Basins in western China produce hydrocarbons from 8,000 m deep and have been penetrated to 10,000 m, but the mechanical and petrophysical properties of deep and ultra-deep rocks are unclear and the origins of porosity and permeability remain a mystery. Our research used core samples from a depth of 7,600 m and mechanical tests to document the likely structural and porosity evolution of sandstone due to burial to 10,000 m. During triaxial tests, we characterized microstructure evolution using micro-CT scanning images and acoustic emissions and monitored stress and strain characteristics in high-temperature and high-pressure fluid environments. Under ultra deep-burial conditions, our samples deformed by pore collapse and pore distortion and brittle and ductile fracture, independently or concurrently. Under increasing triaxial stress, temperature and fluid pressure, sandstones initially lose porosity and permeability by pore collapse and compaction then develop a network of interconnected pores and fractures. Consequently, porosity can reach 8

In New York State and Pennsylvania, USA, Precambrian metamorphic and intrusive rocks and Cambrian to Lower Ordovician sedimentary rocks are reservoir targets for deep direct-use geothermal development. Evaluation of natural fractures and structures in the potential reservoir units at the Cornell University Borehole Observatory site was conducted through cross-scale evaluation of oriented sidewall cores, borehole image (BHI), and far-field acoustic survey data. Oriented sidewall cores in the basement complex (Cayuta Formation) reveal metasediments containing foliations, lineations, mineral-filled fractures, and breccia intervals. Basement sidewall core fracture data aid identification of fractures in BHI surveys riddled with borehole breakouts. In contrast, sidewall and image log data for the Cambrian-Ordovician sedimentary section show that open fractures are present and allow orientation and abundance to be estimated. At various depths sandstone and dolostone sidewall cores contain quartz-filled or carbonate-filled bed-normal and -parallel microfractures. Four subvertical microfracture sets, formed sequentially, strike NW-SE (F1), NE-SW (F2), N-S (F3), and WSW-ENE (F4). Microfracture set orientations F1, F2, and F4 match interpretations of acoustic fracture anomalies (open fractures) located tens of meters from the wellbore. In the uppermost Galway Formation sandstone, common microfracture apertures are 0.001 to 0.01 mm. The widest microfractures transition to quartz-lined and bridged open macrofractures. An open vertical macrofracture in Galway sandstone is observed in BHI surveys and a sidewall core, effectively ground-truthing the F4 fracture set. Based on comparison of core fractures with borehole image survey features, differentiation of natural from drilling-induced fractures reveals three sedimentary rock zones of elevated natural fracture frequency.

The Barra Velha Formation is a prolific Aptian (Lower Cretaceous) oil and gas producer located in the Santos Basin, southeastern Brazilian coast. We used structural core description, petrography, geochemistry, and geochronology to show evidence from drill cores that the Barra Velha Formation was affected by several events of brittle deformation and interacting hydrothermal alteration. The U-Pb geochronology provides an age of ca. 116 Ma for depositional host rock calcites, although recrystallization is possible. An early brecciation event occurred at ca. 108 Ma. Recracked fractures (>10 cm wide) are interpreted as spring mound vents. A dolomite fault mineralization is dated at ca. 95 Ma. Events affecting fractures include widespread dissolution, with secondary porosity filled by barite, quartz, and solid bitumen. Fluid inclusion geochemistry suggests that diagenesis occurred with the participation of seawater. We show a timeline of diagenetic events and infer that fracture porosity is more likely to be preserved where deep faults enabled fluids from greater depths to ascend into the reservoir, where they created large vuggy cavities by dissolution of host rock.

