Normal Faulting Stress Regime Research Articles

A preliminary study of the present-day in-situ stress state in the Ahe tight gas reservoir, Dibei Gasfield, Kuqa Depression

Knowledge of the present-day in-situ stress state has significant applications in the exploration and development of tight gas reservoirs. The Ahe Formation is an important tight gas reservoir in the Dibei Gasfield of Kuqa Depression. However, prior to this study, little attention has been paid to the present-day in-situ stress field within the formation. In the present study, the in-situ stress orientation and magnitudes were investigated based on well log calculations and geomechanical modeling. The horizontal maximum principal stress (SHmax) orientation was determined from interpretations of drilling-induced tensile fractures (DITFs) and borehole breakouts in imaging logs, which showed variations between NNW-SSE-trending and NNE-SSW-trending in the Dibei Gasfield. The in-situ stress magnitudes were calculated in four wells based on well logs, the results indicated a normal faulting stress regime within the Ahe tight gas reservoir. Numerical simulation of the present-day in-situ stresses showed that the magnitudes of vertical stress (Sv), SHmax and horizontal minimum principal stress (Shmin) were −105.5 MPa∼-191.0 MPa, −88.9 MPa∼-142.9 MPa, and −79.1 MPa∼-127.7 MPa within the Ahe Formation, respectively. In addition, considering the present-day in-situ stress state in the Ahe Formation of Dibei Gasfield, natural fractures in directions parallel/sub-parallel to the SHmax orientation with high fracture angles showed great contributions to subsurface fluid flow. Borehole instability may become a potentially significant problem when drilling vertical wells and horizontal wells deviated toward the SHmax orientation in the Ahe tight gas reservoir of Dibei Gasfield.

Numerical modelling of coal seam depressurization during coal seam gas production and its effect on the geomechanical stability of faults and coal beds

This coupled geomechanical-fluid flow numerical modelling study investigates the geomechanical impact of depressurization of coal beds on fault and coal seam geomechanical stability during coal seam gas production, using a simplified 3D reservoir structure based on the Gloucester Basin, Eastern Australia. The model examines transient stress, pore pressure, and fluid flow patterns during gas production under three different background regional stress scenarios, with the aim of understanding possible geomechanical failure states in a fault and coal beds. The model results show that depressurization leads to shear stress increase in normal-faulting stress regimes, but decreases in reverse-faulting (dominant in eastern Australia) and strike-slip faulting stress regimes. This is in combination with an overall effective stress increase associated with pore pressure reduction. Depressurization-resultant fault failure appears to be possible only under normal-stress regimes. In addition, a critically-stressed fault segment is likely to be a precursor requirement for fault reactivation to occur as a result of depressurization/fluid depletion within normal-faulting stress conditions. Results further show fluid flow is dominated by lateral flow along coal beds, converging towards depressurization locations (“wells”). Fault permeability governs fluid transport along/across the fault and development of pore pressure compartmentalisation across the fault. Under all three background stress scenarios, flow velocities in coal beds are governed by coal permeability.

The state of stress in SW Iran and implications for hydraulic fracturing of a naturally fractured carbonate reservoir

The first part of this study provides an interpretation of the present-day stress state and mechanical parameters for the giant Bangestan reservoir in SW Iran. A total of 28 drilling induced tensile fractures (DITF) and 172 breakouts were mapped in seven wells in the Bangestan reservoir using image logs. The average maximum principal horizontal stress (SH) direction for each well is given a quality ranking according to the World Stress Map (WSM) criteria. The mean orientation of SH finds to be N44°(±9.95°). A correlation is seen between this data and stress orientations from earthquake focal mechanism solution. This suggests that stresses are related to tectonic forces generated by Arabia–Eurasia collision. Estimates of vertical principal stress (Sv) gradients vary between 23.3 and 24.7 kPa/m (1.03–1.09 psi/ft), based on empirical methods and density log data, respectively. Minimum horizontal stress (Sh) gradient estimates vary from 15.2 to 17.4 kPa/m (0.67–0.77 psi/ft) using extended leak-off test (XLOT) and step-rate test results, respectively. Uncertainties remain in estimating SH from acid fracturing in the deep naturally fractured carbonate reservoirs, even though numerous papers have been written on the subject. Herein, an approach called Deformation/Diffusion/Thermal (DDT) is suggested to estimate SH in the Bangestan Reservoir. The method combines poroelastic theories for the circular underground cavities, bilinear theory for fluid flow through the preexisting induced fracture and thermal stress. Using this method, SH gradient is calculated to be 18.9 kPa/m (0.83 psi/ft). For comparison, horizontal stress magnitudes and mechanical parameters were computed from sonic wave velocity using poroelastic-tectonic stress equations over the full reservoir layer thickness. All data have been calibrated with field tests and lab results. The measured SH was in good agreement with the previous analytical solution. The study suggests that the contemporary state of stress in the reservoir is characterized by a normal faulting (NF) stress regime. Next, current and prospective hydraulic fracturing operations in the field are discussed. Based on the NF stress regime, a vertical plane fracture is expected and the wells drilled in the direction of SH are likely to intersect the most natural fractures.

