Variations In Fracture Density Research Articles

Seismic signatures of partial steam saturation in fractured geothermal reservoirs: Insights from poroelasticity

Detecting the presence of gaseous formation fluids, estimating the respective volumes, and characterizing their spatial distribution are important for a wide range of applications, notably for geothermal energy production. The ability to obtain such information from remote geophysical measurements constitutes a fundamental challenge, which needs to be overcome to address a wide range of problems, such as the estimation of the reservoir temperature and pressure conditions. With these motivations, we compute the body wave velocities of a fractured granitic geothermal reservoir formation with varying quantities of steam to analyze the seismic signatures in a partial saturation context. We use a poroelastic upscaling approach that accounts for mesoscale fluid pressure diffusion (FPD) effects induced by the seismic strain field, and, thus, describes the governing physical processes more accurately than standard representations. Changes in seismic velocities due to steam saturation are compared with changes associated with fracture density variations, as both are plausible results of pressure changes in geothermal reservoirs. We find that steam saturation has a significant impact on P-wave velocities while affecting S-wave velocities to a significantly lesser extent. This contrasting behavior allows us to discriminate between fracture density and steam saturation changes by means of P- and S-wave velocity ratio analyses. To evaluate the potential of seismic methods to provide this information, a canonical geothermal reservoir model is used to compute the Rayleigh wave velocity dispersion and seismic reflection amplitude variation with angle (AVA) curves. These studies reveal that AVA analyses allow differentiating changes in fracture density from changes in steam saturation. We also note that the Rayleigh-wave-based techniques are much less sensitive to steam content changes than to fracture density changes. Comparisons with elastic approaches indicate that including FPD effects through the use of a poroelastic model is crucial for the reliable detection and characterization of steam in fractured geothermal reservoirs.

What Do P-Wave Velocities Tell Us About the Critical Zone?

Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information—the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

Fracture density variations within a reservoir-scale normal fault zone: A case study from shallow-water carbonates of southern Italy

Fault zones can often display a complex internal structure associated with antithetic faults, branch and tip points, bed rotations, bed-parallel slip surfaces, and subordinate synthetic faults. We explore how these structural complexities may affect the development of fault-related fractures as displacement accumulates. We analysed in detail an incipient fault zone within well-bedded, shallow-water carbonates of the southern Apennines thrust belt (Italy). The fault zone crops out with quasi-complete exposure at a reservoir scale on an inaccessible sub-vertical cliff face, and fault and fracture mapping were carried out on a 3D virtual outcrop model of the exposure built for this study using photogrammetry. Comparing the structure of the fault zone and the density of 9444 mapped fractures allowed us to unravel their spatial relationships. Our results show that the areas of denser fractures coincide with: (i) rock volumes bounded by antithetic faults developed within the fault zone, (ii) branch points between these antithetic faults and fault zone-bounding fault segments, (iii) fault zone-bounding fault segments associated with significant displacement gradients, and (iv) relay zones between subordinate synthetic faults. These findings may aid locating sub-seismic resolution volumes of dense fracturing and associated enhanced permeability within faulted reservoirs.

Accurate Computation of Fracture Density Variations: A New Approach Tested on Fracture Corridors

Fracture density is an important parameter for characterizing fractured reservoirs. Stochastic object-based simulation algorithms that generate fracture networks commonly rely on a fracture density to populate the reservoir zones with individual fracture surfaces. Reservoirs, including fracture corridors, represent particular challenges in petroleum reservoir studies. Indeed, it is difficult to identify fracture corridor zones objectively and precisely along one-dimensional well data, which are characterized by high fracture densities compared to diffuse fractures. To estimate fracture density, a common practice is to graphically depict only fracture corridors on fracture cumulative intensity curves. In this paper, an approach is proposed to formalize this technique using hypothesis testing. This method precisely compartmentalizes the well data into several zones having specific fracture densities. The method consists of the following steps: (i) dividing the diagram into zones depending on a priori drastic changes in density, (ii) computing the local accurate fracture density for each zone and (iii) clustering the zones characterized by similar densities statistically. The key point is to couple regression and hypothesis testing. The regression aims at computing local average fracture density and the hypothesis testing aims at clustering zones for which the densities are statistically the most similar. The proposed approach is dedicated to one-dimensional fracture surveys, such as well data and outcrop scanlines. First, a synthetic case study is presented to prove the ability to highlight changes in fracture density. Second, the procedure is applied on a scanline dataset collected in a quarry (Calvisson, SE France) to show the usefulness of characterizing fracture corridors.

