Hydrostatic Pore Pressure Research Articles

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.

The slip tendency of 3D faults in Germany

Abstract. Fault reactivation potential is a crucial aspect for many underground utilizations, such as the construction and long-term safety of a nuclear waste repository, as seismic events can endanger these operations. An estimation of the fault reactivation potential requires information about the stress field, but stress data are only available pointwise and are not evenly distributed throughout Germany. Geomechanical–numerical modeling can be used to derive a spatially continuous description of all six independent components of the stress tensor as shown by the model of Germany by Ahlers et al. (2022). Information about the geometry of faults extending several kilometers in depth is provided for most areas in Germany by the geological models of the federal states and geological models created in the framework of projects such as GeoMol (Assessing subsurface potentials of the Alpine Foreland Basins for sustainable planning and use of natural resources) or GeORG (Geopotenziale des tieferen Untergrundes im Oberrheingraben). We use the 3D fault geometries provided by such models and map the stress data from the Germany model by Ahlers et al. (2022) onto these faults. Then, assuming hydrostatic pore pressure, we calculate the so-called slip tendency (TS), the ratio between resolved shear stress and the effective normal stress on the fault plane as a measure of fault reactivation potential. A fault is considered critical when its TS value exceeds its coefficient of friction. In general, TS ranges between 0 and 0.7 for the analyzed faults. The highest overall TS values are observed along the NNE–SSW-striking Upper Rhine Graben, where TS routinely reaches and exceeds values of 0.7. In the North German Basin, the Ore Mountains and Saxony only very few TS values exceed 0.7. The area with the lowest overall TS is the Molasse Basin, where the TS of the mostly WSW–ENE-striking faults only rarely exceeds values of 0.4. In general, N–S- to NNE–SSW- and NW–SE-striking faults show the highest TS values, whereas WSW–ENE-striking faults show the overall lowest values. With increasing depth, TS decreases. Pore pressure and overpressure have the potential to significantly influence the resulting TS.

Numerical Simulation of Pore Pressure Change Caused by Hydrocarbon Generation in Chezhen Sag and Its Influence on Hydrocarbon Accumulation

The pore fluid pressure is important for the generation, migration, and accumulation of hydrocarbons. The Chezhen Sag region in the Bohai Bay Basin is typically characterized by pore fluid overpressure, which is the difference between the pore fluid pressure and the hydrostatic pore pressure. The formation mechanisms of pore overpressure and the accumulation regularity of the “upper source-lower reservoir” type in this region remain unknown. In order to investigate these problems, based on the existing seismic, logging data, and regional tectonic stress environment, we established a two-dimensional finite element model to simulate the fluid–solid coupling processes in the Chegu 25 block of the Chezhen depression. We calculated the abnormal overpressure generated at the source rock during hydrocarbon generation and the processes of hydrocarbon migration and accumulation along the faults and analyzed the dynamic conditions of the hydrocarbon downward accumulation. The results showed that overpressure could accelerate the migration of hydrocarbon and improve the efficiency of hydrocarbon accumulation. When the overpressure was too large, tensile fractures and shear fractures could occur, resulting in hydrocarbon dissipation, and changing the results of the oil and gas accumulation. The overpressure at the source rock was mainly caused by hydrocarbon generation, while the overpressure at the reservoir was primarily created by unbalanced compaction. As the dominant channel of hydrocarbon migration that exists, overpressure will change the direction and path of hydrocarbon migration in the fault. Therefore, the high permeability of the fault and the existence of pore fluid overpressure can explain the “upper source-lower reservoir” hydrocarbon accumulation model strongly explained the high permeability of faults and the presence of overpressure. The simulated overpressure results were also in good agreement with the mud weight equivalent overpressure and the drill stem tests (DSTs).

