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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).

Superdeep fault-karst carbonate reservoirs discovered over 7 km deep are controlled by strike-slip fault zones and karst collapses in Tarim Basin, Northwest China. The resulting fracture-cave system provides favorable migration channels and reservoir spaces for hydrocarbon, while the characterization of the internal fault structures remains enigmatic. Based on seismic imaging data, we conducted an integrated study on fault damage zones by seismic curvature attributes, velocity anisotropies, and seismic attenuations. The results show that three typical fault-zone patterns can be identified in the study area, including paratactic multiple fault cores, interactive fault cores and one primary-several subsidiary fault cores. These typical patterns can be clearly characterized via curvature attributes. The elastic characteristics of fault damage zones are significantly affected by seismic frequencies, which are manifested from velocity anisotropies and seismic attenuations. The maximum seismic attenuation occurs along with the orientation of fault cores. There is a strong anisotropic characteristic of P-wave phase velocity with incident angle of three fault-zone models. It appears that seismic attributes associated with geological steering are an effective tool for the subsurface characterization of fault damage zones.

We have investigated the distribution and thickness of damage zones for a system of secondary normal faults in the subsurface of the Wangxuzhuang oilfield, China. Based on seismic variance analysis, we find (1) four isolated faults with approximately 2 km length and approximately 200 m damage-zone thickness. The damage zones of these isolated faults reveal a decaying intensity of deformation from the fault core to the protolith, which fits a power-law form [Formula: see text] similar to that observed in the field. (2) A merged fault with approximately 400 m thickness. (3) A bifurcated fault with approximately 400 m thickness and three linked segments. Damage zones that consist of several subsidiary faults are thicker than those of isolated faults. The displacement-length analyses of the four isolated faults suggest the constant-length growth of the limestone in this case. We determine the potential to apply seismic variance to systematically characterize damage zones as potential fluid migration conduits on the basin scale.

Fracture mechanical properties of shales have gained continued interest recently due to their critical role in many shale-related applications such as unconventional petroleum systems and subsurface carbon and waste disposal. However, due to their strong reactive nature to fluids, fracture mechanical properties of shales have not been extensively studied like other rock types. A more comprehensive understanding of fracture system development in shales under reactive fluids is needed for subsurface applications. We have measured fracture mechanical properties of Longmaxi shale outcrop samples in the Sichuan Basin, China, under different environments of ambient air, deionized water, saline fluids of NaCl and KCl at 0.5 M concentration, and acidic HCl fluid at pH of 5. All aqueous fluids tested show strong weakening effects on fracture propagation compared to the air environment, with the fracture toughness reduced by 75% and the subcritical fracture growth index reduced by 50%. Microstructural analysis reveals the predominantly grain boundary opening cracking mode for all tests, but the fracture traces branch more in reactive aqueous fluids. Natural fractures are comparable with artificial fractures in morphology. Bedding-perpendicular opening mode fractures with multiple-stage fracture fillings of calcite, quartz, and organic matters develop well in natural fractures. Our results suggest that clay mineral hydration and expansion are the main cause for the fluid weakening effect in Longmaxi shale, which has substantial implications for subsurface shale failure processes.

Fractures within fault damage zones are crucial for the migration of subsurface fluids, which is challenging the characterization of the fractures and damage zones in the subsurface due to the lack of subsurface data. We have investigated the fractures and fault damage zones in the tight sandstone gas-bearing Xujiahe Formation in the northeast Sichuan Basin, China, based on a comprehensive study of seismic data, well log, core, as well as field observations. We have demonstrated the structure and distribution of damage zones from the basin-scale down to the microscale. The core samples find mostly opening-mode microfractures that can be subcritically generated under low tectonic stress fields since the Late Triassic-Middle Jurassic. These opening-mode microfractures in such ultra-low-permeability sandstone are likely to provide a storage volume for coal gas to accumulate. Macrofractures from image logging and outcrop display well-developed joint networks within the sandstone. The basin-wide distribution of such macrofractures implies potential conduits for gas migration during basin uplift in the Cretaceous. The damage zones are formed and controlled by a system of reverse faults, with thicknesses ranging 100–1500 m. The multiscale analysis of fractures implies that, for the tight sandstone within the Xujiahe Formation, the fractures and damage zones are likely to control the gas migration and accumulation during the tectonic transformation from the Late Cretaceous to the Quaternary Period. The overpressure history, coupled with the fracture system, enhances the redistribution of the gas reservoir. This approach and these data lead to a more in-depth understanding of the fractures and damage zones on a regional scale, which could extend to hydraulic and mechanical characterization of damage zones in the upper crust.

