John N. Hooker

A universal power-law scaling exponent for fracture apertures in sandstones

A high-resolution data set of kinematic aperture (opening displacement) of opening-mode fractures, from large (up to 2 m long) quartz-cemented sandstone samples, shows that microfractures are ubiquitous and that most natural-fracture sets are better fit by power-law size distributions than by exponential, normal, or log-normal distributions. The data set includes 3822 fractures within 68 scanlines from eight formations on three continents. Kinematic apertures were measured along scanlines using scanning electron microscope–based cathodoluminescence (SEM-CL) and, for field data, using a hand lens. Microtextural evidence from SEM-CL shows that power law–distributed fractures typically have crack-seal texture and are composed of opening increments having a narrow (characteristic) aperture size range. In contrast, rare non-power-law–distributed fracture populations lack crack-seal texture. Power-law exponents, as measured in one dimension, have values of −0.8 ± 0.1. Most variation among fracture sets results from power-law coefficients, which constitute a scale-invariant measure of fracture intensity. We show how observed scaling patterns can be used to improve estimations of large-fracture spacing in cases where fracture sampling is limited, as by the width of cores. The low (

Effects of diagenesis (cement precipitation) during fracture opening on fracture aperture-size scaling in carbonate rocks

Abstract A correlation is demonstrated between the presence of crack-seal texture and power-law kinematic aperture-size (width) distributions among opening-mode fractures in rocks of dominantly carbonate mineralogy. Crack-seal opening increments (opening-displacement increment sizes or ‘gaps’) within individual fractures follow narrow normal or log-normal size distributions, suggesting that fracture widening accumulates in characteristic (usually micrometre-scale) size increments. The scale invariance in overall fracture width distributions present in some fracture sets most likely arises from grouping of these increments (localization) to form larger fractures (millimetre- to centimetre-scale widths). Such localization could be a consequence of the tendency for larger, less cemented fractures to break preferentially during subsequent deformation. Cement accumulation patterns thus provide a mechanism for positive feedback whereby large-fracture growth exceeds small-fracture growth. Using characteristically sized growth increments, a fracture growth model accurately simulates fracture arrays having power-law fracture-width distributions. Model parameters can be altered to produce characteristic-width fracture size distributions. The results have implications for how fracture porosity and permeability evolve in carbonate reservoirs.

Fracture size, frequency, and strain in the Cambrian Eriboll Formation sandstones, NW Scotland

Synopsis We investigated fracture intensity of the Cambrian Eriboll Formation sandstone exposures west of the Palaeozoic Moine Thrust Belt (MTB) in NW Scotland, by measuring the number and size of fractures encountered per unit length along lines of observation orientated perpendicular to fracture strike. Eriboll Formation sandstones have at least five fracture sets, each of which includes penetrative arrays of opening-mode microfractures (quartz-filled micro-veins). The size distributions of the fractures can be described by power laws over more than three orders of magnitude. The observed power-law equation parameters suggest that strain is dominated by the largest fractures present in each set; however, field evidence suggests that opening-mode fractures with apertures greater than 1 cm are rare. Strain from microscopic fractures is heterogeneous compared with strain caused by fractures that are sufficiently wide (0.1 mm and above) to be measured on exposures, reflecting greater variation in fracture size apparent at microscopic scales. Although size patterns are from sets formed at different times, consistent cumulative frequency versus aperture slopes suggests that a common mechanism governs organization of fracture sizes. The penetrative sets of natural fractures within the Eriboll Formation record a strain history spanning burial, Caledonian orogenesis, and uplift.

