Principal Stress Orientations Research Articles

Geology and Structural Studies of Precambrian Basement Rocks Around Mahuta, Zuru Schist Belt, Northwestern Nigeria

The study of geology and structures in part of Zuru schist belt northwestern Nigeria was aimed at understanding the rock units, mineralogy, structural configuration, distribution and deformation. This was achieved through lineament analysis, petrographic study and structural analysis (geometric, kinematic and dynamic). Lineament analysis revealed the distribution, relative abundance and orientations of linear-curvilinear features. Three (3) main lithologic units were revealed; medium-grained granite, quartz-mica schist, and quartzite while minor ones are quartz vein, quartzofeldspathic pegmatite and aplitic dyke; this was arrived at because of minerals assemblages like quartz, feldspars, biotite, and muscovite. Structural analysis revealed the presence of normal fault, strike-slip fault and systematic joint (orthogonal). The joints trend is dominantly NE-SW, N-S, E-W. The normal faults reveal maximum principal stress (σ1) orientation in 087, the intermediate stress (σ2) was oriented in 222 while the minimum stress (σ3) was oriented in 312 directions. The kinematic axes revealed from the fault data are P-axis (Compressional) is 87/087; B-axis (Tensional) is 02/222 while T-axis (Transpressional) is 02/312. The component of deformation of the normal fault is elongated by 10-30 cm, while the vertical deformation effect is neutral. The area experienced multiple phases of deformation. The study will aid in mineral exploration.

Prediction of Fracture Opening Pressure in a Reservoir Based on Finite Element Numerical Simulation: A Case Study of the Second Member of the Lower Triassic Jialingjiang Formation in Puguang Area, Sichuan Basin, China

All stages of oil and gas exploration and development involve the study of in-situ stress. Since the traditional two-dimensional and three-dimensional homogeneous models can no longer fulfil the requirements of research and production, numerical simulation of the stress field has become an effective study method. In this study, we took the Jia 2 member in Puguang area as a case to establish a geological model and a mechanical model based on the tectonic framework and the distribution characteristics of the rock mechanical parameters, respectively, and loaded the model with the present-day in-situ stress state calculated from the logging data as the boundary conditions. The simulation results show that 1) the orientation of the maximum horizontal principal stress in the study area is near E-W, and the in-situ stress orientation is locally deflected due to the influence of faults; and 2) the magnitude of in-situ stress is predominantly affected by the burial depth and lithology, and the minimum horizontal principal stress, maximum horizontal principal stress, and differential stress are mainly concentrated in the ranges of 30–60, 50–80, and 10–40 MPa, respectively. We also analysed the opening sequence of the multiple fracture systems during development, using the present-day stress field model. The analysis revealed that the E-W fractures will open first, and the continuously increasing operating pressure will lead to formation breakdown, producing a fracture network.

4D-stress evolution of tight sandstone reservoir during horizontal wells injection and production: A case study of Yuan 284 block, Ordos Basin, NW China

To investigate the 4D stress change during injection and production in tight sandstone reservoirs, a multi-physical fields modeling method is proposed considering the reservoir heterogeneity, hydraulic fracture and complex injection-production system. The 4D stress evolution of tight sandstone reservoir in Yuan 284 block of Huaqing oilfield, Ordos Basin, during injection-production in horizontal well network is investigated by modeling coupled flow and geomechanics. Results show: (1) Induced by injection and production, the 3D stress increases near the injectors but decreases near the producers, and the horizontal stresses are distributed in obvious strips along their respective stress directions. (2) The horizontal stress difference is the highest at the horizontal wellbore beside injectors during injection and production, while it is the lowest in undeveloped zone between the injectors, and the orientation of maximum horizontal principal stress changes the most near the injectors, which is distributed radially. (3) The hydraulic fracture in re-fracturing well was observed to be asymmetrical in geometry and deflected as the stress changed. The results provide theoretical guidance for horizantal well network modification and re-fracturing optimization design in tight sandstone reservoir.