Partially Penetrated Well Solution of Fractal Single-Porosity Naturally Fractured Reservoirs

In the oil industry, many reservoirs produce from partially penetrated wells, either to postpone the arrival of undesirable fluids or to avoid problems during drilling operations. The majority of these reservoirs are heterogeneous and anisotropic, such as naturally fractured reservoirs. The analysis of pressure-transient tests is a very useful method to dynamically characterize both the heterogeneity and anisotropy existing in the reservoir. In this paper, a new analytical solution for a partially penetrated well based on a fractal approach to capture the distribution and connectivity of the fracture network is presented. This solution represents the complexity of the flow lines better than the traditional Euclidean flow models for single-porosity fractured reservoirs, i.e., for a tight matrix. The proposed solution takes into consideration the variations in fracture density throughout the reservoir, which have a direct influence on the porosity, permeability, and the size distribution of the matrix blocks as a result of the fracturing process. This solution generalizes previous solutions to model the pressure-transient behavior of partially penetrated wells as proposed in the technical literature for the classical Euclidean formulation, which considers a uniform distribution of fractures that are fully connected. Several synthetic cases obtained with the proposed solution are shown to illustrate the influence of different variables, including fractal parameters.

High-resolution stratigraphic characterization of natural fracture attributes in the Woodford Shale, Arbuckle Wilderness and US-77D Outcrops, Murray County, Oklahoma

Natural fracture aperture-size, spacing, and stratigraphic variation in fracture density are factors determining the fluid-flow capacity of low-permeability formations. In this study, several facies were identified in a Woodford Shale complete section. The section was divided into four broad stratigraphic zones based on interbedding of similar facies. Average thicknesses and percentages of brittle and ductile beds in each stratigraphic foot were recorded. Also, five fracture sets were identified. These sets were split into two groups based on their trace exposures. Fracture linear intensity (number of fractures normalized to the scanline length [[Formula: see text]]) values were quantified for brittle and ductile beds. Individual fracture intensity-bed thickness linear equations were derived. These equations, along with the average bed thickness and percentage of brittle and ductile lithologies in each stratigraphic foot, were used to construct a fracture areal density (number of fracture traces normalized to the trace exposure area [[Formula: see text]]) profile. Finally, the fracture opening-displacement size variations, clustering tendencies, and fracture saturation were quantified. Fracture intensity-bed thickness equations predict approximately 1.5–3 times more fractures in the brittle beds compared with ductile beds at any given bed thickness. Parts of zone 2 and almost entire zone 3, located in the upper and middle Woodford, respectively, have high fracture densities and are situated within relatively organic-rich (high-GR) intervals. These intervals may be suitable horizontal well landing targets. All observed fracture cement exhibit a lack of crack-seal texture. Characteristic aperture-size distributions exist, with most apertures in the 0.05–1 mm (0.00016–0.0032 ft) range. In the chert beds, fracture cement is primarily bitumen or silica or both. Fractures in dolomite beds primarily have calcite cement. The average fracture spacing indices (i.e., bed thickness-fracture spacing ratio) in brittle and ductile beds were determined to be 2 and 1.2, respectively. Uniform fracture spacing was observed along all scanlines in the studied beds.

