Fault Stability Research Articles

Role of Plasticity in Induced Seismicity Risk Mitigation: A Case of the Groningen Gas Field

Summary Earthquakes induced by fluid extraction from deep underground reservoirs are not well understood in rocks that are deforming plastically. The problem grows in importance when seismicity risk mitigation strategies, such as gas injection into a depleting hydrocarbon reservoir, attempt to reverse the declining pressure trend of a poromechanical system deforming irreversibly. This is the case at the Groningen gas field and similar fields worldwide. Poroplasticity associated with half a century of seasonally fluctuating gas production makes it challenging to predict Groningen’s state, especially with hundreds of faults compartmentalizing the reservoir. We provide new insights into the role of plasticity in depletion-induced seismicity and its mitigation via injection. The irreversibility of plastic deformation is key to predicting stress and fault stability when the pressure trend is reversed by fluid injection. The elastic deformation assumption predicts unrealistically high Coulomb failure stresses on the faults, implying a higher risk of induced seismicity than possible under plastic deformation. The inaccuracy in the elastic model’s predicted stress cannot be discerned from the reservoir pressure or subsidence measurements in the field. Therefore, rock’s plasticity must be considered in assessing and mitigating the risk of induced seismicity. The probability distribution of the change in Coulomb failure stress over 115 faults in the field reveals a multimodal shape that emerges from the stabilization and destabilization of different faults depending on the fault’s geometry and position relative to the wells.

Оценка рисков реактивации разломов при бурении горизонтальной скважины по результатам одномерного геомеханического моделирования

The article presents main factors affecting the fault stability. Concepts of critically stressed faults and critical pore pressure are discussed. The methodologies for evaluation of stressed-deformed state of rocks in the vicinity of fault zones during reservoir depletion and injection, well drilling are analyzed. 1D geomechanical models of fishbone sidetracks and main borehole of the well N are constructed. The fault stability evaluation is made using Mohr – Coulomb failurecriterion from data of insitustresses and near-wellborestressescalculated from Kirsch wellbore solution. Optimal mud weight ranges are determined for safe drilling of fishbone boreholes accounting fault reactivation risks. Basic recommendations for optimal drilling through the interval of unstable shales are made.

Frictional strength of siliciclastic sediment mixtures in fault stability assessment

Frictional strength of fault zones is a key parameter for evaluation of fault stability and reactivation. We measure friction using the direct shear test (DST) for (i) sand-clay mixes mimicking fault gouges, and (ii) strength of fault zone interfaces. The sand-clay mixing ratio is linked to the established Shale Gouge Ratio (SGR) used in fault seal analysis, suggesting an approach for linking the measured frictional properties to the established subsurface fault characterization methods and risk assessment. We use powdered caprock material from the Draupne Formation mixed with sand to prepare the fault gouge and discuss application of results for fault zones on the Horda Platform, Norwegian North Sea.Shearing of fault gouge show a systematic decrease in residual friction coefficient with increasing clay content from 0.6 (φ = 31°) in pure sand (SGR 0%) to 0.4 (φ = 22°) for clay rich mixtures (SGR 100%). Interface testing mimicking fault gouge on sand surface is less systematic and shows residual friction coefficients ranging from 0.53 to 0.6 (φ = 28–31°) for SGR 0–50%. Detailed interpretation of the shear testing results shows changes in drainage properties, volumetric changes during shearing and shear responses to normal stresses indicating threshold values for sand versus clay dominated material behaviour. However, the results are non-conclusive on the question if a linear variation of friction with clay content or a threshold between sand dominated versus clay dominated friction provides the best approach for linking friction to fault clay content.

Geothermal field development optimization under geomechanical constraints and geological uncertainty: Application to a reservoir with stacked formations

In this work, numerical optimization based on stochastic gradient methods is used to assist geothermal operators in finding improved field development strategies that are robust to accounted geological uncertainties. Well types, production rate targets and well locations are optimized to maximize the economics of low-enthalpy heat recovery in a real-life case with stacked reservoir formations. Significant improvements are obtained with respect to the strategy designed by engineers. Imposing fault stability constraints impacts significantly the optimal configurations, with coordinated well rates and placement playing a key role to boost efficiency of geothermal production while keeping stress change effects to acceptable limits.