Effectiveness of different conditioning procedures for moisture susceptibility evaluation of foamed bitumen stabilised mixes

ABSTRACT The primary objective of the present study is to evaluate the effectiveness of the widely used moisture conditioning methods for moisture susceptibility evaluation while designing the Foamed Bitumen Stabilised Mixes (FBMs). The effectiveness of three different moisture conditioning methods, such as the conventional 24 h of water soaking, vacuum saturation (AASHTO T283), and ASTM D7870 conditioning (hydrostatic pore pressure) were compared based on their ability to distinguish between moisture susceptible and moisture damage resistant FBMs. It was observed that the ASTM D7870 moisture conditioning method could distinguish between moisture-resistant and moisture-susceptible mixes more effectively compared to the existing 24 h of water-soaking method, and can potentially be a more suitable conditioning method for moisture susceptibility evaluation of FBMs. The study also recommended threshold moisture susceptibility criteria in terms of retained tensile strength (TSR) for the ASTM D7870 conditioning method based on the existing moisture susceptibility criteria for 24 h of water soaking by performing Receiver Operation Characteristics (ROC) analysis on the TSR results. For different TSR criteria of 80%, 70%, and 60% for 24 h of water soaking, the equivalent TSR threshold for ASTM D7870 conditioning is recommended as 60%, 55%, and 50% respectively.

In Situ Stress Determination Based on Acoustic Image Logs and Borehole Measurements in the In-Adaoui and Bourarhat Hydrocarbon Fields, Eastern Algeria

The study of in situ stress from image logs is a key factor for understanding regional stresses and the exploitation of hydrocarbon resources. This work presents a comprehensive geomechanical analysis of two eastern Algerian hydrocarbon fields to infer the magnitudes of principal stress components and stress field orientation. Acoustic image logs and borehole measurements were used in this research to aid our understanding of regional stress and field development. The studied In-Adaoui and Bourarhat fields encompass a combined thickness of 3050 m of Paleozoic and Mesozoic stratigraphy, with the primary reservoir facies in the Ordovician interval. The Ordovician sandstone reservoir interval indicates an average Poisson’s ratio (v) of 0.3, 100–150 MPa UCS, and 27–52 GPa Young’s modulus (E). Direct formation pressure measurements indicate that the sandstone reservoir is in a hydrostatic pore pressure regime. Density-derived vertical stress had a 1.1 PSI/feet gradient. Minimum horizontal stress modeled from both Poisson’s ratio and an effective stress ratio-based approach yielded an average 0.82 PSI/feet gradient, as validated with the leak-off test data. Drilling-induced tensile fractures (DITF) and compressive failures, i.e., breakouts (BO), were identified from acoustic image logs. On the basis of the DITF criterion, the maximum horizontal stress gradient was found to be 1.57–1.71 PSI/feet, while the BO width-derived gradient was 1.27–1.37 PSI/feet. Relative stress magnitudes indicate a strike-slip stress regime. A mean SHMax orientation of N130°E (NW-SE) was interpreted from the wellbore failures, classified as B-quality stress indicators following the World Stress Map (WSM) ranking scheme. The inferred stress magnitude and orientation were in agreement with the regional trend of the western Mediterranean region and provide a basis for field development and hydraulic fracturing in the low-permeable reservoir. On the basis of the geomechanical assessments, drilling and reservoir development strategies are discussed, and optimization opportunities are identified.

ENGINEERING AND GEOLOGICAL STUDIES OF THE TENGIZ DEPOSIT

In this article, a study of geological and ecological loads on the geological environment at the Tengizfield, located in the Zhyloy district of Atyrau region of the Republic of Kazakhtan, is carried out. Soil tests by dynamic sounding (STDS) were performed on all drilling wells for sandy soils. In the process of performing engineering and geological exploration, static sounding of soils was performed at the projected sites of GDP-1 (gas distribution plant) and Torch, with the measurement of hydrostatic pore pressure. Physical and mechanical properties of soils, chemical properties of soils and chemical properties of groundwater were determined during laboratory studies. In the studied territory, the soils composing the engineering-geological section to a depth of up to 25 m are saline with an average degree of salinity. All lithological-facies groups of soils also contain a significant amount of carbonates and gypsum in their composition. For loamy silt (IGE-1b), according to the combination of physico-chemical and mechanical characteristics (fluid consistency, high degree of salinity, high degree of compressibility and low strength, the possibility of manifestation of thixotropic properties under dynamic influences), this soil belongs to the category of weak water-saturated clay soils. Weak soil (IGE-1b) is observed at a shallow depth in some areas. The available geotechnical data is not enough to accommodate areas of weak soils. At the same time, heavy sandy loam (IGE1a) and light sandy clay (IGE-8) have swelling properties of a weak degree. Chemical analysis of groundwater samples taken within the studied territories showed a high degree of mineralization, groundwater belongs to the brackish group, a subgroup of strongly brackish waters. Dangerous geological processes that could negatively affect the construction conditions are characterized by the processes of secondary salinization of soils.

