Alan P. Morris

Fault zone deformation controlled by carbonate mechanical stratigraphy, Balcones fault system, Texas

Normal faults in Cretaceous carbonates in the Balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide. Mechanical layering is a fundamental control on carbonate fault zones. Relatively planar faults with low-displacement gradients develop in massive, strong, clay-poor limestones and dolomites. In less competent clay-rich strata, shale beds impede fault propagation, resulting in fault-related folding, and locally steep bedding dips. Faults in clay-poor massive limestones and dolomites tend to be steep (70 or more), whereas weaker, clay-rich limestones develop faults with shallower dips (60 or less). Fault zone rocks show evidence of cataclasis, cementation, deformation of cement by mechanical twinning and pressure solution, and multiple generations of cement with differing degrees of deformation, indicating contemporaneous cementation and fault slip. In stratigraphic sequences consisting of both competent and incompetent strata, the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation. Low displacement-to-propagation ratios associated with competent strata generate low-displacement gradients, inhibiting fault-related folding. Conversely, high displacement-to-propagation ratios associated with incompetent strata promote high-displacement gradients and fault-related folding.

Impact of interlayer slip on fracture prediction from geomechanical models of fault-related folds

Understanding and interpreting the timing, location, orientation, and intensity of natural fractures within a geologic structure are commonly important to both exploration and production planning activities. Here we explore the application of finite-element-based geomechanical models to fracture prediction. Our approach is based on the idea that natural fractures can be interpreted or inferred from the geomechanical-model-derived permanent strains. For this analysis, we model an extensional fault-tip monocline developed in a mechanically stratified limestone and shale sequence because field data exist that can be directly compared with model results. The approach and our conclusions, however, are independent of the specific structural geometry. The presence or absence of interlayer slip is shown to strongly control the distribution and evolution of strain, and this control has important implications for interpreting fractures from geomechanical models.

Fault zone deformation and displacement partitioning in mechanically layered carbonates: The Hidden Valley fault, central Texas

The Hidden Valley fault is exposed in Canyon Lake Gorge (central Texas) and cuts the Cretaceous Glen Rose Formation. This exposure provides an opportunity to explore the relationship between deformation mechanisms and fault displacement along 830 m (2723 ft) of a normal fault typical of those in carbonate reservoirs and aquifers around the world. The fault zone has five domains: gently deformed footwall damage zone, intensely deformed footwall damage zone, fault core, intensely deformed hanging-wall damage zone, and gently deformed hanging-wall damage zone. Footwall deformation is more intense and laterally extensive than hanging-wall deformation, and the intensely deformed hanging-wall damage zone is narrow and locally absent. The fault core contains thin clay-rich gouge or smear in most places but is locally represented by only a slickensided surface between limestone layers. The 55- to 63-m (180–207-ft) fault throw across a 43- to 98-m (141- to 322-ft)-wide fault zone is accommodated by slip along the fault core, layer tilting (synthetic dip development) in footwall and hanging-wall damage zones, and distributed faulting in footwall and hanging-wall damage zones. Total offset across the fault overestimates actual stratigraphic offset by 8 to 12 m (26–39 ft) or about 14 to 21%. In our interpretation, the Hidden Valley fault zone records both early extensional folding of the Glen Rose Formation and subsequent normal faulting that propagated downward from the overlying competent Edwards Group. The damage zone width is thus established before fault breakthrough.

Control of mechanical stratigraphy on bed-restricted jointing and normal faulting: Eagle Ford Formation, south-central Texas

