Calcite-filled Fractures Research Articles

Role of Fracture Roughness in Fracture Surface Evolution During Acidized Corefloods of Calcareous Shales

Summary During the hydraulic fracturing process, an acidic hydraulic fracturing fluid (HFF) is injected at high flow rates to break the rock and enhance its flow potential. This rock-fluid interaction induces both physical and chemical alterations on the fracture surface, resulting in the formation of a “reaction-altered zone.” Recent research has revealed that the depth of reaction penetration is minimal, and most changes occur on the fracture surface. To gain a deeper understanding of how fracture roughness affects fracture aperture change, in this work we adopt an experimental approach. Two similar samples of carbonate-rich Wolfcamp shale with calcite-filled fractures are selected. One sample is cut through the center creating a smooth fracture (SF), while the other is fractured by parting along the calcite-filled fracture, generating a rough fracture (RF). The fracture surface topography, mineral distribution, fracture aperture, and rock hardness are characterized before a reactive coreflood using an equilibrated acidic brine is conducted. The pressure drop across the core is measured, and the effluent is periodically collected and analyzed using mass spectrometry. The temporal changes in the fracture surface are observed by conducting physicochemical surface characterization after the coreflood. The results indicate that calcite dissolution is the primary chemical reaction occurring on the fracture surface, weakening it. Furthermore, this dissolution decreases the fracture roughness, which results in fracture closure and ultimately a decrease in the fracture conductivity. The most significant change in the fracture aperture is observed near the inlet. These results highlight the potential impact of fracture roughness on the mechanism of fracture evolution during acidized corefloods. Higher fracture roughness is associated with increased fines migration and a more significant overall change in fracture aperture during injection. This research provides valuable insights into the intricate processes at play during hydraulic fracturing and aids in understanding the dynamics of fracture growth in such conditions.

A Regional Shallow Décollement at the Front of the Mexican Fold and Thrust Belt: Microstructure of the Santiago Shale

Abstract This contribution analyses the role played by the mechanical properties of a decollément shale layer in the evolution of the Mexican Fold and Thrust Belt (MFTB). The mobility of overpressured shales can accommodate large strains by grain-scale plastic mechanisms, and affect the folding and thrusting styles of the overburden. Research on shale deformation mechanisms is necessary to improve the knowledge of these processes and their influence on the structural style of fold and thrust belts. The ductile behavior of rocks involving grain-scale plasticity was documented in the Jurassic Santiago shale sequence using geological mapping, microstructural observations on thin-oriented sections, and scanning electron microscopy (SEM) imaging. Structural styles such as detachment folding, fault-bend folding, and shale-cored fold-thrusts were observed at the regional scale. At the outcrop scale, the shale developed strong foliation and pencil cleavage, with immersed packstone boudins. Observed structures include thrusting, soft and open folds, and buckle folding. In thin section, the ductile textures include a strong penetrative foliation with lenticular and wavy-parallel laminae composed of carbonates, ribbons of reoriented clays and organic matter (clay+OM), s-c structures, porphyroblasts microtextures, development of oblique cleavage concerning folded foliation (crenulation cleavage), and carbonates dissolution. The Santiago shale shows also evidence of brittle deformation including calcite-filled fractures and cataclastic gouges. X-ray diffraction (XRD) analysis of the clay size fraction suggests that the authigenic calcareous shale was deformed in conditions of the deep diagenetic zone (between 100 and 200°C) and fluid overpressure (>70 MPa). The results help to improve the understanding of ductile microstructure and its role in shale deformation cretaceous cover, promoting the formation of localized fault propagation folds in the overburden. This study aims to open new perspectives in the kinematics and rheology interpretations for this sector of the MFTB, highlighting the role of the décollement layers during the progression of the orogen.

Formational stages of natural fractures revealed by U-Pb dating and C-O-Sr-Nd isotopes of dolomites in the Ediacaran Dengying Formation, Sichuan Basin, southwest China

