The Kuqa Depression boasts rich cretaceous ultra-deep hydrocarbon resources. However, it is in complex geological conditions. At present, sufficient understandings on the in situ stress distribution and influencing factors are lacking, which restricts the process of hydrocarbon exploitation. Therefore, in this study, the Bozi gas field is selected as an example, and a 1D mechanical earth model (1D MEM) is established with the drilling data and logging data through the geomechanical method to clarify the in situ stress distribution of the wellbore. A 3D heterogeneous geomechanical model (3D HGM) is established with the constraint of 1D HEM to clarify the distribution characteristics of the 3D in situ stress field in the Bozi gas field and discuss its influencing factors. The results show that: 1) the Bozi gas field is in an extremely strong in situ stress condition with high stress values. The minimum horizontal principal stress (S_h) of the cretaceous system is 153∼180 MPa, and the maximum horizontal principal stress (S_H) is nearly 200 MPa; 2) the in situ stress in the Bozi gas field has obvious vertical stratification characteristics, which can be divided into three stress sequences of “low–high–low”, with great differences in interlayer stress; 3) the in situ stress distribution of the Bozi gas field is greatly affected by the types of faulted anticline. Different types indicate different stress distribution; 4) within the influence range of overthrusts, the in situ stress in the footwall is lower than that of the hanging wall. The greater the fault offset, the greater the in situ stress difference between the hanging wall and footwall. Moreover, the lower the stress in the footwall, the higher is the degree of overthrust, and the larger is the range of footwall stress area; and 5) the means of highly deviated wells is more helpful to the Bozi gas field for hydrocarbon exploitation.

Effective natural fractures are those that contribute to reservoir storage and conductivity. In this study, the effectiveness of fractures in deep and ultradeep reservoirs (depths > 6 km) is evaluated by using geophysical image logs, cores, microstructures, mechanical tests, and geologic evidence of the tectonic context (including current in situ stress) and sedimentary faces and diagenesis. Intraformational open-mode fractures are the most common fracture type in this tectonically active region, and their effectiveness is affected by the coupling between past tectonic activity, in situ stress, and chemical reactions such as mineral precipitation or dissolution (diagenesis). Our results show that the effectiveness of fractures is mainly related to the aperture and filling, and fractures formed by early tectonic movement are more likely to be filled by minerals, reducing their effectiveness. Strata near orogenic belts in strongly cemented, tight diagenetic facies exhibit a greater abundance of mineral-filled fractures, whereas dissolution increases the fracture apertures, produces secondary pores in host rocks, and roughens the fracture walls. The effective normal stress acting on the fracture surface decreases when the fracture is parallel to the maximum horizontal stress component, so the fracture has a large aperture and, therefore, is more effective. The results of our laboratory measurements reveal that the contribution of fractures to the reservoir space is limited, and the reservoir space provided by fractures accounts for only 13% of all types of reservoir space, whereas the presence of fractures increases reservoir permeability by 1–2 orders of magnitude. Fractures improve the effectiveness of the reservoir by connecting the dispersed pores in the reservoir matrix. Fractures with a low angle to the maximum horizontal stress component have greater apertures and permeabilities and are the main channels for fluid flow.

To improve the development efficiency of ultradeep fault‐controlled carbonate reservoirs, the deformation and connectivity mechanism of fractures was revealed through large‐scale rock mechanical experiments. The in situ stress field and fracture activity distribution law of fault‐control carbonate reservoirs were clarified through geomechanical modeling. It is revealed that the fracture activity in different directions and the connectivity of the fracture and cavity in different parts are significantly different, the development effect of different well trajectories is analyzed, and the integrated working method of geological engineering is proposed to scientifically guide the well trajectory design and water injection scheme optimization. The results show that: (1) Large‐scale fractures and high‐angle fracture systems in the deformation of strike–slip faults are the key factors affecting reservoir quality. High‐pressure water injection can activate preexisting fractures, and it can extend on the basis of preexisting fractures, and even generate new fractures, which promote the interconnection of fault‐controlled fracture‐cave bodies in the vertical and horizontal directions. (2) During the process of high‐pressure water injection, the coupling change between mechanics and flow occurs inside the fracture body, the seepage environment is improved, and the oil and gas recovery factor is improved through cyclic lifting. (3) According to the shape and occurrence of the fault body and the dynamic shear deformation connectivity of the fault surface, the best well point and well trajectory of the directional well can be optimized, and the water injection scheme can be optimized. (4) The single‐well production increased by 1.45 × 10⁴ t through high‐pressure water injection, the cumulative oil increase of the same fracture‐cavity body increased by 4.63 × 10⁴ t. This method is highly efficient for ultradeep fault‐controlled oil reservoirs. The development provides a good theoretical basis and technical support, and plays a leading and exemplary role.

In order to clarify the fracture distribution characteristics of the Bozi gas field in the Kuqa Depression, based on the statistics of fracture parameters from core data and imaging logging, the three-dimensional (3D) structural model of the complex thrust structure is established by using the voxel-based morphometry (VBM) structural framework modeling technology; the 3D heterogeneous rock mechanics field in the study area is constructed by well-to-seismic integration. The relationship between in situ stress field and fracture parameters is established under the consideration of rock fracture criterion, and fracture prediction of the Cretaceous reservoir in the Bozi gas field is carried out with the finite element numerical simulation method. Considering the influence of in situ stress field on fracture parameters, fracture activity is analyzed. The results show that the Cretaceous reservoirs in the Bozi gas field generally develop fractures, most of which are high-angle shear fractures of tectonic origin and are semi-filled or unfilled. The fracture distribution of the Bozi gas field is obviously controlled by local structures such as folds and faults. From north to south, the fracture development gradually weakens, and the fracture density of Bozi 104 and Bozi 102 fault blocks in the north is the highest. The fracture in the northern fault block of the Bozi gas field shows high activity, while that in the southern fault block shows low activity. The fractures in the higher structural parts of most fault blocks show high activity.