Forming Mechanism Research On High Quality Reservoir Of Lower Xu4Member In Xinchang Area, Western Sichuan Depression

The high quality reservoir and its forming mechanism of lower Xu4member inXinchang area in western Sichuan depression was studied in the research, which wasguided by the theory of sedimentology and reservoir sedimentology. According to thereferences, the field data, indoor analysis and well core data, the types of major rocks,reservoir characters, types of diagenesis and elements which affect the developmentof high quality reservoir were discussed.Lower Xu4member in Xinchang area developed fan delta front deposition, thesediments were mainly conglomerate and medium-coarse grained sandstone. Basedon the thin section observation and statistical result, reservoir sandstones of lower Xu4member in Xinchang area were mainly litharenite, lithic quartz sandstone andconglomerate. Full of debris，poor feldspar is the the main features of lower Xu4member sandstone. After compaction，pressure-solution，cementation，replacement，alteration，corrosion，filling effect of authigenic minerals and cataclasm. The reservoirwas mainly low porosity and low permeability, the reservoir space was mainlysecondary pore with poor sorting of pore throat. The permeability contribution wasmainly from small pore throat which belonged to small pore-fine throat. The researchindicated that lithic (quartz) sandstone which was located in high-energy environment,was the primary source of high quality reservoir. Sandstone in this environment hadthe characters of coarse grain size, good sorting, high textural maturity and mineralmaturity. In diagenetic process, it had strong capacity of resist compression, and thecharacters of high energy, high quartz, high porosity, low lithic fragment and lowmatrix. On the basis of Sinopec reservoir dividing standard, the lower limit ofsandstone reservoir porosity was5.5%, and the conglomerate porosity was2%.Reservoir was divided into three types, lower Xu4member were mainly type Ⅱ andtype Ⅲ, with poor permeability.According to research, we find that by the impact of compaction and cementation,primary porosity wiped out, high-quality reservoir space dominated by secondary porosity, secondary porosity from feldspar, shale and other debris of aluminumDissolution of silicate materialof lower Xu4member in Xinchang area was affectedby deposition, diagenesis, and structural factor. Deposition provided material basis forhigh quality reservoir, tectonism offered channels for later stage acid water,kaolinization and dissolution directly improved the physical property of reservoir.Therefore, high quality reservoir was closely related with the development of faultzone in the vertical, and controlled by favorable sedimentary facies in the plane.