New constraints on the neotectonic stress pattern of the Flinders and Mount Lofty Ranges, South Australia

The majority of published in-situ stress information in the Australian continent is confined to petroleum provinces where industry technologies facilitate the capture of contemporary crustal stress information. In contrast, the stress pattern of non-petroleum regions such as the Flinders and Mount Lofty Ranges in South Australia, where intraplate deformation is localised, has not been investigated comprehensively so far. The ongoing activities of the mining industry in South Australia has enabled us to access recently drilled boreholes for image logging techniques that have typically been under-utilised by the mineral sector. Herein, we conduct stress analysis by analysing borehole image logs from 16 boreholes in the basement rocks of South Australia as well as two geothermal wells and one coal seam gas well in South Australia’s northern Flinders Ranges, the Gawler Craton and the Eyre Peninsula. The resulting dataset of stress orientations is further accomplished by including recent seismological observations, which provide crustal stress information derived from focal mechanism solutions. The results of this study suggest a regional E–W orientation of the maximum horizontal compressive stress that is consistent with numerous observed neotectonic structures in this region. The focal mechanism solutions in this study suggest that the majority of events occur in a thrust faulting stress regime, which is consistent with the observed Quaternary fault scarps. However, our data compilation also indicates the presence of strike-slip and normal faulting stress regimes in the region, which has not been suggested extensively before this study.

Tectonic implications of the 2017 Ayvacık (Çanakkale) earthquakes, Biga Peninsula, NW Turkey

The west to southwestward motion of the Anatolian block results from the relative motions between the Eurasian, Arabian and African plates along the right-lateral North Anatolian Fault Zone in the north and left-lateral East Anatolian Fault Zone in the east. The Biga Peninsula is tectonically influenced by the Anatolian motion originating along the North Anatolian Fault Zone which splits into two main (northern and southern) branches in the east of Marmara region: the southern branch extends towards the Biga Peninsula which is characterized by strike-slip to oblique normal faulting stress regime in the central to northern part. The southernmost part of peninsula is characterized by a normal to oblique faulting stress regime. The analysis of both seismological and structural field data confirms the change of stress regime from strike-slip character in the center and north to normal faulting character in the south of peninsula where the earthquake swarm recently occurred. The earthquakes began on 14 January 2017 (Mw: 4.4) on Tuzla Fault and migrated southward along the Kocaköy and Babakale's stepped-normal faults of over three months. The inversion of focal mechanisms yields a normal faulting stress regime with an approximately N-S (N4°E) σ3 axis. The inversion of earthquakes occurring in central and northern Biga Peninsula and the north Aegean region gives a strike-slip stress regime with approximately WNW-ESE (N85°W) σ1 and NNE-SSW (N17°E) σ3 axis. The strike-slip stress regime is attributed to westward Anatolian motion, while the normal faulting stress regime is attributed to both the extrusion of Anatolian block and the slab-pull force of the subducting African plate along the Hellenic arc.