Frictional Properties of Feldspar‐Chlorite Gouges and Implications for Fault Reactivation in Hydrothermal Systems

AbstractAs a particularly common mineral in granites, the presence of feldspar and feldspar‐chlorite gouges at hydrothermal conditions has important implications in fault strength and reactivation. We present laboratory observations of frictional strength and stability of feldspar (K‐feldspar and albite) and feldspar‐chlorite gouges under conditions representative of deep geothermal reservoirs to evaluate the impact on fault stability. Velocity‐stepping experiments are performed at a confining stress of 95 MPa, pore pressures of 35–90 MPa, and temperatures of 120–400°C representative of in situ conditions for such reservoirs. Our experiment results indicate that the feldspar gouge exhibits strong friction (μ ∼ 0.71) at all experimental temperatures (∼120–400°C) but when T > 120°C, the frictional response transitions from velocity‐strengthening to slightly velocity‐weakening. At constant confining pressure and temperature, increasing the pore pressure increases the friction coefficient (∼0.70–0.85) and the gouge remains slightly velocity weakening. The presence of alteration‐sourced chlorite leads to a transition from velocity weakening to velocity strengthening in the mixed gouge at experimental temperatures and pore pressures. As a ubiquitous mineral in reservoir rocks, feldspar is shown to potentially contribute to unstable sliding over ranges in temperature and pressure typical in deep hydrothermal reservoirs. These findings emphasize that feldspar minerals may increase the potential for injection‐induced seismicity on pre‐existing faults if devoid of chloritization.

Geomechanical risk assessment for CO2 storage in deep saline aquifers

We study CO2 injection into a saline aquifer intersected by a tectonic fault using a coupled modeling approach to evaluate potential geomechanical risks. The simulation approach integrates the reservoir and mechanical simulators through a data transfer algorithm. MUFITS simulates non-isothermal multiphase flow in the reservoir, while FLAC3D calculates its mechanical equilibrium state. We accurately describe the tectonic fault, which consists of damage and core zones, and derive novel analytical closure relations governing the permeability alteration in the fault zone. We estimate the permeability of the activated fracture network in the damage zone and calculate the permeability of the main crack in the fault core, which opens on asperities due to slip. The coupled model is applied to simulate CO2 injection into synthetic and realistic reservoirs. In the synthetic reservoir model, we examine the impact of formation depth and initial tectonic stresses on geomechanical risks. Pronounced tectonic stresses lead to inelastic deformations in the fault zone. Regardless of the magnitude of tectonic stress, slip along the fault plane occurs, and the main crack in the fault core opens on asperities, causing CO2 leakage out of the storage aquifer. In the realistic reservoir model, we demonstrate that sufficiently high bottomhole pressure induces plastic deformations in the near-wellbore zone, interpreted as rock fracturing, without slippage along the fault plane. We perform a sensitivity analysis of the coupled model, varying the mechanical and flow properties of the storage layers and fault zone to assess fault stability and associated geomechanical risks.

Study on fault-slip process and seismic mechanism under dynamic loading of hard roof fracture disturbance

The dynamic stress disturbance induced by mining is a significant factor triggering fault-slip. To better understand the impact of mining-induced hard roof fracture on fault-slip, combined with the geological conditions of a longwall face in contact with the fault, a dynamic and static numerical model was established using FLAC3D. The influence of dynamic loading from hard roof fracture on fault stability was investigated. The characteristics of changes in fault shear stress, shear displacement, slip area, and seismic moment under dynamic loading were analyzed. Simultaneously, numerical results were compared and analyzed against microseismic (MS) monitoring records in the mining, validating the accuracy of the numerical results. The results indicate that dynamic loading is a critical factor in inducing fault-slip. As a geological weak surface, the fault impedes the transmission of shock waves and causes their attenuation. Under dynamic loading, when the fluctuation of fault shear stress and friction coefficient exceeds the threshold, the fault-slip. During the fault-slip process, accompanied by a sharp increase in shear displacement, the seismic moment increases. Finally, the influence of changes in seismic parameters (e.g. longwall–fault distance, seismic energy, and seismic location) on the fault-slip mechanism was analyzed. This study contributes to a better understanding of the disturbance mechanism of mining-induced dynamic loading on fault-slip and provides a theoretical basis for fault-slip rock bursts.

