| Ansai oil field is located in the area of Yan’ an city, Shaanxi Province, and structurally located in the central of Yishan Slope in the Ordos Basin. In recent years, exploration of chang10oil-bearing formation of which the prospection’s degree was relatively low has been stepped up because of increasing production.According to logging curve response to the tuff layer developing widely in study area combining with references of well drilling,logging and outcrop, methods of layer uniform thickness,sedimentary cycle and contrast of adjacent well were used to divide chang10into chang-101, chang-102and chang-103through contrasting large section first and single layer then. Next,chang-101oil-rich layer was divided in to three diffferent sub-layers as chang-1011, chang-1012and chang-1013, in addition, chang-1012was divided in details into chang-1012-1, chang-1012-2and chang-1012-3sub-layers.Based on the facies marker of outcrop,rock type,log facies and sedimentary tectonics combining with regional geological background, provenance analysis and geochemistry sign, the sedimentary facies of chang10was confirmed to delta plain subfacies,and the depositional microfacies were distributary channel sandstone of delta plain, mudstone of flood plain, siltstone of natural levee and flood fan.The rock of chang101are mianly medium and fine-grained feldspar sandstone in which dissolution intergranular pores of clastic particles develop,and the filling are calcite and chlorite thin film.The contact form is mainly line contact and the cementing way is porous cementation. Reservoir physical property is well and porosity ranges from10-12%, permeability is1-2×10-3μm2. Throat radius ranges from0.1-0.4μm and the types are small pore-micro fine throat and micro pore-micro fine throat.The distribution of chang101reservoir is controlled by chang7and chang9source rock. On one hand, chang101reservoir need the residual pressure of chang7and chang9to provide motivation, so the residual pressure decrease gradually with the increase of downward migration distance. On the other hand, oil and gas migrate from high potential zone to low potential with the actuation of fluid potential. Accumulation of oil and gas are cotrolled by favorable sedimentary facies belt,high permeability of sand body,nose structure in region and favorable source-reservoir-cap rock assemblage. Based on these factors, the main reservoir combination types are structure-lithologic reservoir and lithologic reservoir.According to characteristics of sedimentary facies, sand distribution,structure, ancient fluid potential and reservoir analysis as well as production performance data, favorable areas of chang-1011, chang-1012and chang-1013are optimized. The favorable areas in which the sand is in great thickness,good physical property and high oil potential locate in partial high structure and are suitable for reservoirs of oil-gas because of the low ancient fluid potential and residual pressure. The results show that favorable areas are identified I class for eight and Ⅱ class for three and chang-1012is main oil-producing formation. |
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