Fault Rocks: A Fourth Class of Rocks

The effect of weathering on cohesion in granitic fault rocks: A case study from the Yeongdeok Fault, South Korea

Fault seal analysis: successful methodologies, application and future directions

During the late Oligocene – middle Miocene, an extensive deformation took place in the Western Mediterranean. In this process, main faults of NE-SW striking direction bordering horsts and grabens of the Catalan margin and minor faults perpendicular to the former were formed. We have studied some of these minor faults affecting Miocene conglomerates in the Barcelona Plain. The fault zone is formed by an absent damage zone and a poorly developed fault core. Their fault rocks are obliterated by later pedogenetic processes that have changed fault and fault rock properties and consequently fault rock classification. Nowadays fault rock is a cataclasite but because it is cemented by later pedogenetic processes, in fact, in origin was a gouge. When fault rock was a gouge (stage 1), and considering fault zone architecture, cross-fault flow could exist. During the second stage, after fault rock pedogenesi, new minerals reduced effective porosity and so permeability of the fault rock. The consequence was the seal of faults and compartmentalisation of flow. In conclusion, faults occurring in the upper meteoric environment can have other kind of processes responsible of their impermeabilization than deeper faults.

Faults are important to the economic geologist in that they can either control the formation of some classes of ore deposits or modify the position and geometry of ore bodies. Fault rock classification is an important part of understanding the structural geology of an area. In this paper practical modifications are suggested to the most commonly used classifications of fault rock, to make it more applicable for the economic geologist. The system recommended is based in the first instance on the classification according to cohesiveness of the rock. The second defining attribute is whether the matrix is foliated or has a random fabric, providing the subdivision of cohesive fault rocks into the mylonitic and cataclastic series. The third attribute used to categorize fault rocks is the proportion of matrix to clasts, and differs in detail from those previously published. The terminology relating to crush breccias used by earlier systems is considered confusing and superfluous and has been eliminated. Although of lesser importance to the economic geologist, the terminology evolving for mylonites and mylonitc gneisses is included and is based on internal fabrics and textures. Application of this simplified classification to geological mapping in underground exploration development and logging of faults in core will enhance the geologist’s ability to interpret the structure of ore deposits.

<p>The GeoTief EXPLORE project aims to explore the geothermal potential and quantify the geothermal resources of the Vienna Basin (Austria) and the underlying Northern Calcareous Alpine basement. The main target of geothermal interest is the massive and tectonically remolded Hauptdolomite facies that has been identified as potential geothermal reservoir in previous studies. Now, this formation is studied using outcrop analogues for the investigation of their petrophysical characterization and specific thermal properties (thermal conductivity and thermal diffusivity).</p><p> </p><p>Here, we report new measurements on a total of 60 samples from 6 outcrops in and around the area of Vienna applying different methods for the laboratory measurement of thermal and hydraulic rock properties. The petrophysical analysis considers the impact of deformation along and across fault zones, which introduces heterogeneity of storage properties and consequently in the thermophysical properties. Using the standard fault core and damage zone model, outcrop samples were grouped into unfractured and fractured protoliths, as well as in fault rocks, like breccias and cataclasites. Rock samples are then classified by their fracture density (m² fracture surface per m³ rock) and by their matrix content and differences in grain sizes, respectively.</p><p> </p><p>The measured thermal rock properties vary significantly between the selected rock groups. The total range [90 % of values] is between 3.2 and 5.0 W/(mK) for thermal conductivity and between 1.3 and 2.7 mm²/s for thermal diffusivity. The results generally met the expected trend for fractured rocks as conductivity and diffusivity decreases with increasing porosity under unsaturated and saturated conditions. The total porosities are less than 5%. The variability of thermal conductivity under saturated conditions shows complex trends depending on the different rock classifications where fault rocks and highly fractured rocks of the damage zone show lower increase in thermal conductivities.</p><p> </p><p>The new petrophysical characterization will be the base for further numerical investigations of the hydraulic and thermal regime as well as for the analysis of the geothermal resources of the Hauptdolomite.</p><p> </p><p> </p><p> </p><p> </p>

J. F. Magloughlin comments: I would like to compliment Woodcock & Mort on an important attempt to bring more order to the complex and under-attended world of fault rock classification. The authors aptly point out several of the persistent difficulties in fault rock terminology, such as the original cohesive-versus-incohesive dichotomy in Sibson's (1977) classification, and they appropriately attempt to create a more nongenetic classification scheme.