Evaluating propensity for fugitive gas migration from integrity compromised oil and gas wells in the Peace Region, British Columbia, Canada, with a petrophysical approach

Oil and gas wells can suffer integrity failure resulting in the release of fugitive gas, formed primarily of methane, into the subsurface and atmosphere. Factors affecting fugitive gas migration in the subsurface, and in particular within and between shallower units intersected by energy wells (i.e. <100 m depth), are usually poorly considered with little focus on specific lithological characteristics that control gas movement, determine environmental impacts and influence how best to detect, monitor and quantify fugitive gases. To better understand and identify properties that can control fugitive gas migration in shallow sedimentary rocks that form the Peace Region of British Columbia, Canada, we developed a workflow by which a series of rock cores from units present in the area were obtained and their flow properties characterised. Insights on propensity for fugitive gas migration within and between these rocks were achieved through laboratory techniques and analytical calculations that are usually applied to the study of deeper hydrocarbon migration and seal integrity. These included Mercury Intrusion Porosimetry, petrographic analysis, X-ray computed tomography, as well as estimations of threshold capillary pressure, gas-column heights, hydrostatic pore pressure and total displacement pressure. Results show a congruent correlation between the examined petrophysical rock characteristics and flow propensity. We find that high capillary pressure will act as an effective mechanism for limiting vertical gas migration in the region, even when potential barrier facies have porosities of more than 10% and permeability of 0.26 mD. Additionally, results suggest that the propensity of studied shallow sediments to transmit fugitive gases will be controlled by heterogeneity, permeability anisotropy and the presence of secondary structural features, such as microfractures. Our approach for evaluating the propensity of fugitive gas migration could be applied elsewhere to enhance understanding of potential migration pathways and aid in development of effective monitoring and detection strategies that can better anticipate gas behaviour in the subsurface and prognosticate reactive transport, impacts and fate.

Geomechanical assessment of the Lower Turonian AR-F limestone Member, Abu Gharadig Field, Egypt: Implications for unconventional resource development

This study evaluates the unconventional reservoir geomechanical characteristics of the Lower Turonian Abu Roash-F (AR-F) carbonates from the Abu Gharadig field, onshore Egypt, which has not been attempted before. The interval dominantly consists of planktic foraminifera and micrite matrix. The AR-F marine carbonate is organic-rich (0.59–3.57 wt% total organic carbon), thermally mature (435–441°C Tmax) and falls within the oil generation window. The studied interval is very tight with up to 2.6% porosity and 0.0016–0.0033 mD permeability with the wireline log-based brittleness index ranging between 0.39–0.72 which indicates a less brittle to brittle nature. AR-F exhibits a hydrostatic pore pressure gradient with minimum horizontal stress (Shmin) varying between 0.66–0.76 PSI/ft. Safe wellbore trajectory analysis was performed for deviated and horizontal wells to infer the mud pressure gradients required to avoid wellbore instabilities. Based on the inferred in-stress magnitudes and considering an NNE regional maximum horizontal stress orientation, none of the fractures are found to be critically stressed at present day. To produce from the AR-F, hydraulic fracturing is necessary, and we infer a minimum pore pressure increment threshold of 1390 PSI by fluid injection to reactivate the vertical fractures parallel to regional minimum horizontal stress azimuth.