Investigation on the influence of multiple fracture interference on hydraulic fracture propagation in tight reservoirs

The combination of horizontal well drilling and staged fracturing technology is one of the most effective ways to increase unconventional oil and gas production. The orientation of the horizontal wellbore is mostly considered to be consistent with the minimum horizontal principal stress in the numerical simulation. However, horizontal wellbores are usually at an angle to the minimum principal stress in field operations. In this case, the dynamic growth of the fractures will be limited during subsequent perforation and fracturing operations. Therefore, the objective of this paper is to build an effective numerical model to investigate the propagation of hydraulic fractures in the case of a horizontal wellbore that is not co-linear with the minimum horizontal principal stress. The extended finite element method is applied for multi-stage fracturing flow-solid coupling in horizontal wells. In addition, the effect of sequential and alternate fracturing on the propagation of fractures in horizontal wells was compared. The simulation results show that fracture growth on one side of the horizontal wellbore is severely restricted as the pinch angle increases. It is most evident in sequential fracturing mode while increasing the fracture spacing is one of the effective ways to reduce inter-fracture interference. Meanwhile, compared to sequential fracturing, alternate fracturing is more effective in reducing inter-fracture interference, allowing for effective fracturing at closer fracture spacing. Finally, the density of the fracturing section has been increased, and a higher reservoir reconstruction area can be obtained. This work, for the first time, evaluates the dynamic propagation of fractures when the horizontal wellbore is not collinear with the minimum horizontal principal stress orientation and helps operators to optimize the fracturing process.

The Influence Mechanism of In Situ Stress State on the Stability of Deep-Buried-Curved Tunnel in Qinghai-Tibet Plateau and Its Adjacent Region

In China, rockburst disaster occurs mostly in construction of underground engineering in Qinghai-Tibet Plateau and its adjacent region. Previous research on deep-buried tunnels has indicated that tunnels stability is related to in situ stress state. To quantify these relationships, three-dimensional finite element modeling was done to analyze the influences that the angle φ between the maximum horizontal principal stress orientation and tunnel axis, and the lateral pressure coefficient KH, had on the tangential stress σ θ in a deep-buried-curved tunnel. Based on the in situ stress condition in Qinghai-Tibet Plateau and its adjacent region, 50 different simulation conditions were used to analyze the relationship that φ and KH had on σ θ for the rock mass surrounding the tunnel. With the simulation data produced, predictive equations were generated for σ θ as a function of φ and KH using multivariate regression analysis. These equations help estimate σ θ at various key positons along the tunnel boundary at Qinghai-Tibet plateau and its adjacent region. The equations were then proved by a set of typical tunnels to ensure validity. The results concluded that the change in φ has a significant impact on σ θ , and thus, the stability of the tunnel, when 30° < φ < 60°, with the most obvious influence being when φ is about 45°. With the equations, the rockburst potential at a certain location within a curved tunnel can be quickly estimated by calculating φ and KH on σ θ , without need of geo-stress background knowledge and heavy simulation, allowing for the practical value in engineering at design phase for the projects in Qinghai-Tibet Plateau and its adjacent region.

Fault Geometry and Mechanism of the Mw 5.7 Nakchu Earthquake in Tibet Inferred from InSAR Observations and Stress Measurements

Different types of focal mechanism solutions for the 19 March 2021 Mw 5.7 Nakchu earthquake, Tibet, limit our understanding of this earthquake’s seismogenic mechanism and geodynamic process. In this study, the coseismic deformation field was determined and the geometric parameters of the seismogenic fault were inverted via Interferometric Synthetic Aperture Radar (InSAR) processing of Sentinel-1 data. The inversion results show that the focal mechanism solutions of the Nakchu earthquake are 237°/69°/−70° (strike/dip/rake), indicating that the seismogenic fault is a NEE-trending, NW-dipping fault dominated by the normal faulting with minor sinistral strike-slip components. The regional tectonic stress field derived from the in-situ stress measurements shows that the orientation of maximum principal compressive stress around the epicenter of the Nakchu earthquake is NNE, subparallel to the fault strike, which controlled the dominant normal faulting. The occurrence of seven M ≥ 7.0 historical earthquakes since the M 7.0 Shenza earthquake in 1934 caused a stress increase of 1.16 × 105 Pa at the hypocenter, which significantly advanced the occurrence of the Nakchu earthquake. Based on a comprehensive analysis of stress fields and focal mechanisms of the Nakchu earthquake, we propose that the dominated normal faulting occurs to accommodate the NE-trending compression of the Indian Plate to the Eurasian Plate and the strong historical earthquakes hastened the process. These results provide a theoretical basis for understanding the geometry and mechanics of the seismogenic fault that produced the Nakchu earthquake.