The structure of an exhumed intraplate seismogenic fault in crystalline basement

Abstract The 600 m-thick Gole Larghe Fault Zone (GLFZ) is hosted in jointed crystalline basement and exposed across glacier-polished outcrops in the Italian Alps. Ancient seismicity is attested by the widespread occurrence of cataclasites associated with pseudotachylytes (solidified frictional melts) formed at 9–11 km depth and ambient temperatures of 250–300 °C. Previous work focused on the southern part of the fault zone; here we quantitatively document fault zone structure across the full width of the GLFZ and surrounding tonalite host rocks by using a combination of structural line transects and image analysis of samples collected across fault strike. These new datasets indicate that the GLFZ has a broadly symmetric across-strike damage structure and contains distinct southern, central and northern zones distinguished by large variations in fracture density, distribution of pseudotachylytes, volume of fault rock materials, and microfracture sealing characteristics. The c. 100 m wide central zone is bound by two thick (~ 2 m) and laterally continuous (> 1 km) protocataclastic to ultracataclastic horizons. Within and immediately surrounding the central zone, fracture density is relatively high due to cataclastic fault–fracture networks that reworked earlier-formed pseudotachylytes. The fault–fracture networks were associated with pervasive microcracking and fluid–rock interaction, resulting in the development of a c. 200 m thick alteration zone delimited by lobate fluid infiltration fronts. In the c. 250 m thick southern and northern zones, fracture densities are much lower and pseudotachylytes systematically overprint cataclastic faults that exploited pre-existing magmatic cooling joints. Analysis of the structure of the GLFZ suggests that it shares certain characteristics with the seismogenic source responsible for the 2002 Au Sable Forks intraplate earthquake sequence in the northeastern USA, including seismicity distributed across a fault zone 500–1000 m thick and large (> 100 MPa) static stress drops associated with frictional melting.

Quantification of fold curvature and fracturing using terrestrial laser scanning

Terrestrial laser scanning is used to capture the geometry of three single folded bedding surfaces. The resulting light detection and ranging (LIDAR) point clouds are filtered and smoothed to enable meshing and calculation of principal curvatures. Fracture traces, picked from the LIDAR data, are used to calculate fracture densities. The rich data sets produced by this method provide statistically robust estimates of spatial variations in fracture density across the fold surface. The digital nature of the data also allows resampling to derive fracture parameters that are more traditionally measured manually from outcrops (e.g., one-dimensional line transects of fracture spacing). The fracture statistics derived from the LIDAR data are compared with the calculated principal and Gaussian curvatures of the surface to assess whether areas of extreme curvature correlate with high-fracture density. For the folds studied, all the fracture spacing distributions showed an exponential distribution, and no significant correlation between fracture density and surface curvature was observed. This questions the validity of using curvature as a proxy for high brittle strains and highlights the need for a complete understanding of fold and fracture mechanics that include considerations of other factors including lithology, strain rate, and confining pressure, not just finite strain. The three case studies also illustrate how terrestrial laser scanning can be used to gather detailed quantitative data sets on fracture and fold distributions from outcrop analogs.

Quantitative estimate of fracture density variations in the Nordegg with azimuthal AVO and curvature: A case study

We investigated the accuracy of surface seismic attributes in predicting fracture density variations within the Nordegg Formation in west central Alberta. We know from core, drill samples, well-log, and drilling data that the Nordegg zone is fractured to some degree. These fractures are of interest because the reservoir has very low permeability, and therefore natural fractures may materially affect well performance. 3D surface seismic techniques such as amplitude variation with azimuth or azimuthal AVO (AVAz), variation of velocity with azimuth (VVAz), curvature, and coherence techniques are all tools that have been used to predict fractures in a qualitative fashion. In this study, we wanted to understand how well these attributes predicted the reservoir quality in a quantitative fashion. Previous quantitative studies have used image log orientation data or estimated ultimate recoveries (EUR) in vertical wells as validation data. The conclusiveness of these studies has been subject to several problems: firstly, the limited sample statistics provided by vertical wells applied to the validation of lateral variations, and secondly by the potential nonuniqueness of the EUR to fracture density relationship.

Application of modern 2-D and 3-D seismic-reflection techniques for uranium exploration in the Athabasca BasinThis article is one of a series of papers published in this Special Issue on the themeLithoprobe — parameters, processes, and the evolution of a continent.

