The Description Of Chang 8 Oil-bearing Beds Of Yanchang Formation In Xifeng Oilfield

Applied with Polarizing microscope, cathodoluminescence, Scanning Electron Microscope, image analysis and other analytic techniques, combining geophysical logging, core description and outcrop observation, the sedimentary facies, reservoir petrology, reservoir digenesis, pore texture and influences on reservoir of Chang 8, Yanchang Formation of the Triassic period in Xifeng Oilfield are studied.Yanchang Formation of Triassic in Xifeng Oilfield developed Lake Delta deposit. Shallow lake facies are mainly developed and delta plain can be found in the area. The main sedimentary microfacies comprise distributary channel, mouth bar, distal bar, blanket sand, natural levee and crevasse splay. Chang 8 is typical delta deposit which has reverse grain size order in ascending order. 6~ 7 sandbodies of Chang 8 that deposited from southwest to northeast penetrate into deep lake region.The reservoir sandstone is fine-medium grained lithic arkose and felspathic lithoclast sandstone. The main constituents of Clastic are feldspar, quartz and lithoclast and the high mica and eruptive rock is typical feature of Chang 8 reservoir. Diagenetic authigenic mineral are chlorite, silicon and ferreous calcspar. Diagenetic authigenic mineral assemblages are different in different area. Main authigenic mineral are bridge-liked illite and chlorite in Zhenyuan area, chlorite in Baima area, kaolinite in Dongzhi area and chlorite, silicon and iron calcspar in Heshui -Ningxian area. The main influences on the reservoir are deposition and diagenesis. Distributary channel sandstone is the best reservoir which has coarse granularity, good sorting, and low mica and develops intergranular pores. The fine granularity of sandstone and high mica content make the blanket sandstone, mouth bar and distal bar the bad reservoir.The reservoirs of Chang 8 belong to low-particular low permeability in research area, which includes intergranular pore reservoir and dissolution pore reservoir. The average porosity is lower than 12%, the average permeability is lower than 1.0×10~-3μm~2 and the average bottleneck radius is lower than 0.2μm, closed pressure is over 0.5MPa. Pore texture of reservoir can be classified four large and six little types, Ⅰ and Ⅱ types of reservoir can produce high commercial oil and Ⅲ types of reservoir can produce lower commercial oil after compression and acidization. Thesuperior reservoir mainly developed in Baima area, Xi 49 and Xi 33, Xi 49 in Dongzhi area. Based on sedimentary facies, distribution of sandstone, porosity and permeability, pore combination and zone of diagenetic characteristics, three superior exploration areas are forecasted, which are Tongchuan area, Baina - Dongzhi area and Heshui - Guchengchuan area.