Measurement of Physical Parameters for Transient Seepage Assessment of Levees

Abstract An embankment was instrumented and monitored as part of a multiyear project to assess appropriate pore-water pressure distributions and soil water retention curves for use as initial conditions in transient seepage analyses. The increased use of unsaturated soil mechanics and transient seepage has led to the need for practical methods of assigning accurate properties and initial conditions in these types of analyses. Monitoring results were validated against published pore-water pressure measurements collected from multiple embankment studies. The collected distributions showed nonlinear behavior in near-surface embankment soils due to infiltration and evapotranspiration processes. Seasonal variations in pore-water pressure and the soil moisture deficit were shown to correlate at the monitoring site. The seasonal response of the embankments exhibited a large variation in pressures between summer and winter. Assessment of flood occurrence showed the distribution of pore pressures during flood loading was typically near −10 kPa at a depth of 2 m at the onset of the flood. Laboratory soil water retention testing of undisturbed samples collected from the embankment showed that when compared with field measurements, they overestimated the field response for the considered initial water content and dry density values. Also, comparisons between hydrostatic pore pressures above the phreatic surface showed that at soil moisture deficits greater than 20 mm, the difference between the measured and hydrostatic pore pressures greatly increased. The laboratory and monitoring results were used to provide practical methods of assigning representative pore-water pressure distributions and material properties necessary for performing transient seepage analyses. These results will aid in more accurate transient seepage analyses of flood control embankments.

A Novel Permeability Model of Coal Considering Gas Slippage and Gas Sorption-Induced Strain

As an unconventional natural gas, coalbed methane (CBM) has been recognized as a significant fuel and chemical feedstock that should be recovered. Permeability is a key factor that controls CBM transport in coal. The slippage effect is an influential phenomenon that occurs during gas penetration processes, especially in low-permeable media. Apparent permeability may differ greatly from intrinsic permeability due to gas slippage. However, the gas slippage effect has not been considered in most analytical permeability models. Based on the cubic law, a new analytical model suited for the permeability analysis of coal under different stress conditions is derived, taking into consideration gas slippage and matrix shrinkage/swelling due to gas desorption/adsorption. To enhance its application, the model is derived under constant hydrostatic stress and pore pressure. The new analytical model is then compared with the existing models, and its reliability is verified by the comparison between the analytical prediction and the experimental permeability data under different stress conditions.

Rock-mechanical properties and in situ stress state of the Upper Cenomanian Mishrif limestone reservoir, Zubair oil field, southern Iraq

Rock-mechanical properties and tectonic stress state have crucial implications for reservoir stability, drilling, and production optimization. We have developed the first ever in situ stress analysis focused on the Mishrif reservoir from the supergiant Zubair oil field, southern Iraq. We interpreted an overall hydrostatic pore pressure gradient of 9.8 MPa/km and a vertical stress ([Formula: see text]) gradient of 22.17 MPa/km in the Upper Cenomanian Mishrif limestone formation. We inferred a Poisson’s ratio of 0.27–0.31, a Young’s modulus of 27–38 MPa, a coefficient of internal friction range of 0.66–0.76, and estimated a uniaxial compressive strength (UCS) range of 35–70 MPa with a low cohesive strength of 7.8–9.8 MPa from compressional sonic slowness data. We established fitting relationships between the dynamic and static elastic properties from the core-based triaxial test measurements and used the same for estimating the minimum ([Formula: see text]) and maximum ([Formula: see text]) horizontal stress magnitudes by poroelastic horizontal strain model. We validated the reservoir [Formula: see text] with the mini-frac-based fracture closure pressure and interpreted its gradient as 14.5–15.9 MPa/km, whereas we calculated the [Formula: see text] gradient as 19.5–23 MPa/km. We interpreted C-quality breakouts from four-arm caliper data and deciphered a northeast–southwest [Formula: see text] orientation (N45°E–N70°E). Using the compressive failure criteria, we constrained the reservoir [Formula: see text] by stress polygon analysis against the estimated UCS ranges and concluded a normal fault to strike-slip transitional tectonic stress state ([Formula: see text] ≥ [Formula: see text] &gt; [Formula: see text]) in the Mishrif reservoir.

Estimation of horizontal stresses from wellbore failures in strike-slip tectonic regime: A case study from the Ordovician reservoir of the Tinzaouatine field, Illizi Basin, Algeria

We present a geomechanical analysis of the Ordovician reservoir from the Tinzaouatine field situated in the prolific Illizi Basin, eastern Algeria. The sandstone reservoir has a hydrostatic pore pressure gradient (9.95 MPa/km). We analyzed a cumulative of 300 m of acoustic image log data and identified the coexistence of B-quality extensive drilling-induced tensile failures (DITFs) and compressive failures, i.e., breakouts (BOs), indicating a mean maximum horizontal stress ([Formula: see text]) orientation of N140°E. We used a combined BO and DITF-based solution to estimate horizontal stress magnitudes when the two failure types coexist. Based on the C-quality minifrac measurements, we interpreted the minimum horizontal stress ([Formula: see text]) gradient as 17.4–17.47 MPa/km, whereas the new approach indicates an [Formula: see text] range of 17.31–18.67 MPa/km. Using the BO width and DITF-based approaches, we inferred an [Formula: see text] gradient range of 28.37–38.59 MPa/km within the studied reservoir. Based on the relative stress magnitudes, we infer a strike-slip tectonic stress regime in the studied field.