Faulting processes have created large damage zones with complex structures in the field; however, estimating the width and geometry of such fault structures in the subsurface is challenging due to a lack of data. Seismic attributes (e.g., coherence and variance) from seismic surveys have been used for the characterization of faults, but most cases do not detail the effectiveness of this approach. By using forward modeling and the associated seismic attributes of variance, four fault models of idealized damage zones are characterized and the frequency effect is evaluated on the width estimation of fault damage zones in the subsurface. The main results indicate that (1) the general geometric pattern of damage zones could be identified by using simulated amplitude and seismic variance with main frequencies of 10, 25, and 40 Hz; (2) the estimated widths of damage zones at a low frequency of 10 Hz are larger (up to twofold) than those at frequencies of 25 and 40 Hz; for large damage zones (>400 m), the width is best estimated by a frequency of 25 Hz; and (3) scattering noise and diffraction around the fault are found in data at a high frequency of 40 Hz, which results in width overestimation of the damage zones by approximately 17%. The internal structures are difficult to distinguish as scattering noise and chaotic reflections dominate seismic signals. More factors that may influence the accuracy of damage zone width estimation via seismic attributes, include the bedding thickness, fracture density, and velocity. An in-depth understanding of this approach is useful in the application of seismic variance to characterize fault damage zones that may significantly control the fluid migration in the subsurface.

SUMMARYThe cumulative displacement by multiple slip events along faults may generate composite damage zones (CDZ) of increasing width, and could modify the hydraulic and mechanical properties of the faults. The internal architecture and fracture distribution within CDZs at the subsurface are analysed here by using seismic attributes of variance, curvature and dip-azimuth of the 3-D seismic reflection data from tight sandstone reservoirs in northeast Sichuan, China. The analysed faults intersect the reservoir within a depth range of 2.4–3.0 km. The damage intensity mapping revealed multiple CDZs with thicknesses approaching 1 km along faults ranging 3–15 km in length, and up to 1000 m of cumulative slip. The identification of numerous fault cores and associate damage zones led us to define three classes of CDZs: banded shape, box shape and dome shape. The mechanical strength contrasts and distortion of fault cores suggest potential weakening and strengthening (healing) mechanisms for formation of CDZs that can be extended to faulting processes and earthquake simulations.

Seismic survey data collected for coal gas exploration show that there are many collapse columns distributed in the subsurface of Qinshui Basin, China. The interesting features of the collapse columns are observed by the seismic attributes, including the circular discontinuous patches on the horizon of the Shanxi Formation and multiple parallel discontinuities in vertical profiles of amplitudes. We speculate that the wide presence of these collapse columns are point constraints for the migration and accumulation of coal gas on a large scale. Geological feature: Collapse columns within coal reservoirs Seismic appearance: The coherence illuminates circular/oval discontinuities on the horizon of the Shanxi Formation; the vertical amplitude profiles show cylindrical/funnel-shaped discontinuities. Alternative interpretations: Fault damage zones; velocity pulldown from the overburden Features with similar appearance: Fault-karst in carbonate reservoir; reef pinnacles Formation: Permian Shanxi Formation and Carboniferous Taiyuan Formation Age: Late Permian Location: Qinshui Basin in Shanxi, north-central China Seismic data: Provided by PetroChina Huabei Oilfield Company Contributors: Zonghu Liao, Lin Zhang, and Lianbo Zeng Analysis tools: The seismic amplitude and attribute of coherence from the seismic survey (prestack time migrated)

Fault damage zones may significantly affect subsurface fluid migration and the development of unconventional resources. Most analyses of fault damage zones are based on direct field observations, and we expand these analyses to the subsurface by investigating the damage zone structure of an approximately 32-km (∼105-ft)-long right-lateral strike-slip fault in Oklahoma. We used the three-dimensional (3-D) seismic attribute of coherence to first define its regional and background levels, and then we evaluated the damage zone dimensions at multiple sites. We found damage zone thickness of approximately 1600 m (∼5300 ft) at a segment that is dominated by subsidiary faults, and it is slightly thicker at a segment with a pull-apart basin. The damage zone intensity decays exponentially with distance from the fault core, in agreement with field observations and distribution of seismic events. The coherence map displays a strong asymmetry of the damage zone between the two sides of the 3-D fault, which is related to the subsidiary structures of the fault zone. We discuss the effects of heterogeneous stress field on damage zone evolution through the detected subsidiary structures. It appears that seismic coherence is an effective tool for subsurface characterization of fault damage zones.

