Non-Tectonic Fracture Characteristics of Lower Paleozoic Shale in Southeast Chongqing and North Guizhou Area (South China) and Its Main Controlling Factors

Natural non-tectonic fracture is an important reservoir space for shale gas. However, there are currently insufficient researches on the formation mechanism, control factors, and development characteristics of non-tectonic fractures. It characterizes natural non-tectonic fractures of Lower Silurian Longmaxi Shale and Lower Cambrian Niutitang Shale in study area. The non-tectonic micro-fractures of Lower Palaeozoic Shale were identified and their interior structures were characterized by analyzing SME images. The main controlling factors on the characteristics and distribution of non-tectonic fractures were analyzed by researching sedimentary environment, organic matter abundance and types, thermal evolution degree, hydrocarbon generation history, clay minerals, paleosalinity and diagenesis. The results show that natural non-tectonic fractures, which produced because of diagenetic shrinkage, dissolution and abnormal high pressure, are abundant in both Longmaxi Shale and Niutitang Shale. Natural non-tectonic fractures, of which the microstructure is in the irregular shape of silk-thread and curly sheet, cut shallowly in longitudinal direction, with width ranging from 10 nm to 50 nm, even more than 1 μm. The porosity and permeability of shale could be improved due to excellent extensibility and connectivity of non-tectonic fractures. The lower part of Niutitang Shale and Longmaxi Shale is deep-water shelf facies with numerous horizontal bedding, therefore, it is a favorable facies for the development of non-tectonic fractures. Undercompaction and hydrocarbon generation can produce large-scale overpressure fractures. In the early stage of burial, undercompaction was the main cause of formation overpressure. Hydrocarbon generation pressurization, that has a good corresponding relationship with thermal evolution, could release organic acids to promote the development of secondary dissolution fractures. While the destruction and adjustment of tectonic movement released the abnormal overpressure, the overpressure fractures shrunk or even closed. The lower Paleozoic shale was deposited in the water environment with medium paleosalinity, in addition, high clay content is conducive to the generation of diagenetic shrinkage fractures. Longmaxi Shale was in middle-late or late diagenesis stage, while the smectite transformed to illite gradually, the volume of diagenetic shrinkage fractures was close to the maximum. Niutitang Shale was in late diagenesis stage, the generation rate of shrinkage fractures became slow, moreover, the volume of diagenetic shrinkage fractures had approached or reached the maximum.