South China Sea Typhoon Hagibis enhanced Xinfengjiang Reservoir seismicity

There was an evident increase in the number of earthquakes in the Xinfengjiang Reservoir from June to July 2014 after the landing of Typhoon Hagibis. To understand the spatial and temporal evolution of this microseismicity, we built a high-precision earthquake catalog for 2014 and relocated 2275 events using recently developed methods for event picking and catalog construction. Seismicity occurred in the southeastern part of the reservoir, with the preferred fault plane orientation aligned along the Heyuan Fault. The total seismic energy peaked when the typhoon passed through the reservoir, and seismicity correlated with typhoon energy. In contrast, a limited seismic response was observed during the later Typhoon Rammasun. Combining data regarding the water level in the Xinfengjiang Reservoir and seismicity frequency changes in the Taiwan region during these two typhoon events, we suggest that typhoon activity may increase microseism energy by impacting fault stability around the Xinfengjiang Reservoir. Whether a fault can be activated also depends on how close the stress accumulation is to its failure point.

Coupled geomechanical analysis of irreversible compaction impact on CO2 storage in a depleted reservoir

The utilization of depleted gas reservoirs for carbon storage offers substantial advantages. However, it raises concerns related to the geomechanical effects of historic gas extraction on the CO2 sequestration operation. In this study, a coupled thermo-hydro-mechanical investigation is conducted on a multi-layered sedimentary system that includes sealed bounding faults. Our simulations exhibit various levels of reservoir compaction during gas extraction under different pre-consolidation conditions of the sediments, highlighting the pivotal role of reservoir compaction in geomechanical analysis. The results demonstrate the occurrence of strain-hardening compaction behavior in the reservoir during post-yield depletion. This compaction is accompanied by a significant reduction in porosity and permeability, as well as irreversible surface subsidence. Hysteresis in the stress state is induced by the aforementioned irreversible reservoir compaction through two primary mechanisms: poro-elastoplastic stressing and differential compaction on each side of the sealing fault. These mechanisms alter the magnitude and orientation of stress inside and outside the depleted reservoir. Moreover, caprock compaction is impeded and delayed by the irreversible reservoir compaction owing to poro-elastoplastic stressing. This implies that conventional methods relying on the poro-elasticity theory alone may overestimate pressure required to fracture the caprock by approximately 2 MPa. When considering the combined effect of poro-elastoplastic stressing and differential compaction, neglecting irreversible reservoir compaction may lead to underestimation of the critical pressure for inducing fault slip by up to 4.9 MPa. Additionally, regardless of whether plastic void compaction is considered, we recommend increased attention be focused on the sub-vertical faults to mitigate the risk of significant slip that potentially could also lead to upward CO2 leakage. In scenarios where a slip event occurs in a fault during reservoir depletion, the results show that a subsequent CO2 injection operation tends to stabilize the faults. Nevertheless, it is crucial to consider that fault stability could deteriorate rapidly over time, potentially leading to a second slip event during carbon sequestration.

Faults locating of power distribution systems based on successive PSO-GA algorithm

As the terminal of the power system, the distribution network is the main area where failures occur. In addition, with the integration of distributed generation, the traditional distribution network becomes more complex, rendering the conventional fault location algorithms based on a single power supply obsolete. Therefore, it is necessary to seek a new algorithm to locate the fault of the distributed power distribution network. In existing fault localization algorithms for distribution networks, since there are only two states of line faults, which can usually be represented by 0 and 1, most algorithms use discrete algorithms with this characteristic for iterative optimization. Therefore, this paper combines the advantages of the particle swarm algorithm and genetic algorithm and uses continuous real numbers for iteration to construct a successive particle swarm genetic algorithm (SPSO-GA) different from previous algorithms. The accuracy, speed, and fault tolerance of SPSO-GA, discrete particle swarm Genetic algorithm, and artificial fish swarm algorithm are compared in an IEEE33-node distribution network with the distributed power supply. The simulation results show that the SPSO-GA algorithm has high optimization accuracy and stability for single, double, or triple faults. Furthermore, SPSO-GA has a rapid convergence velocity, requires fewer particles, and can locate the fault segment accurately for the distribution network containing distorted information.