Seismological and structural constraints on the 2011–2013, Mmax 4.6 seismic sequence at the south-eastern edge of the Calabrian arc (North-eastern Sicily, Italy)

Between June 2011 and September 2013, the Nebrodi Mountains region was affected by a seismic swarm consisting of >2700 events with local magnitude 1.3≤ML≤4.6 and located in the 5–9km depth interval. The seismic swarm defines a seismogenetic volume elongated along the E-W direction and encompasses the NW-SE-oriented tectonic boundary between the Calabrian arc (north-eastward) and the Sicilide units (south-westward). By exploring the recent tectonic deformation and the seismic behavior of the region, this study aims at providing additional constraints on the seismogenetic faults at the southern termination of the Calabrian arc. Waveform similarities analysis allowed observing that ~45% of the whole dataset can be grouped into six different families of seismic events. Earthquake multiplet families are mainly located in the eastern part of the seismogenetic volume. We suggest that such a feature is responsive to the lateral lithological variations as highlighted by geology (at the surface) and P-wave seismic tomography (at depth of 10km). Stress tensor inversions performed on FPSs indicate that the investigated region is currently subject to a nearly biaxial stress state in an extensional regime, such that crustal stretching occurs along both NW-SE and NE-SW directions. Accordingly, mesoscale fault geometries and kinematics analyses evidence that a younger normal faulting stress regime led to a tectonic negative inversion by replacing the pre-existing strike-slip one. Based on our results and findings reported in recent literature, we refer such a crustal stretching to mantle upwelling process (as evidenced by diffuse mantle-derived gas emissions) coupled with a tectonic uplift involving north-eastern Sicily since Middle Pleistocene. Moreover, seismic swarms striking the region would be related to the migration of mantle and sub-crustal fluids toward the surface along the complex network of tectonic structures cutting the crust and acting as pathways.

In-situ stress distribution and coalbed methane reservoir permeability in the Linxing area, eastern Ordos Basin, China

Understanding the distribution of in-situ stresses is extremely important in a wide range of fields such as oil and gas exploration and development, CO2 sequestration, borehole stability, and stress-related geohazards assessment. In the present study, the in-situ stress distribution in the Linxing area of eastern Ordos Basin, China, was analyzed based on well tested parameters. The maximum horizontal principal stress (SHmax), minimum horizontal principal stress (Shmin), and vertical stress (Sv) were calculated, and they were linearly correlated with burial depth. In general, two types of in-situ stress fields were determined in the Linxing area: (i) the in-situ stress state followed the relation Sv>SHmax>Shmin in shallow layers with burial depths of less than about 940 m, indicating a normal faulting stress regime; (ii) the SHmax magnitude increased conspicuously and was greater than the Sv magnitude in deep layers with depths more than about 940 m, and the in-situ stress state followed the relation SHmax>Sv>Shmin, demonstrating a strike-slip faulting stress regime. The horizontal differential stress (SHmax–Shmin) increased with burial depth, indicating that wellbore instability may be a potentially significant problem when drilling deep vertical wells. The lateral stress coefficient ranged from 0.73 to 1.08 with an average of 0.93 in the Linxing area. The coalbed methane (CBM) reservoir permeability was also analyzed. No obvious exponential relationship was found between coal permeability and effective in-situ stress magnitude. Coal permeability was relatively high under a larger effective in-situ stress magnitude. Multiple factors, including fracture development, contribute to the variation of CBM reservoir permeability in the Linxing area of eastern Ordos Basin.

Hydraulic fracturing initiation and propagation in deep inclined open hole for Enhanced Geothermal System

This paper describes the development of a generic geomechanical model for estimating the fracture initiation in open hole section and the overall fracturing propagation during the hydraulic stimulation in Enhanced Geothermal System. General studies on the effects in situ stress and open hole trajectory on hydraulic fracturing indicate that 1) the upward growth of vertical fracture is expected for normal faulting and strike slip stress regimes; 2) the fracture initiation at casing shoe section prevails for common stress range at deep formation; 3) an inclined open hole tends to decrease fracture pressure gradient for normal faulting and reverse faulting stress regimes and 4) an open hole with building up trajectory may shift fracture initiation location from casing shoe to well toe by a lower breakdown pressure. The proposed model predicts fracture initiation at well toe location for Jolokia-1 stimulation by NaBr brine.