Seismic-reflection techniques have been applied in several studies over the last 20 years as a uranium-exploration tool within the Athabasca Basin and have been utilized to provide the larger structural context for known uranium deposits within the basin. At the crustal scale, deposits within the eastern Athabasca Basin are shown to be associated with deep-seated shear zones that originated during Trans-Hudson orogeny and have subsequently been reactivated during and subsequent to deposition of the basin-fill sandstones. Seismic properties of the Athabasca sandstones and underlying basement have been determined through in situ borehole measurements. Reflectivity within the sandstones is generally weak. Seismically recognizable signatures are primarily associated with variations in fracture density, porosity, and degree of silicification. The basement unconformity and regolith, a prime target of exploration, is widely imaged as it is characterized by variable but generally distinct reflectivity. Results from the McArthur River mine site suggest that the spatial coincidence of seismically imaged high-velocity zones and deep-seated faults that offset the unconformity may be a more broadly applicable exploration targeting tool. Three-dimensional (3-D) seismic imaging near existing ore zones can define the local structural controls on the mineralization and point the way to new targets, thus leading to more efficient exploration drilling programs. Furthermore, seismically generated structural maps of the unconformity and rock competence properties may play a significant role at the outset of mine planning.

Structural controls of fracture orientations, intensity, and connectivity, Teton anticline, Sawtooth Range, Montana

The Teton anticline is a multiple hinge anticline containing fractured Mississippian–Devonian carbonates in the frontal part of the Sawtooth Range in Montana. The structure serves as a good surface analog for fracture patterns and connectivities within subsurface-folded carbonate reservoirs. The primary fracture sets are longitudinal and transverse relative to the axis of the fold, although two additional oblique sets are also present. The length and density of the longitudinal fracture sets are strongly controlled by position relative to multiple hinges. The transverse fractures are related to changes in fold plunge and exhibit less variation in fracture density. Fracture connectivity is dependent on the number of fracture sets, their orientations and dispersions, and the densities of the fracture sets. The connectivity is measured using two parameters: the fractional connected area (FCA), which represents the fraction of the total sample area that is connected by fractures, and the distribution of clusters of different sizes in any given area. Because the longitudinal fractures represent the dominant fracture set and also show the most variation with structural position, the fracture connectivity, as measured by both the FCAs and the distribution of cluster sizes, is greater in the vicinity of the fold hinges. The results and approaches used in the study have some important implications for subsurface-folded fractured carbonate reservoirs. The analysis of sparsely distributed fracture data from wells must be integrated with an understanding of the controls of the macroscopic structure on fracture parameters to effectively simulate fracture patterns and connectivities around subsurface structures.

Geologic and Edaphic Controls on a Serpentine Forest Community

This study examined woody vegetation, edaphic factors, bedrock geochemistry, petrography, and outcrop structure to evaluate some of the community-structuring factors in an ultramafic terrain of Maryland. Analyzing the dynamic nature of combined geological and ecological processes can detect correlative relationships between factors that are typically considered as independent such as tectonically driven bedrock fracturing and ecological community interaction. This study provides evidence for structural variation in fracture density of bedrock as a partial control on tree species distribution in an ultramafic woodland/forest ecosystem. Increases in the number of bedrock fractures correlates negatively with plot-level volumetric soil moisture. Additionally, the degree of serpentinization of the ultramafic parent material results in compositional variation in Ca, Mg, and Ni of parent materials and soils. The combination of these factors provides a significant level of control on the distribution of xeric tree species.

Seismic characterization of reservoirs containing multiple fracture sets

ABSTRACTNatural fractures in reservoirs play an important role in determining fluid flow during production and knowledge of the orientation and density of fractures is required to optimize production. Variations in reflection amplitude versus offset (AVO) are sensitive to the presence of fractures but current models used to invert the seismic response often make simplified assumptions that prevent fractured reservoirs from being characterized correctly. For example, many models assume a single set of perfectly aligned fractures, whereas most reservoirs contain several fracture sets with variable orientation within a given fracture set. In addition, many authors only consider the azimuthal variation in the small offset amplitude versus offset and azimuth response (the variation in AVO gradient with azimuth), while the effect of fractures on amplitude versus offset and azimuth increases with increasing offset. In this paper, the variation in the reflection coefficient of seismic P‐waves as a function of azimuth and offset due to the presence of multiple sets of fractures with variable orientation within any fracture set is used to determine the components of a second‐rank fracture compliance tensor αij. The variation in the trace of this tensor as a function of position in the reservoir can be used to estimate the variation in fracture density with position in the reservoir and may be used to choose the location of infill wells in the field. The principal axes of αij reveal the most compliant direction within the reservoir and may be used to optimize the trajectory of deviated wells. The determination of the principal axes of αij requires wide azimuth acquisition and the use of the small‐offset amplitude versus offset and azimuth (the azimuthal variation of the AVO gradient) may give misleading results.