Large earthquakes initiate at small nucleation sites and propagate as rupture fronts along the host fault. It is inherently challenging to resolve the complexity of fault rupture at depth, and therefore, the evolution of frictional strength during earthquakes is commonly analyzed in laboratory experiments. We experimentally demonstrate here that this evolution depends simultaneously on all slip kinematic components: displacement, velocity and acceleration. We incorporate these components in shear experiments with slip-histories that resemble the theoretical expectations for earthquake slip. These experiments led to a new friction law that fits fault behavior during high-velocity/long-displacement slip. Our numerical simulations of dynamic rupture along a planar fault that obeys this friction law reproduced a range of earthquake source features including slip-pulse, Yoffe function, Gaussian velocity, and spontaneous slip arrest. Finally, we demonstrate that this experimentally-based friction law can realistically simulate the propagation and arrest of natural earthquakes.

The structure of subsurface fault networks, which has a significant effect on hydrocarbon migration, is inherently difficult to recognize and map. 3D seismic surveys provide an opportunity to map the fault system within a carbonate reservoir of Xinchang in the western Sichuan Basin, China. We calculated attributes from the seismic reflection data that covers an area of 1330 km2, including variance, edge detection, dip-magnitude and dip azimuth. We focused on the Triassic Leikoupo Formation of dolostone, at ∼6 km depth, adjacent to the Longmen Shan thrusting range. The mapped attributes display (1) a primary network of faults in an orthogonal pattern in the anticline, striking East-West and North-South, and (2) a secondary, conjugate fault set in the fold limb, striking Northeast and Northwest with a length range of 1–10 km. The latter set of conjugate faults are confined to a 300 m-thick layer of dolostone, diminishing into underlaying unit of anhydrite. We envision that the conjugate fault networks and associated fractures enhance gas charging and reflect a model of self-sourced migration and accumulation in the Leikoupo Formation. These results are significant for sweet-spot evaluation of carbonate reservoirs in the Sichuan Basin, and provide insights for understanding the migration of subsurface fluids.

Fault damage zones around faults have a significant influence on fluid flow in tight-oil sandstones because they commonly act as localized conduits. Faults are developed in the tight-oil sandstones of the Upper Triassic Yanchang Formation in the southwest Ordos Basin, China. We integrate cores, image logs, and conventional logs from vertical wells to characterize subsurface fault damage zones in the tight-oil sandstones of the Upper Triassic Yanchang Formation in the southwest Ordos Basin, China. The results indicate that fault damage zones are intensively fractured or intensely broken in the cores. These fault damage zones present borehole collapse and widen sinusoidal curves in the image logs. The fractures in fault damage zones are predominant high dip angles. The fracture intensity decays with the increasing orthogonal distance from the faults within a fault damage zone. In fault damage zones, acoustic log (AC) values and compensated neutron log (CNL) values increase; density log (DEN) values decrease, dual induction log (ILD and ILM) and laterolog 8 (LL8) values decrease, the caliper log (CAL) presents borehole enlargement, and comprehensive fracture index log (CFI) values are greater than 0.43 and average 0.78. To identify fault damage zones by conventional logs in vertical wells, it is critical to distinguish fault damage zones from the background fractured zones. The ILM, CNL, ILD, LL8, and AC logs would be more useful than DEN logs for the distinction between background fractured zones and fault damage zones. The responses of fault damage zones in conventional logs are more intensive than those of background fractured zones, and the heights of fault damage zones are much greater than those of background fractured zones, which can be used for the distinction between fault damage zones and background fractured zones.