Kinematic analysis of fault-slip data in the Nanling Tectonic Belt and Cretaceous to Paleogene tectonic evolution of the Central South China Block

• Six tectonic events were inversed from fault-slip analysis in the Nanling Tectonic Belt. • Four Cretaceous tectonic events resulted from the Paleo-Pacific subduction . • Two Paleogene tectonic events were due to the far field effect of the India-Asia collision. Cretaceous tectonic events and Cenozoic geodynamics of the central South China Block are still under debate. A long-term extensional regime or two extension-contraction tectonic cycles were proposed for the Cretaceous tectonic evolution. While Cenozoic geodynamics of the central South China Block were ascribed to the Pacific subduction or the far field effect of the India-Asia collision. Kinematic analysis of fault-slip data were carried out for the Cretaceous to Paleogene Chaling-Yongxing Basin and the Lianshan-Yangshan Domain in the Nanling Tectonic Belt. Kinematic analysis of fault-slip data (409 groups of faults and striations) reveal four Cretaceous tectonic events and two Paleogene tectonic events. i) The early Early Cretaceous extensional regime is represented by a NW-SE horizontal minimum principal stress and a vertical maximum principal stress, which generated NE-striking boundary normal faults of Early Cretaceous basins. ii) The late Early Cretaceous strike-slip regime is characterized by a N-S horizontal maximum principal stress and a E-W horizontal minimum principal stress, which led to tectonic inversion of Early Cretaceous extensional basins and nearly N-S-striking folding of the Lower Cretaceous. iii) The early Late Cretaceous extensional regime is represented by a NW-SE horizontal minimum principal stress and a vertical maximum principal stress, which activated the NE-striking boundary normal faults of Late Cretaceous basins. And iv) the latest Late Cretaceous strike-slip regime is characterized by NW-SE horizontal maximum principal stress and a NE-SW horizontal minimum principal stress, which led to tectonic inversion of Late Cretaceous extensional basins and NNE-striking folding of the Upper Cretaceous. Orientations of maximum or minimum principal stresses imply that two extension-contraction tectonic cycles during the Cretaceous resulted from the Paleo-Pacific subduction. Two Paleogene tectonic events are v) the strike-slip regime characterized by NE-SW horizontal maximum principal stress and a NW-SE horizontal minimum principal stress, and vi) the subsequent extensional regime represented by a NE-SW minimum principal stress and a vertical maximum principal stress. The former generated NW-striking reverse faults and reactivated strike-slipping of NE- and NW-striking conjugate faults whereas the latter formed NW-striking normal faults in the Lower Paleogene clastic rocks. Orientations of maximum or minimum principal stresses indicate that two Paleogene tectonic events in the central South China Block were attributed to the far field effect of the India-Asia collision and subsequent stress relief.

Tectonic fracture prediction and its implication for hydrocarbon development with a case study in ultra-deep reservoirs of Kuqa Depression, Tarim Basin, NW China

Tectonic fractures can be important reserving space and the main contributor to permeability in ultra-deep reservoirs of Kuqa Depression. In this study, a workflow based on geomechanical method is presented. Firstly, a 3D heterogeneous mechanical field is constructed by core-logging-seismic data; then the finite element in-situ stress simulation is carried out, which is established using heterogeneous mechanical parameters instead of uniform parameters. Based on strain energy theory and energy conservation principle, the relationship between tectonic fracture parameters and in-situ stress field is established by a suitable rock fracture criterion for ultra-deep tight sandstone. Characteristics of tectonic fractures are predicted and revealed by the above stress field simulation and the established relationship between stress and tectonic fracture parameters. The results show that the maximum horizontal principal stress (SH) orientation is generally N-S-trending in Bozi-1 gas reservoir. The present-day in-situ stress magnitudes are generally high with the minimum horizontal principal stress (Sh) is about 110∼160MPa, and the stress difference is over 35MPa. The characteristics of present-day in-situ stress largely vary among different structural styles. The in-situ stress magnitude and burial depth generally indicates a linear relationship. The in-situ stress gradient is stratified with burial depth and is consistent with fold stress stratification. Controlled by the effect of fold bending and protection of salt layer, the development of tectonic fracture in pop-up structures is better with larger fracture aperture and higher permeability, and fracture strikes are generally consistent with the SH orientation. Tectonic fractures in imbricated structures indicate great heterogeneity because of the development of faults and folds, and the rebuilding effect (permeability reduction) under the present-day in-situ stress field. Optimization of favourable zone for in-situ stress and tectonic fracture is of practical effects during well drilling and completion works.