Mechanisms for Microseismicity Occurrence Due to CO2 Injection at Decatur, Illinois: A Coupled Multiphase Flow and Geomechanics Perspective

ABSTRACT We numerically investigate the mechanisms that resulted in induced seismicity occurrence associated with CO2 injection at the Illinois Basin–Decatur Project (IBDP). We build a geologically consistent model that honors key stratigraphic horizons and 3D fault surfaces interpreted using surface seismic data and microseismicity locations. We populate our model with reservoir and geomechanical properties estimated using well-log and core data. We then performed coupled multiphase flow and geomechanics modeling to investigate the impact of CO2 injection on fault stability using the Coulomb failure criteria. We calibrate our flow model using measured reservoir pressure during the CO2 injection phase. Our model results show that pore-pressure diffusion along faults connecting the injection interval to the basement is essential to explain the destabilization of the regions where microseismicity occurred, and that poroelastic stresses alone would result in stabilization of those regions. Slip tendency analysis indicates that, due to their orientations with respect to the maximum horizontal stress direction, the faults where the microseismicity occurred were very close to failure prior to injection. These model results highlight the importance of accurate subsurface fault characterization for CO2 sequestration operations.

Shale fault reactivation during hydraulic fracturing in the Sichuan Basin: Evidence from shear experiments and stress perturbation calculations

Induced seismicity triggered during hydraulic fracturing for shale gas exploitation in the Sichuan Basin has aroused wide public concern with these earthquakes closely correlated with the reactivation of faults within the reservoir. The target shale reservoirs in the Sichuan Bain are currently located in the Longmaxi formation. To explore instability on these faults, we conducted shear experiments on simulated fault gouges under hydrothermal conditions to assess frictional stability and also evaluated the stress perturbations resulting from multistage hydraulic fracturing. Experimental results show that the frictional instability on these faults is primarily controlled by both mineral composition and the applied temperatures. With a decrease in the contents of phyllosilicate minerals or an increase in applied temperatures, the shale faults exhibit a transition from velocity-strengthening to velocity-weakening behaviour, indicating the potential for unstable fault slip. The stress perturbations resulting from multistage hydraulic fracturing were calculated from a dislocation model of the fluid-driven fracture. These results show that the stress changes resulting from multistage hydraulic fracturing are sufficient to reactivate adjacent unstable faults in the shale reservoir and trigger the seismicity. Our experimental and modelling results highlight the importance of shale composition, temperature, and stress perturbations on shale fault stability. These results have significant implications for understanding the shale fault instability and the potential triggering of induced seismicity during hydraulic fracturing in the Sichuan Basin.

Post-mining related reactivation potential of faults hosted in tight reservoir rocks around flooded coal mines, eastern Ruhr Basin, Germany

The cessation of hard coal mining in the Ruhr Basin in 2018 marked the region's transition to the post-mining phase. Controlled mine water rebound induces changes in the subsurface stress conditions, as pore pressure increases locally. Presently, mine water rebound is observed in the eastern Ruhr Basin (water province “Haus Aden”) along with associated microseismicity. Furthermore, post-mining challenges might comprise the potential risk of fault reactivation, which is addressed in this study by conducting a fault slip assessment. Based on subsurface coal seam mapping data, a 3D structural model for the NE part of the “Haus Aden” water province has been constructed to serve as the basis for identifying the most vulnerable fault trends and types of the structural inventory. Slip tendency analysis, considering normal faulting conditions, revealed NW-SE to NNW-SSE trending normal faults to be most susceptible to reactivation. Probabilistic fault slip assessment, focused on NW-SE to NNW-SSE trending normal faults mapped within the “Heinrich-Robert” colliery, show no fault reactivation potential for a mine water rebound up to a level of 640 m below ground. Assuming hydrostatic conditions in the vicinity of the faults, friction coefficients are only partially exceeded for high differential stresses. In addition, a novel workflow is used to model the spatial variability of the frictional fault strength as input for a fault stability analysis, exemplified for a selected NNW-SSE trending normal fault. For considering hydrostatic pore pressure, results show that the fault consists mainly of stable, but also unstable, horizontally elongated patches. These findings question the conventional simplified approach of using a single constant friction coefficient for fault stability analysis.

