Tectonically controlled fracture networks in shale: implications for hydraulic fracturing in the Longmaxi Formation, Luzhou area, southern Sichuan Basin

Hydraulic fracturing technology is key for shale gas development. The effectiveness of hydraulic fracturing is considerably affected by the characteristics of natural fractures within the reservoir, whereas shale fractures are considerably controlled by tectonic location. Therefore, understanding the development characteristics of natural fractures and their effects on the propagation of hydraulic fractures, even when natural fractures are effectively exploited, is critical for optimizing fracturing effects. Within this context, by examining the Longmaxi Formation in the Luzhou area in southern Sichuan in a case study, the development patterns and factors that control fractures in faulted anticlines, slopes and synclines are systematically analyzed through core and thin section observations, numerical simulations, and microseismic monitoring. The effects of different fracture characteristics on the propagation of hydraulic fractures and fracturing efficiency were investigated. The results revealed that (1) the fracture density in the study area decreases in the order of faulted anticlines, slopes and synclines; faulted anticlines are dominated by high-angle shear fractures and fault-associated fractures, which are characterized by high density, long propagation, and satisfactory connectivity, easily forming a network of fracture systems. Slopes are dominated by medium- to low-angle shear fractures, which are mostly arranged in parallel, and synclines are characterized by well-developed medium- to high-angle shear fractures intersecting with shale bedding-parallel fractures. The difference in the tectonic stress field is the main cause of the morphology and scale of fractures at different tectonic locations. (2) High-angle fractures are readily activated by hydraulic fracturing and extend horizontally, thereby forming a complex fracture network, which results in the optimal degree of fracturing stimulation; the hydraulic spreading effect is optimal when the fracture lengths range from 20 to 40 cm, whereas excessively long or short fractures limit vertical or horizontal propagation. The combination of horizontal and vertical fractures can enhance lateral propagation and vertical connectivity, which increases fracturing efficiency. (3) Fracture characteristics dominate the fracturing effect at different tectonic locations: in the syncline area, because shale bedding-parallel fractures intersect high-angle fractures and the fracture length is optimal, the fracturing stimulation area is the largest. Microseismic monitoring revealed that the propagation paths of hydraulic fractures are highly consistent with the simulation results, which validates the coupling mechanism among tectonic location, natural fractures and fracturing effectiveness. This study provides a geological basis for optimizing well placement and fracturing design in the Longmaxi Formation shale gas reservoir, Luzhou area, by highlighting the superior gas potential of syncline structures.