鄂尔多斯盆地吴起地区上三叠统延长组长71亚段砂质夹层孔喉结构及渗流主控因素OA

The sandy interbeds in the Chang 71 sub-member of the Upper Triassic Yanchang Formation,Wuqi area,Ordos Basin,are characterized by strong heterogeneity of pore-throatstructure,ambiguous relationship between pore development and seepage capacity,and insufficient understanding of whether dissolution pores can be transformed into effective seepage spaces.To address these issues,this paper takes sandstone samples collected from shale-reservoir sandy interbeds of the Chang 71 sub-member,Yanchang Formation,at well A within the study area as the research object.Comprehensive methods including bulk-rock X-ray diffraction,porosity and permeability testing,scanning electron microscopy observation,rate-controlled high-pressure mercury intrusion experiments,and nano-CT three-dimensional pore network reconstruction are adopted to analyze the mineral com-position,pore-throat structure characteristics,and their controlling effects on seepage capacity.The results show that the sandy interbeds of the Chang 71 sub-member generally present low porosity and ultra-low permeability features.The porosity ranges from 0.60％to 7.92％,with an average of 2.01％,and permeability ranges from 0.004 84× 10-3 μm2 to 0.131× 10-3 μm2,with an average of 0.018 4× 10-3 μm2.Porosity shows a significant posi-tive correlation with permeability.The reservoir spaces are mainly residual intergranular pores,feldspar dissolu-tion pores,and clay intercrystalline micropores.Carbonate cements and clay minerals are widely developed with obvious heterogeneity.The displacement pressure of representative samples is 0.119-0.306 MPa,and there are remarkable discrepancies in the main throat interval and fine throat tailing characteristics.High-permeability samples have a high proportion of connected pores,large average coordination number,and well-connected pore networks,while low-permeability samples feature a low proportion of connected pores and sparse pore network branches,and are prone to connectivity interruption.The seepage capacity of sandy interbeds in the Chang 71 sub-member is mainly controlled by the development degree of dominant throats,throat threshold,and three-dimensional con-nected framework.Carbonate cementation-dissolution and clay filling exert remarkable coupled controlling effects over pore-throat structure.Early carbonate cementation and late re-precipitation can narrow and partition pore throats,and clay filling further aggravates throat refinement and weakens connectivity.The pore increment formed by feldspar dissolution can only be converted into seepage advantages under the condition of preserving an effec-tive connected framework.The research results can provide a basis for sweet spot identification of shale oil sandy interbeds and the optimization of fracturing intervals in the Chang 7 Member.