Caractérisation de la structure interne et de l'état hydrique de glissements argilo-marneux par tomographie géophysique : l'exemple du glissement-coulée de Super-Sauze (Alpes du Sud, France)

Geophysical methods such as seismic surveying and electrical resistivity imaging appear to be well-adapted to investigate and understand landslide mechanisms. They allow direct and non-intrusive measurement of acoustic velocity and electrical resistivity, two fundamental parameters used to define the physical properties of the reworked moving materials. Both methods were applied at the Super-Sauze site in the French South Alps, where a typical example of an intra-material earthflow-mudslide can be observed. Measurements were taken simultaneously along two orthogonal profiles: one 325 m in length is perpendicular to the axis of the earthflow; the other 235 m in length is located along the axis of the earthflow. The results show a correlation between the seismic and electrical resistivity data, confirming that the simultaneous use of both methods gives complementary information about landslide mechanisms. The seismic data provide information on fracture density variations, whereas the electrical resistivity data provide information on water content variations. To cite this article: G. Grandjean et al., C. R. Geoscience 338 (2006).

AVO and AVA inversion for fractured reservoir characterization

Seismic wave reflection amplitudes are used to detect fluids and fracture properties in reservoirs. This paper studies the characterization of a vertically fractured fluid‐filled reservoir by analyzing the reflection amplitudes of P‐waves with varying incident and azimuthal angles. The reservoir is modeled as a horizontal transversely isotropic medium embedded in an isotropic background, and the linearized P‐waves reflection coefficient are considered. The conditioning of the inverse problem is analyzed, and fracture density is found to be the best conditioned parameter. Using diffraction tomography under the Born approximation, an inversion procedure is proposed in the transformed k–ω domain to detect fracture density variations within the reservoir. Seismic data are rearranged in pairs of incident and reflected plane waves, enlightening only one spectral component of the fracture density field at a time. Only the observable spectral components are inverted. Moreover, working in the transformed domain, picking reflection amplitudes is not required. An example of the inversion applied to a synthetic data set is presented. The limitation of source and receiver numbers and the finite bandwidth of the wavelet produce a loss of resolution, but the overall fracture density variations are recovered correctly.

Shallow seismic reflection profiling over the Brevard zone, South Carolina

Shallow reflection profiles over crystalline rocks of the Brevard zone show events that correlate with projections of mapped lithologic contacts, in spite of strong attenuation and statics effects associated with a zone of severe chemical weathering (saprolite). The profiles, which were collected in western South Carolina about 5 km from Appalachian ultra Deep Core Hole (ADCOH) regional lines 1 and 3, straddle the contact between the Brevard fault zone and the wider Brevard mylonitic shear zone to the southeast. The principal goal of these experiments was to test the feasibility of using a hammer source and 24-channel seismograph to image near‐surface brittle and ductile structures to a depth of several hundred meters. We recorded two dip lines and one strike line with a total common midpoint (CMP) coverage of 1 km. Processing focused on eliminating static shifts and separating reflections from overlapping refracted S-waves. A statics approach based on the alignment of first arrivals on common‐offset gathers effectively removed most of the statics variations. Suppressing S-waves by apparent velocity filtering of shot gathers caused considerable smearing in the stacked section because of the short receiver spreads. Moreover, for shot gathers with high levels of incoherent noise and/or residual statics, velocity filtering imposed a level of coherency on the output that was not present in the original gather. Better results were obtained by applying severe surgical mutes to isolate the most reliable portions of the velocity‐filtered shot gathers prior to CMP stacking. Final CMP stacked sections show distinct packages of moderately dipping reflections with a lateral continuity of 25 to 100 m over a depth range of 85 to 400 m. The reflections are attributed to compositional layering, variations in degree of anisotropy, near‐horizontal expansion joints, and variations in fracture density. Southeast‐dipping reflections imaged for a 450-m dip line straddling the southeast flank of the brittle Brevard fault zone correlate with projected lithologic contacts of thrust‐faulted units mapped at the surface. Reflections show poorer continuity for lines recorded over a thick sequence of more uniform lithology within the adjacent Brevard mylonite zone. Reflections with apparent dips to the northwest are consistent with images in the ADCOH regional profiles and may reflect late‐stage folding and normal faulting.

