Abstract The aim of this study is to evaluate what seismic attributes are best able to highlight porous non-stratabound dolostone geobodies set in low porosity limestone. For this purpose three dolostone geobody volume scenarios were defined using outcrop based three-dimensional models to define the range of dimensions of dolostone geobodies and their association with particular fault populations. Three porosity scenarios were created using a global compilation to assign porosities to three lithologies: host limestone, bulk dolostone geobodies, and dolostone geobodies adjacent to faults. The combination of porosity and geobody volume scenarios yielded nine non-stratabound dolostone geobody scenarios. These include models in which the properties of near-fault dolostones were enhanced or degraded relative to the bulk dolostone geobody values. This allows for the effects of processes such as overdolomitization or dissolution to be implicitly explored, since those processes can degrade or enhance near-fault properties such as porosity, although in all scenarios dolostone porosities are greater than host limestone porosity. Density and compressional velocity ( V p ) were assigned to the scenarios based on a global compilation of the density, porosity, and V p in limestones and dolostones to allow for the calculation of acoustic impedance volumes that are representative of the range of values that could exist at depth. Synthetic seismic cubes and a suite of 14 seismic attributes were generated for each of the nine dolostone scenarios. Each attribute response was evaluated for its potential to highlight porous non-stratabound dolostone geobodies. Attributes that are most sensitive to lateral changes in acoustic properties rank the highest in the evaluation, followed by amplitude attributes, followed in turn by frequency attributes. Continuity attributes rank poorly in this example because fault offset is relatively small and is obscured by dolomitization.

Abstract Hydrothermal dolomite (HTD) bodies are known as high-quality hydrocarbon reservoirs; however few studies focus on the geometry and distribution of reservoir characteristics. Across the platform-to-basin transition of the Ramales Platform, fault-controlled HTD bodies are present. Three kinds of bodies can be distinguished based on their morphology, that is, elongated HTD corridors, a massive HTD body (Pozalagua body) and an HTD-cemented breccia body. The differences in size and shape of the HTD bodies can be attributed to differences in local structural setting. For the Pozalagua body, an additional sedimentological control is invoked to explain the difference in HTD geometry. A (geo)-statistical investigation of the reservoir characteristics in the Pozalagua body revealed that the HTD types (defined based on their texture) show spatial clustering controlled by the orientation of faults, joints and the platform edge. Porosity and permeability values are distributed in clusters of high and low values; however, they are not significantly different for the three HTD types. Two dolomitization phases (i.e. ferroan and non-ferroan) can be observed in all HTD bodies. In general, the HTDs resulting from the second non-ferroan dolomitization phase have lower porosity values. No difference in permeability is found for the ferroan and non-ferroan dolomites.

Abstract Basin modelling tools are now more efficient to reconstruct palinspastic structural cross sections and compute the history of temperature, pore-fluid pressure and fluid flow circulations in complex structural settings. In many cases and especially in areas where limited erosion occurred, the use of well logs, bottom hole temperatures (BHT) and palaeo-thermometers such as vitrinite reflectance (Ro) and Rock-Eval (Tmax) data is usually sufficient to calibrate the heat flow and geothermal gradients across a section. However, in the foothills domains erosion is a dominant process, challenging the reconstruction of reservoir rocks palaeo-burial and the corresponding calibration of their past thermal evolution. Often it is not possible to derive a single solution for palaeo-burial and palaeo-thermal gradient estimates in the foothills, if based solely on maturity ranks of the organic matter. Alternative methods are then required to narrow down the error bars in palaeo-burial estimates, and to secure more realistic predictions of hydrocarbon generation. Apatite fission tracks (AFT) can provide access to time–temperature paths and absolute ages for the crossing of the 120 °C isotherm and timing of the unroofing. Hydrocarbon-bearing fluid inclusions, when developing contemporaneously with aqueous inclusions, can provide a direct access to the pore-fluid temperature and pressure of cemented fractures or reservoir at the time of cementation and hydrocarbon trapping, on line with the tectonic evolution. Further attempts are also currently made to use calcite twins for constraining reservoir burial and palaeo-stress conditions during the main deformational episodes. Ultimately, the use of magnetic properties and petrographical measurements can also document the impact of tectonic stresses during the evolution of the layer parallel shortening (LPS). The methodology integrating these complementary constraints will be illustrated using reference case studies from Albania, sub-Andean basins in Colombia and Venezuela, segments of the North American Cordillera in Mexico and in the Canadian Rockies, as well as from the Middle East.