Predicting Fluid Flow Regime, Permeability, and Diffusivity in Mudrocks from Multiscale Pore Characterisation

In geoenergy applications, mudrocks prevent fluids to leak from temporary (H2, CH4) or permanent (CO2, radioactive waste) storage/disposal sites and serve as a source and reservoir for unconventional oil and gas. Understanding transport properties integrated with dominant fluid flow mechanisms in mudrocks is essential to better predict the performance of mudrocks within these applications. In this study, small-angle neutron scattering (SANS) experiments were conducted on 71 samples from 13 different sets of mudrocks across the globe to capture the pore structure of nearly the full pore size spectrum (2 nm–5 μm). We develop fractal models to predict transport properties (permeability and diffusivity) based on the SANS-derived pore size distributions. The results indicate that transport phenomena in mudrocks are intrinsically pore size-dependent. Depending on hydrostatic pore pressures, transition flow develops in micropores, slip flow in meso- and macropores, and continuum flow in larger macropores. Fluid flow regimes progress towards larger pore sizes during reservoir depletion or smaller pore sizes during fluid storage, so when pressure is decreased or increased, respectively. Capturing the heterogeneity of mudrocks by considering fractal dimension and tortuosity fractal dimension for defined pore size ranges, fractal models integrate apparent permeability with slip flow, Darcy permeability with continuum flow, and gas diffusivity with diffusion flow in the matrix. This new model of pore size-dependent transport and integrated transport properties using fractal models yields a systematic approach that can also inform multiscale multi-physics models to better understand fluid flow and transport phenomena in mudrocks on the reservoir and basin scale.

Slopes Analyses - Case Study, Slope Stability of Bypass Project

The scope of this study was to compare various stability evaluation methods. Accordingly, most common LE approaches were compared with the advanced LE (M‐P) method. Similarly, the differences in FOS computed from LE and FE analyses were compared based on a simple slope considering various load cases. In addition, two real slopes in a case study were analysed for the recorded minimum‐maximum GWT, pseudo‐static and dynamic conditions. Moreover, the stability evaluations of these slopes were based on both LE (M‐P) and FE (PLAXIS) calculation approaches, which both utilized shear strength parameters from advanced triaxle tests. Similarly, Mohr‐Coulomb model was applied in both approaches. The following conclusions are hence derived based on the reported work on both idealized and real slopes. To fulfil one of the aims of the study, the LE based methods are compared based on the factor of safety (FOS) obtained for various load combinations. The comparison is mainly based on simplified slope geometry and assumed input parameters. Among the LE methods, the Bishop simplified (BS), Janbu simplified (JS) and Janbu GPS methods are compared with the Morgenstern‐Price method (M‐PM). These LE methods are well established for many years, and thus some of them are still commonly used in practice for stability analysis. Moreover, the M‐PM has been compared with results from the FE analyses. Compared with theFE (PLAXIS) analyses, the LE (M‐PM) analyses may estimate 5 – 14percent higher FOS, depending on the conditions of a dry slope and a fully saturated slope with hydrostatic pore pressure distributions. For fully saturated conditions in the slope, inaccurate computation of stresses in LE methods may have resulted in larger difference in the computed FOS. Since, the FE software is based on stress‐strain relationship, stress redistributions are surely better computed even for a complicated problem. This has been found one of the advantages in FE simulations. A parameter study shows that the application of a positive dilatancy angle in FE analysis can significantly improve the FOS (4 ‐ 10percent). On contrast, the shear surface optimization in LE (M‐PM in SLOPE/W) analysis results in lower FOS, and thus minimizing the difference in FOS compared with FE analysis

