Differential Analysis Of High-quality Reservoir Genesis On The First Member Of Benxi Formation In Ordos Basin

The Ordos Basin is one of the utmost important natural gas producing basins in China.At present,geologists have discovered tight gas with proven reserves as high as13320×10⁸m³ in the Upper Paleozoic.Along with the continuous exploration in recent years,146 exploration wells in the first Member of Benxi formation have obtained industrial gases.Thus,opening up a good prospect for gas exploration in tight sandstone.However,the research on this member is still in the initial stage,especially the research on diagenetic evolution and porosity prediction of the reservoir.Therefore,our study investigates both aspects to deepen the understanding of the differences in the formation of qualified reservoirs between Shenfu and Gaoqiao regions.Based on the comparison,this study summarizes the diagenetic models of favourable reservoir development to further the prediction of favourable exploration zones.According to the formation-test results of exploration wells,gas reservoirs in the Shenfu area are mainly distributed in the underwater distributary channel of the delta,with pay thickness ranging from 16 to 20 m.The high-quality reservoirs in the Shenfu area are mainly coarse-moderate sorted,moderate rounded quartz arenite reservoirs.The primary intergranular pores contribute significantly to the pore space.The average porosity in the Shenfu area is 8.10%,while permeability is 0.51×10^-3?m².The connected-pore-ratio of reservoirs in the Shenfu is about 43.75%.In contrast,gas reservoirs in the Gaoqiao area are primarily distributed in the tide barrier channel in barrier island,with the pay thickness ranging from 2.4 to 16 m.The high-quality reservoirs in the Gaoqiao area are mainly coarse-moderate sorted,subangular grain quartz sandstone reservoirs,with different kinds of pore types.Secondary pores,dominated by cuttings dissolution pores and kaolinite intergranular pores,are relatively developed.The average porosity in the Gaoqiao area is 5.96%,while permeability is 0.97×10^-3?m².The connected-pore-ratio in reservoirs in the Gaoqiao area is 89.57%.Porosity and permeability in the Shenfu area are relatively larger.Also,the study on the connectivity of small pores and throats in the Gaoqiao area is better.Based on the pore evolution process and the characterization of diagenesis sequence,Shenfu area is currently in the middle diagenetic stage B as it is dominated by the compaction-dissolution phase.Gaoqiao area is in the late diagenetic stage as it is dominated by solutional phase.According to the established porosity quantitative prediction model,the original porosities of the reservoirs in Shenfu and Gaoqiao areas are restored to 5.90%and 5.50%,respectively.Based on the analysis of factors affecting the reservoir quality,overall,the hydrocarbon enrichment is ruled by a combination of the distribution of source rocks and storage factors in the study area.Among them,the distribution of source rocks determines the enrichment range of tight gas,while the storage factors control the productivity of tight gas.Besides,the tight gas reservoir in the Shenfu area is mainly controlled by the source-reservoir assemblage and physical properties of reservoirs.In contrast,physical properties,especially porosity,mainly control the tight gas reservoir in the Gaoqao area.Finally,this paper establishes the main control factors model for high-quality reservoirs.It predicts the perspective regions of the first member of Benxi formation in Shenfu and the Gaoqiao area by the instrumentality of overlay main controlling factors.