Mechanism for the formation of natural fractures and their effects on shale oil accumulation in Junggar Basin, NW China

Despite the importance of natural fractures in shales, few techniques are currently available to examine their formation mechanism and effects on shale oil accumulation directly. The Middle Permian Lucaogou Formation in the Jimsar Sag of the Junggar Basin is a typical fractured shale. The Jimsar Sag has experienced multiple phases of tectonic movement since the Paleozoic Era, forming numerous natural fractures. However, controversy still surrounds the timing of formation, genetic mechanisms, and effects on shale oil accumulation of these fractures. Based on C-O-Nd isotopes, rare earth elements, fluid inclusions, SmNd isochron dating of fracture cements, rock acoustic emission, and triaxial rheological experiment, this study precisely define the fracture formation time, trace the provenances of fluids filling the fractures, elucidate the genetic mechanism of fractures and their effects on shale oil accumulation. The results suggest that these fractures were mainly formed in the Late Permian, Late Jurassic, and Early Cretaceous. Palaeotectonic stress field simulations further show that the tectonic movements in the Late Permian formed nearly N-S-trending fractures due to severe north-south extrusion. High-temperature fluids originating from the post-collision mantle intruded the Lucaogou Formation between ~258.1 Ma and ~ 257.6 Ma. Subsequently, tectonic movements in the Tianshan Mountains during the Late Jurassic resulted in extrusion from southwest to northeast, forming NEE-trending fractures. During this tectonic event, basin fluids intruded the Lucaogou Formation between ~150.2 Ma and ~ 146.5 Ma. The persistent fold rollback that occurred in the Bogda piedmont foreland sag in the Early Cretaceous subjected the Jimsar Sag to severe extrusion, forming NNW-trending fractures. This tectonic event influenced the intrusion of basin fluids into the Lucaogou Formation between ~123.0 Ma and ~ 120.3 Ma. The Late Jurassic and Early Cretaceous fractures coincided with the peak of oil generation and expulsion in the Lucaogou shale and created conduits and reservoir volumes for the migration and accumulation of shale oil. Our study introduces a new approach that integrates geophysical and geochemical data to investigate the formation processes of fracture and their impact on shale oil accumulation.