Phase field modeling of multi-cluster hydraulic fracturing in horizontal wellbore with inconsistent direction to minimum principal stress

Zhongjiang gas field is a typical narrow channel tight sandstone gas reservoir. In order to obtain enough horizontal section length in high-quality reservoir, horizontal wellbore azimuth is basically consistent with direction of narrow channel sand body, resulting in a certain included angle θ between wellbore azimuth and in-situ minimum principal stress orientation. In this study, a seepage-stress-damage coupled dynamic fracture propagation model is established through phase field method to investigate the effect of angle θ on multi-cluster fracture simultaneous propagation based on geological engineering data of JS-X well. Propagation of hydraulic fracture is automatically tracked through phase field evolution without additional criteria or grid direction limitation. Simulation results show that: (1) Fracture tip of each cluster would approach to stress diversion zone created by adjacent clusters. Simultaneous propagating fractures of each cluster are prone to deflect, interconnect with each other and form one single fracture eventually. (2) Increasing cluster number or decreasing cluster spacing in the same fracturing stage would further promote diverting of each cluster fracture and accelerates fractures merging into one single fracture. We suggest that azimuth of horizontal wellbore in tight gas reservoir should be consistent with in-situ minimum principal stress orientation to reduce the effect of angle θ.

Variations in vertical stress in the Permian Basin region

Constraining the magnitude of vertical stress (Sv), or overburden pressure, is key in determining a region’s stress state and has implications for reservoir geomechanics and the potential for induced seismicity. Of the principal stress orientations (Sv, minimum horizontal stress [Shmin], and maximum horizontal stress [SHmax]), Sv is the most straightforward to constrain using wire-line log data. The magnitude of Sv varies because of lithology and burial history, potentially causing local perturbations in the in situ stress field. Previous studies on the state of stress in the Permian Basin use a constant Sv, relying on determination of SHmax and Shmin, and yield an interpretation that the faulting regime transitions from normal faulting in the west to normal to strike-slip faulting in the east. Here, we present an interpretation of the spatial and depth variability in Sv trends in the Permian Basin based on density log integration. Where density measurements are absent, values are calculated from compressional velocity logs using a transform that is fit to local data. Notable variations include higher Sv gradient on carbonate platforms and shelves, where high-density carbonates are thicker and are found at shallower depths than in the basins. Within the basins, the magnitude of Sv gradient is as low as 1.06 psi/ft at depth. This work shows the potential for regional interpretations of Sv to gain insight into the effect of variations in Sv on state of stress.

Characteristics of in situ stress field in the Huainan mining area, China and its control factors

Due to the high in situ stresses, dynamic disasters occurred frequently in the Huainan mining area, China. Our understanding of the in situ stresses in this area is still insufficient. In this study, the in situ stresses of 18 sections in two boreholes in the Xinji No. 1 coalfield were measured using the hydraulic fracturing method, and the distribution of in situ stresses in the Huainan mining area were investigated. The relationship between in situ stress and geological structure in the Huainan mining area was summarized and the limitation of fault friction strength on in situ stress was discussed. The result showed that the maximum horizontal principal stress (σH) at Xinji No. 1 mine was 13.95–25.23 MPa, the minimum horizontal principal stress (σh) was 12.16–21.17 MPa. The average azimuth of the maximum horizontal principal stress was N83.61°E. The statistical results showed that the in situ stresses in Huainan mining area were characterized by a strike-slip faulting regime. Both the horizontal and vertical principal stresses increased approximately linearly with the increase of burial depth. The orientation of the maximum principal stress in the study area is closely related to the tectonic movement and the ratio of maximum principal stress to minimum principal stress was primarily limited by the friction strength of the faults. The outcomes of this research can provide some reliable engineering parameters and benefit the roadway layout and support design in the Huainan mining area.