CPT Data Interpretation Employing Different Machine Learning Techniques

The classification of soils into categories with a similar range of properties is a fundamental geotechnical engineering procedure. At present, this classification is based on various types of cost- and time-intensive laboratory and/or in situ tests. These soil investigations are essential for each individual construction site and have to be performed prior to the design of a project. Since Machine Learning could play a key role in reducing the costs and time needed for a suitable site investigation program, the basic ability of Machine Learning models to classify soils from Cone Penetration Tests (CPT) is evaluated. To find an appropriate classification model, 24 different Machine Learning models, based on three different algorithms, are built and trained on a dataset consisting of 1339 CPT. The applied algorithms are a Support Vector Machine, an Artificial Neural Network and a Random Forest. As input features, different combinations of direct cone penetration test data (tip resistance qc, sleeve friction fs, friction ratio Rf, depth d), combined with “defined”, thus, not directly measured data (total vertical stresses σv, effective vertical stresses σ’v and hydrostatic pore pressure u0), are used. Standard soil classes based on grain size distributions and soil classes based on soil behavior types according to Robertson are applied as targets. The different models are compared with respect to their prediction performance and the required learning time. The best results for all targets were obtained with models using a Random Forest classifier. For the soil classes based on grain size distribution, an accuracy of about 75%, and for soil classes according to Robertson, an accuracy of about 97–99%, was reached.

Thermo-Rheological Properties of the Ethiopian Lithosphere and Evidence for Transient Fluid Induced Lower Crustal Seismicity Beneath the Ethiopian Rift

Lower crustal earthquakes at plate boundaries and intraplate settings occur at depth where deformation is normally expected to occur in a ductile manner. Here we use the available earthquake catalogs and compute theoretical predictions for a range of conditions for the occurrence of lower crustal earthquakes beneath the Main Ethiopian Rift (MER) and adjacent north-western (NW) plateau. Yield strength envelops are constructed using information on geothermal gradient, strain rate, and composition constrained by geophysical observations. Our models suggest that away from the MER beneath the NW plateau the depth distribution of earthquakes in the lower crust is best explained by strong mafic lower crustal rheology and hydrostatic fluid pore pressure conditions. In the same region the effective elastic thickness is similar to seismogenic thickness showing that the lower crust has long-term strength and hence can physically support brittle deformation. On the contrary, in the central MER the seismogenic thickness is much larger than the effective elastic layer thickness implying that the lower crust has no long-term strength. Here our models show that both hydrostatic and near-lithostatic fluid pore pressures fail to explain the observed seismicity and instead a combination of near-lithostatic pore fluid pressure and transient high strain rate due to the movement of fluids provide a plausible mechanism for the occurrence of seismicity in the lower crust. Our interpretations are supported by occurrence of swarms of deep earthquakes beneath the MER, as opposed to more continuous background deep seismicity away from the rift. Using time-depth progression of earthquakes, we estimate permeability values of 5.9 × 10−15m2and 1.8 × 10−14m2at lower crustal depth. The range of permeability implies that seismicity can be induced by pore-pressure diffusion, likely from fluids sourced from the mantle that reactivate preexisting faults in the lower crust. Our thermo-rheological models explain the first order differences in lower crustal earthquakes both directly beneath and outboard of the rift valley.

Crustal Stress State in Taiwan: Moderately Strong Crust Supporting Gravitational and Flexural Loading

AbstractIn Taiwan and other contractional tectonic wedges, there is evidence that the basal detachment is very weak and the overlying crust is relatively strong. In this study, we compute the absolute deviatoric 3‐D stress field in Taiwan using focal mechanism data and coseismic stress changes for the 1999 M7.6 Chi‐Chi earthquake. We estimate a minimum coefficient of friction of 0.3 in the upper 15 km crust, assuming hydrostatic pore pressure, consistent with a moderately strong crust. The estimated stress state is consistent with primary tectonic structures of Taiwan. The maximum compressive stress in the fold and thrust belt and the Coastal Range is approximately aligned with relative motion of the Eurasian and the Philippine Sea Plates, but the minimum and intermediate principal stresses vary laterally and with depth. A reverse‐faulting stress regime wraps around the Peikang high in western Taiwan suggesting inherited basement structure influences crustal stress state. Vertical maximum compressive stress is inferred under the high topography of the Central Range mountains. We present a simple stress model for Taiwan assuming spatial variations in gravitational potential energy are supported elastically. The model stress state is broadly consistent with the stress inversion result. The model shows that vertical maximum compressive stress in the Central Range is a result of high topography and thick crustal root. Consistent with a moderately strong crust, the model deviatoric stresses in areas of high seismicity in Taiwan are of the order 50–100 MPa. Lower deviatoric stresses under the Central Range are consistent with low rates of observed seismicity.

