Study On Sedimentary Microfacies And Reservoir Distribution Regulations Of Chang 8 Oil-bearing Of Yanchang Formation In Dingbian Area, Ordos Basin

Recent years, hydrocarbon exploration activities of Chang 8 Oil-bearing of Yanchang Formation in Dingbian area of western Ordos basin has obtained a great improvement, and a giant or super giant oilfield is expected to be developed. Because of its greater depth, higher exploration costs, reservoir distribution regulation had not been found out. Clarifying the sedimentary microfacies and reservoir distribution regulation of Chang 8 oil-bearing of Yanchang Formation in Dingbian area can not only direct the oil exploration, but also has academic interest.On the basis of synthesizing regional geologic information and the earlier researches, guided by leading edging theories of Reservoir Sedimentology, Petroleum Geology, and based on the cores, well logging, laboratory experiments results, the sedimentary provenance direction of the research area is studied in depth according to cluster analysis of light, heavy minerals and palaeocurrent analysis. The types, characteristics and macro-distributio of sedimentary facies and the distribution regulations of sandbodies are studied by means of multidisciplinary analysis, including litho facies, palaeoecologic markers, logging facies, depositional sequence, sandbody thickness, the ratio of sand to land thickness. Petrologic characteristics, physical properties, pore types, pore textures of reservoir sand, and diagenesis which affect reservior are studied systemly, according to laboratory experiments results, such as conventional thin sections, injecting thin sections, grain grade analysis, SEM observation, mercury injection. Reservoir distribution regulations was revealed according to the research results of the sedimentary microfacies, reservoir characteristics and diagenetic facies.The deposition was controlled by the northwest and northeast sedimentary system in the study area. Chang 8 oil-bearing formation in Dingbian area develops the parfacies of delta front and prodelta facies. The parfacies of delta front which belongs to frontal deposition of shallow water terrace type deltas develop mainly the microfacies of subaqueous distributary channel and subaqueous interdistributary embayment. The sand of subaqueous distributary channels of deltas front in different directions set with the characters of local thick sands pod distributed along the river and ringed distributed crosscut the river. The sand bodies taked the shape of the banded distribution from NE-SW and NW-SE were controlled by sedimentary facies.The rock of Chang 8 reservoir of the study area is mainly composed of lithic feldspathic sandstone. Cementing materials is mainly composed of the authigenic clay mineral mainly consisting of kaolinite, chlorite and illite and carbonate cements consisting of ferrocalcite, calcite and dolomite. Pore types of the reservoir develops mainly intergranular dissolved pore. The result of the mercury penetration experiment suggest that the reservoir has a characteristic of low initial expulsion pressure and high median expulsion pressure. The most important pore-throat structure types of reservoir is mainly small pore with micro-thin pore throat type. With a compact lithology, the porosity and permeability of Chang 8 reservoir belongs to the reservoir type of special low pore and low permeability reservoir.The types of diagenetic phases include kaolinite cementation feldspar corrosion facies, laumontite cementation corrosion facies, chlorite cementation facies, carbonate mineral cementation facies. Kaolinite cementation feldspar corrosion facies in the western of region and carbonate mineral cementation facies in the eastern of region taked the shape of the banded distribution is mainly distributed in the axis of the sands of subaqueous distributary channels. Subaqueous distributary channels of deltas front controlled the macro-direction distribution of reservoir. Diagenesis determined the quality of the reservoir zone. Kaolinite cementation feldspar corrosion facies of sheet is the the most favorable diagenetic facies zone.