Abstract Deep core (>4.9 km) from Ediacaran Deng IV Member algal dolomites in the Gaoshiti-Moxi block in the Sichuan Basin, southwest China, reveals multiple generations of dolomite-lined and dolomite-filled opening-mode fractures. Three progressive stages of fracture formation are marked by crosscutting relations visible in the core, by acoustic emission experiments revealing evidence of past stress directions, and by fluid inclusions, U-Pb ages, C-O-Sr-Nd isotope patterns, and rare earth element data for dolomite cements in fractures, which document ages and differing thermal conditions and fluid compositions during fracture. In calcite-filled fractures, U-Pb ages and carbon and oxygen isotope signatures vary greatly, indicating that fractures developed with intensified tectonic activity marked by regional structures and with enhanced diagenetic alteration. In stage I, WNW-striking opening-mode fractures formed that contain dolomite deposits precipitated from basinal fluids between ca. 549 Ma and ca. 532 Ma. At this time, the Sichuan Basin experienced Xingkai taphrogenesis (rifting) from the late Neoproterozoic to early Cambrian. The central Sichuan paleo-uplift was undergoing ENE extension, and preexisting ESE- and nearly E-W–striking faults were oblique to the ENE principal stress orientation. This led to a local stress field favoring dextral shear near fault zones accommodated by the fractures. In stage II, ENE-striking fractures that are younger based on crosscutting relations contain dolomite deposits from basinal fluids with ages from ca. 423 Ma to ca. 411 Ma. Contemporaneous with Xuefeng thrusting, the central Sichuan paleo-uplift was in a NNE-striking transpressional stress field, which likely further generated ENE-striking fractures. In stage III, nearly N-S–striking fractures formed in the Gaoshiti-Moxi block. High-temperature fluids related to the Permian Emeishan large igneous province invaded these fractures from ca. 260 Ma to ca. 256 Ma. At this time, the Sichuan Basin was uplifted under the influence of the Emei taphrogenesis in the late Permian, and the central Sichuan paleo-uplift was subjected to E-W–striking extension. In fractures in these carbonate rocks, micro-computed tomography imaging reveals that macropores (>10 μm, 12.1%–21.8%) and small pores (2–10 μm, 76.6%–86.1%) dominate the dolomite mineral deposits, and that there are few (1.6%–1.8%) micropores or nanopores (<2 μm). Medium-sized throats (1–3 μm) are the main connecting channels. We infer that fractures served as conduits for fluid migration, leading to the dissolution of matrix pores adjacent to the fractures. This secondary porosity not only enhances reservoir storage capacity but also augments reservoir connectivity. Our study shows that in situ U-Pb dating and full-diameter rock acoustic emission data can effectively constrain the timing of fractures. By integrating this information with regional tectonic sequences and fracture diagenetic sequences from combined relative timing, geochemistry, and rock mechanics evidence, we clarify the factors controlling fracture formation.

Anisotropy and petrophysics of Floyd Shale, Alabama

Floyd Shale has a measured anisotropy that is consistent with transverse isotropy but with a magnitude of nearly 40% in Thomsen parameters γ and ε. The measured anisotropy is very strong, with a relation between γ and ε but not δ. This requires incorporating strong anisotropy while locating any microseismic events induced during fracture stimulation. It makes velocities deduced from sonic logs in deviated wellbores considerably different than vertical velocities. Floyd Shale looks very similar to Barnett Shale in many of its petrophysical properties except mineralogy. Production to date has been disappointing. We review key petrophysical and rock-physics properties of 103 ft of Floyd Shale recovered from the McShan Timberlands well. Based on these analyses, Floyd looks very promising (i.e., total organic carbon is in excess of 4 wt%, and porosities average 6%). However, mineralogy is dominated by clays (typically greater than 50 wt%), and Barnett has generally less than 29% clay. High-pressure mercury injection capillary pressure and nuclear magnetic resonance indicate that pore throats and pore bodies are small (i.e., less than 15 nm). Physical inspection of the recovered core reveals fresh slickensides (i.e., evidence of hydraulic fracturing) and compromised calcite-filled fractures. If this core is representative of Floyd Shale, we suspect that any hydrocarbons generated during maturation likely migrated during later tectonic fracturing.

Quantifying Centimeter- to Microscale Heterogeneities in Sedimentary, Compositional, and Geomechanical Properties of Siltstone Deposits in the Lower Triassic Montney Formation, Northeastern British Columbia, Canada

Abstract Geological and geomechanical heterogeneities exist at multiple scales in fine-grained rocks; however, the complexity of characteristics at the centimeter- to microscale heterogeneities remains poorly understood. In this study, 10 representative samples composed of three centimeter-scale sedimentary fabrics (massive siltstone (F1), stratified siltstone (F2), and bioturbated siltstone (F3)) were analyzed from the Lower Triassic Montney Formation in the Western Canada Sedimentary Basin to describe sedimentological heterogeneity based on sedimentary fabric, compositional, and geomechanical properties. Sedimentary fabric was determined based on grainsize and the distribution of bedforms, which subdivide the facies into four μm- to mm-scale microfacies (massive siltstone (MF1), pinstriped laminated siltstone (MF2), planar- to cross-stratified siltstone (MF3), and bioturbated siltstone (MF4)). Microscale analysis using a scanning electron microscope was used to characterize microfacies and their respective mineralogical makeup (matrix, cement, and framework grains). To quantify heterogeneity, sedimentary fabric was assessed using a CT scan complemented by elemental composition (using X-ray fluorescence), and geomechanical hardness (using Equotip Piccolo Bambino handheld microhardness tool) was collected within a 1 cm by 1 cm grid within each sample. Datasets were compared using a discriminant analysis (DA) to recognize trends between multiple properties and suggest that sedimentary fabric with the highest centimeter-scale aluminum content from XRF (avg. 11%) comprises microfacies that are comparatively matrix-rich consisting of micas, negligible calcite cement, and exhibit the lowest handheld hardness values (<770). Alternatively, sedimentary fabric with a higher elemental calcium component (avg. 18%) comprises microfacies that are matrix-poor, cemented by carbonate (calcite and dolomite) and quartz, and overall exhibit a positive trend with hardness measurement (770–850). Furthermore, to relate the elemental and geomechanical proxies to controls on rock mechanics, natural calcite-filled fractures within the studied core intervals were characterized. Fractures were subdivided into three types—brecciated, bed-parallel, and vertical to subvertical fractures with each type being constrained to a specific sedimentary fabric. Based on centimeter gridding, microscale analysis and the degree of fabric interbedding play a primary role on the variability in mechanical hardness and the geometry and termination of natural fractures. Collectively, this dataset provides insight into the influence that sedimentary fabric and the distribution of elemental composition has on mechanical properties and natural fractures below well log resolution. These findings can be used to better model and predict fine-grained deposit characteristics before undergoing hydraulic stimulation.

