Analysis of the in situ stress orientation and magnitude in the No. 4 Structure of Nanpu Sag was performed on the basis of data obtained from borehole breakout and acoustic emission measurements. On the basis of mechanical experiments, logging interpretation, and seismic data, a 3D geological model and heterogeneous rock mechanics field of the reservoir were constructed. Finite element simulation techniques were then used for the detailed prediction of the 3D stress field. The results indicated that the maximum horizontal stress orientation in the study area was generally NEE–SWW trending, with significant changes in the in situ stress orientation within and between fault blocks. Along surfaces and profiles, stress magnitudes were discrete and the in situ stress belonged to the Ia-type. Observed inter-strata differences were characterized as five different types of in situ stress profile. Faults were the most important factor causing large distributional differences in the stress field of reservoirs within the complex fault blocks. The next important influence on the stress field was the reservoir’s rock mechanics parameters, which impacted on the magnitudes of in situ stress magnitudes. This technique provided a theoretical basis for more efficient exploration and development of low-permeability reservoirs within complex fault blocks.

The fracture prediction model for low-permeability reservoirs is hard to be got through data statistical method and simplified strain energy density method under high confining pressures. In this research we make an attempt to establish a series of geomechanical models for prediction of fracture distribution in brittle reservoirs, especially for tight sandstone reservoirs. Firstly, we emphasize that energy generated by tectonic stress on brittle sandstone can be distinguished into accumulating elastic strain energy, fracture surface energy, and residual strain energy and natural fractures can be interpreted or inferred from geomechanical-model-derived strains. For this analysis, we confirm 0.85 σc as the key threshold for mass release of elastic strain energy and bursting of macrofractures, then deduce and build a physical relation model between fracture volume density and strain energy density under uniaxial stress state based on Theory of Geomechanics and CT scanning. Then on this basis, using combined Mohr-Coulomb criterion and Griffith’s criterion and considering the effect of filling degree in fractures, we continue to modify and deduce the mechanical models of fracture parameters under complex stress states. Finally, all the geomechanical equations are planted into the FE platform to quantitatively simulate the present-day 3-D distributions of fracture density, aperture, porosity and occurrence based on paleostructure restoration of Keshen anticline. It is concluded that the evolutionary model the Keshen structure present to us is a distinct and intuitive top-pop-up fold with low amplitude and relatively low-density fractures in the top, has only experienced pre-folding stage and strong compression stage, and has not yet been developed to rapid uplift stage. Its predictions also agreed well with present in-situ core observations and FMI interpretations.

It is generally difficult to predict fractures of low-permeability reservoirs under high confining pressures by data statistical method and simplified strain energy density method. In order to establish a series of geomechanical models for the prediction of multi-scale fractures in brittle tight sandstones, firstly, through a series of rock mechanics experiments and CT scanning, we determined 0.85 σ_c as the key thresholds for mass release of elastic strain energy and bursting of micro-fractures. A correlation between fracture volume density and strain energy density under uniaxial stress state was developed based on the Theory of Geomechanics. Then using the combined Mohr-Coulomb criterion and Griffith’s criterion and considering the effect of filling degree in fractures, we continued to modify and deduce the mechanical models of fracture parameters under complex stress states. Finally, all the geomechanical equations were loaded into the finite element (FE) platform to quantitatively simulate the present-day 3-D distributions of fracture density, aperture, porosity, permeability and occurrence based on paleostructure restoration of the Keshen anticline. Its predictions agreed well with in-situ core observations and formation micro-imaging (FMI) interpretations. The prediction results of permeability were basically consistent with the unobstructed flow distributions before and after the reservoir reformation.

This research introduces an innovative fractal–fractional synergy framework for multiscale analysis of stress field dynamics in geo-energy systems. By integrating fractional calculus with multiscale fractal dimension analysis, we develop a coupled approach examining stress redistribution patterns across different geological scales. The methodology combines fractal characterization of rock mechanical parameters with fractional-order stress gradient modeling, validated through integrated analysis of core testing, well logging, and seismic inversion data. Our fractal–fractional operators enable simultaneous characterization of stress memory effects and scale-invariant fracture propagation patterns. Key insights reveal the following: (1) Non-monotonic variations in rock mechanical properties (fractal dimension D = 2.31–2.67) correlate with oil–water ratio changes, exhibiting fractional-order transitional behavior. (2) Critical stress thresholds (12.19–25 MPa) for fracture activation follow fractional power-law relationships with fracture orientation deviations. (3) Fracture network evolution demonstrates dual-scale dynamics—microscale tip propagation governed by fractional stress singularities (order α = 0.63–0.78) and macroscale expansion obeying fractal growth patterns (Hurst exponent H = 0.71 ± 0.05). (4) Multiscale modeling reveals anisotropic development with fractal dimension increasing by 18–22% during multi-well fracturing operations. The fractal–fractional formalism successfully resolves the stress-shadow paradox while quantifying water channeling risks through fractional connectivity metrics. This work establishes a novel paradigm for coupled geomechanical–fluid dynamics analysis in complex reservoir systems.

This study delves into the geomechanical responses of different sedimentary hydrodynamic cycles in deep tight sandstone formations. Employing core observation and thin section analysis, we quantitatively identified and characterized bedding planes, sedimentary microfacies, and tectonic fractures. Then, the intricate relationships between various architectural interfaces and geomechanical parameters were elucidated. Subsequently, utilizing finite element numerical simulation software, in situ stress and fracture parameters were derived. By identifying a fracture facies zone correlated with the sedimentary hydrodynamic cycle and production data, our findings unveil several key insights: (1) Geomechanical parameters (Young’s modulus, Poisson’s ratio, brittleness index) exhibited noteworthy variations within the T₃x²⁻⁵ sand group, indicative of weak elasticity and robust plasticity. (2) The effective distance, influenced by diverse reservoir architecture interfaces, displayed variability, with each transition between peak-valley-peak or valley-peak-valley pinpointed as a distinct sedimentary hydrodynamic cycle. (3) In environments characterized by strong sedimentary hydrodynamics (between two level 3 architecture interfaces), fractures with larger strike angles and lower dip angles were observed to be more prevalent. (4) Three significant fracture faces—level I, level II, and level III—were discerned within the study area. Notably, reservoirs associated with level III exhibited characteristics suggestive of medium porosity and permeability, indicative of a gas layer. By thoroughly understanding the geomechanical response characteristics of formations such as the Xujiahe Formation, it is possible to guide the exploration and development of energy resources such as oil and natural gas. This helps to improve the efficiency and safety of resource extraction, promoting the sustainable utilization of energy.

Due to strong reservoir heterogeneity and low-resolution limit of geophysical data, it is difficult to predict fractures in ultra-deep reservoirs by conventional methods. In this research, we established a novel geomechanical model for prediction of fracture distribution in brittle reservoirs, especially for ultra-deep tight sandstone reservoirs. Methodologically, we intended to introduce the minimum energy dissipation principle considering time variable, combined with the generalized Hooke’s law containing damage variable, and obtained the energy dissipation rate expression corresponding to the energy dissipation process of brittle rocks. Combined with the three-shear energy yield criterion, the Lagrangian multiplier was introduced to deduce and construct the constitutive model and the failure criterion of rocks under the framework of the theory of minimum energy dissipation. Based on the law of conservation of energy, the stress-energy coupling characterization model of fracture density parameter was derived. Finally, all the improved geomechanical equations were incorporated into a finite element software to quantitatively simulate the distributions of tectonic stress filed and fractures based on paleo-structure restoration of Keshen anticline during the middle and late Himalayan periods. Its predictions agreed well with measured fracture density from reservoir cores and image logs.