Effect of faults on stress path evolution during reservoir pressurization

Fluid injection operations, such as CO2 storage and enhanced oil recovery (EOR), imply reservoir pressurization, which changes the effective and total stresses due to poroelastic effects. These stress changes control the geomechanical stability of discontinuities like faults and fractures. Though the effect of these pre-existing discontinuities on stress path is sometimes neglected, the stress state is altered around them. We investigate the effect of a fault on the stress path evolution when pressurizing a reservoir using an in-house hybrid FEM-DEM code called “MDEM”. Simulation results indicate that the stress path is affected by the presence of faults considered to deform elastically, especially in the vicinity of the fault in the reservoir-caprock interfaces. The stress path perturbation is caused by the shear deformation of the fault plane, which is different in the reservoir and the caprock sections. Actually, the magnitude and the extension of the stress path perturbation around a fault become larger for faults with lower shear stiffness. The upper hanging wall and the lower footwall of the fault in the reservoir-caprock interface experience a higher stress path in the horizontal and the vertical directions. Furthermore, the stress paths decrease (negative in the vertical direction) in the upper footwall and the lower hanging wall in the reservoir-caprock interfaces. The fault effect on the stress path increases as the aspect ratio of the reservoir becomes lower. Moreover, the results indicated that both the caprock and the reservoir in the footwall experience a greater change for lower Poisson’s ratio of the caprock.These stress changes are independent of the in situ stress regime as long as the fault deforms elastically. However, the impact of the stress path perturbation on the stability of the reservoir and the caprock is different in a compressional (reverse faulting) and an extensional (normal faulting) stress regimes. The stress state becomes less stable in the vicinity of the fault in the reservoir and in the caprock in a compressional stress regime than in an extensional stress regime. Therefore, a compressional stress regime leads to a less stable situation due to the fault effect on the evolution of the stress path. Overall, the presence of faults alters the stress state around them, which may lead to a stress state that is closer to failure conditions than predicted by models that do not explicitly include faults.

Stress state measured at ~7\u2009km depth in the Tarim Basin, NW China

The in-situ stress state in the Tarim Basin, Northwest China, down to 7 km depth is constrained using the anelastic strain recovery (ASR) method and wellbore failure analysis. Results are consistent between the two methods, and indicate that the maximum principal stresses (σ1) are close to vertical and the intermediate and minimum principal stresses (σ2 and σ3) are approximately horizontal. The states of stress at the studied wellbore is in the normal faulting stress regime within the Tarim Basin rather than in the compressional tectonic stress regime as in the periphery of the Tarim Basin, which explains the presence of the normal faults interpreted in 3-D seismic profiles collected from adjacent areas. Our results demonstrate that the ASR method can be used for rocks recovered from depths as deep as 7 km to recover reliable stress state information. The in-situ stress measurement results revealed in this paper will help future development of the petroleum resources and kinematics study in the Tarim Basin.

3D geomechanical modeling and numerical simulation of in-situ stress fields in shale reservoirs: A case study of the lower Cambrian Niutitang formation in the Cen'gong block, South China

An analysis of the in-situ state of stress in a shale reservoir was performed based on comprehensive information about the subsurface properties from wellbores established during the development of an oil and gas field. Industrial-level shale gas production has occurred in the Niutitang formation of the lower Cambrian Cen'gong block, South China. In this study, data obtained from hydraulic fracturing, drilling-induced fractures, borehole breakout, global positioning system (GPS), and well deviation statistics have been used to determine the orientation of the maximum horizontal principal stress. Additionally, hydraulic fracturing and multi-pole array acoustic logging (XMAC) were used to determine the vertical variations in the in-situ stress magnitude. Based on logging interpretation and mechanical experiments, the spatial distributions of mechanical parameters were obtained by seismic inversion, and a 3D heterogeneous geomechanical model was established using a finite element stress analysis approach to simulate the in-situ stress fields. The effects of depth, faults, rock mechanics, and layer variations on the principal stresses, horizontal stress difference (Δσ), horizontal stress difference coefficient (Kh), and stress type coefficient (Sp) were determined. The results show that the direction of the maximum principal stress is ESE 120°. Additionally, the development zones of natural fractures appear to correlate with regions with high principal stress differences. At depths shallower than 375m, the stress type is mainly a thrust faulting stress regime. At depths ranging from 375 to 950m, the stress type is mainly a strike-slip faulting stress regime. When the depth is >950m, the stress type is mainly a normal faulting stress regime. Depth, fault orientation, and rock mechanics all affect the type of stress. The knowledge regarding the Cen'gong block is reliable and can improve borehole stability, casing set point determination, well deployment optimization, and fracturing area selection.

Geology and in situ stress of the MH-2 borehole, Idaho, USA: Insights into western Snake River Plain structure from geothermal exploration drilling

Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°. The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime. The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.

