Reservoir Characteristics And Oil-water Distribution Of Chang 7 Sandstone In Heshui Area, Ordos Basin

The reservoir of Chang 7 Member in Heshui Area of Ordos Basin is the key target of development in recent years.With the further development of exploration and development,the effective development of the oilfield is severely restricted by the problems such as poor reservoir physical property,complex pore throat structure,low fluid mobility and unclear distribution of oil and water.On the basis of statistical analysis of core data,logging curves,oil test and production test data in the study area,combined with thin section observation,XRD,high-pressure mercury injection,constant rate mercury injection,nuclear magnetic resonance,oil-water relative permeability and other related experiments,this paper analyzes and studies the petrology characteristics,pore throat structure characteristics and seepage characteristics of the reservoir in the Chang 7member of Heshui area,and establishes a comprehensive evaluation standard for the reservoir in the Chang 7 member of Heshui area in combination with the characteristics of oil-water distribution laws,provide guidance for subsequent exploration and development of the oilfield.The recognition mainly includes the following aspects:（1）The Chang 7 reservoir in the Heshui area is mainly developed in the gravity flow sand body formed in the semi-deep lacustrine sedimentary environment.The reservoir has poor physical properties and belongs to an ultra-low porosity ultra-low permeability reservoir.（2）The rock type of the Chang 7 reservoir in the Heshui area is mainly feldspathic lithic sandstone,with various pore types such as intergranular pores,dissolution pores,intergranular pores,and microcracks developed.The average porosity is 1.417%,with the highest content of dissolution pores.According to the curve shape,displacement pressure,and pore throat radius obtained from high-pressure mercury injection,the pore throat structure of the Chang 7 reservoir in the study area is divided into four types:I-IV.As the pore throat structure changes from type I to type IV,the pore and throat radius gradually decrease,and the ratio of the maximum mercury injection into the pore to the total maximum mercury injection tends to decrease.The volume proportion of the throat gradually increases.Based on the conversion results of high-pressure mercury injection and nuclear magnetic resonance T₂ spectrum,during the changing of pore throat structure from class I to class IV,the proportion of small pores（0.01<r<0.1μm）gradually increases,The proportion of mesopores（0.1<r<1μm）gradually decreases.（3）According to fluid mobility parameters obtained by NMR experiment,the average saturation of the movable fluid in the sample is 45.83%,and the average porosity of the movable fluid is 4.84%.As the pore throat structure of the reservoir gradually changes from classⅠto classⅣ,the starting pressure gradient of the reservoir increases gradually,and the fluid mobility deteriorates gradually.There are differences in the occurrence characteristics of fluid in different types of reservoirs,and the movable fluids of classⅠandⅡmainly exist in middle pores（0.1～1μm）,and those of classⅢandⅣmainly exist in small pores（0.01～0.1μm）.The overall performance is characterized by high bound water saturation and high residual oil saturation,and the overall oil displacement efficiency of the reservoir is relatively low.（4）The distribution of Chang 7 oil layer in Heshui area is jointly affected by sedimentation and diagenesis,and the high production area of oil testing is mainly distributed in the area with good relative physical properties of underwater branch channel microfacies and unstable component dissolution facies.On the basis of studying the macroscopic characteristics,microscopic pore throat characteristics,seepage characteristics,and oil water distribution characteristics of the reservoir,and referring to previous classification schemes,the Chang 7 reservoir in the Heshui area is divided into four categories:A～D.Among them,class A reservoirs have large pore throat radius,weak heterogeneity and strong reservoir and percolation capacity,while class B reservoirs have large pore throat radius,weak heterogeneity and good percolation capacity.These two types of reservoirs are the main targets for future exploration and development.Class C reservoir has poor pore throat structure,strong heterogeneity and certain percolation capacity.Class D reservoirs are ineffective reservoirs with small pore throat radius,strong heterogeneity and low seepage capacity.These two kinds of reservoirs cannot be developed temporarily under the existing technical conditions.