The Study On Sedimentary Facies And Reservoir Characteristics Of Chang4+5 Formation From 29-38 Area Of Wuqi Oil Field In Ordos Basin

The Chang 4+5 oil layer subgroup of the Yanchang Formation in the Wuqi Oilfield,Ordos Basin,has always been one of the major oil production zones in the area,but its basic geological studies such as sedimentary microfacies and reservoir characteristics are still weak.In order to meet the need of oilfield development in later period and to provide data support for oil reserves calculation,it is particularly important to study the sedimentary facies and reservoir characteristics of the target zone.In this paper,previous research results,cores,drilling and mud logging,and well-logging data were fully utilized.Combined with experimental analysis data such as cast thin sections,X-ray diffraction,scanning electron microscope,etc.,the target layers in the study area were identified and divided.The typies and distribution of sedimentary facies were analyzed and the reservoir characteristics were studied in detail.The research results are as follows:Based on the auxiliary mark layer "thin neck",the subgroup of the target oil layer Chang 4+5 is divided into Chang 4+51 and Chang 4+52.The response characteristics on the logging curve are 3 to 4 high finger tips at the bottom of Chang 4+51.The strata are distributed stably with a single west-inclined slope and a thickness of about 110m.The 4+5 stratum of the study area mainly developes front delta sediments,mainly is composed of medium-fine sandy sediments filled in underwater distribunary channels and argillaceous-silty mudstones filled in interdistribunary.The source direction of the study area is northeast.The sedimentary microfacies profile along the provenance direction shows that the distribution of microfacies is featured like belts with good continuity.In the direction perpendicular to the sources,Chang 4+52 sand bodies are vertically stacked,and the Chang 4+51 sand bodies are small in size,pinch out at both ends and lens-shaped.The 4+5 reservoirs of the Yanchang Formation in the study area are mainly lithologic reservoirs.The Chang 4+52 strata belong to delta front sediments.Underwater distributary channels are relatively developed,with six main northeast-southwest extending channels and interdistributary sediments are local developed.The Chang 4+51 strata belong to delta front sediments.Underwater distributary channels are relatively developed,with five main northeast-southwest long-extending channels and interdistributary sediments are local developed.In the study area,Yanchang 4+5 mainly deposited fine-silt feldspar sandstones and lithic arkose sandstones,which have good structure and composition maturity with mainly line-contact particles.The main factors that affect physical properties of sandstone reservoirs in Chang 4+5 are from sedimentary and diagenesis-epigenetic aspects.The pore porosity and permeability conditions show good correlation.The higher the quartz content is,the better the porosity and permeability conditions are;the higher the carbonate content is,the denser the cementation of the reservoir and poorer the porosity and permeability conditions are.However,after the carbonate content is less than 5%,the porosity and permeability conditions do not change significantly.The research area mainly develops residual intergranular pores and intercrystalline pores,belonging to the micropore-middle throat type and micropore-micro throat type.There is a good correlation between the parameters of pore throat size and distribution and reservoir properties,especially permeability.The Chang 4+5 oil-bearing subgroup is located in the stage of the late diagenetic stage,and its heterogeneity is strong.The main reservoir of the study area is type ? and type ? reservoir,which is mostly developed on the subaqueous branch channel and flanking sand body of delta front.And its distribution area is gradually decreasing along the direction of the source.Three favorable blocks are predicted in long 4+52 and one in long 4+51.