The description of reservoir fractures constitutes the basis for studying fracture formation and distribution, as well as for evaluating the significance of fracture effects in different formations, conducting effectiveness analysis, and predicting the underground fluid seepage network system. Given that the distribution, development methods, and effectiveness of fractured oil and gas reservoirs are mainly restricted by fracture development, accurately and comprehensively describing fracture types, filling conditions, formations, orientations, densities, and scales is of crucial importance for exploration, well network deployment, development plan design, and subsequent production management. Taking the natural fractures in the Yanchang Formation reservoir of the southern exploration area in the Ordos Basin, China, as the research object, a macroscopic description of core fractures was initially conducted. Combined with thin section identification, scanning electron microscopy, and energy spectrum technology, the scale, occurrence, nature, and filling conditions of natural fractures were clarified. The research findings indicate that a considerable number of micro-fractures, accounting for over 60 % of the total, show widths ranging from 10 to 20 µm within the target reservoir. The manifestation of micro-fractures varies among different lithologies: conglomerates have granular micro-fractures with impermeable cements; micro-fractures within silty conglomerates present a diverse range of shapes, sometimes distributed in bands; and those in mudstone appear as intermittent or spider-web patterns. The results of the water flooding experiment imply that longer and wider fractures decrease water flooding efficiency. During low-pressure water flooding, pore-scale fractures display a relatively weak response and only show their characteristics when the applied pressure reaches their ultimate extension pressure. The characterization and evaluation of natural fractures provide crucial references for the efficient development of tight sandstone reservoirs in the study area and offer valuable guidance for optimizing water flooding techniques and enhancing recovery efficiency.
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