Vein spacing in extending, layered rock: The effect of synkinematic cementation

Cemented fractures (veins) commonly show mm-scale spacing, even in layerbound arrangements in beds of cm- to dm-scale thickness. The relief of tension around such layerbound fractures should preclude nearby fracture propagation and result in cm- to dm-scale fracture spacing. We hypothesize that cement precipitated during vein opening could re-establish tension across veins and lessen the effects of relieved tension, thus decreasing fracture spacing. We test this hypothesis using a computer-based numerical model. The model consists of a 2D triangular lattice of nodes connected by elastic springs. The lattice is stretched by holding the left-boundary stationary and moving the right-boundary to the right at constant velocity. The lattice consists of three layers; springs within the middle fracturing layer fail upon stretching past a given critical length. Springs within the upper and lower matrix layers are indestructible. We tune the model parameters to produce the familiar regular spacing of barren fractures (joints). Then we perturb this system by adding cement within fractures as the fractures propagate and widen. Cementation is simulated by extending failed springs across the space between the nodes on which they are rooted and re-attaching springs once they reach across. Re-attached springs are assigned a new neutral length equal to their current length on re-attachment. The primary effect of cementation is to make fractures narrower and more closely spaced. Thus the model highlights the resistance to fracture widening by cement as a potential reason why veins can be more closely spaced than joints. Individual fractures open and seal multiple times when the stiffness of cemented springs is lower than that of the host-rock springs, suggesting that natural crack-seal vein opening is associated with persistent mechanical weakness at extant fractures. Modeled veins are more irregularly spaced than modeled joints, but the vein patterns remain more regularly spaced than would be expected for a random arrangement. Therefore the model does not explain systematic clustering of natural veins.

Early overpressuring in organic-rich shales during burial: evidence from fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin, UK

Field, petrographic and geochemical analysis of fibrous calcite veins in the Lower Jurassic Shales-with-Beef Member in the Wessex Basin was conducted to investigate the formation mechanism of the veins. Bedding-parallel fibrous calcite veins, including beef veins and tabular cone-in-cone structures, are widespread in the black shales. The calcite veins consist of subvertical fibrous crystals and a dark median zone. The median zone contains scattered clays, pyrite microcrystals, skeletal fragments and amorphous organic matter. The veins exhibit moderate carbon isotope values, ranging from −1.515 to 2.732‰. The oxygen isotope composition ranges from −8.872 to −4.521‰, which is possibly too negative to reflect the primary porewater oxygen isotope signatures and indicates a porewater modification. It is interpreted that the veins mainly derive carbonates from seawater inorganic carbon and bioclasts. The veins formed as closed-system hydraulic fractures in overpressured cells during sediment degassing in the methanogenic zone. The shale beds with a high total organic carbon content could have generated abundant CO2, which may have resulted in either the cementing of the pores in the matrix or overpressure buildup. The skeletal fragments provide a control on the spatial distribution of veins as nuclei for calcite precipitation from supersaturated pore fluids.

Fracturing and fluid flow in a sub-décollement sandstone; or, a leak in the basement

Crack-seal texture within fracture cements in the Triassic El Alamar Formation, NE Mexico, shows that the fractures opened during precipitation of quartz cements; later, overlapping calcite cements further occluded pore space. Previous workers defined four systematic fracture sets, A (oldest) to D (youngest), with relative timing constrained by crosscutting relationships. Quartz fluid inclusion homogenization temperatures are higher within Set B (148 ± 20°C) than in Set C (105 ± 12°C). These data and previous burial history modelling are consistent with Set C forming during exhumation. Fluid inclusions in Set C quartz have higher salinity than those in Set B (22.9 v. 14.2 wt% NaCl equivalent, respectively), and Set C quartz cement is more enriched in 18O (20.2 v. 18.7‰ VSMOW). Under most assumptions about the true temperature during fracture opening, the burial duration, the amount of cement precipitated and fluid-flow patterns, it appears that the fracture fluid became depleted in 18O and enriched in 13C. This isotopic evolution, combined with increasing salinity, suggests that throughout fracture opening there was a gravity-driven influx of fluid from upsection Jurassic evaporites, which form a regional décollement. Fracture opening amid downward fluid motion suggests that fracturing was driven by external stresses such as tectonic stretching or unloading, rather than increases in fluid pressure.

