Craig B. Forster

Fault zone architecture and permeability structure

Fault zone architecture and related permeability structures form primary controls on fluid flow in upper-crustal, brittle fault zones. We develop qualitative and quantitative schemes for evaluating fault-related permeability structures by using results of field investigations, laboratory permeability measurements, and numerical models offlow within andnearfaultzones.Thequalitativeschemecomparesthepercentageofthetotalfaultzone width composed of fault core materials (e.g., anastomosing slip surfaces, clay-rich gouge, cataclasite,andfaultbreccias)tothepercentageofsubsidiarydamagezonestructures(e.g., kinematically related fracture sets, small faults, and veins). A more quantitative scheme is developed to define a set of indices that characterize fault zone architecture and spatial variability.Thefaultcoreanddamagezonearedistinctstructuralandhydrogeologicunits that reflect the material properties and deformation conditions within a fault zone. Whether a fault zone will act as a conduit, barrier, or combined conduit-barrier system is controlled by the relative percentage of fault core and damage zone structures and the inherent variability in grain scale and fracture permeability. This paper outlines a frameworkforunderstanding,comparing,andcorrelatingthefluidflowpropertiesoffaultzones in various geologic settings.

Permeability of Fault-Related Rocks, and Implications for Hydraulic Structure of Fault Zones

Abstract The permeability structure of a fault zone in granitic rocks has been investigated by laboratory testing of intact core samples from the unfaulted protolith and the two principal fault zone components; the fault core and the damaged zone. The results of two test series performed on rocks obtained from outcrop are reported. First, tests performed at low confining pressure on 2.54-cm-diameter cores indicate how permeability might vary within different components of a fault zone. Second, tests conducted on 5.1-cm-diameter cores at a range of confining pressures (from 2 to 50 MPa) indicate how variations in overburden or pore fluid pressures might influence the permeability structure of faults. Tests performed at low confining pressure indicate that the highest permeabilities are found in the damaged zone (10 −16 –10 −14 m 2 ), lowest permeabilities are in the fault core ( −20 –10 −17 m 2 ), with intermediate permeabilities found in the protolith (10 −17 –10 −16 m 2 ). A similar relationship between permeability and fault zone structure is obtained at progressively greater confining pressure. Although the permeability of each sample decays with increasing confining pressure, the protolith sustains a much greater decline in permeability for a given change in confining pressure than the damaged zone or fault core. This result supports the inference that protolith samples have short, poorly connected fractures that close more easily than the greater number of more throughgoing fractures found in the damaged zone and fault core. The results of these experiments show that, at the coreplug scale, the damaged zone is a region of higher permeability between the fault core and protolith. These results are consistent with previous field-based and in-situ investigations of fluid flow in faults formed in crystalline rocks. We suggest that, where present, the two-part damaged zone-fault core structure can lead to a bulk anisotropy in fault zone permeability. Thus, fault zones with well-developed damaged zones can lead to enhanced fluid flow through a relatively thin tabular region parallel to the fault plane, whereas the fault core restricts fluid flow across the fault. Although this study examined rocks collected from outcrop, correlation with insitu flow tests indicates that our results provide inexact, but useful, insights into the hydromechanical character of faults found in the shallow crust.

Structural heterogeneity and permeability in faulted eolian sandstone: Implications for subsurface modeling of faults

We determined the structure and permeability variations of a 4 km-long normal fault by integrating surface mapping with data from five boreholes drilled through the fault (borehole to tens of meters scale). The Big Hole fault outcrops in the Jurassic Navajo Sandstone, central Utah. A total of 363.2 m of oriented drill core was recovered at two sites where fault displacement is 8 and 3-5 m. The main fault core is a narrow zone of intensely comminuted grains that is a maximum of 30 cm thick and is composed of low-porosity amalgamated deformation bands that have slip surfaces on one or both sides. Probe permeameter measurements showed a permeability decline from greater than 2000 to less than 0.1 md as the fault is approached. Whole-core analyses showed that fault core permeability is less than 1 md and individual deformation band permeability is about 1 md. Using these data, we calculated the bulk permeability of the fault zone. Calculated transverse permeability over length scales of 5-10 m is 30-40 md, approximately 1-4% the value of the host rock. An inverse power mean calculation (representing a fault array with complex geometry) yielded total fault-zone permeabilities of 7-57 md. The bulk fault-zone permeability is most sensitive to variations in fault core thickness, which exhibits the greatest variability of the fault components.

