S. Laubach

West-central Ar izona contains the eastern half of the Maria fold and thrust belt, a belt of large-scale folds and maj or thrust faults that trends east-west, across the northerly grain of the North American Cordillera. Structures within the be lt document a complex superposition of multiple episodes of deforma tion. The earliest preserved Mesozoic structures in several ranges are large southeast-to south-vergent folds and locally associated thrust faults that have overturned and attenuated entire sections of Paleozoic sedimentary and Mesozoic sed imentary and volcanic rocks. Thrust faults with southwest, south, north, and northeast vergence discordan tly truncate these folds and have interleaved the Paleozoic and Mesozoic rocks with Proterozoic and Jurassic crystalline rocks. Deformation was generally accompanied by metamorphism and some thrusts are associated with apparent inverse metamorphic gradients in the lower-plate rocks. The details of the tectonic setting of deformation are uncertain, in part because the timing of deformation is poorly constrained and controversial. Deformation occurred after Jurassic volcanism and subsequent deposition of the McCoy Mountains Formation, but mostly before emplacement of Upper Cretaceous and early Te rtiary plutons.

ABSTRACT Natural fractures have long been suspected as a factor in production from shale reservoirs because gas and oil production commonly exceeds the rates expected from low-porosity and low-permeability shale host rock. Many shale outcrops, cores, and image logs contain fractures or fracture traces, and microseismic event patterns associated with hydraulic-fracture stimulation have been ascribed to natural fracture reactivation. Here we review previous work, and present new core and outcrop data from 18 shale plays that reveal common types of shale fractures and their mineralization, orientation, and size patterns. A wide range of shales have a common suite of types and configurations of fractures: those at high angle to bedding, faults, bed-parallel fractures, early compacted fractures, and fractures associated with concretions. These fractures differ markedly in their prevalence and arrangement within each shale play, however, constituting different fracture stratigraphies-differences that depend on interface and mechanical properties governed by depositional, diagenetic, and structural setting. Several mechanisms may act independently or in combination to cause fracture growth, including differential compaction, local and regional stress changes associated with tectonic events, strain accommodation around large structures, catagenesis, and uplift. Fracture systems in shales are heterogeneous; they can enhance or detract from producibility, augment or reduce rock strength and the propensity to interact with hydraulic-fracture stimulation. Burial history and fracture diagenesis influence fracture attributes and may provide more infoimation for fracture prediction than is commonly appreciated. The role of microfractures in production from shale is currently poorly understood yet potentially critical; we identify a need for further work in this field and on the role of natural fractures generally.

... Opening-mode fractures have an important influence over fluid flow in many dolostones (eg, [Montañez, 1997] , [Antonellini and Mollema, 2000] , [Gale et al., 2004] , [Philip et al., 2005] and [Kosa and Hunt, 2006] ). ... 1) ( [Gale et al., 2004] and [Gale and Gomez, 2007] ). ...

Diagenesis and fracture are often linked processes in deformed rock. Empirical observations show that quartz-lined natural fractures are very common in sandstones that have been exposed to temperatures in excess of 90°C. These fractures exhibit crack-seal textures as well as cement bridges propping the fractures open and preserving fracture porosity. These diagenetic effects are examined in the context of detailed fracture characterizations generated by geomechanical modelling. Aperture, length and fracture network geometry are examined in the context of subcritical crack growth and various biaxial loading boundary conditions of varying initial anisotropy. An isotropic initial state results in more polygonal fracture patterns. A small initial anisotropy creates preferential through-going fractures that are later connected by cross-fractures. A larger initial anisotropy results in only one parallel set. The flow connectivity of isotropic and small strain anisotropic patterns appears ...

... deep subsurface, patterns are governed by the maturation state (Berg and Gangi, 1999), structural position (Corbett et al., 1991), and fracture mechanics attributes of the chalk, such as a high subcritical crack index, that cause fractures to tend to cluster in swarms (Olson, 2004). ...