Faden quartz is characterized by a central thread-like fluid-inclusion-rich zone surrounded by a wide, clear, faceted rim. Typically found in fractures (veins) within low-temperature metamorphic rocks, the origins of Faden quartz remain contentious. We use scanning electron microscope-based cathodoluminescence and charge contrast imaging microscopy to reveal that Faden quartz threads consist of closely spaced, narrow microfractures (gap deposits) filled with quartz, which traps fluid inclusions and is surrounded by postkinematic lateral quartz deposits. Gap deposits form by the crack-seal mechanism of sequential breakage, fracture opening, and quartz precipitation. Faden quartz forms by the same mechanism as crack-seal quartz bridges found in some fractures formed under diagenetic conditions. Most fluid inclusions trapped within crack-seal gap deposits of Faden crystals from the Zhob region, Pakistan show a narrow range of homogenization temperatures between 140 degrees C and 147 degrees C and salinities of 3.5-5.0 wt% NaCl equivalents. Spanning quartz results in fracture-normal lengths of several tens of centimeters composed of narrow (5-10 mu m) crack-seal deposits. Rapid widening relative to quartz accumulation and apertures enabled these fractures to remain open and function as fluid conduits in the subsurface for millions of years.

Rock fractures are a key contributor to a broad array of Earth surface processes due to their direct control on rock strength as well as rock porosity and permeability. However, to date, there has been no standardization for the quantification of rock fractures in surface process research. In this work, the case is made for standardization within fracture-focused research, and prior work is reviewed to identify various key datasets and methodologies. Then, a suite of standardized methods is presented as a starting “baseline” for fracture-based research in surface process studies. These methods have been shown in pre-existing work from structural geology, geotechnical engineering, and surface process disciplines to comprise best practices for the characterization of fractures in clasts and outcrops. This practical, accessible, and detailed guide can be readily employed across all fracture-focused weathering and geomorphology applications. The wide adoption of a baseline of data collected using the same methods will enable comparison and compilation of datasets among studies globally and will ultimately lead to a better understanding of the links and feedbacks between rock fracture and landscape evolution.

Fracture lengths govern permeability and are unknowns in geothermal assessment. Along their lengths, fracture widths vary due to growth by linkage. Under the influence of diagenesis, narrow widths seal, breaking porosity continuity and reducing open length. The largest range of widths and thus susceptibility to fill occurs where fractures are linked by narrow segments. Outcrops of a geothermal target, Cambrian Potsdam quartz arenite, contain opening-mode fractures having lengths spanning five orders of magnitude from 0.082 mm to 17.9 m. Combined lengths measured at a range of scales can be described by power laws, but at a given image resolution, lengths are best fit by exponential functions. Owing to preferential sealing of small fractures, open fractures follow exponential functions, but values depend on rules for designating fractures as continuous. En échelon segments, offset 10 mm, are connected by narrow fractures or microfractures (hard linked) not evident on outcrop 1 m-elevation LiDAR or 30 m-height drone images. A rule that identifies where narrow and likely connected segments are located can yield lengths meaningful for flow simulation. Depending on diagenesis, continuity rules can halve or double average and maximum length values. Length values from outcrop for geothermal applications should be adjusted based on wellsite-specific diagenesis information.

To better understand the porosity, strength, chemical reactivity, and patterns of fractures in the subsurface, we used evidence from mineral deposits in open fractures to unravel how fracture growth and diagenesis interact to create and destroy fracture porosity. Quartz cement textures and associated fluid inclusions and thermal histories provided data used to infer the duration and rates at which fractures open.

Fracture spatial patterns can strongly affect fluid flow in the subsurface. Proximity and distribution of fractures control reservoir flow behavior over various length scales. In many studies, however, simplified geometrical patterns are generated for fractures which may lead to unrealistic subsurface models. Here we introduce a new method for characterization and modeling of fracture spatial patterns based on outcrop observations. We use Ripley's K-function to characterize the arrangement of fracture barycenters and intersection points over various length scales. In addition, we use semivariograms to quantify spatial correlation in fracture intensity maps. Using this information, we develop a stochastic algorithm that generates two-dimensional fracture network realizations with spatial properties similar to those of a real fracture network measured in the field. Numerical simulation models indicate that the generated fracture realizations exhibit similar flow behaviors as that of the original fracture network. Such modeling tools expand and improve our capability in building representative fracture models and in quantification of uncertainty in naturally and hydraulically fractured reservoirs.

