Research On The Reservoir Characteristics Of The Chang-8 Member Of The Yanchang Formation In Zhenjing Oilfield, Ordos Basin

Zhenjing area has a huge potentiality of oil resource,the amount of oil resources is 16039.2×10⁴ tons in this area,and the abundance of oil resource has reached 6.38×10⁴ t/km²,so it has a better exploration and development prospect.It can be found from the present exploration that the Chang-8 member is a reservoir with a large area,low oil abundance,and very low permeability,and the heterogeneity is strong on planar and vertical in Zhenjing oilfield.Therefore,it is of practical importance to speed up the step of research on reservoir characteristics of the Chang-8 member of the Yanchang formation in Zhenjing oilfield.On the base of core observation,logging data and the studied results of former geologists,and through statistical analysis of petrophysical data and logging data,this paper systematically studies reservoir characteristics of the Chang-8 member of the Yanchang formation in Zhenjing oilfield,Ordos Basin.Firstly,develop the sequence stratigraphic study and build the sametime sequence pattern,use the method of sedimentary facies control- sametime principle to correlate sandbody groups in the target strata,and build stratigraphic framework.Then,use logging curves to classify logging facies based on the lithology-electric property relation.According to the upper research results,the sedimentary facies is determined to be the front-delta deposition of braided river delta deposits,and five microfacies types are recognized including subaqueous distributary channel,subaqueous natural barrier,subsea crevass-splay, mouth-bar,subaqueous interdistributary embayment.The depositional characterisytics and the structure of microfacies types in sections are discussed,and the lateral distribution of the sedimental facies are predicted.Through the research of petrologic feature,pore types,pore structure and relationships among lithology,poroperm characteristics,electric properties and oilbearing properties,it is thought that the lithology are middle-fine grained feldspathic litharenite sandstone,lithic arkose sandstone.The effective porosity is 8.5%,and effective permeability is 0.3×10^-3μm² in the Chang-8 member.So it is considered after the comprehensive reservoir evaluation that it belongs to the type of reservoirs with low porosity and permeability,the fine pore throat and poor sorting.After the research on the heterogeneity of the reservoir sandstones,it can be concluded that the heterogeneity resulted from sedimentary environment and diagenesis interaction.On the basis,the lower limit of reservoir parameters of the Chang-8 member in this oilfield is determined in the paper:the lower limit of throats is 0.25urn,the low limit of permeability and porosity which are corresponding with the lower limit of throats is 0.1172×10^-3μm² and 8.5%respectively and the low limit size is 5m of fine sandstone.In light of reservoir evaluation,the reservoirs can be classified as four types.The type-â… and type-â…¡are very easy to form reservoir,and not need very thick closure.But type-â…¢must have thicker sand and closure,so it could have the oil producing capability after late reformation,type-â…£is non-reservoir because it must have high H₅₀and prevent secondary migration.The type-â… (excellent reservoirs)is not developed;type-â…¡(better reservoirs)is seldom and type-â…¢is commonly found out in the research region.Compared the thickness of the effective sand bodies of the five sandbodies of the Chang-8 member of the Yanchang formation,it is found out that the thickness of sand body of Chang-8^1-2is the largest,but the smallest in Chang-8^1-3.The difference of the thickness of sand bodies would result in the difference of high-quality reservoir distribution.The high-quality reservoirs are mainly developed in the thick sandstone of subaqueous distributary channel of the front-delta deposition.At last,it is suggested that the most favorable sand body is Chang-8^1-2and Chang-8^2-2,and the exploration targets to survey the type-â…¡reservoir which distributed in the east of the Well-5 and in northern Zhenjing Oilfield in Chang-8^1-2,and in the central part of Zhenjing Oilfield in Chang-8^2-2.