Variations in Forearc Stress and Changes in Principle Stress Orientations Caused by the 2004–2005 Megathrust Earthquakes in Sumatra, Indonesia

Coseismic changes in principal stress orientation in the northern Sumatra subduction zone due to two giant megathrust earthquakes there in 2004 and 2005 are estimated to investigate the in-situ stress. The two megathrust earthquakes, the 2004 Sumatra-Andaman and the 2005 Nias-Simeulue events, are both among the 11 largest earthquakes ever recorded. Previous studies have shown that these giant earthquakes perturbed the stress field in the Sumatra subduction zone enough to alter the principal stress directions there, and here we investigate whether these changes can be used to better understand spatial variations in stress along the subduction zone. We used 330 previously published focal mechanisms to estimate pre- and post-mainshock principal stress orientations in 3 outer forearc segments and assessed whether orientation differences were resolved and what they imply about the pre- and post-mainshock stress fields. Our results agree with previous studies in establishing that coseismic changes in stress orientation in the forearc are resolvable, and consistent with a low level of stress in the outer Sumatran forearc before the earthquake, with almost all the shear stress on the megathrust relieved in the 2004 and 2005 earthquakes. In this study, we reveal that both the stress orientations and coseismic changes in them exhibit along-strike variations, with a decrease in both the pre-mainshock stress and stress drop found in the rupture area of 2005 relative to that of the 2004 earthquake. The forearc segment between the 2004 and 2005 rupture areas, which coincides with a well-known megathrust rupture barrier beneath the island of Simeulue is observed to have a characteristic signature, with lower shear stress relative to the pre-mainshock stress field and higher shear stress relative to the post-mainshock stress field in the adjacent segments.

Using Syntectonic Calcite Veins to Reconstruct the Strength Evolution of an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

AbstractQuantifying the strength evolution of faults that cut the lithosphere is essential to better understand seismicity in continental regions. We estimate differential stresses and principal stress orientations driving rapid slip of ∼10 mm/yr on the active Mai'iu low‐angle normal fault (LANF), SE Papua New Guinea. The fault's mafic footwall hosts a well‐preserved sequence of mylonite, (ultra‐)cataclasite, and gouge. In these fault rocks, we combine stress inversion of fault‐slip data and paleostress analysis of syntectonically emplaced calcite veins with microstructural and clumped‐isotope geothermometry to constrain a syn‐exhumational sequence of deformation stresses and temperatures, and to construct a stress profile through the exhumed footwall of the active Mai'iu LANF. This includes: (a) at ∼12–20 km depth (T ≈ 275–370°C), mylonites accommodated slip on the Mai'iu fault at low differential stresses (>25–135 MPa) before being overprinted by localized brittle deformation at shallower depths; (b) at ∼6–12 km depth (T ≈ 130–275°C) differential stresses in the foliated cataclasites and ultracataclasites were high enough (>150 MPa) to drive slip on a mid‐crustal portion of the fault (dipping 30–40°), and to trigger brittle yielding of mafic footwall rocks in a zone of mixed‐mode seismic/aseismic slip; and (c) at shallower crustal depths (T < 150°C; depth <6 km) on the most poorly oriented segment of the Mai'iu LANF (dipping ∼22°), slip occurred on frictionally weak clay‐rich gouges (μ ≈ 0.15–0.38). Subvertical σ1 and subhorizontal σ3 parallel to the extension direction, with σ1 ≈ σ2 (constriction), reflect vertical unloading and 3‐D bending strain during rolling‐hinge style flexure of the footwall.

A new methodology for measuring residual stress using a modified Berkovich nano-indenter

In this paper, a new methodology, based on nanoindentation, is proposed to estimate the magnitude and orientation of the principal residual stress components, in a non-equi-biaxial residual stress field at the nanoscale. The proposed method exploits the force-penetration response provided by a modified Berkovich tip that, compared to the most traditional one, is characterized by a preferential elongated size. This feature allows in estimating non-equibiaxial residual stress field since the force-penetration response changes according to the indenter orientation. No observation of the residual imprint is required. Principal stress components and orientations are estimated thanks to theoretical and finite element analysis, establishing a linear relation between stress components and load variations respect to the stress-free sample. Finite element analysis is used to calibrate the model coefficients and to verify the accuracy of this approach for isotropic metals. Obtained results revealed that the method is well suitable in estimating both tensile and compressive residual stresses with an average error lower than 6% for the first case and lower than 10% for the second one.

