Evaluation methods and engineering applications of in-situ stress in deep, strong heterogeneity terrestrial shale oil and gas reservoirs: a case study of jurassic shales in the Yingshan-Pingchang area, northeast Sichuan

Introduction The Jurassic terrestrial shale in the Yingshan-Pingchang area of the northeastern Sichuan Basin holds substantial exploration and development potential. However, the area exhibits significant vertical heterogeneity and anisotropy in in-situ stress. Thus, precise vertical evaluation of in-situ stress is urgently required to provide a scientific basis for selecting hydraulic fracturing layers in future operations.Methods This study conducted a detailed in-situstress analysis utilizing paleomagnetic data, velocity anisotropy measurements, differential strain experiments, hydraulic fracturing results, and both conventional and specialized logging data. A transversely isotropic in-situstress prediction model was developed to evaluate the stress distribution, aiming to identify target layers favorable for hydraulic fracturing.Result Comprehensive analysis indicates that the in-situstress orientation of Jurassic shale in the Yingshan-Pingchang area generally aligns with the regional stress orientation (NE90 degrees +/- 10 degrees). Due to the influence of local NW-trending structures, the in-situstress orientation exhibits a clockwise deflection. In the Jurassic formation, the maximum horizontal principal stress ranges from 42.33 MPa to 102.56 MPa, averaging 74.89 MPa; the minimum horizontal principal stress ranges from 39.20 MPa to 84.04 MPa, averaging 67.20 MPa; and the vertical principal stress varies between 31.91 MPa and 91.39 MPa, averaging 60.23 MPa. These findings were corroborated by in-situstress measurements obtained through hydraulic fracturing, demonstrating that the stress magnitudes determined via differential strain analysis are highly accurate. The analysis of the three-dimensional stress relationships indicates that the study area predominantly exhibits a strike-slip faulting regime. Comparative analysis reveals that the minimum principal stress gradient in shale is higher than that in limestone and sandstone. Furthermore, the transverse isotropic in-situstress prediction model demonstrates high accuracy. When comparing its predictions for minimum and maximum horizontal principal stresses to measured in-situstress data, the model exhibits average relative errors of only 3.39% and 3.23%, respectively.Discussion In the study area, vertical high-low-high (HLH) stress difference profiles exhibit the highest oil-bearing potential and a reduced likelihood of fracturing-induced artificial fractures crossing through layers. This makes HLH profiles the optimal structural type for selecting fracturing stages in in-situstress difference fracturing operations.