Outcrops of the middle Eagle Ford Formation in south-central Texas reveal well-developed joint networks in subhorizontal competent carbonate (chalk) beds and less well developed networks in interlayered incompetent calcareous mudrock beds. Northeast-striking bed-perpendicular joints in competent beds have the longest trace lengths and are abutted by northwest-striking joints. All observed joints terminate vertically in incompetent beds. Normal faults are common but less abundant than joints; dominantly dip north, northwest, or southeast; and are abutted by the joint sets and, thus, predated jointing. The faults cut multiple competent and incompetent beds, providing vertical connectivity across mechanical layering. Products of hybrid and shear failure, the dip of these faults is steep through competent beds and moderate through incompetent beds, resulting in refracted fault profiles with dilation and calcite precipitation along steep segments. Fluid inclusions in fault zone calcite commonly contain liquid hydrocarbons. Rare two-phase fluid inclusions homogenized between about (1) 40 and 58°C, and (2) 90 and 100°C, suggesting trapping of aqueous fluids at elevated temperatures and depths on the order of 2 km (6562 ft). Fluid inclusion and stable isotope geochemistry analyses suggest that faults transmitted externally derived fluids. These faults likely formed at depths equivalent to portions of the present-day oil and gas production from the Eagle Ford play in south Texas. Faults connect across layering and provide pathways for vertical fluid movement within the Eagle Ford Formation, in contrast to vertically restricted joints that produce bed-parallel fracture permeability. These observations elucidate natural fractures and induced hydraulic fracturing within the Eagle Ford Formation.

Stratigraphic control on extensional fault propagation folding: Big Brushy Canyon monocline, Sierra Del Carmen, Texas

Abstract Mechanical stratigraphy exerts a first-order control on deformation at a range of scales from oilfield-scale structural style to deformation (e.g. fracturing) within an individual reservoir stratum. This paper explores an outcrop example where mechanical stratigraphy in a limestone and shale sequence directly influenced the structural style and distribution of deformation related to the propagation of a ‘seismic-scale’ normal fault that has maximum displacement on the order of 100–500 m and extends for more than 10 km. A monocline developed in Cretaceous Buda Limestone above tectonically thinned Del Rio Clay and faulted Santa Elena Limestone is here interpreted as an extensional fault propagation fold. Monocline limb dips reach 59°. The Del Rio Clay is thinned from approximately 36 m to 1.5 m, whereas the underlying Santa Elena Limestone is offset vertically by approximately 74 m along a steep (approximately 80°) normal fault. This large fault displacement of the Santa Elena Limestone is not transferred upward to the Buda Limestone because of ductile flow within the intervening Del Rio Clay. Although upward fault propagation has been inhibited, thinning of the Del Rio Clay and the resultant extreme displacement gradient at the tip of the fault have forced the Buda Limestone into a monoclinal fold. Two competent packstone and grainstone beds, 6 m and 2.7 m thick and separated by 10.5 m of less competent calcareous shale, comprise the Buda Limestone at this location. Deformation features within the competent Buda beds include bed-perpendicular veins that accommodate bed-parallel extension, and bedding plane slip surfaces with an up-dip sense of shear that offset the veins. Deformation is concentrated in the monoclinal limb and not in the monoclinal hinge regions. Consequently, bed-parallel extension and shear strain are associated with monoclinal dip, not with curvature. These results show that for this structure, bed dip is a better proxy for bed-parallel extension and related fracture dilation than is curvature.

Mechanical stratigraphy and faulting in Cretaceous carbonates

Normal faults measured in exposures of Cretaceous carbonate rocks in Texas provide the basis for fault-strain determination, analysis of fault displacements, and exploring the function of mechanical stratigraphy in influencing fault-size distributions. Layer competence and competence contrast, measured using a Schmidt hammer, allow the analysis of mechanical stratigraphy. Fault frequency and displacement distributions exhibit patterns that correlate to mechanical stratigraphy. In particular, the average competence contrast is related to the exponent (C) of cumulative frequency versus displacement distributions as described by log(cumulative frequency) = (C) log(displacement) + A. This correlation between competence contrast and C values is interpreted to indicate that, at low competence contrast, there are many potential nucleation sites for faults and no mechanisms by which fault displacement can be filtered. In addition, several frequency versus displacement distributions exhibit steep sections, indicating a clustering of fault displacement(s). Clustering of fault displacement(s) is also interpreted as the result of low-competence layers inhibiting the propagation of faults through the layering until a threshold displacement has been reached. This has the effect of creating a cluster of faults with displacements near the threshold displacement value. These patterns are true both for data sets surveyed along a scan line and along a key bed. An appreciation of these effects of mechanical stratigraphy on fault displacement distributions is important when using observed data to infer subseismic fault populations during reservoir evaluation and modeling.