Influence of Structural Symmetry of Fault Zones on Fluid-Induced Fault Slips and Earthquakes

Subsurface fluid injection and extraction can reactivate faults and induce earthquakes. In current research, faults are typically described as symmetrical structures and the presence of asymmetric structures is often overlooked. The reality is that numerous asymmetric faults exist within the Earth’s crust. The architectural and permeability characteristics of fault zones differ significantly between symmetrical and asymmetrical faults. These differences may have a great influence on fault stability during fluid injection or extraction. In this study, the impact of fault zone structures on fluid-induced slips and seismic activity were investigated through numerical analysis. The findings indicated that symmetrical faults were more likely to induce larger slips and earthquakes during various subsurface fluid operations. For asymmetric faults, larger induced slips occurred when fluid was operated in a hanging wall reservoir than in a footwall reservoir. In symmetrical faults, the opposite was true. When evaluating the stability of a fault in subsurface fluid engineering, the fault structure and fluid pattern and their combined effects must be considered comprehensively.

Frictional Stability and Permeability Evolution of 3D Carved Longmaxi Shale Fractures and Its Implications for Shale Fault Stability in Sichuan Basin

Frictional Stability and Permeability Evolution of 3D Carved Longmaxi Shale Fractures and Its Implications for Shale Fault Stability in Sichuan Basin

Thermal Fracture Simulation in Depleted Gas Field Carbon Capture and Storage: Implications for Injectivity and Flow Assurance

Summary CO2 injection into depleted gas fields causes long-term cooling of the reservoir. Therefore, even if injection pressure stays below the fracture initiation pressure, the cooled volume still creates an extensive stress disturbance that can induce the propagation of large fractures over time. The enhanced injectivity after the onset of this thermal fracturing might jeopardize injection operations due to the risk of hydrate plugging in the injection well caused by the combination of low pressure and low temperature, and large fractures may also increase the risk of loss of containment. Modeling the fracture evolution provides an estimate of the magnitude and timing of these effects. In this study, commercial compositional reservoir simulation software capable of modeling the physical phenomena associated with CO2 injection into depleted natural gas reservoirs has been used. These encompass CO2 mixing with natural gas, water vaporization, thermal effects, and geomechanics. The finite-element geomechanics module used “two-way” coupling, which computes pressure and temperature in the flow simulation module, transmits this information to the geomechanics module to update stress and strain parameters, and uses these parameters to adjust porosity and permeability, thereby enhancing the accuracy and reliability of the overall simulation results. The thermal fracture opening is simulated as increased permeability in the fracture domain by using a fracturing criterion based on the effective stress. The fracture simulations were developed in close relation with flow assurance modeling to determine the operational windows that avoid hydrate formation while maintaining the required injection target. Unlike matrix injection, thermal fracturing shows a substantial reduction in injection bottomhole pressure (BHP), reaching 26 000 kPa (260 bar) in a specific scenario. These findings underscore the crucial consideration of cooling effects and thermal fracturing in carbon capture and storage (CCS) operations, particularly in flow assurance studies where well injectivity significantly influences overall outcomes. Due to the intense cooling-induced stress reduction, thermal fractures may propagate uncontrollably, potentially reaching faults within the reservoir. Temperature distributions along boundary faults may differ markedly from matrix flow conditions, highlighting the need to incorporate these effects into geomechanical studies to mitigate risks associated with fault stability during cooling processes.

Path and Slip Dependent Behavior of Shallow Subduction Shear Zones During Fluid Overpressure

AbstractElevated pore fluid pressure is proposed to contribute to slow earthquakes along shallow subduction plate boundaries. However, the processes that create high fluid pressure, disequilibrium compaction and dehydration reactions, lead to different effective stress paths in fault rocks. These paths are predicted by granular mechanics frameworks to lead to different strengths and deformation modes, yet granular mechanics do not predict their effects on fault stability. To evaluate the role of fluid overpressure on shallow megathrust strength and slip behavior, we conducted triaxial shear experiments on chlorite and celadonite rich gouge layers. Experiments were conducted at constant temperature (130 and 100°C), confining pressure (130 and 140 MPa), and pore fluid pressures (between 10 and 120 MPa). Fluid overpressure due to disequilibrium compaction was simulated by increasing confining and pore fluid pressure synchronously without exceeding the target effective pressure, whereas overpressure due to dehydration reactions was simulated by first loading the sample to a target isotropic effective pressure and then increasing pore fluid pressure to reduce the effective pressure. We find that the effects of fluid pressure and stress path on the mechanical behavior of the chlorite and celadonite gouges can generally be described using the critical state soil mechanics (CSSM) framework. However, path effects are more pronounced and persist to greater displacements in chlorite because its microstructure is more influenced by stress path. Due to its effects on microstructure, the stress path also imparts greater control on the rate‐dependence of chlorite strength, which is not predicted by CSSM.