ABSTRACT Natural opening-mode fractures commonly fall upon a spectrum whose end-members are veins, which have wide ranges of sizes and are mostly or thoroughly cemented, and joints, which have little opening displacement and little or no cement. The vein end-member is common in metamorphic rocks, whose high temperature and pressure of formation place them outside typical reservoir settings; conversely, many uncemented joints likely form near the surface and so too have limited relevance to subsurface exploration. Sampling of cores retrieved from tight-gas sandstone reservoirs suggest that it is intermediate fractures, not true joints or veins, that provide natural porosity and permeability. Such fractures have abundant pore space among fracture-bridging cements, which may hold fractures open despite varying states of stress through time. Thus the more sophisticated our understanding of the processes that form veins and joints, i.e., how natural fracturing varies by depth, the better our ability to predict intermediate fractures. Systematic differences between veins and joints, in terms of size-scaling and lateral and stratigraphic spatial arrangement, have been explained in the literature by the mechanical effects of sedimentary layering, which likely exert more control over fracture patterns at shallower depths. Thus stratabound joints commonly have narrow size ranges and regular spacing; non-stratabound veins have a wide range of sizes and spacings. However, new fieldwork and careful literature review suggest that the effects of mechanical layering are only half the story. Although atypical, veins may be highly stratabound and yet spatially clustered; non-stratabound fractures may nonetheless feature narrow size ranges. These anomalous fracture arrangements are better explained by the presence of precipitating cements during fracture opening than by mechanical layering. Cement is thought to be highly important for fracture permeability, but potential effects of synkinematic cement on fracture size and spacing have been largely overlooked. Such effects are currently poorly understood, but numerical models of fracture widening amid precipitating cements can replicate observed size-scaling patterns. Synkinematic fracture-bridging cements can also potentially account for irregular fracture spacing in stratabound fracture arrays.

ABSTRACT Natural fractures in tight sandstone and shale reservoirs are characterized by partial to complete cementation. In all tight-gas sandstone reservoirs and suitable outcrop reservoir analogs, fractures frequently contain crack-seal quartz and carbonate cement that formed during incremental fracture opening. These synkinematic cements may be followed by blocky postkinematic cement occluding any residual fracture porosity. Fluid inclusion microthermometry combined with Raman analyses demonstrate that synkinematic cement formed under conditions close to maximum burial and incipient exhumation under elevated pore fluid pressures and over time spans of 10-50 m.y.. Fracture opening rates, integrated over the kinematic fracture aperture, are on the order of 10 microm/m.y. Based on the textural evidence of synkinematic cement growth, in combination with kinetic models of quartz cementation, we infer that these rates are comparable to rates of dissolution-precipitation reactions in the host rock, and of mass transfer between host rock and fracture. It is thus suggested that dissolution-precipitation creep is a dominant deformation mechanism allowing accommodation of permanent fracture strain under these deep-burial, diagenetically reactive conditions. Synkinematic mineral reactions in the host rock and precipitation of fracture lining cement guarantee that partially cemented natural fractures remain propped open and thus conductive under production conditions. However, cement linings and bridges can inhibit flow between micro-porous host rock and residual fracture porosity resulting in flow barriers. Complex pore geometry in partially cemented fractures may impede multiphase fracture flow and production. In shale, the interface between host rock and fracture cement is frequently mechanically weak potentially allowing fracture reactivation during well completion. Such artificially reactivated fractures may thus increase flow of production fluids even in formation containing otherwise sealed natural fractures.

ABSTRACT The Cambrian Eriboll Formation quartzarenites contain abundant fractures with varying degrees of quartz cement infill. Fractures exist that are entirely sealed; are locally sealed by bridging cements but preserve pore space among bridges; are mostly open but lined with veneers of cement; or are devoid of cement. Fracture propagation in the Eriboll Formation is highly sensitive to the presence of pre-existing fractures. Fracture reactivation occurs in opening mode as individual fractures repeatedly open and are filled or bridged by syn-kinematic cements. As well, reactivation occurs in shear as opening of one fracture orientation coincides with shear displacement along pre-existing fractures of different orientations. The tendency for pre-existing fractures to slip varies in part by the extent of cement infill, yet we observe shear and opening-mode reactivation even among sealed fractures. Paleotemperature analysis of fluid inclusions within fracture cements suggests some fractures now in outcrop formed deep in the subsurface. Fractures within the Eriboll Formation may therefore affect later fracture propagation throughout geologic time. With progressive strain, fault zones develop within fracture networks by a sequence of opening-mode fracture formation, fracture reactivation and linkage, fragmentation, cataclasis, and the formation of slip surfaces. Cataclasite within fault zones is commonly more thoroughly cemented than fractures in the damage zone or outside the fault zone. This variance of cement abundance is likely the result of (1) continued exposure of freshly broken quartz surfaces within cataclasite, promoting quartz precipitation, and (2) possibly more interconnected pathways for mass transfer within the fault zone. Enhanced cementation of cataclasite results in strengthening or diagenetic strain hardening of the evolving fault zone. Further slip is accommodated by shear localization along discrete slip surfaces. With further linkage of fault segments, new, more favorably oriented slip surfaces form while earlier formed slip surfaces become inactive and cemented. This interaction between mechanical processes of fault-fracture network evolution with cementation leads to complex fault systems with heterogeneous permeability structure.