Fracture development in reservoir rocks enhances the flow and transport properties of the petroleum system, yet fracture in caprocks can significantly risk reservoir seal integrity. Over geological time scales, the slow, subcritical fracture propagation can contribute considerably to the fracture system development, especially under reactive fluid environments during petroleum migration and accumulation. Using the double torsion technique, we measure the subcritical fracture growth properties of carbonate and anhydrite caprock core samples in Sichuan Basin, China, under different environments of ambient air, de-ionized water, and formation water. Experimental results show that the aqueous fluids significantly promote subcritical fracture growth: (1) compared to ambient air, the presence of aqueous fluids dramatically reduces both the fracture toughness (KIC) and subcritical fracture growth index (SCI) of the caprocks; (2) the KIC reduction is more prominent for the anhydrite (50%) caprock than the carbonate (30%) caprock, which suggest that mineral solubility is responsible for the fluid-weakening effect; (3) a systematic change of SCI with varying fluid conditions is observed: SCI is the largest in air, smallest in formation water, and intermediate in de-ionized water. Microstructural analysis suggests a predominantly opening mode for both the natural fractures and induced fractures during fracture mechanical testing. Our results further demonstrate that subcritical fracture growth process is also strongly related to rock composition and stress conditions. These observations provide insights into the mechanisms controlling fracture growth in carbonate and evaporite caprocks, which could extend to brittle failure and fluid-rock interactions occurring during the seismic cycle in the upper crust.

Seismic analyses show that slip during large earthquakes evolves in a slip-pulse mode that is characterized by abrupt, intense acceleration followed by moderate deceleration. We experimentally analyze the friction evolution under slip-pulse proxy of a large earthquake, and compare it with the evolution at loading modes of constant-velocity and changing-velocity. The experiments were conducted on room-dry, solid granite samples at slip-velocities of 0.0006–1 m/s, and normal stress of 1–11.5 MPa. The analysis demonstrates that (1) the strength evolution and constitutive parameters of the granite fault strongly depend on the loading mode, and (2) the slip-pulse mode is energy efficient relatively to the constant-velocity mode as manifested by faster, more intense weakening and 50–90% lower energy dissipation. The results suggest that the frictional strength determined in slip-pulse experiments, is more relevant to simulations of earthquake rupture than frictional strength determined in constant-velocity experiments. Further, for a finite amount of crustal elastic energy, the efficiency of slip-pulse would amplify earthquake instability.

Shallow landslides induced by heavy rainfall events represent one of the most disastrous hazards in mountainous regions because of their high frequency and rapid mobility. Recent advancements in the availability and accessibility of remote sensing data, including topography, land cover and precipitation products, allow landslide hazard assessment to be considered at larger spatial scales. A theoretical framework for a landslide forecasting system was prototyped in this study using several remotely sensed and surface parameters. The applied physical model SLope-Infiltration-Distributed Equilibrium (SLIDE) takes into account some simplified hypotheses on water infiltration and defines a direct relation between factor of safety and the rainfall depth on an infinite slope. This prototype model is applied to a case study in Honduras during Hurricane Mitch in 1998. Two study areas were selected where a high density of shallow landslides occurred, covering approximately 1,200 km2. The results were quantitatively evaluated using landslide inventory data compiled by the United States Geological Survey (USGS) following Hurricane Mitch’s landfall. The agreement between the SLIDE modeling results and landslide observations demonstrates good predictive skill and suggests that this framework could serve as a potential tool for the future early landslide warning systems. Results show that within the two study areas, the values of rates of successful estimation of slope failure locations reached as high as 78 and 75%, while the error indices were 35 and 49%. Despite positive model performance, the SLIDE model is limited by several assumptions including using general parameter calibration rather than in situ tests and neglecting geologic information. Advantages and limitations of this physically based model are discussed with respect to future applications of landslide assessment and prediction over large scales.