Study on Rock Mechanics and Wellbore Stability of Igneous Formation in the Shunbei Area.

While drilling into the igneous rock formations of the Shunbei area, problems such as loss of well circulation and borehole collapse occur frequently, seriously hindering the efficient development of oil and gas resources. Aiming to solve this problem, the physicochemical and mechanical properties of igneous rocks are studied through a series of laboratory tests to determine the main factors influencing formation collapse and instability. In addition to the laboratory results, the weak plane effect of fracture mechanics, the seepage effect of drilling fluid, and the hydration effect of the drilling fluid on the borehole wall stability of igneous rock formations are evaluated and analyzed by establishing a mathematical model. The results show that the microfractures in the igneous rock are relatively developed and can be divided into unfilled fractures and calcite-filled fractures. The mechanical strength of the matrix igneous rock is higher than that of the rock samples with microfractures. The compressive strength of calcite-filled and unfilled fracture samples is 1/3–1/4 that of matrix igneous rocks, and immersion in the drilling fluid has little influence on the mechanical strength of igneous rocks. The fracture weak plane effect has the greatest influence on wellbore stability. With the increase of the number of fractures at different angles, the collapse pressure equivalent density of the formation subject to the mechanical weak plane effect increases by 21% compared with that of the homogeneous formation without fractures. The seepage effect of the drilling fluid on borehole stability is secondary, and the equivalent density of the formation collapse pressure increases by 11%. Because of the low content of clay minerals, the hydration effect of the drilling fluid on borehole stability was minimal and the equivalent density of collapse pressure increased by only 2%. During the drilling process, considering the weak plane effect of the microfractures and the seepage effect of the drilling fluid, the drilling fluid density should be controlled at about 1.82 g/cm3. The effective plugging ability and rheological properties of the drilling fluid should be improved for the formation with microfractures. The research results can provide a theoretical basis for the safe and efficient development of igneous reservoirs.

Natural mineralized fractures from the Montney-Doig unconventional reservoirs (Western Canada Sedimentary Basin): Timing and controlling factors

Characterizing the origin of natural fractures in organic-rich fine-grained deposits is key to constraining permeability evolution in these potential source rocks and tight reservoirs, as well as to assess the hydraulic connectivity of the fluid systems in which they develop.Differently oriented calcite-filled fractures (i.e. veins), hosted by organic-rich mudrocks of the Lower-Middle Triassic Montney-Doig unconventional resource play (Western Canada Sedimentary Basin), were sampled in sub-surface well cores from British Columbia. A multidisciplinary approach (including sedimentology, Rock-Eval pyrolysis, petrography, O–C–Sr isotope geochemistry and fluid inclusion microthermometry) was applied to host-rocks and fracture filling calcites.The results demonstrate the relevance and usefulness of such multidisciplinary studies to gather insights on: 1) the lithology-related factors controlling fracture occurrence; 2) the timing of fracture opening and the origin of the circulating paleo-fluids; 3) the openness of the fluid system through time.More specifically, host-rock facies (particularly grain size) and vertical facies changes appear to be the leading factors controlling fracture occurrence. A less relevant role was played by the occurrence of diagenetic carbonates, while TOC possibly did not control fracture occurrence.Three generations of calcite cemented fractures were identified. Vertical fractures (first generation) post-dated the onset of oil generation (Late Cretaceous). Horizontal, bedding-parallel fractures (second generation) post-dated the onset of gas generation and possibly opened close to maximum burial, corresponding to peak hydrocarbon (CH4) generation (Late Cretaceous - Early Paleogene). Vertical fractures (third generation) post-dated the horizontal ones and opened during basin uplift (Middle to Late Paleogene).The consistent petrographic and geochemical features of all the calcite cements point to parent fluids in equilibrium with the host-rock lithologies, that possibly behaved as a closed hydraulic system during Late Cretaceous to Paleogene time; this would support the hypothesis that, at least in the portion of the basin investigated, the Montney Fm also acted as source rock of the unconventional system.

