Controlling factors of fracture distribution of shale in Lower Cambrian Niutitang Formation in southeast Chongqing

The paper summarized characteristics and main controlling factors for distribution of fractures in shale of Lower Cambrian Niutitang Formation, through observation and description of fractures in shale outcrops, cores and slices, discussed the genetic types, distribution characteristics of fractures, content of brittle minerals, shale thickness, mechanical properties of rock, structure, lamellation development, relationship between the content of organic matter and the thermal evolution degree. The results show that the nearly E-W direction fractures, the NE direction fractures and the nearly S-N direction fractures are the three main groups of tectonic fractures. The nearly E-W direction fractures are mostly developed. Shale fractures are mostly high-angle shear fractures. They have small scale and common reservoir capability, but they play a main role in percolation process. Brittle minerals content is the key internal factor that controls the formation and distribution of shale fractures. Stratum thickness, rock mechanical property, tectonic stress and structure position are the key external factors. The development of lamellation influences the formation and distribution of bedding fractures and low angle decollement fractures. The formation of abnormal high-pressure fractures are related to organic matter content and controlled by thermal evolution of organic matter.