Multistage landform development, with particular reference to a cratonic bornhardt

The multistage concept is explained and illustrated, with particular reference to a bornhardt from the southern Yilgarn Block of southwestern Western Australia. Hyden Rock is a complex inselberg developed on Archaean granite. It probably developed through fracture‐controlled differential subsurface weathering in the Cretaceous, followed in the Eocene by erosion of the lateritic regolith and exposure of the massive compartment as an inselberg. But the variations in fracture density to which Hyden Rock owes its origin developed long before the Cretaceous, probably in the latest Archaean or earliest Proterozoic. The relief amplitude of the residual has increased through the Cainozoic.A rich assemblage of minor forms (basins, runnels, tafoni, pitting, displaced slabs, A‐tents) is developed on the bornhardt and, though their mode of development varies, the origin of many can be traced back to magmatic and tectonic events of Late Archaean and earliest Proterozoic times. Others were initiated at the weathering front during the later Mesozoic, and many have developed since exposure, beginning in the Eocene. A few are related to tectonism during the later Quaternary.The residual is a multistage form. Its development as a major landform can be traced back some 120 million years, and the structures to which the outlines of the bornhardt, and those related to several minor features, originated in later Archaean times. The geomorphological evolution of landforms of the shield lands and cratons extends over vast ages and cannot be regarded as a geologically recent phenomenon.

3-D AVO analysis and modeling applied to fracture detection in coalbed methane reservoirs

Over the years, amplitude variation with‐offset (AVO) analysis has been used successfully to predict reservoir properties and fluid contents, in some cases allowing the spatial location of gas‐water and gas‐oil contacts. In this paper, we show that a 3-D AVO technique also can be used to characterize fractured reservoirs, allowing spatial location of crack density variations. The Cedar Hill Field in the San Juan Basin, New Mexico, produces methane from the fractured coalbeds of the Fruitland Formation. The presence of fracturing is critical to methane production because of the absence of matrix permeability in the coals. To help characterize this coalbed reservoir, a 3-D, multicomponent seismic survey was acquired in this field. In this study, prestack P‐wave amplitude data from the multicomponent data set are used to delineate zones of large Poisson's ratio contrasts (or high crack densities) in the coalbed methane reservoir, while source‐receiver azimuth sorting is used to detect preferential directions of azimuthal anisotropy caused by the fracturing system of coal. Two modeling techniques (using ray tracing and reflectivity methods) predict the effects of fractured coal‐seam zones on angle‐dependent P‐wave reflectivity. Synthetic common‐midpoint (CMP) gathers are generated for a horizontally layered earth model that uses elastic parameters derived from sonic and density log measurements. Fracture density variations in coalbeds are simulated by anisotropic modeling. The large acoustic impedance contrasts associated with the sandstone‐coal interfaces dominate the P‐wave reflectivity response. They far outweigh the effects of contrasts in anisotropic parameters for the computed models. Seismic AVO analysis of nine macrobins obtained from the 3-D volume confirms model predictions. Areas with large AVO intercepts indicate low‐velocity coals, possibly related to zones of stress relief. Areas with large AVO gradients identify coal zones of large Poisson's ratio contrasts and therefore high fracture densities in the coalbed methane reservoir. The 3-D AVO product and Poisson's variation maps combine these responses, producing a picture of the reservoir that includes its degree of fracturing and its possible stress condition. Source‐receiver azimuth sorting is used to detect preferential directions of azimuthal anisotropy caused by the fracturing system of coal.