Abstract A structural analysis and petrographic investigation has been performed on veins in Cretaceous carbonates in the Cordoba Platform in eastern Mexico, which is part of the Laramide foreland fold and thrust belt (FFTB). This chapter focuses on the different episodes of vein formation, vein morphology, and possible mechanisms of vein formation. Vein (fracture) formation is interpreted in relation to the kinematic evolution of the FFTB. Evidence for the development of hydrofractures during this evolution is given. This study documents veins (fractures) related to Laramide FFTB development in the Cordoba Platform. These veins (fractures) are related to the kinematic evolution of the area and the inferred paleostress conditions. The kinematic evolution can be split up into three major stages: a precompression phase with platform development; a Laramide compressional stage, during which the FFTB developed; and finally, a late Basin and Range-related extension phase. Compound veins and densely spaced microveins record multiple fracturing events in a cyclic stress field during burial, most probably caused by changes in fluid pressure. They are interpreted in relation with early foreland flexuring. With rising compressional stress, less well-oriented veins and breccia veins develop because of a lowered differential stress in the prefolding stage. Progressive layer-parallel shortening (LPS) leads to a caterpillar-type scenario of fluid migration toward the foreland, eventually causing hydrofracturing, succeeded by pressure solution and development of vertical stylolitic planes. These LPS stylolites have the potential to be reopened during subsequent folding of the strata. In addition, older LPS-parallel planes and extrados fractures may open in anticlinal hinges. Shear-associated, shallow-dipping veins develop after LPS development, possibly because of bedding-parallel shear and/or thrust migration. Other post-LPS veins are steeply dipping and commonly reuse older vein orientations. Dark, banded veins, which are filled with a silt-sized and clay-sized material and lack significant cementation, are interpreted to reflect fracture planes along which recrystallization of matrix occurred. Many post-LPS dissolution-enlarged veins and breccias relate to telogenetic karstification. Post-LPS multiple brecciation just above a major thrust plane in the buried tectonic front area is interpreted to reflect the damage zone of that fault.

Abstract X-ray computed tomography (CT) is a non-destructive technique with wide applications in various geological disciplines. It reveals the internal structure of objects, determined by variations in density and atomic composition. Large numbers of parallel 2D sections can be obtained, which allows 3D imaging of selected features. Important applications are the study of porosity and fluid flow, applied to investigations in the fields of petroleum geology, rock mechanics and soil science. Expected future developments include the combined use of CT systems with different resolutions, the wider use of related X-ray techniques and the integration of CT data with results of compatible non-destructive techniques.

Abstract High-resolution and ultra-high-resolution X-ray computed tomography are rapid, non-destructive and extremely powerful techniques for three-dimensional examination and measurement of a great variety of geological materials and specimens with sizes from several millimetres to several decimetres. A review of recent applications in petrology, meteoritics and palaeontology, which utilized an instrument optimized for geological studies (High-Resolution X-ray Computed Tomography Facility of the University of Texas at Austin), documents an abundance of novel scientific results and illuminates the potential for still broader application of these techniques in the earth sciences.

Abstract Imaging the distribution of porosity, permeability, and fluid phases is important to understanding single and multiphase flow characteristics of porous media. X-ray computed tomography (CT) has emerged as an important and powerful tool for nondestructive imaging because it is relatively easy to apply, can offer fine spatial resolution and is adaptable to many types of experimental procedures and flow conditions. This paper gives an overview of CT technology for imaging multiphase flow in porous media, the principles behind the technology and effective experimental design. By critically reviewing prior work using this important tool, we hope to provide a better understanding of its use and a pathway to improved analysis of CT-derived data. Because of the wide variety of image processing options, they are discussed in some detail.

Abstract A series of measurements of the structure of a variety of porous materials has been made using synchrotron computed microtomography (SCMT). The work was carried out at the Brookhaven National Synchrotron Light Source (NSLS), the Argonne Advanced Photon Source (APS) and the European Synchrotron Radiation Facility (ESRF). The experiments at Brookhaven and Argonne were carried out on bending magnet beam lines using area detectors to obtain CT images based on determination of X-ray absorption coefficients. The work at the ESRF used an undulator beam line, a 13KeV pencil X-ray beam of 2 μm and an energy dispersive X-ray detector to make tomographic sections of trace element distributions by X-ray fluorescence tomography. Most of the work was done with a pixel/voxel size ranging from 0.002 to 0.010 mm. We examined the structure of unconsolidated estuarine sediments, whose structure is relevant to transport of contaminants in rivers and estuaries. Fluorescent tomography with 2-3 μm resolution was used to ascertain whether or not metals were concentrated on the surface or throughout the volume of a single sediment particle. Sandstone samples were investigated to obtain a set of values describing their microstructures that could be useful in fluid flow calculations relevant to petroleum recovery or transport of environmental contaminants. Measurements were also made on sandstone samples that had been subjected to high-pressure compression to investigate the relation between the microgeometry and the magnitude of the applied pressure. Finally, a Wood's metal-filled sample was scanned for demonstration of resolution enhancement and fluid flow studies.

