Mechanisms of bedding fracturing in the Junggar Basin, northwest China: Constraints from in situ U-Pb dating and C-O-Nd isotopic analysis of calcite cements
Authors: Zhang Chen , Kong Xiangye , Wang Qiqi , She Min , Liang Feng , Dong Yixin , Xu Hao , He Jianhua , Chen Huadong 
Published: February 2025

Frequent natural fracturing along shale bedding interfaces facilitates the formation of bed-parallel fractures. However, these mechanisms are not yet fully understood. The middle Permian Lucaogou Formation in the Jimusar Sag of the Junggar Basin serves as a quintessential example of fractured shale. Complex geological processes have led to the development of numerous bed-parallel fractures, which significantly contribute to the formation’s heterogeneity. In this study, we analyzed rare earth element, fluid inclusion, and C-O-Nd isotope data and in situ U-Pb ages of calcite cement to determine the timing and mechanisms of bed-parallel fracture formation. Our findings revealed two generations of bed-parallel fractures: type A and type B fractures. The crystallization age of calcite cement in type A bed-parallel fractures (group A calcites) ranges from ca. 254.1 Ma to 251.0 Ma. These calcites contain mainly aqueous inclusions with homogenization temperatures between 145.3 °C and 208.9 °C. The δ13C, δ18O, and εNd(t) values for group A calcites range from −13.24‰ to −0.66‰, 4.22‰ to 12.93‰, and 6.14 to 8.61, respectively, indicating the calcite cement formed from high-temperature fluids related to postcollisional, mantle-derived magmatism. In contrast, calcite cement in type B bed-parallel fractures (group B calcites) formed between ca. 15.6 Ma and 4.5 Ma, coinciding with the period of hydrocarbon generation in the Lucaogou Formation. These cements are characterized by the coexistence of hydrocarbon and aqueous inclusions, with homogenization temperatures ranging from 98.9 °C to 158.7 °C. The δ13C, δ18O, and εNd(t) values of the group B calcites are −5.27‰ to 6.73‰, 12.09‰ to 21.64‰, and −3.35 to −1.99, respectively, like those of the surrounding shale, suggesting a connection between calcite cementation and hydrocarbon-generating fluids. We propose that continuous tectonism within the Junggar Basin during the late Permian resulted in N-S shortening, which led to fracturing of weak bedding planes. Fluid overpressure driven by late Permian magmatism triggered the opening of type A bed-parallel fractures. The formation of type B bed-parallel fractures was likely driven by overpressure from hydrocarbon generation and expulsion. Weak bedding planes were opened when the pore pressure reached the fracture threshold. Fracture analysis indicates that bed-parallel fractures are the most prevalent and well-developed fracture type within the Lucaogou Formation. Regions where both bedding-parallel and non-bed-parallel fractures coexist are identified as optimal zones for shale oil accumulation. Our study underscores the importance of integrating petrological, fluid inclusion, geochronological, and geochemical data to understand the diverse mechanisms of bedding fracturing. The significant role of bed-parallel fractures in hydrocarbon accumulation, as revealed in this study, is crucial and should not be overlooked.