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We suggest that faults in high slip tendency orientations tend to develop larger displacements than other faults. Consequently, faults that accumulate larger displacements are more likely to be reliable indicators of the longer term stress field and should be weighted accordingly in paleostress estimation. Application of a stress inversion technique that uses slip tendency analyses and fault displacements to interpret populations of coherent normal faults within the Balcones Fault System of south-central Texas provides stress estimates that are consistent with established regional stress analyses. Although the method does not require measurement of slip directions, these data, where available, and sensitivity analyses of the angular mismatch between measured slip directions and those predicted by inverted stress states provide high confidence in the stress estimates generated using slip tendency analyses. Close inspection of the fault orientation and displacement data further indicates that subpopulations of faults with orientations different from the regional pattern have formed in response to stress perturbations generated by displacement gradients on an adjacent seismic scale fault. [ABSTRACT FROM AUTHOR]

Abstract: The in situ stress state is an important controlling factor for the slip behavior of faults and fractures in the earth''s crust and hence for the productivity of faulted and fractured hydrocarbon reservoirs. Current methods for stress tensor estimation rely on slip vector field data; however, this information is not generally available from data sets that are commonly used in the oil and gas industry. This work presents a new stress inversion approach where slip tendency is used as a proxy for fault displacement, which can easily be extracted from data sets routinely used by the oil and gas industry. The inversion approach is demonstrated using a data set obtained from the Canyon Lake Gorge in Comal County, Texas. [Copyright &y& Elsevier]

Abstract: In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′2). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (T s) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., T s ≥0.6) decreases from a maximum of about 38% where σ′2 = σ′3, to a minimum of approximately 14% where σ′2 is halfway between σ′3 and σ′1, and increases to another high of approximately 29% where σ′2 = σ′1. This analysis illustrates the influence of the magnitude of σ′2 on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity. [Copyright &y& Elsevier]

At low differential effective stress and with minimum principal effective stress near zero or tensile, rocks fail in several modes and with variable failure angles. Under these conditions mechanical stratigraphy exerts a significant influence on initial dip of normal faults. Less competent layers fail in shear mode along fractures that approximate the failure angle predicted by a standard rock mechanics analysis. Deformation of more competent layers, which is driven in part by interaction with the more rapidly deforming incompetent layers, produces hybrid mode failure in which failure angles are smaller than in shear mode. Analyses of small-displacement (<1 m displacement) normal faults cutting Cretaceous carbonate strata in west Texas (USA) indicate that fault geometries resulting from this effect commonly display steep segments where the fault traverses more competent beds. Displacement on these faults has caused dilation of steeper segments. Analogous steep dilational fault segments host ore deposits in the North Pennine Orefield, England. Dilatant segments along faults within carbonates of the Cretaceous age Edwards Group near San Antonio, Texas (USA) have been enlarged by groundwater flow, and are important permeability and shallow groundwater infiltration pathways. [Copyright &y& Elsevier]

Abstract: Understanding stress states and rock mass deformation deep underground is critical to a range of endeavors including oil and gas exploration and production, geothermal reservoir characterization and management, and subsurface disposal of CO2. Geomechanical modeling can predict the onset of failure and the type and abundance of deformation features along with the orientations and magnitudes of stresses. This approach enables development of forward models that incorporate realistic mechanical stratigraphy (e.g., including competence contrasts, bed thicknesses, and bedding planes), include faults and bedding-slip surfaces as frictional sliding interfaces, reproduce the overall geometry of the fold structures of interest, and allow tracking of stress and strain through the deformation history. Use of inelastic constitutive relationships (e.g., elastic–plastic behavior) allows permanent strains to develop in response to the applied loads. This ability to capture permanent deformation is superior to linear elastic models, which are often used for numerical convenience, but are incapable of modeling permanent deformation or predicting permanent deformation processes such as faulting, fracturing, and pore collapse. Finite element modeling results compared with field examples of a natural contractional fault-related fold show that well-designed geomechanical modeling can match overall fold geometries and be applied to stress, fracture, and subseismic fault prediction in geologic structures. Geomechanical modeling of this type allows stress and strain histories to be obtained throughout the model domain. [Copyright &y& Elsevier]

Abstract: The occurrence of hybrid failure – the transition from tensile to shear failure – has been controversial. Production of hybrid failure in laboratory experiments in recent years has strengthened the case for hybrid failure, but field recognition has remained elusive. In this paper, we present data from a small-displacement fault in the Hidden Valley fault zone at Canyon Lake Gorge, Texas, which is refracted through mechanically layered carbonates and exhibits surface characteristics expected for hybrid failure on steep fault segments. We discuss field criteria for identifying hybrid fractures and the deformation conditions under which hybrid failure is to be expected. Documented field examples of hybrid fractures are rare, yet presence of hybrid fractures can be useful indicators of stress conditions at the time of failure. Specifically, hybrid fractures are likely indicators of tensile minimum effective stress and low differential stress at failure. [Copyright &y& Elsevier]