Peter Eichhubl

Natural fractures in shale: A review and new observations

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 information 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.

Natural fractures in shale

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 information 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.

Evolution of a hydrocarbon migration pathway along basin-bounding faults: Evidence from fault cement

Extensive calcite fault cement has resulted from leakage of Santa Barbara basin fluids and hydrocarbons into the Refugio-Carneros fault, a north-bounding structure to the basin. Calcite cements are only found at the end segments of the 24-km (15-mi)-long fault zone, which has less than 150 m (490 ft) of maximum normal offset. The calcite is contemporaneous with fault movement, as evidenced by pervasive crystal twinning and brecciation, as well as textures indicating repeated episodes of rapid fluid flow and calcite cementation. Based on U-Th dates of the calcite, fluid flow along the fault occurred between 110 and greater than 500 ka, indicating that fluid migration was intermittently active during the recent uplift history of the basin flank. Stable carbon isotopic values of the calcite are 13CPDB = 35 to 41, which means that the carbon source is predominantly thermogenic methane. The composition of fluid inclusions in calcite is consistent with mixing of meteoric and saline water in the presence of liquid and gaseous hydrocarbons. Fluid-inclusion homogenization temperatures of about 80–95C suggest that hot water leaked from 2- to 3-km (1.2- to 1.9-mi) depths in the basin and moved up faults on the basin flank at rates rapid enough to transport substantial heat to shallow depths. Finite-element models show that, in this case, this process requires faulting of an overpressured basin and that a single flow event would have lasted for at least 103 yr.Subsurface fluid pressures at comparable depths in the offshore section today are close to hydrostatic, and therefore, only slow hydrocarbon seepage occurs. When combined with the U-Th age data, this suggests that over a 105-yr timescale, basin fluid flow has evolved from the rapid expulsion of hot water and gas being carried up along active, bounding faults derived from overpressured strata to present hydrostatic conditions of slow, buoyancy-driven seepage of hydrocarbons.

A 48 m.y. history of fracture opening, temperature, and fluid pressure: Cretaceous Travis Peak Formation, East Texas basin

Quartz cement bridges across opening-mode fractures of the Cretaceous Travis Peak Formation provide a textural and fluid inclusion record of incremental fracture opening during the burial evolution of this low-porosity sandstone. Incremental crack-seal fracture opening is inferred based on the banded structure of quartz cement bridges, consisting of up to 700 cement bands averaging ∼5 μm in thickness as observed with scanning electron microscope–cathodoluminescence. Crack-seal layers contain assemblages of aqueous two-phase fluid inclusions. Based on fluid inclusion microthermometry and Raman microprobe analyses, we determined that these inclusions contain methane-saturated brine trapped over temperatures ranging from ∼130°C to ∼154°C. Using textural crosscutting relations of quartz growth increments to infer the sequence of cement growth, we reconstructed the fluid temperature and pore-fluid pressure evolution during fracture opening. In combination with published burial evolution models, this reconstruction indicates that fracture opening started at ca. 48 Ma and above-hydrostatic pore-fluid pressure conditions, and continued under steadily declining pore-fluid pressure during partial exhumation until present times. Individual fractures opened over an ∼48 m.y. time span at rates of 16–23 μm/m.y. These rates suggest that fractures can remain hydraulically active over geologically long times in deep basinal settings.

Paleo-fluid flow and deformation in the Aztec Sandstone at the Valley of Fire, Nevada—Evidence for the coupling of hydrogeologic, diagenetic, and tectonic processes