A review of fault sealing behaviour and its evaluation in siliciclastic rocks

The most recent advances in the knowledge of physical properties of rock are summarized. The rock mechanics of dams is approached through an examination of the means used by engineers to design modern dams. The research required, and the interpretation of results of tests and measurements are reviewed. A discussion of the physical properties of rock covers such aspects as deformation in-situ under stress, shear strength determinations, methods of testing rock, and the classification of rock. The problem of water pressure in rock faults and joints and in rock fissures is discussed. This covers the aspects of drainage versus grouting, and practical examples of rock drainage (arch dams). Attempts to calculate rock abutments (mainly for arch dams) are outlined. Typical examples are described of modern dams with different foundations on rock. These include the Malpasset dam, dams with reinforcement of rock foundations, buttress dams with foundation reinforcement, and dams with other foundation problems. The present trend in rock mechanics and concrete dam foundations is summarized.

Despite extensive research on fault rocks, and on their commercial importance, there is no non-genetic classification of fault breccias that can easily be applied in the field. The present criterion for recognizing fault breccia as having no ‘primary cohesion’ is often difficult to assess. Instead we propose that fault breccia should be defined, as with sedimentary breccia, primarily by grain size: with at least 30% of its volume comprising clasts at least 2 mm in diameter. To subdivide fault breccias, we advocate the use of textural terms borrowed from the cave-collapse literature – crackle, mosaic and chaotic breccia – with bounds at 75% and 60% clast content. A secondary breccia discriminant, more difficult to apply in the field, is the ratio of cement to matrix between the clasts. Clast-size issues concerning fault gouge, cataclasite and mylonite are also discussed.

The concentration of sub-seismic deformation adjacent to larger faults forms a complex deformation volume or damage zone which is important to hydrocarbon migration, trapping and reservoir management. The results of a combined study of structural logging of cores, microstructural characterization and physical property measurement of deformation features preserved in cores, with analysis of fault populations from seismic and outcrop studies, are used to evaluate the properties and evolution of damage zones. The detailed internal structure of fault zones is dependent on the conditions of deformation, the lithological architecture present and the position in the fault array. Clusters of deformation within the damage zone are also surrounded by relatively undeformed volumes. Quantification of the geometry of the these zones is critical to understanding the fluid-flow and sealing properties of-the fault zone. Complete evaluation of the impact of the damage zone on reservoir properties requires knowledge of the petro-physical properties and spatial distributions of the fault rocks present. These range from quartz-rich cataclasites, developed from pure sandstones, to phyllosilicate smears developed from shales. An important class of intermediate fault rock is generated from impure sandstones. Localization of cement precipitation within the damage zone will remove the applicability of simple seal analysis basedmore » only on the host-rock lithology and displacement. Realistic prediction of the impact of faults on reservoir behavior requires integration of the detailed geometry with the petrophysical characterization of the fault rocks.« less

A successful case history of exploring for concealed structure using the high-resolution EM method in the investigation of the West-East Gas Pipeline Project’s B Tunnel is presented in this paper. The high frequency electromagnetic image system named STRATAGEM EH4, operating at frequencies ranging from 90 KHz to lHz, was used for data acquisition. The orthogonal components of the electromagnetic field were measured during the field acquisition and the relevant electromagnetic attributes of the object body were extracted from the electromagnetic data. Hybrid sources, consisting of natural and full tensor-controlled sources, were utilized to produce high-quality electromagnetic field data. B Tunnel lies in the western part of Hubei province, at depths of less than 200 m. The geologic setting of B tunnel is very complex. Following an initial geologic investigation, an outcrop considered to be a bedrock interface by investigators, collapsed during tunneling operations. A second investigation applied high-resolution EM and seismic refraction methods to reveal a more complex geologic structure along the tunnel route. The predicted rock classes and fault were encountered during the subsequent tunneling operations.

Quantification of classification of the fault rocks along the GSJ Nojima Drining Core.

Dynamic recrystallization of plagioclase and classification of fault rocks

Engineering geological classification of fault rocks : Zhang Xian-Gong; Han Wen-Feng; Nie De-Xin Proc 5th International Congress International Association of Engineering Geology, Buenos Aires, 20–25 October, 1986V1, P479–486. Publ Rotterdam: A A Balkema, 1986

Engineering geological classification of fault rocks : Zhang Xian-Gong; Han Wen-Feng; Nie De-Xin Proc 5th International Congress International Association of Engineering Geology, Buenos Aires, 20–25 October, 1986V1, P479–486. Publ Rotterdam: A A Balkema, 1986