Distribution characteristics, genesis analyses, and research significance of Triassic regional structural fractures in the Ordos Basin, Central China

摘要

Through systematic investigations and measurements of Triassic structural fractures on outcrops in the Ordos Basin, as well as massive observations of fractures from drill cores in areas of coverage, combined with the data synthetic analyses of fracture logging, core fracture location, and oil-field development performance, it is shown that Triassic structural fractures are well developed and mainly vertical or steeply dipping. These fractures are distributed differently in different areas. The fractures usually appear in pairs and belong to X conjugate shear fracture systems: One set is opening fractures with extension-shear characteristics (striking NE, ENE, or near S-N), and the other set is closing fractures with compression-shear characteristics (striking NW, NNW, or near E-W). Regional horizontal compression as well as the reactivation of basement faults and peripheral faults in the Ordos Basin are responsible for the development of the fracture system. The geodynamic background is the remote tectonic compression produced by the convergence of the Siberia Plate, the Pacific Plate, and the Tethys block during Yanshanian period. Moreover, the Indian Plate converged with the East Asia Continent Plate during Himalayan period. Triassic fractures are mainly formed during Yanshanian, but there are superposition and transformation since the Himalayan period. The distribution, development, and evolution of the fractures in Triassic Yanchang Formation are very important to further study the migration and accumulation of oil and gas as well as exploration and development of low permeable oil-gas deposits in the Ordos Basin. Furthermore, the origin analysis of regional structure fractures is important to explore the relationship between the Ordos Basin and Qinling orogenic belt.