Multi-scale fractures formation and distribution in tight sandstones—a case study of Triassic Chang 8 Member in the southwestern Ordos Basin

Fracture system is an important factor controlling tight oil accumulation in the Triassic Chang 8 Member, southwestern Ordos Basin, China. A systematic characterization of the multi-scale natural fractures is a basis for the efficient tight oil production. Based on outcrops, seismic reflections, well cores, well logs (image and conventional logging), casting thin sections, and scanning electron microscope observation, the multi-scale fractures occurrences and their influences on Chang 8 tight sandstone reservoirs are revealed. The results show that three periods of strike-slip faults and four scales of natural fractures developed, namely mega-scale (length > 7 × 107 mm), macro-scale (3.5 × 105 < length < 7 × 107 mm), meso-scale (10 < length < 3.5 × 105 mm), and micro-scale (length < 10 mm) fractures. The mega- and macro-scale fractures developed by strike-slip faults are characterized by strike-segmentation and lateral zonation, which connect the source and reservoir. These scale fractures also influence the distribution and effectiveness of traps and reservoirs, which directly influence the hydrocarbon charging and distribution. The meso fractures include the tectonic, diagenetic, as well as hydrocarbon generation-related overpressure types. The meso- and micro-scale fractures improve the sandstone physical properties and also the tight oil well production performance. This integrated study helps to understand the distribution of multi-scale fractures in tight sandstones and provides a referable case and workflow for multi-scale fracture evaluation.