Pore pressure and stress regime in a thick extensional basin with active shale diapirism (western Mediterranean)

The Alboran Sea in the Mediterranean is a back-arc basin developed during the Miocene by extensional collapse within an arc-shaped orogen. A major depocenter (>10 km [>6.2 mi]) is located to the west of the basin (West Alboran Basin [WAB]) and contains a diapiric province with overpressured shales and mud volcanoes. Seismic and well data are used to analyze the evolution of the shale structures in the northern margin of the WAB and to estimate the in situ stress tensor. Geomechanical modeling suggests a present-day normal faulting stress regime along the northern WAB, where the maximum horizontal stress is parallel to the coastline. Pore pressure shows a hydrostatic gradient down to 2000 m (6561 ft), where the top of the regional pore pressure ramp is located. Undercompaction is the dominant mechanism generating overpressures in sediments shallower than 5000 m (16,406 ft). At greater depths, thermal mechanisms impose an excess of pore pressure on the sediments feeding the diapirs. This framework is used to discuss the contribution of thermally generated pressures to the triggering of shale diapirism. Increasing thermal pressures in the deepest confined units cause tensile failure of the overburden and subsequently promote mud withdrawal and injection in the overburden. The magnitude of the overpressure conditions the vertical ascent of shale. The more mature structures reuse preexisting normal faults in their ascent toward shallower basin levels. Results provide insights into the current discussion about the triggering factors behind shale diapirism. They also help to explain the differences between shale structures and those shaped by salt tectonics.

State of stress in exhumed basins and implications for fluid flow: insights from the Illizi Basin, Algeria

Abstract The petroleum prospectivity of an exhumed basin is largely dependent on the ability of pre-existing traps to retain oil and gas volumes during and after the exhumation event. Although faults may act as lateral seals in petroleum traps, they may start to become hydraulically conductive again and enable fluid flow and hydrocarbon leakage during fault reactivation. We constrain the present day in situ stresses of the exhumed Illizi Basin in Algeria and demonstrate that the primary north–south and NW–SE (vertical strike-slip) fault systems in the study area are close to critical stress (i.e. an incipient state of shear failure). By contrast, the overpressured and unexhumed Berkine Basin and Hassi Messaoud areas to the north do not appear to be characterized by critical stress conditions. We present conceptual models of stress evolution and demonstrate that a sedimentary basin with benign in situ stresses at maximum burial may change to being characterized by critical stress conditions on existing fault systems during exhumation. These models are supportive of the idea that the breaching of a closed, overpressured system during exhumation of the Illizi Basin may have been a driving mechanism for the regional updip flow of high-salinity formation water within the Ordovician reservoirs during Eocene–Miocene time. This work also has implications for petroleum exploration in exhumed basins. Fault-bounded traps with faults oriented at a high angle to the maximum principal horizontal stress direction in strike-slip or normal faulting stress regimes are more likely to have retained hydrocarbons in exhumed basins than fault-bounded traps with faults that are more optimally oriented for shear failure and therefore have a greater propensity to become critically stressed during exhumation.

Grain Deformation Processes in Porous Quartz Sandstones – Insight from the Clusters of Cataclastic Deformation Bands

Porous sandstones represent classical reservoirs for water or hydrocarbons. Deformation in such granular material is due to tectonic events and occurs through the process of cataclasis, implying the crushing of the grains to a diversity of smaller clasts. Cataclasis is generally accompanied by porosity and permeability decrease. Although it is known that cataclastic deformation localizes to form individual bands and clusters of bands, the parameters controlling the distribution of this deformation were not well understood until recently. We used scanline measurements to show a favoured localization and clustering of the deformation on the case of normal-fault stress regime and potentially in strike-slip fault regime. The reverse regime favours the formation of distributed networks of conjugate deformation bands. At the scale of a cluster, field data reveals that the minimum modal grain size value of the host sandstone(s) controls the band density. Finally, microscopic cathodoluminescence analysis reveals enhanced quartz cementation for high degree of cataclasis. Hence, because band clustering, high degree of cataclasis and band cementation are favoured in normal-fault stress regime, tectonic extension appears to be favourable conditions for the formation of efficient barriers to fluid-flow in porous sandstone reservoirs.

