Ronald N. McGinnis

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.

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.

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.

Deformation analysis of tuffaceous sediments in the Volcanic Tableland near Bishop, California

Small-scale brittle faults and fractures that cut bedded tuffaceous sediments of variable textures and grain sizes were studied in a 110-m-long cutbank exposure of poorly consolidated sediments at the southern erosional boundary of the Volcanic Tableland, Owens Valley, California. This study was motivated by the need to evaluate potential length scales for lateral flow in nonwelded bedded tuffs and tuffaceous sediments at Yucca Mountain, Nevada—the site of a potential high-level radioactive waste repository. Small-displacement ( 20 cm). Vertical fractures are present throughout the exposure, but fracture frequency is generally highest in the vicinity of larger faults. Fault zones are characterized by grain-size reduction and discrete slip surfaces, the number of which increases with increasing displacement. A semiquantitative stress field interpretation, based on tectonic constraints and reconstruction of overburden thicknesses, yields a simple history of burial, deformation, and exhumation under continuous tectonic extension. We interpret the deformation to include shear (faults), hybrid (faults, nonvertical fractures), and tensile (vertical fractures) failure of the tuffaceous sediments under conditions of low overburden stress (<2.5 MPa). The intersecting network of faults and fractures is characterized by grain comminution, cementation, and fracture dilation. These features, in conjunction with stratigraphic layering, likely produce anisotropic permeability, where maximum permeability is parallel to fault and fracture strike.

Fault frequency and strain

Faults are among the most numerous deformation features on Earth and are common on other planetary bodies in the solar system. Small faults, in terms of either displacement or trace length, outnumber large faults, and the distribution is well-described by some form of power-law relationship. However, this observation may be of limited practical use for inferring fault populations in inaccessible locations. Toward developing a practical approach to fault prediction, we measured the spacing, orientations, slip directions, and slip magnitudes (our measure of fault size) of exposed normal faults in outcrops of Cretaceous carbonate rocks in Texas. Using data from these observed faults, we calculated extensional strain for each study locality, and we demonstrate that fault frequency correlates with extensional strain. This approach provides a tool for estimating fault frequency in areas where strain can be inferred or determined and for extrapolating fault frequency from data sets with limited resolution.

Pitfalls of using entrenched fracture relationships: Fractures in bedded carbonates of the Hidden Valley Fault Zone, Canyon Lake Gorge, Comal County, Texas

Characterizing natural fracture systems involves understanding fracture types (faults, joints, and veins), patterns (orientations, sets, and spacing within sets), size distributions (penetration across layering, aperture, and trace length), and timing relationships. Traditionally, observation-based relationships to lithology, mechanical stratigraphy, bed thickness, structural position, failure mode, and stress history have been proposed for predicting fracture spacing along with the relative abundance of opening-mode fracture versus faults in fractured rocks. Developing a conceptual fracture model from these relationships can be a useful process to help predict deformation in a fractured reservoir or other fractured rock systems. A major pitfall when developing these models is using assumptions based on general relationships that are often site specific rather than universal. In this paper, we examine a mixed carbonate-shale sequence that is cut by a seismic-scale normal fault where fracture attributes do not follow commonly reported fracture relationships. Specifically, we find (1) no clear relationship between frequency (or spacing) of opening-mode fractures (joints and veins) and proximity to the main fault trace and (2) no detectable relationship between fracture spacing and bed thickness. However, we did find that (1) the frequency of small-displacement faults is strongly and positively correlated with proximity to the main fault trace, (2) fracture networks change pattern and failure mode (extension versus shear fracture) from pavement to pavement through the mechanically layered stratigraphic section, and (3) faults are more abundant than opening-mode fractures in many areas within the fracture network. We interpret that the major fracturing initiated near maximum burial under relatively high-differential stress conditions where shear failure dominated and that mode-1 extension fracturing occurred later under lower differential stress conditions, filling in between earlier formed shear fractures. We conclude that whenever possible, site-specific observations need to be carefully analyzed prior to developing fracture models and perhaps a different set of fracture network rules apply in rocks where shear failure dominates and mechanical stratigraphy influences deformation.

Fault displacement gradients on normal faults and associated deformation

Faults are important components of hydrocarbon and other reservoirs; they can affect trapping of fluids, flow pathways, compartmentalization, production rates, and through these, production strategies and economic outcomes. Displacement gradients on faults are associated with off-fault deformation, which can be manifest as faulting, extension fracturing, or folding. In this work, displacement gradients—both in the slip direction and laterally—on a well-exposed large-displacement (seismic-scale) normal fault within the Balcones fault system of south-central Texas are correlated with anomalous deformation patterns adjacent to the fault. This anomalous deformation consists of two superimposed small-displacement fault systems, including (1) an earlier set that formed in response to a displacement gradient in the slip direction, and (2) a later set of oblique faults that formed in a perturbed stress-and-strain field in response to a lateral displacement gradient on the fault. Bed dip, fault-cutoff relationships, and small-displacement fault patterns in the adjacent rock volume inform strain and paleostress estimates. Results indicate that seismically resolvable displacement gradients on and bed dips adjacent to the seismic-scale fault provide a means by which the smaller (subseismic-scale and off-fault) deformation features can be predicted both in terms of orientation and intensity. Specifically, lateral displacement gradients on a normal fault with dip-slip displacement will generate fault-strike-parallel extension, causing anomalously oriented (in the far-field stress context) deformation features adjacent to the fault. Displacement gradient analysis can be used to help predict the characteristics of subseismic-scale deformation within a reservoir adjacent to a seismic-scale normal fault.

Hydrogeologic heterogeneity of faulted and fractured Glass Mountain bedded tuffaceous sediments and ash-fall deposits: The Crucifix site near Bishop, California

Lithologic, macrostructural, microstructural, geophysical, and in situ gas permeability data from a natural exposure of highly porous, faulted and fractured tuffaceous sediments and interbedded ash-fall deposits near Bishop, California, are presented and analyzed in relation to published geologic information. This natural analog study was motivated by the need to evaluate potential length scales over which lateral flow diversion might occur above and within the nonwelded Paintbrush Tuff at Yucca Mountain, Nevada. Lateral diversion of flow in the overlying Paintbrush Tuff was previously proposed by others as a natural barrier that might protect a proposed high-level radioactive waste and spent nuclear fuel repository from percolating water. Because the length scale for capillary barrier breakthrough and leakage is decreased in the presence of subvertical structural heterogeneities, we characterized a horst-bounding fault, small-displacement normal faults within a footwall deformation zone, and secondary heterogeneities within two beds dissected by the faults. Critical deformation-related features that may influence fluid flow within bedded tuffaceous sediments include (1) permeability anisotropy imposed by steeply dipping faults and stratigraphic layering; (2) fault zone widths and styles, which are dependent on bed thickness and ash, glass, and clay content; and (3) fracture intensities and overprinting mechanisms (associated with fault deformation and vertical and nonvertical fracture orientations), which strongly influence the hydrogeologic heterogeneity of units they dissect. Microstructural analysis reveals structurally induced porosity variations at the micrometer to millimeter scale, gas permeability data show the influence of deformation on permeability at the centimeter to tens of centimeters scale, and resistivity and ground-penetrating radar data show lateral variations on the meter to tens of meters scale in horizontally bedded layers. All together, these observations and data show heterogeneity over seven orders of magnitude of length scale. Structurally enhanced porosity and permeability heterogeneities will tend to limit the length scale of lateral flow diversion, redirect flow downward, and enhance vertical fluid movement within the vadose zone.