Comparison of carbonate reservoir fractures from core and modern electrical borehole images

Abstract Several methods exist for collecting data about opening-mode fractures (hereinafter fractures) in subsurface reservoir rocks. The datasets, with the exception of core data, are indirect and subject to limited resolutions. This paper includes new results on the detection of fractures in carbonate reservoirs by modern electrical borehole imagers. Barren fractures and calcite-filled fractures (calcite veins) were described and compared from whole core and borehole images. These datasets were obtained from directional drilling into the Arab petroleum reservoirs in a low-relief dome structure in Eastern Arabia. The comparison of fracture abundance resulted in a significant mismatch, because many reservoir fractures in the core could not be detected from corresponding borehole images. Most calcite veins were undetected because of their small widths, below 2 mm. Exceptions include a few calcite veins of 0.5 mm that were detected because of the strong contrast between resistive veins and conductive host rock beds. The barren fractures that are aligned sub-parallel to the maximum horizontal principal stress (SHmax) were detected more abundantly than other non-aligned barren fractures. The present-day SHmax is roughly trending between 60° and 100° azimuth in Eastern Arabia. The aligned fractures have larger apertures in the borehole images than the non-aligned fractures. The apertures diminish in size as the fractures gradually deviate in orientation from the SHmax. Most of the non-aligned barren fractures were undetected, except the fractures with rough and mismatching walls. This indicates that the walls of the non-aligned fractures are mechanically closed by the normal SHmax, and that the remaining apertures were dependent on the availability of roughness mismatches between the closed fracture walls. Electrical borehole images are biased in measuring fracture abundance, but indispensable to detect and measure the orientations and apertures of fractures, especially the large (i.e., more conductive) fractures in reservoir rocks.

Fracture Characteristics and Change of Permeability Under the Influence of Natural Fractures: Experimental Study of Wufeng–Longmaxi Shale

Summary Currently, substantial shale gas reserves have been recognized in many countries, including China and the United States. An improved comprehension of the structure of these shale plays can lead to improved shale gas producibility. Gas is produced through natural fractures (NFs) and through hydraulically induced fractures, together creating the so-called stimulated reservoir volume (SRV). Although NFs are advocated as significantly influencing the size and shape of an SRV, the effects of NFs on fracturing and permeability changes are incomplete. This paper describes experimental studies performed on calcite-filled fractures from outcrops of Wufeng (O3w)-Longmaxi (S1l) shale. These samples are representative of Ordovician-Silurian productive shales in the Fuling Basin, near Chongqing City, China. Basic parameters, such as acoustic wave velocity, porosity, and permeability, were first measured on representative samples. Subsequently, permeability and acoustic emissions were measured during triaxial compression experiments through the post-peak domain. After these evaluations, the fracture modes and the fragment distribution were evaluated. In the measurements made, the natural calcite-filled macroscopic fractures reduced the confined compressive strength of these shale samples from 26 to 47%. Permeability is also affected. During the triaxial compression experiments, different NF orientations affected the overall permeability differently. We divided the collected samples into three groups. Group 1 samples had axial fractures (AFs). The gas permeability reduced from 0.70 to 0.12 md. When the sample was broken, permeability increased to 2.0 md, whereas the axial stress dropped to zero. Samples in Group 2 had transverse natural fractures (TFs). When new fractures were generated, permeability increased to greater than 0.70 md. Because samples in Group 3 had no obvious, natural macroscopic fractures (NFs), permeability was too low to measure before catastrophic failure during triaxial compression. When these samples did fail in compression, significant fractures formed, and permeability increased, resembling Group 2. There was a close relationship between the initial pattern of NFs and the subsequently generated fracture morphologies. For Group 1, the samples broke into several larger pieces along the original AFs. For Group 2, the samples broke into numerous small and relatively uniform pieces. For Group 3, the samples did not break completely, and the constituent fragments were nonuniform. Our results are helpful for understanding fracture-propagation mechanisms and the evolution of permeability under in-situ loading processes (long-term geologic loading, stress alteration during well construction and stimulation, and reservoir modification during depletion).