Arrays of natural opening-mode fractures show systematic patterns in size and spatial arrangement. The controls on these factors are enigmatic, but in many cases the depth of formation appears to be critical. Physical, potentially depth-dependent factors that could account for these variations include confining stress, fluid pressure, and strain rate; these factors are common inputs to existing fracture models. However, temperature-dependent chemical processes likely exert an equally important control on patterns, and such processes have not yet been rigorously incorporated into models of fracture formation. Here we present a spring-lattice model that simulates fracturing in extending sedimentary rock beds, while explicitly accounting for cementation during opening of fractures, and for rock failure via both elastic and time-dependent failure criteria. Results illustrate three distinct fracturing behaviors having documented natural analogs, which we here term fracture facies. “Exclusionary macrofracturing” occurs at shallow levels and produces large, widely spaced, uncemented fractures; “multi-scale fracturing” occurs at moderate depth and produces partially cemented fractures having a wide range of sizes and spacings; and “penetrative microfracturing” occurs at great depth and produces myriad narrow, sealed fractures that are closely and regularly spaced. The effect of depth is primarily to accelerate both dissolution and precipitation reactions via increased temperature and porewater salinity; the specific depth range of each fracture facies will vary by host-rock lithology, grain size, strain rate, and thermal history.

This volume describes progress in understanding brittle structures in deep and ultra-deep (>4 km to > 7 km) sedimentary basins. Under deep conditions in sandstone, carbonate rocks, shale, and other rocks, fluid charge and resource recovery are sensitive to faults and opening-mode fractures. In China, work is in progress on deep, deformed, and tectonically active basins including drilling of wells expected to exceed 10 km in depth. Papers describing fractures in horizontal wells indicate locally highly clustered spatial arrangements. Orientation patterns record protracted superposed deformation. Despite deep settings, open fractures are abundant, and wide (>1 mm) fractures with varying amounts of sealing calcite are common. Differences in cement abundances are due to the diagenetic history of fractures, not their origins (e.g., tectonic loading or elevated pore fluid pressure). In carbonate rocks, solution enhanced strike-slip faults and fractures with cavernous porosity are present, and in sandstone enigmatic enhanced host-rock porosity halos a few mm wide locally surround sealed fractures. Owing to differences in thermal exposure due to recent (>6 Ma to present) rapid deep burial (in some cases >2000 m) and locally low geothermal gradients, some fractures at great depths are less diagenetically altered than those at shallower depths in the North American Cordillera. Contrasts in diagenesis may affect fracture size, spatial arrangement, and connectivity.

<p>Under diagenetic conditions between ca. 50 ℃ to 250 ℃ the systematics of cement precipitation and differential infill makes network porosity, and thus permeability and strength, scale and thermal history dependent. Using examples of regional opening-mode fractures in sandstones from the Cambrian Flathead Formation, Wyoming, a low-enthalpy geothermal outcrop analog, we show that quartz deposits preferentially fill fractures up to ca. 0.05 mm wide with a transition from mostly sealed to mostly open fractures over a narrow size range of opening displacements from 0.05 to 0.1 mm. Scale- and diagenesis- dependent connectivity can be described using use rule-based node descriptions to rapidly measure diagenesis sensitive connections within the context of current field practice.  In our example, although networks have trace connectivity, effective connectivity for fluid flow is greatly reduced by quartz cement. Near some faults, trace connectivity increases as initially wide porous fractures preferentially shear and wing cracks form, increasing fracture intersections (Y-nodes). However, pore space is lost due to the development of quartz-cemented microbreccia. Macro-scale trace connectivity increases, but porous connectivity diminishes and thus potential for fluid flow is markedly lower. We illustrate how diagenesis-sensitive contingent nodes can be used to extrapolate permeability estimates to locations having different thermal histories.</p>

Spatial arrangement of fractures as a function of scale is an important component of fracture quantification for inferential and predictive modeling. Available methods that analyze fracture spatial arrangement are based on one-dimensional spacing data; therefore, they are limited to semi-parallel fractures. Such methods cannot be applied to fracture networks in higher dimensions, particularly when fractures have different orientations. To characterize fracture arrangements in two dimensions, we propose using Ripley's K-function, as a method of point pattern analysis, to quantify spatial arrangement of fracture nodes. Fracture nodes, such as barycenters, intersection points, and tips, are point-based representations of fracture locations and connectivity within the fracture network. We introduce formulations for isotropic as well as directional analyses of spatial arrangement. In addition, we derive formulations for edge correction in circular and rectangular study domains. Finally, we demonstrate applications of Ripley's K-function on two natural fracture datasets. Our proposed method supports quantification and characterization of fracture spatial arrangements that allow practitioners to build representative models of fractures in the subsurface.