Stress heterogeneity in the Changning shale-gas field, southern Sichuan Basin: Implications for a hydraulic fracturing strategy

To evaluate the risk of induced seismicity due to hydraulic fracturing, a detailed stress analysis in a seismically active region, i.e., the Changning shale-gas field in the southern Sichuan Basin, was conducted utilizing sonic wave velocity logs and high-resolution electrical resistivity image logs of shale gas wells. Wells with relatively higher magnitudes of horizontal stresses are spatially located in the vicinity of thrust faults, in agreement with the stress concentration at the upper tip of the basement-involved thrust fault. The prevailing orientation of maximum horizontal principal stress (SHmax) is N110°–120°E on the southwestern Changning anticline, which aligns with the motion of the Sichuan Basin, as driven by the continuous eastward extrusion of the Tibet plateau, which initiated at approximately 50 Ma. However, the SHmax orientation is N30°–70°E along the northern Changning anticline, implying an inheritance of the N–S geodynamic compression during the Jurassic to Late Cretaceous. Drilling-induced tensile fractures primarily appear in compliant layers that are rich in clay or organic matter in wells located adjacent to thrust faults, suggesting that stiff layers can efficiently inhibit the hydrofracture growth height owing to more intense tectonic stress near thrust faults. A natural fault with a strike of approximately 30° to the SHmax orientation can be more easily activated by extreme pumping pressure in wells that are weakly affected by the stress concentration along the thrust fault. Nevertheless, a cyclic injection of fracturing fluid with a gradually increasing target pressure may effectively reduce the magnitude of induced seismicity.

Numerical and experimental study of the presplit blasting failure characteristics under compressive stress

Recently, the application of deep hole presplit blasting (DHPB) in underground-engineering excavation has increased, but the research on deformation characteristics and failure mechanism of DHPB is relatively less, and lack of relevant numerical simulation verification. Owing to the above situation, the deformation characteristics and failure mechanism of DHPB are obtained through numerical simulation and model testing of samples under horizontal compressive stress, compressive vertical stress and no compressive stress, and the shape analysis of presplit cracks after presplit blasting. This work demonstrates when presplit blasting is performed in underground engineering, the angle between the connection orientations of the blasthole and the compressive stress orientation should be avoided at 90° as far as possible. When the deep confining pressure is not uniform, the orientation of the minimum principal stress should be at an angle of 90° to the orientation of the blasthole connecting line as far as possible. Furthermore, the orientation of the maximum principal stress should be consistent with the orientation of the blasthole connecting line as far as possible.

Stress changes associated with the significant first subevent of the 2008 Wenchuan earthquake and implications for the rupture behavior transition

The rupture process of the Wenchuan earthquake demonstrated a transition from thrust-dominated slip to northeastward strike-slip motion along the Longmen Shan Fault Zone. The initial stress has been reported as playing a critical role in this process; however, the stress changes, especially those caused by the significant first subevent of the Wenchuan earthquake are not well understood. Here, we employ a three-dimensional finite element model of the Sichuan-Yunnan region to analyze the stress change caused by the significant first subevent and explore the possible influence on the following ruptures. The results indicate that the auxiliary maximum principal compressive stress (SH) associated with the significant first subevent was horizontal and that the auxiliary stress regime was SH>Sh>Sv, supporting the ongoing regional thrust motion near the southwestern segment of the rupture plane. However, in the northeastern segment, the auxiliary stress regime transitioned to SH>Sv>Sh, demonstrating that the stress changes promoted the transition of the rupture behavior from predominantly thrust motion in the southwest to right-lateral strike-slip motion in the northeastern segment, which was also supported by the dominant shear stress change and the subtle normal stress change along the fault plane in the northeastern segment. In addition, our modeled results also indicate that the orientation of the maximum principal compressive stress changed from SEE to northeastward NEE along the strike of Longmen Shan Fault Zone. This anticlockwise rotation hastened the rupture behavior transition, suggesting that both the initial stress and the stress changes associated with the first subevent jointly controlled the following northeastward rupture of the Mw7.9 Wenchuan earthquake.