Multiscale Imaging of Gas Storage in Shales

Summary Four intact 2.54-cm-diameter cores from different shale plays (Barnett, Haynesville, Eagle Ford, and Permian Basin) were analyzed for their gas-storage capacity by use of a novel multiscale-imaging methodology spanning from centimeter to nanometer scale. Gas-storage (free and sorbed gas) capacity was investigated at the core scale with carbon dioxide (CO2) and krypton (Kr) by use of X-ray computed tomography (CT) with voxel dimensions of 190 × 190 × 1000 µm. Also, 2D tiled images were acquired with a scanning electron microscope (SEM) and stitched together to form 2.54-cm-diameter mosaics with a pixel resolution of 1.5 µm. Multiscale-image registration was then carried out to align the CT data with the SEM mosaics. Energy-dispersive spectroscopy (EDS) generated elemental spectra maps and subsequent component maps for regions with either substantial or minimal gas storage to assess the interplay of structural features (e.g., fractures) and matrix composition with respect to gas accessibility and storage. Registration of CT scans (vacuumed and gas-filled) as well as 190-µm-resolution CT-derived gas-storage maps with 1.5-µm-resolution SEM mosaics is straight forward for samples with dense features (such as calcite-filled fractures) that are resolvable by CT imaging. Alignment methods were developed for samples lacking these features, including registration marks by use of silver paint and intermediate-resolution microCT scans with cubic voxel dimensions of 27 µm. After alignment, the relationship of enhanced storage zones with open fractures and reduced storage regions with secondary mineralization (such as nodules) is apparent for the carbonaceous samples. For the clay-rich Barnett sample, fracture-filling calcite is associated with reduced storage similar to the other samples; however, secondary carbonate cementation within the clay matrix aligns with regions with substantial Kr- and CO2-gas storage. In contrast, clay-rich matrix regions lacking secondary carbonate cementation exhibit minimal gas-storage potential. Causes for this unexpected result include reduced gas accessibility and, possibly, low organic-matter content in the clay-rich matrix compared with secondary cemented matrix. These gas-sorption experiments prove the feasibility of dynamic core- to nanometer-scale CT/SEM/EDS image registration to improve sample characterization. To our knowledge, this is the first investigation of core-scale CO2-gas storage using multiscale imaging. CT and SEM image registration reveal spatial details regarding gas accessibility and storativity at the core scale. This work also supports the potential of carbon storage in shale formations and guides engineers toward optimal CO2-injection zones for enhanced gas recovery.

Major factors controlling fracture development in the Middle Permian Lucaogou Formation tight oil reservoir, Junggar Basin, NW China

Natural fractures in seven wells from the Middle Permian Lucaogou Formation in the Junggar Basin were evaluated in light of regional structural evolution, tight reservoir geochemistry (including TOC and mineral composition), carbon and oxygen isotopes of calcite-filled fractures, and acoustic emission (AE). Factors controlling the development of natural fractures were analyzed using qualitative and/or semi-quantitative techniques, with results showing that tectonic factors are the primary control on fracture development in the Middle Permian Lucaogou Formation of the Junggar Basin. Analyses of calcite, dolomite, and TOC show positive correlations with the number of fractures, while deltaic lithofacies appear to be the most favorable for fracture development. Mineral content was found to be a major control on tectonic fracture development, while TOC content and sedimentary facies mainly control bedding fractures. Carbon and oxygen isotopes vary greatly in calcite-filled fractures (δ13C ranges from 0.87‰ to 7.98‰, while δ18O ranges from −12.63‰ to −5.65‰), indicating that fracture development increases with intensified tectonic activity or enhanced diagenetic alteration. By analyzing the cross-cutting relationships of fractures in core, as well as four Kaiser Effect points in the acoustic emission curve, we observed four stages of tectonic fracture development. First-stage fractures are extensional, and were generated in the late Triassic, with calcite fracture fills formed between 36.51°C and 56.89°C. Second-stage fractures are shear fractures caused by extrusion stress from the southwest to the northeast, generated by the rapid uplift of the Tianshan in the Middle and Late Jurassic; calcite fracture fills formed between 62.91°C and 69.88°C. Third-stage fractures are NNW-trending shear fractures that resulted from north-south extrusion and thrusting in a foreland depression along the front of the Early Cretaceous Bogda Mountains. Calcite fracture fills formed between 81.74°C and 85.43°C. Fourth-stage fractures inherited the tectonic framework of the third stage, resulting in fractures with the same orientation, but without calcite filling. By differentiating the various stages of fracture development, we were able to better understand the origin of fractures in tight oil reservoirs and their significance for exploration and development.