Pore pressure and in-situ stress magnitudes in the Bhiret Hammou hydrocarbon field, Berkine Basin, Algeria

A recently drilled exploratory well encompassing over 5 km of Mesozoic and Paleozoic sediments has been studied from the Bhiret Hammou field of the Berkine Basin, Algeria. Geophysical logs and downhole measurements have been integrated to ascertain rock strength, elastic properties, pore pressure and principal in-situ stress magnitudes. Vertical stress has an average 1.02 PSI/feet gradient in the studied field, as estimated from the density log. The Devonian shales are mildly over-pressured, while the Triassic and Carboniferous hydrocarbon reservoirs are in a hydrostatic pore pressure regime. Minimum and maximum horizontal stresses are quantified from a poroelastic strain model. The Sh gradient varies between 0.59 and 0.80 PSI/feet, whereas the SH gradient is interpreted as 0.86–1.26 PSI/feet. Based on the relative stress magnitudes (SH≥Sv>Sh), a present-day normal to strike-slip transitional tectonic regime is inferred.

Present-day stress state and fault stability analysis in the capital area of China constrained by in situ stress measurements and focal mechanism solutions

This work proposes to determine the present-day in situ stress state across both known and suspected faults in the capital area of China and to assess fault stability and consequent seismic hazards. The population of main active faults in the capital area is summarized, and 168 sets of hydraulic fracturing and overcoring data and 84 sets of earthquake focal mechanism solutions are gathered and studied. The available data indicate that reverse and strike-slip faulting stress regimes are favored at reservoir depths in this region. The ratio of maximum horizontal stress to vertical stress (KH) and the ratio of minimum horizontal stress to vertical stress (Kh) at a depth of 500 m show complex spatial variations in stress magnitudes. Generally, KH appears to increase southward, which might suggest that stress is accumulating in the southern part of the region. The relative stress magnitude R appears independent of the depth and mainly varies between 0.5 and 1.0. The observation of stress data suggests a relatively uniform direction (nearly E-W) for the maximum horizontal principal stress. The influence of hydrostatic pore pressure on critical stresses is discussed, and the presence of hydrostatic pore pressure makes faults more conducive to reactivation. Most locations in this region are critically stressed based on the Coulomb frictional failure criterion and Byerlee’s law with hydrostatic pore pressure; thus, the present-day stress state appears to be high enough to initiate slip. In addition, the relationship between the tectonic stress field and earthquakes is discussed.

Methane Hydrate Stability and Potential Resource in the Levant Basin, Southeastern Mediterranean Sea

To estimate the potential inventory of natural gas hydrates (NGH) in the Levant Basin, southeastern Mediterranean Sea, we correlated the gas hydrate stability zone (GHSZ), modeled with local thermodynamic parameters, with seismic indicators of gas. A compilation of the oceanographic measurements defines the &gt;1 km deep water temperature and salinity to 13.8 °C and 38.8‰ respectively, predicting the top GHSZ at a water depth of ~1250 m. Assuming sub-seafloor hydrostatic pore-pressure, water-body salinity, and geothermal gradients ranging between 20 to 28.5 °C/km, yields a useful first-order GHSZ approximation. Our model predicts that the entire northwestern half of the Levant seafloor lies within the GHSZ, with a median sub-seafloor thickness of ~150 m. High amplitude seismic reflectivity (HASR), correlates with the active seafloor gas seepage and is distributed across the deep-sea fan of the Nile within the Levant Basin. Trends observed in the distribution of the HASR are suggested to represent: (1) Shallow gas and possibly hydrates within buried channel-lobe systems 25 to 100 mbsf; and (2) a regionally discontinuous bottom simulating reflection (BSR) broadly matching the modeled base of GHSZ. We therefore estimate the potential methane hydrates resources within the Levant Basin at ~100 trillion cubic feet (Tcf) and its carbon content at ~1.5 gigatonnes.