Geomechanical modeling of hydraulic fracturing: Why mechanical stratigraphy, stress state, and pre-existing structure matter

The increasing exploration and production in unconventional resource plays in the past decade has been accompanied by a greater need for understanding the effectiveness of multistage hydraulic fracturing programs, particularly in long (>1500 m or 5000 ft) subhorizontal boreholes (laterals). Traditional (analytical) analysis techniques for estimating the size and orientation of fractures induced by fluid injection typically result in predictions of relatively long and planar extension (mode I) bi-wing fractures, which may not be representative of natural systems. Although these traditional approaches offer the advantage of rapid analysis, neglect of key features of the natural system (e.g., realistic mechanical stratigraphy, pre-existing natural faults and fractures, and heterogeneity of in situ stresses) may render results unrealistic for planning, executing, and interpreting multimillion-dollar hydraulic stimulation programs. Numerical geomechanical modeling provides a means of including key aspects of natural complexity in simulations of hydraulic fracturing. In this study, we present the results of two-dimensional finite element modeling of fluid-injection-induced rock deformation that combines a coupled stress–pore pressure analysis with a continuum damage-mechanics-based constitutive relationship. The models include both the natural mechanical stratigraphic variability as well as the in situ stress-state anisotropy, and permit tracking of the temporal and spatial development of shear and tensile permanent strains that develop in response to fluid injection. Our results show that simple, long planar fractures are unlikely to be induced in most mechanically layered natural systems under typical in situ stress conditions. Analyses that assume this type of fracture geometry may significantly overestimate the reach of hydraulically induced fractures and/or effectively stimulated rock volume.

Geomechanical modeling of an extensional fault-propagation fold: Big Brushy Canyon monocline, Sierra Del Carmen, Texas

Field structural data from the Big Brushy Canyon monocline developed in Cretaceous strata of west Texas are combined with nonlinear finite element modeling to help bridge the gap between geometric, kinematic, and mechanical analysis techniques for understanding the deformation history of reservoir-scale geologic structures. The massive Santa Elena Limestone is offset along a steep normal fault, and fault displacement is accommodated upward by the folding of the Buda Limestone and Boquillas Formation and the thinning in the intervening Del Rio Clay. Mesostructures within competent Buda Limestone beds are concentrated in the monocline limb instead of the hinge and include bed-perpendicular veins that accommodate bed-parallel extension and bedding-plane slip surfaces that offset the veins and accommodate flexural slip. Finite element models were constructed to reproduce the monocline geometry and deformation distribution as well as to assess the effect of material properties and boundary conditions on structural evolution. The initial model configuration replicated the assumed predeformational geometry, included frictional sliding surfaces to allow for bedding-parallel slip, and used a displacement boundary condition at the base of the Santa Elena footwall to simulate fault motion. Geometry and strain evolution were tracked so that (1) fold shape, (2) cumulative extension, and (3) layer-parallel shear strain could be compared to field observations. Iterative model runs successfully matched field data and revealed the importance of benchmarking the model results against monocline geometry, layer-parallel extensional strain, and bedding slip in the natural example. Our results illustrate the potential use of this modeling approach whereby calibration is performed using available data and is followed by strain measurement throughout the model domain to aid in prediction of subseismic faults and fractures. This geomechanical modeling approach provides a powerful tool for site-specific subsurface deformation prediction in hydrocarbon reservoirs that incorporates details of the local mechanical stratigraphy and structural setting.

Stress inversion using slip tendency

The in situ stress state is an important controlling factor for the slip behavior of faults and fractures in the earths 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.

Precise positioning with wireless sensor nodes: Monitoring natural hazards in all terrains

Prediction, assessment, and mitigation of surface-affecting natural hazard processes such as landslides, avalanches, earthquakes, and floods call upon geoscientists to rapidly deploy instruments and accurately characterize these earth processes, often with little lead time and under dangerous working conditions. Affected areas may have heavy tree canopies, or high atmospheric dust loads (volcanic eruptions), precluding the use of traditional location techniques like Global Positioning System (GPS). The proliferation of inexpensive radio systems provides a technology that has the potential to redefine the approach to rapid characterization of hazardous earth processes. The research effort described in this paper developed and demonstrated an inexpensive, cooperative radar-like technology for precise distance measurement between intelligent radio nodes.