Paleo-fluid flow conditions are reconstructed for an exhumed faulted and fractured sandstone aquifer, the Jurassic Aztec Sandstone at Valley of Fire, Nevada. This reconstruction is based on detailed mapping of multicolored alteration patterns that resulted from syndepositional reddening of the eolian sandstone and repeated episodes of dissolution, mobilization, and reprecipitation of iron oxide and hydroxide. A first stage of bleaching and local redeposition of hematite is attributed to upward migration of reducing basinal fluid during and subsequent to Late Cretaceous Sevier thrusting and foreland deposition of clastic sediments. A second stage of bleaching and iron remobilization, precipitating predominantly goethite and minor iron sulfates, occurred during Miocene strike-slip faulting associated with Basin and Range tectonics. This second stage is explained by mixing of reducing sulfide-rich basinal fluid with meteoric water entering the aquifer. The distribution of alteration patterns indicates that regional-scale fluid migration pathways were controlled by stratigraphic contacts and by thrust faults, whereas the outcrop-scale focusing of 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 is consistent with their measured hydraulic properties, demonstrating the significance of structural heterogeneities for focused fluid flow in a porous sandstone aquifer.

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.

Focused fluid flow along faults in the Monterey Formation, coastal California

Fluid flow in fractured siliceous mudstone of the Miocene Monterey Formation of California is inferred to be highly focused toward map-scale faults that locally contain extensive amounts of carbonate and minor silica cement. The distance of cross-stratigraphic flow, as inferred based on the strontium isotopic composition of carbonate fault cement, is close to the thickness of the Monterey Formation of 700 m in one of two study locations, Jalama Beach, and less than the formation thickness at another location, Arroyo Burro Beach. Fluid is thus derived from within the Monterey Formation rather than from underlying older units. Based on mass-balance estimates of the fluid volume required for fault cementation at Jalama Beach, the minimum distance of formation-parallel flow into the fault zone is 4 km and possibly >12 km. The inferred distance of flow parallel to the formation into this fault thus exceeds the distance of cross-formational upward flow along the fault by at least a factor of six. The mass-balance estimate requires that fluid flow along the fault is channeled into a pipe-shaped conduit rather than distributed along fault strike. Fluid flow from the surrounding formation into fault pipes is inferred to follow a radial rather than uni- or bilateral flow symmetry, using bedding-confined sets of extension fractures and stratabound breccia bodies. Radial fluid flow toward fault pipes requires fairly isotropic fracture permeability for flow along bedding and a low permeability across bedding. The inferred flow geometry illustrates the combined effect of fault permeability structure, permeability anisotropy of the surrounding formation, and hydraulic head distribution in controlling basinal fluid flow in faulted 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.

Structural control of fluid flow: offshore fluid seepage in the Santa Barbara Basin, California

Abstract Evidence of active and dormant fluid seepage in the Santa Barbara Basin is observed as active venting of gas and oil, bacterial mats, precipitates of authigenic carbonate, and mud and tar volcanoes. Fluid seepage occurs preferentially in the proximity to faults and faulted anticlines, and to slump scarps. Seepage next to faults and anticlines indicates that hydrocarbon migration and pore fluid expulsion is controlled structurally, with faults acting as preferred conduits for fluid flow across units of low matrix permeability.

Testing the basin-centered gas accumulation model using fluid inclusion observations: Southern Piceance Basin, Colorado

The Upper Cretaceous Mesaverde Group in the Piceance Basin, Colorado, is considered a continuous basin-centered gas accumulation in which gas charge of the low-permeability sandstone occurs under high pore-fluid pressure in response to gas generation. High gas pressure favors formation of pervasive systems of opening-mode fractures. This view contrasts with thatofothermodelsoflow-permeabilitygasreservoirsinwhich gas migrates by buoyant drive and accumulates in conventional traps, with fractures an incidental attribute of these reservoirs. We tested the aspects of the basin-centered gas accumulation model as it applies to the Piceance Basin by determining the timing of fracturegrowth and associated temperature,pressure, and fluid-composition conditions using microthermometry and Raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth. Trapping temperatures of methane-saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185°C and then decrease to approximately 158°C over time, which indicates fracture growth during maximum burial conditions. Calculated pore-fluid pressures for methanerich aqueous inclusions of 55 to 110 MPa (7977–15,954 psi) indicate fracture growth under near-lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge. Lack of systematic pore-fluid–pressure trends