Characteristics and openness evaluation of natural fractures in deep marine shale gas reservoirs: A case study of the Longmaxi Formation in Zigong area, Sichuan Basin, China

The Zigong area in the southern Sichuan Basin has been subjected to multi-stage and multi-directional tectonic stresses, resulting in the development of natural fracture systems with multiple scales and types. These fractures have a significant impact on the enrichment, preservation, and development of shale gas. By comprehensively utilizing data from core samples, thin sections, dipole sonic logs, and image logs, the types and development characteristics of natural fractures in the first sub-member of the Long 1 Member (S1l11) shale reservoir in the Zigong area were quantitatively characterized. Combined with experimental analyses, including fluid inclusion analysis and acoustic emission testing, the fracture formation mechanisms were clarified, and a comprehensive evaluation of fracture openness was conducted. The results indicate that the S1l11 shale reservoir in the study area has developed structural fractures, diagenetic fractures, and overpressure fractures. Among these, high-angle structural fractures trending in the NNE and NEE directions are dominant, exhibiting a high degree of development and good effectiveness. Across different tectonic units, fracture development patterns show systematic variation. The S1l11 shale has undergone three stages of fracturing and three stages of cementation. The early two stages of structural fractures experienced multiple episodes of filling and thus exhibit relatively poor effectiveness, whereas the late-stage structural fractures are minimally filled and demonstrate relatively good effectiveness. The Coulomb Failure Function (CFF) values of fractures in the study area are generally less than 0, indicating that, under the in-situ stress conditions, the fractures remain in a stable state. Spatially, the CFF values increase progressively from the northwest to the southeast and then to the north, reflecting a corresponding increase in fracture openness. Two principal stress mechanisms—normal faulting and strike-slip faulting—are predominant in the study area. Under different stress mechanisms, the orientations of preferentially opened fractures vary. Under the normal faulting stress mechanism, NWW–SEE trending fractures, which are parallel to the current maximum principal stress direction, are preferentially opened. Under the strike-slip faulting mechanism, high-angle fractures trending NEE–SWW and NW–SE, oriented at approximately 30° to the current maximum principal stress, are preferentially opened. These findings provide a theoretical basis for optimizing shale gas sweet spot selection, preventing and controlling fracture stringing and casing deformation, and optimizing horizontal well trajectories in the study area.