Characteristics And Evaluation Of Chang 6 Tight Sandstone Reservoir In Huaishuzhuang Area,ordos Basin

The Huaishuzhuang area is located in Fuxian,south of the northern Shaanxi slope in the Ordos Basin.The Chang 6 member is the main exploration target.In this paper,guided by the theory of tight reservoir geology,using various test and analysis methods such as cast thin slices,scanning electron microscope,high-pressure mercury intrusion,nuclear magnetic resonance,etc,combined with well logging data and core data,etc,the Chang 6 member in the study area was deposited facies and reservoir characteristics have been systematically studied,and the results show that:According to the core color,sedimentary structure,grain size and logging facies signs,combined with the regional sedimentary background,it is believed that the Huaishuzhuang area during the Chang 6 period mainly developed delta front subfacies deposits,including underwater distributary channels,underwater natural dikes,and diversion bays.There are three kinds of sedimentary microfacies.The reservoir rock type is fine-grained feldspar sandstone,and the interstitial materials are mainly calcite and chlorite.The average porosity is 8.37%and the average permeability is 0.27×10-3?m2.The pore types mainly include residual intergranular pores and dissolved pores,and the total face rate is about 2.5%.The pore size distribution interval of the reservoir is 0.0001-16.7352?m,mainly distributed in 0.001-l?m.The physical properties of the reservoir are controlled by the microscopic distribution characteristics of pores and throats.The permeability is<0.05×10-3?m2,Nano-scale pore-throat systems are mainly developed,pores are mainly controlled by<0.1?m throat,permeability is mainly contributed by<0.1?m throat;permeability is(0.05-1)×10-3?m2,micro-and nano-scale storage spaces coexist,its pores are mainly controlled by 0.1-0.5 ?m throat,and permeability is mainly contributed by 0.1-0.25?m throat;permeability>1×10-3?m2,pores are mainly controlled by>0.5?m throat,permeability is mainly contributed by>0.25?m throat.The Chang 6 reservoir has entered the stage A of diagenesis.Pore evolution simulation reveals that the original porosity of the reservoir is 36.1%on average,the compacted porosity loss rate is as high as 58.5%,the cemented porosity loss rate is 25.8%,and the dissolution and porosity increase rate is only 5.4%.Compaction is the main reason for the loss of porosity in Chang 6 reservoir,and cementation is the key to the formation of tight reservoirs.The paleoburial depth at which the reservoir started to be dense is about 1600m,and the completely dense paleoburial depth is about 2100m,corresponding to geological time of 150Ma(Late Jurassic)and 120Ma(Mid Early Cretaceous).The Chang 6 reservoir can be divided into three types:good,medium and poor reservoir.The lithology of type I reservoirs is mainly medium-fine-grained and fine-grained feldspar sandstone,with reservoir quality index>1.73,fractal dimension<2.58,porosity greater than 9%,and permeability greater than 0.3 ×10-3?m2.The lithology of type ? reservoirs is mainly fine-grained and very fine-grained feldspar sandstone,the reservoir quality index is 1.05-1.73,the fractal dimension is between 2.58-2.69,the porosity is between 7%-9%,the permeability is between 0.05×10-3?m2 and 0.3×10-3?m2;the lithology of type III reservoirs is mainly silty sandstone and siltstone,the reservoir quality index is less than 1.05,the fractal dimension is greater than 2.69,and the porosity is less than 7%.The permeability is less than 0.05×10-3?m2.