Electromagnetic images of crustal structures in southern and central Canadian Cordillera

Data from more than 400 magnetotelluric soundings, made since the early 1980's in the Canadian Cordillera over a 300 000 km2area between 49 and 53.5°N, are used to image qualitatively regional three-dimensional crustal variation in electrical conductivity by means of phase maps, phase–frequency sections, and maps of resistivity at depth. Two hundred of the soundings were acquired as part of Lithoprobe Southern Canadian Cordillera Transect activities, and their locations were coordinated with the seismic reflection and refraction experiments. The lower crust has a generally pervasive, low resistivity (1–100 Ω·m) throughout the Cordillera west of the Foreland Belt. Within this "Canadian Cordilleran Regional" conductor, the magnetotelluric data reveal both two-dimensional structures, with highest conductivities along the Coast Belt and Omineca Belt, and three-dimensional variation along geological strike. This conductor, mapped over a volume in excess of 106 km3, is most probably caused by fluids–saline waters and silicate melts–in fractures and along interconnected grain boundaries. The observed lateral variations in conductivity may result from variations in fracture density, temperature, and the sources of hot fluid, such as the subducting Juan de Fuca plate under the Coast Belt, and mantle upflow under the Omineca Belt. In addition, we report a major east–west-trending geophysical discontinuity in the upper and middle crust of the Omineca Belt at a latitude of 50°N, with highly resistive rocks (>1000 Ω·m) to the south and more conductive rocks to the north (30–300 Ω·m). Seismic refraction models, residual gravity, and filtered magnetic maps correlate changes in compressional-wave velocity, density, and magnetization along this cross-strike discontinuity.

Fluid-rock interaction between syenites and marbles at Stephen Cross Quarry, Qu�bec, Canada: petrological and stable isotope data

Grenville dolomitic marbles and calc-silicates at Stephen Cross Quarry, Quebec, underwent contact metamorphism and metasomatism associated with the intrusion of the Wakefield syenite at ambient pressures of ∼0.4GPa at 1090–1070Ma. Fluid infiltration produced exoskarns, calcite+periclase+forsterite±diopside±orthoclase assemblages in the marbles, and quartz±calcite±wollastonite±diopside±anorthite assemblages in the calc-silicates. Phase-equilibria in the CaO−MgO−Al2O3−SiO2−H2O−CO2 system suggest that fluid infiltration occurred close to the thermal peak of contact metamorphism (715–815°C) and that the fluids hadXCO2≤0.15. In the metasediments, δ18O values of calcite (Cc) are as low as 8.6‰, suggesting that the fluids were in isotopic equilibrium with the syenites (δ18O =8.8–10.2‰). Marble δ13C(Cc) values are-0.1 to-3.2‰; the lack of correlation between δ13C(Cc) and δ18O(Cc) is consistent with the infiltration of water-rich fluids. The resetting of stable isotopes and the mineralogical changes can be explained by time-integrated fluid fluxes of up to 110 m3/m2 (4×106 mol/m2), corresponding to actual fluxes of 3×10-11 to 3×10-12 m3/m2-s and intrinsic permeabilities of 10-18 to 10-20 m2 for fluid flow lasting 0.1-1Ma. Marble δ18O(Cc) values do not correlate well with distance from the syenite, and fluids were probably channelled across lithological layering. The correlation between the degree of resetting of marble δ18O(Cc) values with the abundance of submillimetre-wide veins, suggests that fluid focussing may have resulted from variations in fracture density. Late, lower temperature (<500°C), fluid flow formed serpentine (Serp) and brucite (Br) from periclase and forsterite. δ18O(Br) and δ18O(Scrp) values correlate with δ18O(Cc), suggesting that retrogression involved only limited volumes of fluid. The observation that Δ18O(Cc-Br) and Δ18O(Cc-Serp) values are higher in marbles that have lower Δ18O(Cc) values is interpreted as indicating that fluid flow persisted to lower temperatures in those rocks due to higher intrinsic permeabilities. Calcite in the syenite was also formed by the influx of fluids during cooling. Syenite Δ18O(Cc) values are approximately in isotopic equilibrium with the high-temperature silicate minerals, suggesting that again only minor volumes of fluid were involved. In detail fluid flow was prolonged and complex, creating problems for the application of quantitative fluid flow models.