Influence of lithofacies and structures on fracture development in the Wulalike Formation shales in the western margin of the Ordos Basin

Based on core observation statistics, combined with MaipSCAN mineral scanning, dual-energy CT scanning and rock mechanics logging, the lithofacies, structure and fracture geometry characteristics of the Wulalike Formation shale in the western margin of the Ordos Basin were systematically described, and the influence of lithofacies and structure on fracture development was discussed. It is found that the shales in the third member of the Wulalike Formation in the study area mainly develops lamellar shale lithofacies, and limestone, gray lamina and silicon-rich lamina are frequently interbedded on the core-micro scale. The horizontal cracks are the major type (accounting for more than 70 %), the width is generally less than 0.5 mm, followed by high-angle and low-angle cracks; high-angle fractures are mostly filled with calcite, and low-angle and horizontal fractures are mostly unfilled. Fractures are most developed in laminated shale, and there is a strong positive correlation between lamina density and horizontal fracture density. According to the research, the development of laminae aggravates the heterogeneity of shale mechanical properties. The laminated shale has low Poisson's ratio and high Young's modulus. The laminae interface is a mechanical weak surface. Horizontal and high-angle fractures are intertwined into a network to form a complex fracture network, which improves the seepage capacity of the rock. The laminated shale is the dominant reservoir lithofacies of the Uralike Formation and is a "dessert" for shale gas exploration.