Laminar controls on bedding-parallel fractures in Permian lacustrine shales, Junggar Basin, northwestern China

The lamina-rich structures formed during slow sedimentation in lacustrine shales are the foundation for the widespread development of bedding-parallel fractures (BPFs). Considering the differences in type, composition, and characteristics of laminae, BPFs are developed following various tectonic and diagenetic evolutions that exhibit significant heterogeneity. This study analyzed multi-scale data from lacustrine shales in the Permian Fengcheng Formation of the Junggar Basin of northwestern China to understand how laminae control BPF initiation and growth. The results show fine-grained mixed laminae would develop the most BPFs with large lateral extensions, which is attributed to their enriched organic matter and clay minerals fostering outstanding stratification and being conducive to diagenetic evolution to enhance rock cohesion. In felsic and carbonate laminae, the dissolution would change the mineral crystal lattice to develop more BPFs, while these fractures in alkali mineral laminae are less developed and easily pinched out. Moreover, variations in mineral composition, arrangement, and size lead to apparent mechanical anisotropy and weak plane at lamina interfaces, which promote the formation and lateral extent of BPFs. Additionally, hydrocarbons produced during the thermal degradation and cracking of organic matter would generate abnormally high pore fluid pressure, which instigates vertical seepage force and a tensile stress state and contributes to fracture formation. Consequently, the development degree of BPFs and the organic matter content in laminae are positively correlated. These insights into the controls of laminae on BPFs can provide a theoretical basis for revealing the formation mechanism of BPFs and elucidating their development patterns in lacustrine shales.