Natural fractures within unconventional reservoirs of Linxing Block, eastern Ordos Basin, central China

Unconventional reservoirs are generally characterized by low matrix porosity and permeability, in which natural fractures are important factors for gas production. In this study, we analyzed characteristics of natural fractures, and their influencing factors based on observations from outcrops, cores and image logs. The orientations of natural fractures were mainly in the ∼N-S, WNW-ESE and NE-SW directions with relatively high fracture dip angles. Fracture densities were calculated based on fracture measurements within cores, indicating that natural fractures were not well-developed in the Benxi-Upper Shihezi Formations of Linxing Block. The majority of natural fractures were open fractures and unfilled. According to the characteristics of fracture sets and tectonic evolution of the study area, natural fractures in the Linxing Block were mainly formed in the Yanshanian and Himalayan periods. The lithology and layer thickness influenced the development of natural fractures, and more natural fractures were generated in carbonate rocks and thin layers in the study area. In addition, in the Linxing Block, natural fractures with ∼N-S-trending strikes contributed little to the overall subsurface fluid flow under the present-day stress state. These study results provide a geological basis for gas exploration and development in the Linxing unconventional reservoirs of Ordos Basin.