Abstract Fractures in tight gas sandstone remain challenging to characterize or predict accurately. Here we recapitulate recent work on continuity of fracture porosity and its important effect on fluid flow. Natural cement precipitation (diagenesis) in fractures can preserve fluid conduits by propping fractures open or otherwise reducing stress sensitivity of fracture permeability. It can also impede fluid flow by reducing effective fracture length, or occluding porosity. We report patterns of natural fracture growth and decay that are extensively influenced by diagenesis. These patterns typify many fractured siliciclastic and carbonate rocks. We show how appreciation of diagenetic effects can be used to improve accuracy of predictions of fracture attributes and illustrate implications for fluid-flow simulation. Our results also imply that fractures will not tend to close under subsurface loading conditions in many tectonic settings. Chemical alteration and the interactions of diagenetic reactions with rock properties and the in situ stress dictate the location of open fractured flow conduits.

Abstract The interaction among brittle deformation, fluid flow, and diagenesis is displayed at Valley of Fire, southern Nevada, where diagenetic iron oxide and hydroxide stains provided a visible record of paleofluid flow in Jurassic Aztec Sandstone. Deformation features include deformation bands, joints, and faults composed of deformation bands and sheared joints. Faults formed by shear along joints, formation of splay fractures, and linkage of fault segments. Measurements of fault permeability, combined with numerical permeability upscal-ing, indicate that these faults impede cross-fault fluid flow, with cross-fault permeability reduced by two orders of magnitude relative to the host sandstone, whereas fault-parallel permeability is enhanced by nearly one order of magnitude. A reconstruction of paleofluid flow in the Aztec Sandstone is based on detailed mapping of multicolored alteration patterns and their cross-cutting relations with brittle structures. These patterns resulted from syndepositional reddening of the eolian sandstone and repeated episodes of dissolution, mobilization, and reprecipita-tion of iron oxide and hydroxide. The distribution of alteration patterns indicates that regional-scale fluid migration pathways were controlled by stratigraphic contacts, thrust faults, and high-angle oblique-slip faults. Outcrop-scale focusing of fluid flow was controlled by structural heterogeneities such as joints, joint-based faults, and deformation bands as well as the sedimentary architecture. The complex interaction of structural heterogeneities with alteration in this exhumed analog of a fractured and faulted sandstone aquifer is consistent with their measured hydraulic properties demonstrating the significance of structural heterogeneities for focused fluid flow in porous sandstone aquifers.

Abstract Opening-mode fractures in clinker and opal-CT chert spheroids form by growth and coalescence of pores, and are associated with extensive textural and compositional changes in the host material. Extensive inelastic deformation outside the immediate vicinity of fracture tips characterizes these fracture processes as ductile. Fracture formation in clinker is concurrent with high-temperature combustion alteration of diatomaceous mudstone. Fracture formation in chert spheroids is associated with the opal-CT to quartz transition in the same host material during early marine diagenesis. In both cases, growth of elongate pores is attributed to the combined effects of diffusive-fracture growth and flow by solution-precipitation creep. Pore growth and coalescence occur preferentially ahead of fracture tips along two directions oblique to the mean macroscopic fracture direction. This growth process, referred to as side-lobe damage, is interpreted to reflect the shear-stress dependence of pore growth by solution-precipitation creep. The tendency for oblique fracture growth is suppressed by global stress and strain-boundary conditions forcing the fracture along a characteristic zig-zag propagation path that is macroscopically perpendicular to the loading direction. These examples of ductile fracture demonstrate that macroscopic fracture formation is not uniquely associated with damage processes by microfracture at low-temperature ‘brittle’ subsurface conditions. Instead, fracture is a deformation process that can occur due to various inelastic-deformation mechanisms under diverse crustal environments, which include high-temperature conditions.