Research On Characteristics And Comprehensive Evaluation Of Chang 6 Reservoir In Yangdaomao Area Of Ordos Basin

Yangdaomao block is located in the middle of Northern Shaanxi Slope in Ordos Basin and is one of the important oil and gas producing areas in Ordos Basin.The oil-bearing strata in the area are mainly Chang 6 reservoir.With the gradual expansion of production scale,a series of serious problems have appeared in the development process,such as the imbalance between injection and production,the rapid rise of water cut,the difficulty in determining the water injection limit and so on,the reason lies in the lack of a comprehensive understanding of the reservoir,so it is necessary to further study the characteristics of Chang 6 reservoir and establish a comprehensive reservoir classification and evaluation criteria,which has important guiding significance for the next exploration and development of Chang 6 reservoir in this area.Based on the principle of stratigraphic division,this paper divides the Chang 6formation into fine sub-layers,on this basis,analyzes the types of sedimentary microfacies in combination with the regional sedimentary background.It is considered that the delta front subfacies developed in the Chang 6 formation,and the main microfacies types are underwater distributary channel and interdistributary bay.Based on the study of the"four properties"relationship of Chang 6 reservoir,the interpretation model of reservoir porosity,permeability and oil saturation is established.Finally,the lower limit of reservoir physical property is determined,and the logging identification standard of effective reservoir is established.Core observation and various experimental analyses show that the lithology of Chang 6 reservoir in the study area is mainly gray fine-grained and medium-fine-grained feldspar sandstone,and the interstitial materials are mainly chlorite,calcite,laumontite,siliceous and feldspar,among which chlorite content is the highest.The pore types are mainly intergranular pores,followed by feldspar dissolved pores,and throat types are mainly flaky.The microscopic pore characteristics as a whole show that the pore throat is small and the sorting is medium,the reservoir performance is good,but the percolation ability is poor,belonging to the reservoir with extra small throat.Diagenesis mainly includes four types:compaction,pressure solution,cementation and dissolution,of which cementation is the most common,and the main body of diagenetic evolution has entered the middle diagenetic stage A.The analysis of oil-water phase permeability experiment shows that the irreducible water saturation of Chang 6 reservoir is relatively high,and the oil-water two-phase permeation zone is relatively narrow.In the seepage experiment of real sandstone model,displacement pressure,pore type and pore throat radius are the main factors for oil displacement,the piston displacement is the main style for water displacement.The ultimate displacement efficiency averages 42.9%.The displacement efficiency is mainly affected by injection pressure,injection multiple and pore throat heterogeneity.The heterogeneity of Chang 6 reservoir is analyzed,and the reservoir heterogeneity is considered as medium.Based on the clustering analysis method,combined the sedimentation and reservoir characteristics,the classification and evaluation criteria of Chang 6 reservoir in Yangdaomao area are established,and the reservoirs are divided into types I to IV from good to bad.The Chang 6 reservoir is mainly classified into class II and class III,and the class I reservoir is mainly developed in the sub-layers of Chang 6₁ and 6₂.From a plane point of view,the reservoirs of class I and class II are generally distributed in the distributary channel of underwater distributary where the sand body is thicker and has better connectivity;The reservoirs of class III and class IV are distributed in thin sand bodies which near the distributary bay area.