Laboratory characterization of hydromechanical properties of a seismogenic normal fault system

Abstract The Stillwater seismogenic normal fault in Dixie Valley, Nevada has been historically active and is located in an area of high heat flow and hydrothermal activity. Three primary structural elements are identified in the fault zone: a relatively wide fault core (with breccia pods embedded in cataclasites), a damage zone (with arrays of mesoscopic fractures), and protolith. Hydromechanical properties of representative core samples were characterized in the laboratory, and microstructural analyses were conducted using optical and scanning electron microscopy. When deformed in conventional triaxial compression, dilatancy and brittle fracture were observed in each sample. Samples from the core of the fault were relatively weak, with strengths similar to that of unconsolidated fault gouge, whereas granodiorite samples from the protolith were as weak as the core and damage zone samples were stronger. Permeability is dependent on effective pressure, porosity and connectivity of the pore space, with values ranging over four orders of magnitude among the core samples. The lowest permeability of 3×10 −20 m 2 was measured in a fault core sample with a microstructure indicative of implosion brecciation. In conjunction with field measurements, the laboratory data suggest that fluid flow and changes in fluid storage are concentrated in the damage zone, with permeability several orders of magnitude higher than the protolith and fault core. Permeability contrast (one order of magnitude) at the core sample scale exists between the cataclasite and implosion breccia in the fault core. Because of dilatancy and poor drainage in the breccia pods, anomalously low pore pressures may develop in localized clusters due to dilatancy hardening during the preseismic period. These clusters of low pore pressure can act similarly to fault jogs, locally inhibiting fault rupture and inducing brecciation when the delayed failure finally occurs by catastrophic implosion.

Hydrogeology of thrust faults and crystalline thrust sheets: Results of combined field and modeling studies

Field, laboratory, and modeling studies of faulted rock yield insight into the hydraulic character of thrust faults. Late-stage faults comprise foliated and subparallel faults, with clay-rich gouge and fracture zones, that yield interpenetrating layers of low-permeability gouge and higher-permeability damage zones. Laboratory testing suggests a permeability contrast of two orders of magnitude between gouge and damage zones. Layers of differing permeability lead to overall permeability anisotropy with maximum permeability within the plane of the fault and minimum permeability perpendicular to the fault plane. Numerical modeling of regional-scale fluid flow and heat transport illustrates the impact of fault zone hydrogeology on fluid flux, fluid pore pressure, and temperature in the vicinity of a crystalline thrust sheet.

Detailed internal architecture of a fluvial channel sandstone determined from outcrop, cores, and 3-D ground-penetrating radar: Example from the middle Cretaceous Ferron Sandstone, east-central Utah

Ideally, characterization of hydrocarbon reservoirs requires information about heterogeneity at a submeter scale in three dimensions. Detailed geologic information and permeability data from surface and cliff face outcrops and boreholes in the alluvial part of the Ferron Sandstone are integrated here with three-dimensional (3-D) ground-penetrating radar (GPR) data for analysis of a near-surface sandstone reservoir analog in fluvial channel deposits. The GPR survey covers a volume with a surface area of 40 x 16.5 m and a depth of 12 m. Five architectural elements are identified and described in outcrop and well cores, using a sixfold hierarchy of bounding surfaces. Internally, the lower four units consist of fine-grained, parallel-laminated sandstone, and the upper unit consists of medium-grained, trough cross-bedded sandstone. The same sedimentary architectural elements and associated bounding surfaces are distinguished in the GPR data by making use of principles developed in seismic stratigraphic analysis. To facilitate comparison of geologic features in the depth domain and radar reflectors in the time domain, the radar data are depth migrated. The GPR interpretation is carried out mainly on migrated 100 MHz data with a vertical resolution of about 0.5 m. Measures of the spatial continuity and variation of the first- and second-order bounding surfaces are obtained by computing 3-D experimental variograms for each architectural element (each radar (Begin page 1584) facies). The maximum correlation length of the dominant internal features ranges between 4 and 6 m, and the anisotropy factor ranges between 0.6 and 0.95.

3‐D characterization of a clastic reservoir analog: From 3‐D GPR data to a 3‐D fluid permeability model

A three‐dimensional (3‐D) 100 MHz ground‐penetrating radar (GPR) data volume is the basis of in‐situ characterization of a fluvial reservoir analog in the Ferron Sandstone of east‐central Utah. We use the GPR reflection times to image the bounding surfaces via 3‐D velocity estimation and depth migration, and we use the 3‐D amplitude distribution to generate a geostatistical model of the dimensions, orientations, and geometries of the internal structures from the surface down to ∼12 m depth. Each sedimentological element is assigned a realistic fluid permeability distribution by kriging with the 3‐D correlation structures derived from the GPR data and which are constrained by the permeabilities measured in cores and in plugs extracted from the adjacent cliff face. The 3‐D GPR image shows that GPR facies changes can be interpreted to locate sedimentological bounding surfaces, even when the surfaces do not correspond to strong GPR reflections. The site contains two main sedimentary regimes. The upper ∼5 m co...