Using image logs from horizontal wells in the asymmetric East Painter Reservoir anticline in Wyoming, scanlines and petrographic observations from nearby outcrops, and methods that allow statistical quantification of fracture spatial arrangement patterns (including clustering), we show that in early Jurassic Nugget Formation sandstone degree of clustering and intensity of fractures varies with structural position. In the backlimb, fractures are markedly more clustered than random, and hierarchical and regularly spaced clusters are present, but intensity is low. In the forelimb, arrangements are mostly indistinguishable from random, but intensity is high. Based on image log response, the backlimb contains a higher proportion of open fractures, although the forelimb has greater numbers of open fractures. Spatial arrangement and intensity patterns are similar in a forelimb outcrop. Petrography reveals that quartz seals fractures less than 0.15 mm wide. Wider fractures are open with quartz rinds or are quartz lined and sealed with calcite or filled by quartzose cataclasite. The proportion of open fractures depends on quartz and calcite deposits and closure by reactivation (shearing) of pre-existing quartz -lined fractures. Based on increased abundance and greater strike dispersion, results suggest that in forelimbs, high fracture intensities reflect shear on preexisting fractures, possibly resulting in more numerous but less spatially correlated open fractures. The most prolific gas and water production wells are in the backlimb, sug-gesting that clustered but sparse quartz-lined open fractures are more effective fluid conduits than closely spaced partly damaged/sheared arrays.

Abstract Improved understanding of hydraulic fractures is needed to optimize petroleum well drilling and completion strategies. Yet direct observations of hydraulic fractures are rarely made, and reliance is placed on indirect methods such as microseismic monitoring, interference tests, fibre optic detection of fracturing in adjacent wells and numerical modelling. While these techniques provide useful insights, verification of such studies is commonly lacking; core taken through stimulated intervals offers a robust option for verification. We make the case that core can provide complementary information at a different scale from other data types. Core from a slant well adjacent to two stimulated wells at the Hydraulic Fracture Test Site (HFTS1) in West Texas revealed 375 hydraulic fractures, striking 090°±20°, subparallel to present-day S Hmax . There are more hydraulic fractures than expected, and clustering across a range of scales from approximately 1 cm to 50 m. The largest cluster correlates with high microseismic event density. Diversion, bifurcation, and segmentation structures may account for the large number of fractures observed and the orientation spread. Reactivation of sealed natural fractures and bedding planes is relatively uncommon. Proppant sand packs and patches occur in a few locations, particularly where hydraulic fractures are complex.

Using examples of regional opening-mode fractures in sandstones from the Cambrian Flathead Formation, Wyoming, we show that quartz deposits preferentially fill fractures up to ca. 0.05 mm wide and fractures transition from being mostly sealed to mostly open over a narrow size range of opening displacements from 0.05 to 0.1 mm. In our example, although isolated (I-node) dominated networks have some trace connectivity, the effective connectivity for fluid flow is likely greatly reduced by quartz cementation. Trace connectivity at microscopic and outcrop scale is similar, but most porosity is found in outcrop-scale fractures. Near faults, trace connectivity increases as initially wide porous fractures preferentially shear and wing cracks form, increasing fracture intersections (Y-nodes). However, pore space is lost due to the development of microbreccia. Macroscale trace connectivity increases, but porous connectivity diminishes and thus potential for fluid flow is markedly lower. Connectivity descriptions should include accurate measures of widths and lengths and use nodes that reflect scale and diagenesis. We propose new rule-based node descriptions to measure diagenesis sensitive connections within the context of current field practices. Under diagenetic conditions between ca. 50 degrees C-250 degrees C differential infill makes network porosity, and thus permeability and strength, scale dependent.

