Research On Sedimentary Microfacies Of Chang4+5 Oil-bearing Beds Of Yanchang Formation In Ganguyi Oilfield

Ganguyi oilfield is located in Shanbei slope of the south-east Ordos Basin.The thickness of Chang4+5 oil-bearing beds of Yanchang Formation is from 90 to 110m in Ganguyi oilfield.The Chang4+5 oil-bearing beds is the minor oil producing bed comparing with the Chang6 oil-bearing beds in the area.In the process of formation analysis ,the marker formation must be ascertained frist .Based on this, the stratum is divided into Chang4+5¹ and Chang4+5² .In study , Chang4+5¹ is subdivided into Chang4+5^1-1 and Chang4+5^1-2,meanwhile, Chang4+5² is subdivided into Chang4+5^2-1 and Chang4+5^2-2. And the time correlation in sequence stratigraphic unit was realized according to the temporal rock marker.According to the analysis of the facies marks ,such as petrosal kinds with the features, sedimentary structure ,fossil, logging facies and other criteria in the study area, the analysis of the facies shows that Chang4+5 oil-bearing beds is composed of plain-delta sedimentary system. The sedimentary microfacies include distributary channel, swamp, natural barrier, crevasse splay, interdistributary deposits.On the basis of regional geological background, the distribution pattern of sedimentary microfacies in Chang4+5 oil-bearing beds was distinguished and the palaeogeographic environment was reconstructed by the analysis of sedimentary facies ,logging facies ,sandstone thinckness and sandstone percent and other sedimentary character.The sedimentary microfacies of the Chang4+5 oil-bearing beds controls the characteristic of the reservoir distribution, and the reservoir distribution controls the oil enrichment mainly. Based on the observation from cast thin section and scanning electron microscope in Ganguyi oilfield , it is held that ,Chang4+5 reservoir are mainly composed of grey feldspathic sandstone. The porosity is mainly the kind of intergranular pore and intragranular dissolved pore. On the basis of porosity and permeability data ,the results show that sandstone reservoir in this area are the typical low porosity and typical extra low permeability .Diagenesis is the main factor that affects the porosity and permeability of reservoir in the study area.