Observations concerning the vigor of hydrothermal circulation in young oceanic crust

A detailed suite of seafloor heat flow measurements and seismic reflection profiles has been completed in a young (circa 1 Ma) area on the eastern flank of the Juan de Fuca Ridge that is characterized by unusually smooth basement topography and uniform sediment cover. Measurements spaced nominally 100 m apart along one 5-km-long line segment define a coherent pattern of heat flow variation. The profile exhibits a series of four maxima and minima with an average half-wavelength of 600 m and an amplitude of variation of 35 mW m−2, roughly 15% of the average background heat flow of 270 mW m−2. The heat flow variations are uncorrelated with local basement topography or sediment thickness variations and may reflect cellular convection in the extrusive layer of the igneous oceanic crust. This layer, which is bounded above by low-permeability sediments and below by low-permeability intrusive rocks, is imaged locally along a multichannel seismic reflection profile and estimated to be about 600 m thick. Temperatures estimated at the top of this layer by extrapolation of the seafloor heat flow measurements average roughly 40°C and vary between adjacent heat flow maxima and minima by only about 7 K. These observations, together with a series of numerical simulations of hydrothermal circulation in a confined, permeable upper crustal layer, provide quantitative insights into the convection process. Values of upper crustal permeability used in the simulations ranged from below the critical value required for convective instability to roughly 2 orders of magnitude above. Results show the amplitudes of lateral seafloor heat flow variability, upper basement temperature variability, and fluid-pressure variability to be strongly dependent on permeability. For example, the amplitude of heat flow variations is predicted to increase rapidly above critical conditions, from zero at a subcritical permeability (k) of 2 × 10−14 m2 to 75 mW m−2 at k = 5 × 10−14 m2 and to a maximum of 90 mW m−2 at k = 8 × l0−14 m2. Predicted variability then falls roughly as k−1/2, reaching the level observed in the Juan de Fuca Ridge flank study area at a permeability of 2 × 10−12 m2. Although a value at near-critical conditions is also allowed by the results, the higher value is probably correct, for it is known from other observations that hydrothermal circulation persists in crust of much greater age, despite the effect of chemical alteration, which reduces permeability, and thermal aging, which reduces buoyancy driving forces. The value of average permeability thus estimated for the upper oceanic crust is more than 1 order of magnitude greater than values determined in deep-ocean boreholes. The borehole values may be lower because they are representative of older and/or more highly altered crust or because they do not correctly represent the permeability at the full scale of the convective system.

Impact of seafloor sediment permability and thickness on off‐axis hydrothermal circulation: Juan de Fuca Ridge eastern flank

Sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust. Interlayered silt-rich and clay-rich sediments almost completely bury basement on the eastern flank of the Juan de Fuca Ridge. Laboratory measurements indicate that silt-rich sediment permeability is 1 to 2 orders of magnitude greater than clay-rich sediment permeability at similar depths. Numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor. The layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data. Two distinct patterns of hydrothermal circulation are obtained, depending on whether silt-rich or clay-rich sediments compose the sediment layer above a permeable upper basement aquifer. A clay-rich sediment column nearly isolates circulation within the basement aquifer, resulting in closed convection. Computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the Juan de Fuca Ridge. Computed vertical fluid fluxes across the sediment column are small, in agreement with fluid fluxes estimated from sediment geochemical profiles. When the sediments are silt-rich, 17% of the flow within a convection cell crosses the sediment column. Open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt. Each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow. A transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt-rich column. Thus the addition of only a small amount of clay to an otherwise silt-rich sediment column may effectively isolate the crustal aquifer from the ocean.

Mediated Modeling: Using Collaborative Processes to Integrate Scientist and Stakeholder Knowledge about Greenhouse Gas Emissions in an Urban Ecosystem

Technical experts often find they need to include critical information from local residents to address environmental issues. To access this information while simultaneously educating participants, we used a participatory process called mediated modeling. Mediated modeling provides a structured framework for enabling diverse stakeholders to use systems modeling to understand the multidimensional, dynamic, and interactive aspects of environmental problems. We invited scientists, decision makers, and stakeholders from the Salt Lake City area to participate in a series of workshops designed to create a relatively simple model of urban ecosystem processes operating in the Salt Lake Valley, UT. This article describes the mediated modeling process, including four activities used to help participants think systemically about the local airshed and enable them to identify key issues, concerns, and interrelationships between variables affecting emissions. Our analysis indicates that participants gained a greater understanding of complexity and system dynamics related to the urban airshed.