Three-dimensional quantitative fracture analysis of tight gas sandstones using industrial computed tomography

Tight gas sandstone samples are imaged at high resolution industrial X-ray computed tomography (ICT) systems to provide a three-dimensional quantitative characterization of the fracture geometries. Fracture networks are quantitatively analyzed using a combination of 2-D slice analysis and 3-D visualization and counting. The core samples are firstly scanned to produce grayscale slices, and the corresponding fracture area, length, aperture and fracture porosity as well as fracture density were measured. Then the 2-D slices were stacked to create a complete 3-D image using volume-rendering software. The open fractures (vug) are colored cyan whereas the calcite-filled fractures (high density objects) are colored magenta. The surface area and volume of both open fractures and high density fractures are calculated by 3-D counting. Then the fracture porosity and fracture aperture are estimated by 3-D counting. The fracture porosity and aperture from ICT analysis performed at atmospheric pressure are higher than those calculated from image logs at reservoir conditions. At last, the fracture connectivity is determined through comparison of fracture parameters with permeability. Distribution of fracture density and fracture aperture determines the permeability and producibility of tight gas sandstones. ICT has the advantage of performing three dimensional fracture imaging in a non-destructive way.

Impact of laminations and natural fractures on rock failure in Brazilian experiments: A case study on Green River and Niobrara formations

Natural fractures and laminations are common features in many unconventional resources which require hydraulic fracturing for economic production. To better understand the effect of such features on rock failure, we have conducted indirect tensile or Brazilian testing on several rock samples from Green River and Niobrara formations. Brazilian experiments were conducted at various orientations of laminations or natural fractures with respect to the loading direction. In contrast to isotropic rocks, we observed that rock failure can occur away from the vertical central line of rock parallel to lamination or natural fracture when the approaching angle between the lamination and loading direction is less than 30° in the tested samples. It was also determined that the tensile strength parallel to laminations or calcite-filled fractures is lower than the tensile strength perpendicular to lamination and/or the natural fractures; for instance the tensile strength of Niobara calcite-filled fractures were almost one third of the rock matrix.

Imaging-Based Characterization of Calcite-Filled Fractures and Porosity in Shales

SummaryGas- and liquid-rich shales exhibit structural and compositional features across a broad range of length scales from meters to nanometers. This laboratory characterization effort of a shale sample aids hydrocarbon resource and reserves estimation, and improves understanding of flow behavior including potential geological carbon dioxide storage. Multiscale laboratory-imaging techniques were applied to characterize pore and microfracture structure of a Barnett Shale sample including connectivity and heterogeneity. X-ray computed tomography (CT) illuminated the krypton (Kr)-accessible porosity of centimeter-sized shale cores. Transmission X-ray microscopy (TXM) imaged micrometer-sized shale samples, and high-resolution scanning electron microscopy (SEM) revealed pore, fracture, and textural features. Registration of 190 μm resolution CT images with micrometer to nanometer resolution TXM and SEM images improved physical understanding of transport through organic-rich shale. Results focus on calcite-filled fractures and the calcite/shale-matrix interface as well as the distribution of micrometer- and nanometer-scale porosity. Fractures are likely both natural and induced. For the sample studied, pore accessibility determined by CT imaging corresponds with open microfractures that cross calcite-filled fractures and adjacent shale matrix. Such observations are made with corresponding micrometer- to nanometer-scale SEM images as well as compositional data. Taken together, these data indicate that calcite-filled fractures in this core act as a barrier to flow parallel to bedding except where breached by numerous open fractures. In contrast, these filled fractures enhance vertical flow, that is, flow between laminations. A region containing porosity and organic matter (with dimensions of tens to hundreds of nanometers) determined by 3D nanocharacterization with TXM and focused ion beam/SEM at the filled-fracture/shale-matrix interface facilitates this observed gas transport along the wall of the fracture fill. Areas adjacent to calcite-filled fractures and carbonaceous laminations within the shale matrix of the study sample are most readily accessed by Kr and may therefore be more readily produced than comparatively clay-rich laminations. The numerous open fractures and sheet pores within the calcite fracture fill, as well as the inherent weakness of the porosity and organic matter at the fracture-fill/shale-matrix interface, are indicative of its susceptibility to reopening and fracturing.

Fluid circulation and deformational gradient in north-Pyrenean flyschs: Example from the Saint-Jean-de-Luz basin (France)

Abstract The relationships between fluid circulation and deformation are one of the issues of the sedimentary basin study. In the Cretaceous flysch of the Saint-Jean-de-Luz basin, the evolution of folds geometry and the increasing volume of calcite-filled fractures and veins evidence a northward deformational gradient along the French Basque coast. A combined approach is proposed to assess the corresponding physical and chemical conditions: the microthermometric study of fluid inclusions in calcite sampled in different generations of veins and fractures and the X-ray diffraction analysis of clay minerals from adjacent marl layers. Salinity of the trapped H2O–CaCl2–NaCl fluids increases with depth in the series, in good agreement with salinity gradients reported in sedimentary basins. Dispersion of the data also increases with depth from 0.3 to 1.3 wt.% NaCl eq. in the shallowest formation (Haizabia flysch) to 9.1 to 23.0 wt.% NaCl eq. in the deepest formation (Guethary flysch). Minimal trapping temperatures of the fluids in the Haizabia and Socoa flyschs (79 and 102 °C, respectively) are consistent with the temperatures estimated from the depth of burial, which did not exceed 5 km, in good agreement with the stability of the smectite–illite–kaolinite assemblage found in marls. In addition, the kaolinite proportion significantly decreases with depth in the series, as a potential consequence of climate changes and diagenetic transformations, whereas the increasing dispersion of illite crystallinity data might indicate fluid–mineral interactions. We propose a synthetic model of fluid circulation in the folded series that involves the mixing of mainly-horizontal fluid circulation (potentially meteoric) with an upward flow of high-salinity fluid throughout the deepest formations (potentially related to underlying evaporite-rich layers). The northward deformational gradient, as exposed along the French Basque coast, is likely to be responsible for such a vertical circulation, by increasing the volume of fracture (particularly cross-cutting fractures) in the deeply buried formations.

