Nicholas C. Davatzes

Structural and diagenetic control of fluid migration and cementation along the Moab fault, Utah

The Moab fault, a basin-scale normal fault that juxtaposes Jurassic eolian sandstone units against Upper Jurassic and Cretaceous shale and sandstone, is locally associated with extensive calcite and lesser quartz cement. We mapped the distribution of fault-related diagenetic alteration products relative to the fault structure to identify sealing and conductive fault segments for fluid flow and to relate fault–fluid-flow behavior to the internal architecture of the fault zone. Calcite cement occurs as vein and breccia cement along slip surfaces and as discontinuous vein cement and concretions in fault damage zones. The cement predominates along fault segments that are composed of joints, sheared joints, and breccias that overprint earlier deformation bands. Using the distribution of fault-related calcite cement as an indicator of paleofluid migration, we infer that fault-parallel fluid flow was focused along fault segments that were overprinted by joints and sheared joints. Joint density, and thus fault-parallel permeability, is highest at locations of structural complexity such as fault intersections, extensional steps, and fault-segment terminations. The association of calcite with remnant hydrocarbons suggests that calcite precipitation was mediated by the degradation and microbial oxidation of hydrocarbons. We propose that the discontinuous occurrence of microbially mediated calcite cement may impede, but not completely seal, fault-parallel fluid flow. Fault-perpendicular flow, however, is mostly impeded by the juxtaposition of the sandstone units against shale and by shale entrainment. The Moab fault thus exemplifies the complex interaction of fault architecture and diagenetic sealing processes in controlling the hydraulic properties of faults in clastic sequences.

Overprinting faulting mechanisms during the development of multiple fault sets in sandstone, Chimney Rock fault array, Utah, USA

Abstract The deformation mechanisms producing the Chimney Rock normal fault array (San Rafael Swell, Utah, USA) are identified from detailed analyses of the structural components of the faults and their architecture. Faults in this area occur in four sets with oppositely dipping fault pairs striking ENE and WNW. The ENE-striking faults initially developed by formation of deformation bands and associated slip surfaces (deformation mechanism 1). After deformation band formation ceased, three sets of regional joints developed. The oldest two sets of the regional joints, including the most prominent WNW-striking set, were sheared. Localized deformation due to shearing of the WNW-striking regional joints formed WNW-striking map-scale normal faults. The formation mechanism of these faults can be characterized by the shearing of joints that produces splay joints, breccia, and eventually a core of fault rock (deformation mechanism 2). During this second phase of faulting, the ENE-striking faults were reactivated by shear across the slip surfaces and shearing of ENE-striking joints, producing localized splay joints and breccia (similar to deformation mechanism 2) superimposed onto a dense zone of deformation bands from the first phase. We found that new structural components are added to a fault zone as a function of increasing offset for both deformation mechanisms. Conversely, we estimated the magnitude of slip partitioned by the two mechanisms using the fault architecture and the component structures. Our analyses demonstrate that faults in a single rock type and location, with similar length and offset, but forming at different times and under different loading conditions, can have fundamentally different fault architecture. The impact by each mechanism on petrophysical properties of the fault is different. Deformation mechanism 1 produces deformations bands that can act as fluid baffles, whereas deformation mechanism 2 results in networks of joints and breccia that can act as preferred fluid conduits. Consequently, a detailed analysis of fault architecture is essential for establishing an accurate tectonic history, deformation path, and hydraulic properties of a faulted terrain.

Overprinting faulting mechanisms in high porosity sandstones of SE Utah

Normal faults in sandstone of the Jurassic Entrada Formation, South-East Utah formed by two mechanisms: (1) deformation band faulting overprinted by (2) jointing and subsequent shearing along joints. Fundamental structural elements of deformation band faults are single deformation bands, zones of deformation bands, and slip surfaces. Joint-based faults are composed of joints, sheared joints, splay fractures, fragmentation zones, breccia, and fine-grained fault rock. We demonstrate that both mechanisms contribute to slip in a single fault zone with joint-based faulting consistently postdating deformation band faulting at a given location along a fault. The occurrence, distribution, and geometric arrangement of structures formed by the two mechanisms resulted in faults with distinct fault architecture. This fault architecture is related to the relative contributions of each deformation mechanism to the total offset and to their relative timing. Overprinting of a deformation band-based fault by a joint-based mechanism introduces extensive localized structural heterogeneity with a distinct hydraulic signature. Whereas deformation bands tend to act as fluid baffles, joints may act as preferred fluid conduits. Therefore, fluid flow properties such as the permeability of the faults with overlapping mechanisms are expected to change over time accompanying the overprinting process.