Neogene Kinematics of the Potwar Plateau and the Salt Range, NW Himalayan Front: A Paleostress Inversion and AMS study

ABSTRACT We provide new kinematic data from the Potwar Plateau (Pakistan) to evaluate the tectonic evolution of the region during the Neogene. The plateau is bound by two major strike-slip faults in the west and the east, accommodating its southwards translation. We have recognized two Neogene deformation phases in the plateau, based on paleostress inversion and Anisotropy of Magnetic Susceptibility (AMS) tensors. The first phase lasted until the early Pliocene and was characterized by vertical minor stress and N-S compression, implying thrust tectonics. The second deformation phase is characterized by a near-vertical intermediate principal stress and near-horizontal major and minor stresses, interpreted to be associated with strike-slip tectonics since the late Pliocene. Kint vectors from 21 sites are relatively compatible with the major principal stress orientations (σ1) and indicate two distinct domains. This is possibly because Kmin orientations are related to compaction, whereas Kint orientations were always parallel to tectonic shortening and hence compression direction during both strike-slip (post-late Pliocene) and thrusting (pre-late Pliocene) phases. These phases are characterized by swapping of (σ2) and (σ3) orientations while (σ1) maintained its orientation. The most prominent change occurs at the western part of the Potwar Plateau, where major principal stress directions (σ1) and Kint axes fan out south-westwards. The eastern domain is dominated by NE-SW trending folds and thrust faults, which are absent in the western domain. These structural features are interpreted to be the result of the distribution of deposits of the Neoproterozoic Salt Range Formation as a substratum below the Potwar Plateau. The Salt Range Formation is very thick and widespread in the west area and almost absent in the east. This factor led to unconstrained southwards gliding of the Potwar Plateau over the salt deposits in the west as opposed to frictional sliding and substantial internal deformation in the east.

Stress Reversals near Hydraulically Fractured Wells Explained with Linear Superposition Method (LSM)

Prior studies have noted that the principal stress orientations near the hydraulic fractures of well systems used for energy extraction may wander over time. Typically, the minimum and maximum principal stresses—in the horizontal map view—swap their respective initial directions, due to (1) fracture treatment interventions, and (2) pressure depletion resulting from production. The present analysis shows with stress trajectory visualizations, using a recently developed linear superposition method (LSM), that at least two generations of stress reversals around hydraulic fractures occur. The first generation occurs during the fracture treatment; the second occurs immediately after the onset of so-called flow-back. During each of these stress swaps in the vicinity of the hydraulic fractures, reservoir directions that were previously in compression subsequently exhibit extension, and directions previously stretching subsequently exhibit shortening. The pressure change in the hydraulic fractures—from over-pressured to under-pressured (only held open by proppant packs)—caused the neutral points that separate domains with different stress states to migrate from locations transverse to the fracture to locations beyond the fracture tips. Understanding such detailed geo-mechanical dynamics, related to the pressure evolution in energy reservoirs, is extremely important for improving both the fracture treatment and the well operation, as future hydrocarbon and geothermal energy extraction projects emerge.

Stability of basement-rooted faults in the Delaware Basin of Texas and New Mexico, USA

Since 2009 the Delaware Basin of Texas and New Mexico has experienced increased seismicity related to oilfield operations. Available, near present-day principal stress orientations, relative magnitudes, and vertical stress estimates are integrated to characterize 20 stress domains in the Delaware Basin. Data density, variability, and quality inform classification of most-likely, less likely, and least likely stress tensor fields. A new interpretation of Delaware Basin, 3D basement-rooted faults is analyzed for stability using most-likely stress tensors. Fault stability is expressed in terms of slip tendency (Ts) and critical pore pressure (ΔPfc). Individual faults are compared in terms of their stability and potential seismic capability, and stress scenarios can be compared for their effects on fault stability. Faults range from stable to critical under the conditions investigated, 8% of total mapped fault area may become critically stressed by small pore-pressure increases (1 MPa, 145 psi) above ambient. Comparison of stability measures with focal mechanisms of recent pore-pressure-induced seismicity indicates that Ts and ΔPfc have predictive value. This work provides critical information for earthquake hazard and mitigation studies in the Delaware Basin.