The different effects of polished and post-slip roughnesses on fault stability

Linking fault morphology to its mechanical properties in laboratory experiments may shed some light on analyzing the seismogenic potential of a natural fault. Many previous experiments suggested that increasing the roughness of a polished fault tends to stabilize the fault. Furthermore, the roughness of faults, both experimental and natural, spontaneously evolves during slip. Here, we investigate the effects of the polished roughness and the post-slip roughness on experimental fault stability. The experiments were conducted on a granodiorite fault under initial normal stresses of 5–6 MPa and total slip distances of 1.5–2.5 mm. The results reveal that the polished fault with a higher roughness exhibits stick-slip, while the post-slip fault with a lower roughness exhibits stable slip. Moreover, a decrease in the roughness of a post-slip fault tends to stabilize the fault. The monitored macroscopic loading data indicate that the critical slip distance is significantly greater in a post-slip fault than in a polished fault. These results imply that the critical slip distance of young, rough faults may evolve dramatically with slip, subsequently affecting the fault stability.

Chemical compositions of Longmaxi shales and implications for fault stability during the extraction and storage of fuels and energy

Shales of the Longmaxi formation are currently the most important target zone for gas recovery in the Sichuan Basin, southwest China. Considering the frequent occurrence of hydraulic fracturing implicated induced earthquakes, it is important to understand the mineralogical controls on fault stability – across this transitional basin. We recover a full stratigraphic sequence of the Longmaxi and complete XRF and TOC analyses on these shales. Major components include SiO2, CaO, Fe2O3, Al2O3, K2O, MgO, TiO2 and TOC, and a transition from Ca-to Si-dominant contents from the top to the base of the stratigraphic column. This variation in chemical composition reflects a change in sea level during deposition with both biogenic and hydrothermal sediments contributing to the sedimentary record. Chemical compositions are closely related to shale fault stability with high tectosilicate and/or carbonate contents promoting the potential for both instability and thus triggered earthquakes. The highest Si- and Ca-contents in sub-section 1-1 (S1l1-1) and section 2 (S1l2) of the Longmaxi contribute the highest contents of tectosilicates and carbonates, with these minerals potentially enabling and promoting fault instability during hydraulic fracturing. Our results have important implications for understanding the relationship of shale composition and fault stability during hydraulic fracturing for shale gas exploitation.

Stress disturbance around Xianshuihe fault zone in the eastern Qinghai–Tibet Plateau and implication for fault stability

The Xianshuihe fault zone in the eastern Qinghai–Tibetan Plateau is an important active tectonic boundary. Understanding its stress state is important for characterizing the dynamic evolution of the Qinghai–Tibet Plateau and the mechanism of the frequent occurrence of large earthquakes. Using 30 years of in-situ stress data from the Xianshuihe active fault zone, we statistically analyzed the spatial distribution characteristics of the stress in the region. The study area is generally characterized by a strike-slip stress field. Nevertheless, the stress state is vulnerable to topography and shows high spatial variation near the Earth’s surface at a depth of 0–400 m. The local stress near the fault zone varies from the far-field stress. The orientations of the maximum horizontal principal stress possess an elliptical shape around the fault zone, while its magnitudes become hump-like as the distance increases from the fault. The large difference in properties between the fault zone and its adjacent rocks contributes to the differentiation of the direction of the local stress field near the fault. The results allow us to formulate a preliminary hypothesis that a rigid lateral extrusion model may control the nonuniformity of the local stress field in the Xianshuihe fault zone and preferentially interpret the tectonic uplift of the southeastern margin of the Qinghai–Tibet Plateau. Further, the stress accumulation in the shallow crustal regions of the Xianshuihe fault zone is relatively high, indicating that some segments of the fault zone are critically unstable. Kangding area (the Zheduotang segment and the Yalahe segment) and Luhuo segment hold relatively high potential for large earthquakes. The results of this study are of great significance for revealing the mechanism of fault–stress field interactions and for understanding the dynamic evolution mechanism of the uplift of the Qinghai–Tibet Plateau.