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.

尽管页岩中天然裂缝很重要，但目前很少有技术可以直接研究其形成机制和对页岩油聚集的影响。准噶尔盆地吉木萨尔凹陷中二叠统芦草沟组是典型的裂缝性页岩。吉姆萨尔凹陷自古生代以来经历了多期构造运动，形成了众多天然裂缝。然而，关于这些裂缝的形成时间、成因机制以及对页岩油聚集的影响仍然存在争议。基于 CO-Nd 同位素、稀土元素、流体包裹体、Sm对裂缝胶结物进行钕等时测年、岩石声发射和三轴流变实验，准确确定裂缝形成时间，追溯裂缝充填流体物源，阐明裂缝成因机制及其对页岩油成藏的影响。结果表明，这些裂缝主要形成于晚二叠世、晚侏罗世和早白垩世。古构造应力场模拟进一步表明，由于强烈的南北挤压，晚二叠世的构造运动形成了近南北向的裂缝。来自碰撞后地幔的高温流体在~258.1 Ma~257.6 Ma之间侵入芦草沟组。随后，晚侏罗世天山的构造运动导致自西南向东北的挤压，形成北东东向的断裂。在这次构造事件中，盆地流体在~150.2 Ma~146.5 Ma之间侵入芦草沟组。早白垩世博格达山前前陆凹陷的持续褶皱回滚使吉姆萨尔凹陷受到严重挤压，形成北西向断裂。这一构造事件影响了~123.0 Ma~120.3 Ma之间盆地流体侵入芦草沟组。晚侏罗世和早白垩世断裂恰逢芦草沟页岩生排油高峰期，为页岩油运聚形成了通道和储集层。