Disturbance characteristics of in-situ stress field within ultra-deep tight sandstone reservoirs in thrust-nappe structures: a case study from Cretaceous reservoirs in Bozi-Dabei area, Tarim Basin

Abstract

A series of large-scale north-dipping faults and imbricate folding structures have developed in the Cretaceous ultra-deep tight sandstone reservoirs in Bozi-Dabei area of Kuqa Depression of Tarim Basin under the north-to-south thrust-nappe movement in this area. This complex structural morphology results in highly variable in-situ stress fields, leading to significant differences in reservoir modification effectiveness. Therefore, it is urgent to clarify the disturbance characteristics of the in-situ stress caused by the complex structures in the study area. In this study, multiple methods were combined for the accurate interpretation of the current in-situ stress of a single well. The disturbance effects of faults, folds, and fault-fold composite structures on the in-situ stress were analyzed separately. The relevant disturbance mechanisms were identified, and a zoning map of the disturbance characte-ristics of the in-situ stress for the study area was presented. Based on these disturbance characteristics, models of in-situ stress disturbance for different structures, as well as recommendations for well deployment and trajectory, were proposed. Faults exhibit an unloading effect on the in-situ stress, leading to varying degrees of reduction in the horizontal principal stress gradient near the faults, with the maximum reduced by about 0.3 MPa/hm. Near EW-oriented faults, the regional stress direction near SN exhibits a clockwise deflection, with the maximum deflection angle reaching 60°. The disturbance range of faults with different scales is approximately 60% of the fault throw. A disturbance of the in-situ stress appears when the strata curvature exceeds 0.4 km-1. The in-situ stress is lower than the regional stress in the upper tensile disturbance zone of the folded strata, while it increases in the lower compressive disturbance zone. In the tensile disturbance zone, the maximum decrease in the horizontal principal stress gradient is approximately 0.3 MPa/hm, with the stress direction deflecting counterclockwise, reaching a maximum deflection angle of 70°. The greater the fold deformation curvature, the thicker the tensile disturbance zone, and the more significant the disturbance. Under fault-fold composite structures, the superposition of fault disturbance zones and fold tensile disturbance zones further reduces the magnitude of the in-situ stress. After offsetting the disturbance effects of both, the in-situ stress direction deviates less or does not deviate at all from the regional stress. Considering the difficulty of reservoir modification and the characteristics of tight gas enrichment, drilling should be prioritized in the overlapping areas of faults and fold tensile disturbance zones within the fault-fold composite structure zone. It is recommended that the drilling depth should not exceed the neutral plane of the folds, and the horizontal well trajectory should be designed along the EW direction.

Keywords

• disturbance of in-situ stress
• thrust-nappe structure
• ultra-deep formation
• tight sandstone reservoir
• fault-fold composite structure
• tarim basin