Spatial variations of current tectonic stress field and its relationship to the structure and rheology of lithosphere around the Bohai Sea, North China

The tectonic stress field in the middle-upper crust is closely related to the structure and rheology of the lithosphere. To determine the stress field in the deep crust, we inversed the focal mechanism solutions (FMSs) of 62 earthquakes that occurred between 2009 and 2015 in the Bohai Sea and its surrounding areas using broadband seismic waveforms collected from 140 stations. We then derived the tectonic stress field using the software SATSI (Spatial And Temporal Stress Inversion) based on the damped linear inversion method. The inversion results show that both the maximum (σ1) and minimum (σ3) principle stress axes throughout the entire region are nearly horizontal except in the Tangshan and Haicheng areas, suggesting that the study area is predominantly under a strike-slip faulting stress regime. The σ1 and σ3 axes are found to be oriented in the NEE-SWW or nearly E-W and NNW-SSE or nearly S-N directions, respectively. These results indicate that the stress field in the North China Craton is controlled by the combined effects of the Pacific Plate westward subduction and the India-Eurasia Plate collision. However, localized normal faulting stress regimes (where the vertical stress σv≈σ1) are observed in the Tangshan and Haicheng areas, where low viscosity bodies (LVBs) were identified using geophysical data. Based on the analysis of focal mechanism solutions, active faults and lithosphere rheology characteristics in the Tangshan and Haicheng areas, we speculate that the anomalous stress regime is caused by the local extension resulting from the movement of strike-slip faults under the action of the regional stress field. The existence of LVB may indicate weakness in the crust that favors the accumulation of tectonic stress and triggers large earthquakes.

Present‐day stress states underneath the Kumano basin to 2 km below seafloor based on borehole wall failures at IODP site C0002, Nankai accretionary wedge

AbstractWe constrain the state of stress to 2 km below seafloor in the Nankai accretionary prism at the Integrated Ocean Drilling Program (IODP) site C0002F, southwest Japan, based on borehole wall failures and rock strengths. The logging‐while‐drilling resistivity images from 872.5 to 2005.5 m below seafloor show that drilling‐mud control in riser drilling worked properly to minimize borehole wall failures. Available breakouts indicate a consistent maximum compression orientation subparallel to the subducting plate margin. Breakout analysis with drill logs suggests that breakouts occurred only when borehole pressure was slightly lowered and time lag between hole cutting and image logging was several hours. This indicates that the observed breakouts are not immediate stress‐induced failure but brought up into shape gradually with time due to other mechanisms. Laboratory investigations on deformation and failure of the cores suggest that the time‐delayed breakout might be a result of progressive rock spall‐out in borehole wall damage zones that occur at a stress level close to failure condition. We constrain stress magnitudes assuming that the stress state is sufficient to bring about the damage zones at the borehole wall. An integrated method utilizing breakouts, drilling‐induced tensile fractures, and a leak‐off test suggests that the stress states are on the boundary between strike‐slip faulting and normal faulting stress regimes, and somewhat variable depending on depth. The stress magnitudes in the accretionary wedge appear to be controlled by frictional strength of the rock, such that the differential stresses are constrained by the laboratory determined frictional coefficients.

Late Cenozoic stress state distributions at the intersection of the Hellenic and Cyprus Arcs, SW Turkey

The history of the Late Cenozoic stress regime was determined for an area between the gulfs of Fethiye and Antalya. Fault kinematic analysis and inversion of focal mechanisms of shallow earthquakes reveal significant evolution of the regional stress regime in SW Anatolia, i.e., the area of interaction between the Hellenic and Cyprus arcs, from the Mio-Pliocene to the present time. Fault kinematic analysis yields two different normal faulting stress regimes along the southwestern part of Fethiye-Burdur Fault zone, e.g., in and around Çameli Basin (Zone A1) and two different strike-slip to normal faulting stress regimes characterized by a roughly orthogonal set of extensional axes between Fethiye and Demre (Zone B) with an older NW–SE σ3 axis for Mio-Pliocene and a younger NE–SW σ3 axis for Plio-Quaternary time. Inversion of focal mechanisms of the earthquakes occurring in Zone A1 provides an extensional stress state with approximately N-S σ3 axis. Inversion of those occurring in Zone B, south of Zone A1, yields a dominantly strike-slip stress state with a NE-SW σ3 axis and a NW-SE σ1 axis respectively. The inversion slip vectors from fault planes yield a consistent normal faulting stress regime in Burdur Basin and its surroundings (i.e., along the northeastern part of Fethiye-Burdur Fault Zone, (Zone A2)) during Plio-Quaternary, continuing into recent time as indicated by earthquake focal mechanism inversions. Both states have a consistent NW–SE σ3 axis. Fault kinematic analysis indicates NW-SE extension acting in Zone C (subarea between Demre and Antalya), south of Zone A2, during Mio-Pliocene time. The inversion of focal mechanisms yields normal faulting also characterized by a consistent NW-SE σ3 axis. The nearly orthogonal extensional stress regimes (NW-SE and NE-SW) obtained by inversion of both measured and seismic faults seem to have been acting contemporaneously with each other at different intensities from the Mio-Pliocene onwards in SW Turkey. This may be attributed to the geodynamic effects related to the subduction of the African plate beneath Anatolia diffusing along the Hellenic and Cyprus arcs and in the west-southwestward extrusion of Anatolia. The cause of the early NW–SE extension is the slab-pull force due to the subduction process along the Cyprus arc, considered to be dominant until the Plio-Quaternary in the western part of the study area in zones A1 and B. The dominant status of the Cyprus arc continues today in the eastern part of study area in zones A2 and C. The later NE–SW to present day approximately N–S extension, dominant since the Plio-Quaternary, is related to the combined forces of the Anatolian extrusion and the subduction process along the Hellenic arc.

