the smallest(only about 4%).,which gradually increases to the west and north as a ring.The distribution of permeability is:the ?,? sand group permeability is relatively good,with an average value of about 0.4×10-3 um2,the plane distribution characteristics and the porosity distribution characteristics are similar,showing the distribution characteristics of "lower in the north and high in the south".The gas content and abundance of sandstone reservoirs in the eighth member of Shihezi formation in the Linxing area are calculated in detail.On the basis of this,combined with the characteristics of reservoir,the favorable areas in the study area are predicted.It is believed that LX-9-LX-10 well areas and LX-4-LX-6-LX-17 well areas are the most favorable zones in the eighth member of Shihezi formation in the study area.