Abstract Porosity of reservoir rocks is an important petrophysical characteristic, used as a basic parameter in simulation studies for predicting reservoir quality. An extensive debate continues about the techniques that are available for porosity measurements and visualization. One aspect is the fact that petrophysical measurements are performed on volumetric samples, whereas classical geological petrography using a petrographical microscope is restricted to 2D analysis. This leads to a discrepancy between petrographical and petrophysical studies. This paper aims to evaluate microfocus X-ray computed tomography (μCT) as a technique that can link petrography and petrophysics. A short overview of the μCT technique is given, together with a discussion of its limitations, mainly due to artefacts. Optimization of image quality and procedures for quantification are outlined. (μCT results for porosity measurements of a limestone and a sandstone are compared with results obtained by other techniques.

Abstract Microfocus X-ray computed tomography (μCT) was used as a tool to determine the apertures of a fracture in a cylindrical sample of crinoidal limestone. After scanning, artefacts were removed from the images. Phantom objects were used to establish a calibration relationship between real fracture apertures and fracture aperture measurements on the μCT images. The performance of different procedures for quantitative fracture determination was examined. The calibration relationship was then used to determine the fracture apertures in a naturally fractured sample. A comparison of the μCT technique and a microscope technique shows a good agreement between their results.

Abstract A series of experiments on cubic blocks of quartzite were performed to create fractures with three-dimensional characteristics so that they could serve as verification examples for numerical models that are being developed to analyse the fracture processes around mining excavations. The experiments proved to be very successful for creating 3D fracture patterns that have characteristics similar to those observed underground. The shape and position of the fracture surface is determined by the mining geometry and by the interaction with pre-existing discontinuities. However, variations of the fracture planes within the sample could not be determined from the visual study of the block surfaces. The application of a state-of-the-art medical X-ray computed tomography scanner and the development of automatic surface reconstruction software provided a method of producing a full three-dimensional, digital view of fractures within laboratory test samples. Software was developed to provide an interactive graphical method for studying the scans in three orthogonal planes simultaneously. By contouring below a selected density threshold, three-dimensional images of the fracture surfaces were produced. X-ray computed tomography was found to provide a unique means of visualizing the fractures within rock test samples, which can greatly assist the study of rock fracture processes.

Abstract Semivariograms of samples of Culebra Dolomite have been determined at two different resolutions for gamma ray computed tomography images. By fitting models to semivariograms, small-scale and large-scale correlation lengths are determined for four samples. Different semivariogram parameters were found for adjacent cores at both resolutions. Relative elementary volume (REV) concepts are related to the stationarity of the sample. A scale disparity factor is defined and is used to determine sample size required for ergodic stationarity with a specified correlation length. This allows for comparison of geostatistical measures and representative elementary volumes. The modifiable areal unit problem is also addressed and used to determine resolution effects on correlation lengths. By changing resolution, a range of correlation lengths can be determined for the same sample. Comparison of voxel volume to the best-fit model correlation length of a single sample at different resolutions reveals a linear scaling effect. Using this relationship, the range of the point value semivariogram is determined. This is the range approached as the voxel size goes to zero. Finally, these results are compared to the regularization theory of point variables for borehole cores and are found to be a better fit for predicting the volume-averaged range.

Abstract The porosity and transfer properties of a very low porosity material (granite) are measured. A new procedure is defined using a capillary test and X-ray computed tomography (CT) scanning. Injected volumes are very low, i.e. a few cm 3 for a sample volume of 1 dm 3 , using a fluid/rock ratio lower than 0.1%. This technique allows monitoring of the anisotropy of fluid flow during the test. Flow along the injection direction is higher than along the perpendicular direction. Saturation depends on the specific saturation of each mineral zone. Multiscale analysis allows defining the flow conditions as being controlled at both the mineral and the sample scale. Results indicate the specific role for various constituting parts of the material. High speed flow occurs in the crack network of K-feldspar, while the storage function is localized in the reaction zone forms by quartz and muscovite.