The role of the stress regime on microseismicity induced by overpressure and cooling in geologic carbon storage

AbstractFluid injection in deep geological formations usually induces microseismicity. In particular, industrial‐scale injection of CO2 may induce a large number of microseismic events. Since CO2 is likely to reach the storage formation at a lower temperature than that corresponding to the geothermal gradient, both overpressure and cooling decrease the effective stresses and may induce microseismicity. Here, we investigate the effect of the stress regime on the effective stress evolution and fracture stability when injecting cold CO2 through a horizontal well in a deep saline formation. Simulation results show that when only overpressure occurs, the vertical total stress remains practically constant, but the horizontal total stresses increase proportionally to overpressure. These hydro‐mechanical stress changes result in a slight improvement in fracture stability in normal faulting stress regimes because the decrease in deviatoric stress offsets the decrease in effective stresses produced by overpressure. However, fracture stability significantly decreases in reverse faulting stress regimes because the size of the Mohr circle increases in addition to being displaced towards failure conditions. Fracture stability also decreases in strike slip stress regimes because the Mohr circle maintains its size and is shifted towards the yield surface a magnitude equal to overpressure minus the increase in the horizontal total stresses. Additionally, cooling induces a thermal stress reduction in all directions, but larger in the out‐of‐plane direction. This stress anisotropy causes, apart from a displacement of the Mohr circle towards the yield surface, an increase in the size of the Mohr circle. These two effects decrease fracture stability, resulting in the strike slip being the least stable stress regime when cooling occurs, followed by the reverse faulting and the normal faulting stress regimes. Thus, characterizing the stress state is crucial to determine the maximum sustainable injection pressure and maximum temperature drop to safely inject CO2.

Characteristics of Focal Mechanisms and the Stress Field in the Southeastern Margin of the Tibetan Plateau

Crustal earthquake focal mechanisms are investigated in the southeastern margin of the Tibetan Plateau, where the Tibetan Plateau and stable South China Block merge. An updated database of focal mechanisms has been compiled by selecting 132 Global Centroid Moment Tensor solutions and by adding the 173 new solutions (3.5 ≤ Ms ≤ 7.4) estimated by waveform inversion in this study. A total of 305 mechanisms are included in this database. These solutions show regionally specific distributions with dominant strike-slip faulting and some normal and reverse faulting. Focal mechanism solutions have also been inverted for the stress tensor orientation to obtain the principal stress axes over the study region. Results show that the horizontal maximum principal σ 1 axes rotate clockwise with a wider range than the geodetically measured surface motion in the east, which is not limited to the Xianshuihe–Xiaojiang fault, but has some overlap with the Zhaotong–Lianfeng fault. Localized normal faulting stress regimes are observed in the Jinshajiang–Litang fault areas and the Baoshan sub-block. The minimum principal axes are oriented with a gradually changing trend from north–south to northwest–southeast, from north to south, indicating diverse compression stress patterns. Significant changes in the crustal stress field after the Wenchuan earthquake are preliminarily observed in the Baoshan sub-block where orientations of two principal axes have changed, and in the Jinggu–Ximeng sub-block areas where the strike-slip faulting stress pattern has transformed to normal faulting.