Developmental characteristics and formational stages of natural fractures in the Wufeng - Longmaxi Formation in the Sangzhi Block, Hunan Province, China: Insights from fracture cements and fluid inclusions studies

The developmental characteristics and formational stages of natural fractures are of great significance for shale oil and gas accumulation in complex structural areas. We determined the developmental characteristics and formational stages of fractures in the Wufeng - Longmaxi Formation in the Sangzhi Block, Hunan Province, China based on outcrops, cores, optical microscopy, cathodoluminescence, scanning electron microscopy, energy spectrum analysis, fluid inclusion homogenization temperature, micro laser Raman spectrum analysis, and acoustic emission analysis. Furthermore, we deduced the possible fracture formational depth, filling time, and stress magnitude of structural fractures in rocks, and established the development model of structural fractures of time series. The results show that the Wufeng - Longmaxi Formation fractures can be divided into tensile fractures, compression-torsion fractures, and strike-slip fractures in terms of the combination characteristics of fracture plane and the roughness of fracture plane. The fractures mainly occurred in the late Yanshanian - Himalayanian period after the violent uplift. In the process of burial-hydrocarbon generation, it experienced at least six stages of repeated process of splitting, healing, re-splitting, which reflected the periodic release of local stress caused by the continuous westward push of Xuefeng uplift. The micmcracks of different periods have obvious differences in orthogonal polarization and cathodoluminescence reflection characteristics. The cathodoluminescence spectrum ranges from orange red to bright yellow, and some even do not glow, which is a comprehensive performance of mineral filling composition, crystal size, crystallization degree and alteration degree. The gas generated in the early stage are easy to enter the early opened fractures. With the continuous opening and healing, the shale gas has a continuous dynamic process of enrichment, dispersion, re-enrichment, and re-dispersion, which affects the distribution and preservation of shale gas.

Automated summary

The study on fractures in the Wufeng-Longmaxi Formation in Hunan Province reveals that these fractures can be divided into different types based on their combination characteristics and roughness. The fractures mainly occurred in the late Yanshanian-Himalayan period and experienced repeated processes of splitting, healing, and re-splitting. The gas migration and enrichment in these fractures play a key role in the distribution and preservation of shale gas.