Integration of hydraulically induced microseismic event locations with active seismic attributes: A North Texas Barnett Shale case study

Hydraulic fractures are delineated by induced microseismic event distributions and typically propagate perpendicular to the regional minimum stress direction. However, at a smaller scale, varying mineralogical composition and existing fault and fracture networks can influence developing fracture networks. We integrated microseismic event locations with seismic attributes from multichannel seismic reflection data, including inversion results for impedance and Lamé parameters, and seismic curvature attributes. We found that microseismic event locations consistently correlate to zones of low seismic impedance and low [Formula: see text] and [Formula: see text] values, describing characteristic material properties of fracture-prone zones within the North Texas Lower and Upper Barnett Shale. Additionally, event locations showed a weak correlation with anticlinal structures as defined by volumetric curvature attributes. We suggest that the low impedance, low [Formula: see text] and [Formula: see text] zones were related to the boundary between calcite-filled fractures and the host rock.

The Transfiguration continental red-bed Cu–Pb–Zn–Ag deposit, Quebec Appalachians, Canada

The Transfiguration Cu–Pb–Zn–Ag deposit, enclosed within reduced grey sandstone, is associated with continental red beds of the Lower Silurian Robitaille Formation in the Quebec Appalachians, Canada. The Robitaille Formation rests unconformably on foliated Cambro-Ordovician rocks. The unconformity is locally cut by barite veins. The basal unit of the Robitaille Formation comprises green wacke and pebble conglomerate, which locally contain calcite nodules. The latter have microstructures characteristic of alpha-type calcretes, such as “floating” fabrics, calcite-filled fractures (crystallaria) and circumgranular cracks. Massive, grey sandstone overlies the basal green wacke and pebble conglomerate unit, which is overlain, in turn, by red, fine-grained sandstone. Mineralisation occurred underneath the red sandstone unit, chiefly in the grey sandstone unit, as disseminated and veinlet sulphides. Chalcopyrite, the most abundant Cu sulphide, replaced early pyrite. Calcrete, disseminated carbonate and vein carbonate have stable isotope ratios varying from −7.5‰ to −1.1‰ δ13C and from 14.7‰ to 21.3‰ δ18O. The negative δ13C values indicate the oxidation of organic matter in a continental environment. Sulphur isotope ratios for pyrite, chalcopyrite and galena vary from −19‰ to 25‰ δ34S, as measured on mineral concentrates by a conventional SO2 technique. Laser-assisted microanalyses (by fluorination) of S isotopes in pyrite show an analogous range in δ34S values, from −21‰ to 25‰. Negative and positive δ34S values are compatible with bacterial sulphate reduction (BSR) in systems open and closed with respect to sulphate. We interpret similarly high δ34S values for sulphide concentrates (25.1‰) and for vein barite (26.2‰) to result from rapid and complete thermochemical reduction of pore-water sulphate. Two early to late diagenetic stages of mineralisation best explain the origin of the Transfiguration deposit. The first stage was characterised by the ponding of groundwater over the Taconian unconformity, recorded by calcrete and early pyrite formation via BSR in grey sandstone. Early pyrite contains up to 2 wt.% Pb, which is consistent with Pb fixation by sulphate-reducing bacteria. The second stage (II) is defined by the replacement of early pyrite by chalcopyrite, as well as by sulphide precipitation via either BSR or thermochemical sulphate reduction (TSR) in grey sandstone. This event resulted from the synsedimentary fault-controlled percolation and mixing of (1) an oxidising, sulphate-bearing cupriferous fluid migrating per descensum from the red-bed sequence and (2) a hydrocarbon-bearing fluid migrating per ascensum from the Cambro-Ordovician basement. Mixing between the two fluids led to sulphate reduction, causing Cu sulphide precipitation. The positive correlation between Cu and Fe3+/Fe2+ bulk rock values suggests that Fe acted as a redox agent during sulphate reduction. Stage II diagenetic fluid migration is tentatively attributed to the Late Silurian Salinic extensional event.