Abstract Faults profoundly affect patterns and rates of fluid flow and solute transport in the geological environment. They may act as conduits, barriers or combined conduit-barrier systems. In order to elucidate the relationship between fluid flow properties and deformation mechanisms of fault-related rocks, we applied X-ray CT during laboratory permeameter measurements for direct imaging of fluid flow during permeability testing. A KI solution, which has high X-ray attenuation values, was used as a contrast medium for the advection imaging. Three-dimensional fluid flow distributions were measured for the studied fault-related rocks. Fault zones characterized by independent particulate flow as deformation mechanism act as conduits for fluid flow, whereas cataclastic fault zones act as barriers.

Abstract Drying experiments were conducted under controlled conditions (relative humidity, temperature, motionless atmosphere) using Fontainebleau sandstone. X-ray computed tomography images acquired at various stages of the drying show that the location of water in the pore network depends on the method of initial saturation. After capillary absorption, the trapped air, or free porosity, allows the water distribution to be homogeneous and independent of the pore structure. This is because it homogenizes the tensions that apply to the water in the pores, regardless of the pore dimensions. However, this is not the case after a total saturation under vacuum. These differences are visible from the beginning to the end of the drying experiments. These results are important for curators or restorers of the architectural heritage, because they show that before testing a conservation product for stones, the method of saturation of these stones must be considered and attention should be given to the required effect.

Abstract Water plays a fundamental role in rock weathering processes. Its penetration and movement inside rocks greatly influences the nature and intensity of damage affecting building stones. X-ray computed tomography (CT) is a useful technique for non-destructive mapping of water penetration. For the reported study, CT was used to investigate the internal structure and water penetration patterns for the Piedramuelle Stone, a Cretaceous limestone used as building material in the Oviedo Cathedral (North of Spain). X-ray CT provides good images of the internal structure of the samples: the sedimentary layering due to differences in composition and porosity is clearly seen, as well as other textural features. The movement and penetration rate of water was monitored during standard free absorption water tests. The images that were obtained show a clear difference between dry and wet zones in the interior of the sample. Water movement is related to the petrographic characteristics of the rock, mainly to layering which controls the direction of water penetration. Hounsfield Unit numbers provide a quantitative approach for assessing the penetration rate of water.

Abstract Heterogeneities are common in natural porous media and are present on different scales. Use of X-ray computed tomography (CT) may provide a tool for quantifying small-scale heterogeneities in porosity and hydraulic conductivity in porous media. Porosity and saturated hydraulic conductivity distributions were estimated using CT for a series of undisturbed soil core samples taken from a field site. CT measurements were collected during breakthrough experiments using an iodide tracer. Techniques were developed to estimate porosity and hydraulic conductivity from solute breakthrough data. Results were compared with bulk sample measurements. CT-measured porosity compared well with laboratory-measured porosity. Hydraulic conductivity estimated from CT methods slightly overestimated laboratory-measured values. These techniques provide a method to quantify the spatially variable porosity and hydraulic conductivity on a millimetre scale rather than on a core-averaged scale. Chemical transport through the soil was predicted using a finite element method for each core using the CT-measured soil properties. Comparisons between measured and predicted chemical transport suggest that small-scale heterogeneities cause departures between measured and simulated solute breakthrough curves, and that a smaller grid size may be needed to improve the simulation.

Abstract Assessment of soil structure, characterized by complex morphological and functional properties, is difficult because most conventional soil physical investigations are destructive and variable in spatial resolution. The use of X-ray computed tomography, as a non-destructive technique, presents significant progress. It can be used to study soil structure at the millimetre scale, e.g. with a resolution of 0.25 mm in the horizontal direction and 1 mm in the vertical direction for the reported study. The measured Hounsfield Unit (HU) values characterize X-ray attenuation for each volume element of the soil core samples. From HU values, soil physical properties of soil cores or their subunits can be derived. They enable: (i) visual assessment of the soil structural condition through inspection of the X-ray CT images; (ii) 3D visualization of air-filled macropores; and (iii) calculation of the mean dry bulk density and standard deviation of voxel-related HU values for successive slices of soil cores. The degradation of structure of loamy and silty soils by tillage could be assessed by CT through quantification of decreased air-filled porosity, destroyed macropore connectivity, increased dry bulk density and decreased standard deviation of HU values in horizontal slices. Small-scale compactions near earthworm burrows could also be detected.