Fluid evolution in fracturing black shales, Appalachian Basin

Opening-mode veins in cores drilled from the mudrocks overlying and underlying the major Silurian salt decollement in the Appalachian plateau (Tioga and Lawrence Counties, Pennsylvania) have mineralogic and isotopic compositions generally matching those of their host mudrocks, suggesting opening and filling amid little cross-stratal fluid motion. Calcite and most trace minerals probably entered the veins via dissolution–reprecipitation from nearby host rock. Consistent with this interpretation are the observations that (1) trace minerals within the veins, including quartz, pyrite, and dolomite, are invariably also present within the layers hosting the veins, with vein cement minerals generally reflecting the abundance and solubility of minerals in the host rock, and (2) carbon and oxygen isotopic compositions of vein-filling calcite are similar to those of calcite within the host rock, with vein-filling δ18O slightly depleted and δ13C slightly enriched. Modeling the fluid isotopic evolution, assuming vein opening and filling amid immobile connate formation water, accounts for these minor but systematic differences, which are attributable to increasing temperature and hydrocarbon maturation. An exception to the above trend is barite, which, despite its low solubility, is systematically enriched in veins with respect to the host rock. It is unclear whether barite precipitation resulted from the influx of external fluids—perhaps deriving from Silurian salt—or from barium mobilized at depth from local clays or organic material.

Lithological control on fracture cementation in the Keuper Marl (Triassic), north Somerset, UK

The spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the Triassic Keuper Marl Formation (UK), a caprock for hydrocarbon reservoirs and CO 2 sequestration. The marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins. The nodular gypsum in evaporite horizons provides excess gypsum for nodule-rooted horizontal gypsum veins. Our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls. We interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks, based on stratigraphic correlations, and much lower Sr concentrations than gypsum nodules. Gypsum was transported to adjacent veins mainly through diffusion in the low-permeability marls. The localization of gypsum veins and varied Sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non-evaporite beds. This results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements. The study implies that the cementation of natural fractures in low-permeability rocks can highly depend on the presence of cement minerals in the host rock.

Dolomite overgrowths suggest a primary origin of cone-in-cone

A long-debated aspect of cone-in-cone structures is whether the mineral aggregates composing the structure precipitated with their conical form (primary cone-in-cone), or whether the cones formed after precipitation (secondary cone-in-cone). A calcite deposit from the Cretaceous of Jordan bears all the defining characteristics of the structure. Trace dolomite within the sample supports the primary cone-in-cone hypothesis. The host sediment is a biosiliceous mudstone containing abundant rhombohedral dolomite grains. Dolomite rhombohedra are also distributed throughout the calcite of the cone-in-cone. The rhombohedra within the calcite locally have dolomite overgrowths that are aligned with calcite fibres. Evidence that dolomite co-precipitated with calcite, and did not replace calcite, includes (i) preferential downward extension of dolomite overgrowths, in the presumed growth-direction of the cone-in-cone, from the dolomite grains on which they nucleate, and (ii) planar, vertical borders between dolomite crystals and calcite fibres. Because dolomite overgrows host-sediment rhombohedra and forms part of the cones, it follows that the host-sediment was incorporated into the growing cone-in-cone as the calcite precipitated, and not afterward. The host-sediment was not injected into the cone-in-cone along fractures, as the secondary-origin theory suggests. This finding implies that cone-in-cone in general does not form over multiple stages, and thus has greater potential to preserve the chemical signature of its original precipitation.

Regional‐scale development of opening‐mode calcite veins due to silica diagenesis

The formation and distribution of natural fractures in Cretaceous–Paleogene strata in Jordan are strongly tied to diagenetic processes, which in turn reflect the lithology of the host material. Observations collected from subsurface cores show that widespread fracturing began before compaction of the host sediment was complete, based on ptygmatic folding of one set of mineral-filled fractures (veins). Non-folded veins are preferentially developed within heavily cemented layers. Calcium carbonate is the greatest volumetric component of the host sediment, and most fractures are at least partially filled by calcite. Dolomite- and silica-bearing fractures are present in dolomitized and silicified host beds, respectively. Horizontal veins are filled by cone-in-cone calcite or, rarely, silica or dolomite. The stratigraphic arrangement and degree of compaction around ptygmatically folded calcite veins and chert nodules suggest that silica diagenesis was an important driver of early fractures. Nevertheless, those fractures were filled with carbonate cements as they opened, based on crack-seal texture of the vein fill. The volume loss associated with silica diagenesis created fracture porosity, which was filled coevally by carbonate cements. The distribution of later veins reflects embrittlement of host layers by cementation and is consistent with crustal deformation as the primary fracture driver.