Structural evolution of fault zones in sandstone by multiple deformation mechanisms: Moab fault, southeast Utah

Faults in sandstone are frequently composed of two classes of structures: (1) deformation bands and (2) joints and sheared joints. Whereas the former structures are associated with cataclastic deformation, the latter ones represent brittle fracturing, fragmentation, and brecciation. We investigated the distribution of these structures, their formation, and the underlying mechanical controls for their occurrence along the Moab normal fault in southeastern Utah through the use of structural mapping and numerical elastic boundary element modeling. We found that deformation bands occur everywhere along the fault, but with increased density in contractional relays. Joints and sheared joints only occur at intersections and extensional relays. In all locations, joints consistently overprint deformation bands. Localization of joints and sheared joints in extensional relays suggests that their distribution is controlled by local variations in stress state that are due to mechanical interaction between the fault segments. This interpretation is consistent with elastic boundary element models that predict a local reduction in mean stress and least compressive principal stress at intersections and extensional relays. The transition from deformation band to joint formation along these sections of the fault system likely resulted from the combined effects of changes in remote tectonic loading, burial depth, fluid pressure, and rock properties. In the case of the Moab fault, we conclude that the structural heterogeneity in the fault zone is systematically related to the geometric evolution of the fault, the local state of stress associated with fault slip, and the remote loading history. Because the type and distribution of structures affect fault permeability and strength, our results predict systematic variations in these parameters with fault evolution.

Differentiating induced and natural seismicity using space‐time‐magnitude statistics applied to the Coso Geothermal field

A remarkable characteristic of earthquakes is their clustering in time and space, displaying their self-similarity. It remains to be tested if natural and induced earthquakes share the same behavior. We study natural and induced earthquakes comparatively in the same tectonic setting at the Coso Geothermal Field. Covering the preproduction and coproduction periods from 1981 to 2013, we analyze interevent times, spatial dimension, and frequency-size distributions for natural and induced earthquakes. Individually, these distributions are statistically indistinguishable. Determining the distribution of nearest neighbor distances in a combined space-time-magnitude metric, lets us identify clear differences between both kinds of seismicity. Compared to natural earthquakes, induced earthquakes feature a larger population of background seismicity and nearest neighbors at large magnitude rescaled times and small magnitude rescaled distances. Local stress perturbations induced by field operations appear to be strong enough to drive local faults through several seismic cycles and reactivate them after time periods on the order of a year.

Distribution and Nature of Fault Architecture in a Layered Sandstone and Shale Sequence: An Example from the Moab Fault, Utah

We examined the distribution of fault rock and damage zone structures in sandstone and shale along the Moab fault, a basin-scale normal fault with nearly 1 km (0.62 mi) of throw, in southeast Utah. We find that fault rock and damage zone structures vary along strike and dip. Variations are related to changes in fault geometry, faulted slip, lithology, and the mechanism of faulting. In sandstone, we differentiated two structural assemblages: (1) deformation bands, zones of deformation bands, and polished slip surfaces and (2) joints, sheared joints, and breccia. These structural assemblages result from the deformation band-based mechanism and the joint-based mechanism, respectively. Along the Moab fault, where both types of structures are present, joint-based deformation is always younger. Where shale is juxtaposed against the fault, a third faulting mechanism, smearing of shale by ductile deformation and associated shale fault rocks, occurs. Based on the knowledge of these three mechanisms, we projected the distribution of their structural products in three dimensions along idealized fault surfaces and evaluated the potential effect on fluid and hydrocarbon flow. We contend that these mechanisms could be used to facilitate predictions of fault and damage zone structures and their permeability from limited data sets.

Linear complementarity formulation for 3D frictional sliding problems

Frictional sliding on quasi-statically deforming faults and fractures can be modeled efficiently using a linear complementarity formulation. We review the formulation in two dimensions and expand the formulation to three-dimensional problems including problems of orthotropic friction. This formulation accurately reproduces analytical solutions to static Coulomb friction sliding problems. The formulation accounts for opening displacements that can occur near regions of non-planarity even under large confining pressures. Such problems are difficult to solve owing to the coupling of relative displacements and tractions; thus, many geomechanical problems tend to neglect these effects. Simple test cases highlight the importance of including friction and allowing for opening when solving quasi-static fault mechanics models. These results also underscore the importance of considering the effects of non-planarity in modeling processes associated with crustal faulting.

Quantifying the heterogeneity of the tectonic stress field using borehole data

The heterogeneity of the tectonic stress field is a fundamental property which influences earthquake dynamics and subsurface engineering. Self-similar scaling of stress heterogeneities is frequently assumed to explain characteristics of earthquakes such as the magnitude-frequency relation. However, observational evidence for such scaling of the stress field heterogeneity is scarce. We analyze the local stress orientations using image logs of two closely spaced boreholes in the Coso Geothermal Field with sub-vertical and deviated trajectories, respectively, each spanning about 2 km in depth. Both the mean and the standard deviation of stress orientation indicators (borehole breakouts, drilling-induced fractures and petal-centerline fractures) determined from each borehole agree to the limit of the resolution of our method although measurements at specific depths may not. We find that the standard deviation in these boreholes strongly depends on the interval length analyzed, generally increasing up to a wellbore log length of about 600 m and constant for longer intervals. We find the same behavior in global data from the World Stress Map. This suggests that the standard deviation of stress indicators characterizes the heterogeneity of the tectonic stress field rather than the quality of the stress measurement. A large standard deviation of a stress measurement might be an expression of strong crustal heterogeneity rather than of an unreliable stress determination. Robust characterization of stress heterogeneity requires logs that sample stress indicators along a representative sample volume of at least 1 km.