Fluid flow reconstruction in hanging and footwall carbonates: Compartmentalization by Cenozoic reverse faulting in the Northern Oman Mountains (UAE)

In this paper, the diagenesis from either side of a major Cenozoic reverse fault in the Northern Oman Mountains is documented. Detailed petrographical and geochemical analysis of calcite-filled fractures in carbonate strata of Late Triassic and Early Cretaceous age in the hanging wall and footwall in Wadi Ghalilah reflect a different diagenetic history. In both hanging wall and footwall most of the fractures are pre-burial, extensional in origin, formed by a crack-seal mechanism, and the calcite vein infill has a host-rock buffered signature. In the hanging wall, the fluid responsible for calcite precipitation of these extensional fractures was a marine fluid at 60 °C. These veins predate deep burial and contractional tectonic deformation and consequently do not provide any information about syntectonic fluid flow. Neither do the pre-burial extension fractures in the footwall which are also host-rock buffered. The fractures post-dating the tectonic stylolitization in the footwall, by contrast, show evidence of syntectonic migration of saline formation waters at temperatures between 80 and 160 °C during contractional deformation. These fluids probably were sourced from the subsurface via the reverse fault, which acted as a fluid conduit. At the same time, however, this fault functioned as a permeability barrier towards the hanging wall, since no evidence of syntectonic fluid flow is present here. In this way compartmentalization of the hanging wall and footwall block was realized.

The Mechanics of Earthquakes and Faulting: (C. H. Scholz)

Have you wondered why those short, en echelon, calcite-filled fractures that all geologists have seen have a mild “S” shape? Why are the en echelon fractures oriented at an angle to the trend of the fracture zone? Which came first, the fault or the shear zone? Why are map traces of strike-slip faults straighter than dip-slip faults? How are tension cracks and shear cracks related? Why are fault-bend folds more pronounced on thrust faults than on other fault types? The answers to these and other questions can be found in Christopher Scholz's 2003 book The Mechanics of Earthquakes and Faulting , published by Cambridge University Press. The second edition of 2002 was reprinted with corrections in 2003. This upper-division/graduate level reference book is organized into seven chapters. Chapter 1 addresses brittle fracture of rock, including pore-fluid effects on fracture and brittle-plastic transitions caused by pressure and temperature, and extrapolates laboratory results to geological conditions. The three modes of crack propagation from fracture mechanics are described early in the chapter. These modes should be understood by all geologist working on ground-deformation problems, including landslides and subsidence, where distribution and nature of cracks is fundamental to interpretation of process. Rock friction, the subject of Chapter 2, is a contact property of faults that already exist; it is not a bulk property. This chapter sets the stage for an understanding that seismic or aseismic fault motion is governed by the stability of friction. Fault surfaces have topography and touch only at asperities. Asperity area increases as shearing occurs, but other aspects of interaction contribute to actual friction: ploughing, riding up, and interlocking. Regular stick slip is dynamic instability with all sliding during the instability. A mechanism proposed for earthquakes is recurring slip instability on pre-existing faults that remain stationary between earthquakes. Unstable sliding …

The Mechanics of Earthquakes and Faulting, Second Edition: (C. H. Scholz)

Have you wondered why those short, en echelon , calcite-filled fractures that all geologists have seen have a mild “S” shape? Why are the en echelon fractures oriented at an angle to the trend of the fracture zone? Which came first, the fault or the shear zone? Why are map traces of strike-slip faults straighter than dip-slip faults? How are tension cracks and shear cracks related? Why are fault-bend folds more pronounced on thrust faults than on other fault types? The answers to these and other questions can be found in Christopher Scholz's 2003 book, The Mechanics of Earthquakes and Faulting , published by Cambridge University Press. The 2002 second edition was reprinted with corrections in 2003. This upper-division/graduate-level reference book is organized into seven chapters. Chapter 1 addresses brittle fracture of rock, including pore-fluid effects on fracture and brittle-plastic transitions caused by pressure and temperature, and extrapolates laboratory results to geological conditions. The three modes of crack propagation from fracture mechanics are described early in the chapter. These modes should be understood by all geologists working on ground-deformation problems, including landslides and subsidence, where the distribution and nature of cracks are fundamental to interpretation of process. Rock friction, the subject of Chapter 2, is a contact property of faults that already exists; it is not a bulk property. This chapter sets the stage for an understanding that seismic or aseismic fault motion is governed by the stability of friction. Fault surfaces have topography and touch only at asperities. Asperity area increases as shearing occurs, but other aspects of interaction contribute to actual friction: ploughing, riding up, and interlocking. Regular stick slip is a dynamic instability with all sliding occurring during the instability. A mechanism proposed for earthquakes is recurring slip instability on preexisting faults that remain stationary between earthquakes. …