The key to advancing the predictability of rainfall-triggered landslides is to use physically based slope-stability models that simulate the transient dynamical response of the subsurface moisture to spatiotemporal variability of rainfall in complex terrains. TRIGRS (transient rainfall infiltration and grid-based regional slope-stability analysis) is a USGS landslide prediction model, coded in Fortran, that accounts for the influences of hydrology, topography, and soil physics on slope stability. In this study, we quantitatively evaluate the spatiotemporal predictability of a Matlab version of TRIGRS (MaTRIGRS) in the Blue Ridge Mountains of Macon County, North Carolina where Hurricanes Ivan triggered widespread landslides in the 2004 hurricane season. High resolution digital elevation model (DEM) data (6-m LiDAR), USGS STATSGO soil database, and NOAA/NWS combined radar and gauge precipitation are used as inputs to the model. A local landslide inventory database from North Carolina Geological Survey is used to evaluate the MaTRIGRS’ predictive skill for the landslide locations and timing, identifying predictions within a 120-m radius of observed landslides over the 30-h period of Hurricane Ivan’s passage in September 2004. Results show that within a radius of 24 m from the landslide location about 67% of the landslide, observations could be successfully predicted but with a high false alarm ratio (90%). If the radius of observation is extended to 120 m, 98% of the landslides are detected with an 18% false alarm ratio. This study shows that MaTRIGRS demonstrates acceptable spatiotemporal predictive skill for landslide occurrences within a 120-m radius in space and a hurricane-event-duration (h) in time, offering the potential to serve as a landslide warning system in areas where accurate rainfall forecasts and detailed field data are available. The validation can be further improved with additional landslide information including the exact time of failure for each landslide and the landslide’s extent and run out length.

An early warning system has been developed to predict rainfall-induced shallow landslides over Java Island, Indonesia. The prototyped early warning system integrates three major components: (1) a susceptibility mapping and hotspot identification component based on a land surface geospatial database (topographical information, maps of soil properties, and local landslide inventory, etc.); (2) a satellite-based precipitation monitoring system (http://trmm.gsfc.nasa.gov) and a precipitation forecasting model (i.e., Weather Research Forecast); and (3) a physically based, rainfall-induced landslide prediction model SLIDE. The system utilizes the modified physical model to calculate a factor of safety that accounts for the contribution of rainfall infiltration and partial saturation to the shear strength of the soil in topographically complex terrains. In use, the land-surface “where” information will be integrated with the “when” rainfall triggers by the landslide prediction model to predict potential slope failures as a function of time and location. In this system, geomorphologic data are primarily based on 30-m Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, digital elevation model (DEM), and 1-km soil maps. Precipitation forcing comes from both satellite-based, real-time National Aeronautics and Space Administration (NASA) Tropical Rainfall Measuring Mission (TRMM), and Weather Research Forecasting (WRF) model forecasts. The system’s prediction performance has been evaluated using a local landslide inventory, and results show that the system successfully predicted landslides in correspondence to the time of occurrence of the real landslide events. Integration of spatially distributed remote sensing precipitation products and in-situ datasets in this prototype system enables us to further develop a regional, early warning tool in the future for predicting rainfall-induced landslides in Indonesia.

A cost-effective physical model (SLope-Infiltration-distributed Equilibrium—SLIDE) has been developed to identify the spatial and temporal occurrences of rainfall-induced landslides, employing a range of remotely sensed and in situ data. The main feature of SLIDE is that it takes into account of some simplified hypotheses on water infiltration and defines a direct relationship between the factor of safety and the rainfall depth on an infinite slope model. This prototype has been applied to two case studies in Indonesia and Honduras during heavy rainfall events brought by typhoon and hurricane, respectively. Simulation results from SLIDE demonstrated good skills in predicting rainfall-induced shallow landslides by assimilating the most important dynamic triggering factor (i.e., rainfall) quantitatively. The model’s prediction performance also suggested that SLIDE could serve as a potential tool for the future landslide early-warning system. Despite positive model performance, the SLIDE model is limited by several assumptions including using general parameter calibration rather than in situ tests and neglecting geotechnical information and some of the hydrological processes in deep soil layers. Advantages and limitations of this physically based model are also discussed with respect to future applications of landslide assessment and prediction over large scales.

In this special section, we include investigations of different aspects of fault damage zones, including seismic interpretation, field observations, laboratory experiments, theoretical models, numerical simulations, and case studies of exploration and production of reservoirs. Fault damage zones are

Earthquakes are associated with slip along fault-zones in the crust, and the intensity of dynamic-weakening is one of the central questions of earthquake physics (Dieterich, 1979; Reches and Lockner, 2010). Since it is impossible to determine fault friction with seismological methods (Kanamori and Brodsky, 2004), the study of fault friction and earthquake weakening has been usually addressed with laboratory experiments (Dieterich, 1979) and theoretical models (Ohnaka and Yamashita, 1989).

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