Characteristics, controlling factors and mechanisms of natural fractures formation in lacustrine shale oil reservoirs: The Chang 7 member in Ordos Basin, China

Lacustrine shale oil reservoirs of the Upper Triassic Chang 7 Member in the Ordos Basin have demonstrated significant potential for hydrocarbon resources. Natural fractures play a crucial role in hydrocarbon enrichment and production. Outcrops, cores, borehole image logs, thin sections, and FE-SEM images were used to investigate the types and characteristics of natural fractures in the Chang 7 Member. The factors controlling fracture development and the mechanisms of bedding-parallel fracture formation were revealed by integrating TOC analysis, XRD analysis, and rock pyrolysis. Results show that natural fractures in the study area include high-angle tectonic fractures and nearly horizontal bedding-parallel fractures. Brittle minerals and bed thickness control the occurrence and attributes of tectonic fractures. High TOC content and thermal maturity positively affect the development of bedding-parallel fractures, formed through the conversion of organic matter to hydrocarbons or the smectite-to-illite transformation. Additionally, the dominant orientations of tectonic fractures intersect the present-day maximum horizontal principal stress at a small angle, resulting in large apertures and good effectiveness. Bedding-parallel fractures contribute to enhance porosity and provide favorable pathways for lateral hydrocarbon migration. Collectively, this study could provide valuable insights for finding promising exploration areas in lacustrine shale oil reservoirs in the Ordos Basin and worldwide.