Here we present a methodology that quantifies fracture arrangement in space as a function of position (barycenters), size (trace length in 2D), and orientation (variation in strike). We use point process statistics to analyze distances between barycenters and estimate a confidence interval for randomness that is compared with input data. The methodology can discriminate at least four types of spatial arrangements and identifies preferential associations of distances with fracture length and orientation. We apply our method to two naturally fractured formations and show that it can determine fracture arrangements under a significant statistical confidence.

Relative timing of fracturing is a key input for predictive fracture models, but timing information for fractures is commonly diffi-cult to obtain. In this study, we used crosscutting relations and fluid inclusion assemblage temperatures from fracture cements from a few well-documented sampled fractures, combined with a one-dimensional burial history model, to establish timing for three generations of opening-mode fractures in a Barnett Shale core from the southern part of the Delaware Basin, Pecos County, West Texas. A burial history model is presented for the cored well and matched to measured vitrinite reflectance in sam-ples from the core, and bottomhole temperature in the well. The earliest fractures (group 1) likely formed due to early fluid-expulsion events (ca. 300 Ma) and were folded during host-rock compaction. Later group 2 fractures are sealed with fibrous barite containing primary, liquid hydrocarbon inclusions (mean homogenization temperature [Th] =-9 degrees C) and aqueous fluid inclusions (mean Th = 108.1 degrees C). Group 2 fractures likely formed in response to fluid overpressure associated with crack-ing of type II kerogen to oil. Group 3 vertical fractures are up to 2 m in height with kinematic apertures ranging from less than 0.05 to 1.4 mm, partly open, and strike dominantly 010 degrees-020 degrees. Sequentially trapped aqueous fluid inclusions in fracture-spanning quartz cement bridges (mean Th = 110 degrees C in crack-seal texture and 128 degrees C in post-crack-seal fracture cement) record fracture opening under increasing temperature, inferred to reflect increas-ing burial, with continued overpressuring during the Triassic to Late Cretaceous. Some group 3 fractures may have continued to fill during Cenozoic uplift.

Natural fractures are widespread in rock. Fracture development depends on a wide range of factors such as burial history, temperature, strain rate, mechanical properties, layer geometry, and chemical reactions. We developed a three-dimensional (3D) simulator that couples tectonic strain with diagenesis (cement accumulation) to model the development and preservation of fracture aperture more fully. Displacement boundary conditions represent realistic large-scale tectonic strain through geological time and concurrent cement precipitation at fracture surfaces represents diagenesis. In a coupled model, cement effects on fracture opening can be conceptualized (and simplified) as; 1) fully cemented, 2) bridged, and 3) fully open. The mechanical effects of these conditions are simulated by different normal stiffness in fractures. Results of 3D models with diagenesis compare more favorably to field observations than those without diagenesis, exemplified by the reproduction of power-law aperture distributions.

Exposed in large, continuous outcrops in Provence, southern France, the Castellas fault formed as a normal fault in the Upper Cretaceous and was reactivated as a left-lateral fault in the Eocene. Structural, petrographical, isotopic, and geochronological analyses shed light on the sequence of deformational-diagenetic stages of the fault zone and help identify properties that controlled fluid-flow behavior through time. Abrupt contrasts in fracture abundance compared to the undeformed host rock define a damage zone containing fractures arranged in regularly spaced clusters. We identified eleven episodes of calcite cementation within opening-mode fractures and host rock primary pores. Cement fabrics, sediment fills, and geochemistry show evidence of shallow burial environments. U-Pb geochronology of calcite cements indicate two main deformational phases of the fault zone during the Albo-Cenomanian Durancian uplift and Eocene Pyrenean orogeny at ca. 90 Ma and 50-40 Ma, respectively. Deformation created poro-permeability, but cementation followed shortly after, pervasively occluding most of the fault zone porosity by the end of the Eocene.