Research On Heterogeneity And Reservoir Geological Characteristics Of Chang 6 Reservoir Of Yanchang Formation In Ansai Heishanliang Oilfield

Heishanliang area of Ansai Oilfield is located in the middle of Yishan slope in Ordos Basin,and its main production layer is Chang 6 reservoir of Yanchang formation,which has entered the stage of water injection development.however,some problems are gradually exposed,such as rapid increase of water cut,serious production decline,poor overall water flooding effect and so on.The previous basic geological research can not well match the current development situation.Therefore,it is necessary to finely describe the current reservoir heterogeneity and reservoir characteristics of the study area,so as to provide a geological basis for adjusting development measures and improving the effect of oilfield development in later stage.Based on the data of drilling,logging and production of more than 400 wells in heishanliang area,combined with the core observation,analysis results of casting thin section,SEM and mercury injection test,this article makes a basic research on the sedimentary facies and reservoir microscopic features of Chang 6 oil-bearing formation in the Heishanliang area,so as to analyze the reservoir heterogeneity and reservoir geological characteristics.The Chang 6 oil-bearing formation in Heishanliang area is the shallow-water delta front sub-facies,which mainly develops the underwater distributary channel microfacies,followed by the underwater distributary bay and mouth bar,and the extension direction is NE-SW.The rock type is mainly fine feldspar sandstone,and the main cements are laumontite and chlorite.The reservoir's pore type is mainly intergranular dissolved pores,in which laumontite dissolved pore is the most important oil and gas reservoir space of Chang 6 reservoir in the study area.The displacement pressure and median displacement pressure of the reservoir are medium to large,belonging to the medium pore with micro-throat type.The lowest limit of the effective porosity of reservoir is 7%,and the permeability is 0.2×10^-3?m²,which belongs to low-porosity and ultra-low permeability reservoir type as a whole.The permeability heterogeneity of each oil-bearing layer in Chang 6 reservoir of the study area is strong,and the interlayer is well developed,which reflects the strong intra-layer heterogeneity.Meanwhile,the Chang 6 reservoir in the study area shows strong interlayer heterogeneity,which is reflected in different development degree of sand bodies and distribution of interlayer.In plane,the heterogeneity of reservoir is weak in underwater distributary channel,while it is stronger in the edge of channel and underwater distributary bay.In addition,the partial reservoir in the overlapping area of underwater distributary channels has low physical properties and strong heterogeneity.The heterogeneity of Chang6 reservoir is mainly controlled by its sedimentation and diagenesis.The Chang 6 oil-bearing formation in Heishanliang area is a typical lithologic reservoir with obvious malconformation in distribution of the hydrocarbon.In plane,the oil accumulation area of Chang 6 reservoir gradually migrates from the southwest to the north and northeast.vertically,there are low-high-low gradations in the degree of oil accumulation from the lower to upper strata,among which the Chang 6^2-2 reservoir is the most oil enrichment layer and the largest energy storage coefficient,followed by Chang6^1-2 reservoir,but the oil production capacity of the former is slightly worse than that of Chang 6^1-2 layer.The distribution of Chang 6 reservoir in study area is mainly controlled by spread characteristics of framework sand body and lateral shielding of tight rock,and the local structure has little influence on distribution of oil and gas.