Atilla Aydin

Effect of Faulting on Fluid Flow in Porous Sandstones: Petrophysical Properties

Fault zone permeability in outcrop is quantified by detailed geologic mapping and by measurements using a minipermeameter. Deformation bands, zones of deformation bands, and slip planes are structural elements associated with successive stages in the evolution of a fault zone in porous sandstones. Deformation bands have a porosity about one order of magnitude less than the surrounding host rock and, on average, a permeability three orders of magnitude less than the surrounding host rock. The intensity of cataclasis and the clay content control the amount of permeability reduction as measured perpendicular to a band. The wall rock in proximity to slip planes can have permeabilities more than seven orders of magnitude less than the pristine sandstone. Capillary pressure wit in deformation bands is estimated to be 10-100 times larger than that in the surrounding host rock. Thus, deformation bands and slip planes can substantially modify fluid flow properties of a reservoir and have potential sealing capabilities with respect to a nonwetting phase, as evident in outcrop exposure.

Fractures, faults, and hydrocarbon entrapment, migration and flow

Abstract This paper presents an overview of the role of structural heterogeneities in hydrocarbon entrapment, migration and flow. Three common structural heterogeneity types are considered: (1) dilatant fractures (joints, veins, and dikes); (2) contraction/compaction structures (solution seams and compaction bands); and (3) shear fractures (faults). Each class of structures has a different geometry, pattern, and fluid flow property, which are described by using analog outcrop studies, conceptual models, and, in some cases, actual subsurface data. Permeability of these structures may, on average, be a few orders of magnitude higher or lower than those of the corresponding matrix rocks. Based on these differences and the widespread occurrence of fractures and faults in rocks, it is concluded that structural heterogeneities should be essential elements of hydrocarbon migration and flow as well as entrapment and that they should be included in large-scale basin models and reservoir-scale simulation models. This proposition is supported by a number of case studies of various reservoirs presented in this paper.

Development of faults as zones of deformation bands and as slip surfaces in sandstone

Three forms of fault are recognized in Entrada and Navajo Sandstones in the San Rafael Desert, southeastern Utah; deformation bands, zones of deformation bands, and slip surfaces. Small faults occur asdeformation bands, about one millimeter thick, in which pores collapse and sand grains fracture, and along which there are shear displacements on the order of a few millimeters or centimeters. Two or more deformation bands adjacent to each other, which share the same average strike and dip, form azone of deformation bands. A zone becomes thicker by addition of new bands, side by side. Displacement across a zone is the sum of displacements on each individual band. The thickest zones are about 0.5 m and total displacement across a thick zone rarely exceeds 30 cm. Finally,slip surfaces, which are through-going surfaces of discontinuity in displacement, form at either edge of zones of highly concentrated deformation bands. In contrast with individual deformation bands and zones of deformation bands, slip surfaces accommodate large displacements, on the order of several meters in the San Rafael Desert.The sequence of development is from individual deformation bands, to zones, to slip surfaces, and each type of faulting apparently is controlled by somewhat different processes. The formation of zones apparently involves strain hardening, whereas the formation of slip surfaces probably involves strain softening of crushed sandstone.

Microstructure of deformation bands in porous sandstones at Arches National Park, Utah

Abstract At Arches National Park it is possible to distinguish three kinds of deformation bands on the basis of their distinctive microstructure: (1) deformation bands with little or no cataclasis; (2) deformation bands with cataclasis; and (3) deformation bands with clay smearing. The micromechanics of deformation band development consist of initial dilatancy followed by grain crushing and compaction. This process may be developed to different stages according to the interplay of porosity, confining pressure, clay content and amount of strain. Low porosities and low confining pressures promote the formation of dilatant bands with no cataclasis. High porosities and high confining pressures promote compaction and cataclasis. Two generations of deformation bands were documented. The older generation has little or no cataclasis and formed in relatively undisturbed sandstone probably under conditions of low confining pressure. The younger generation exhibits cataclasis, appears to be localized in proximity to major faults and seems to have developed under conditions of high confining pressure. The temporal sequence of deformation band development can be related to the regional geology of the area; where the first generation probably formed during growth of the salt anticline, and the second generation during its collapse.

Analysis of faulting in porous sandstones

Abstract Faults in porous sandstones occur in three forms: deformation bands about 1-mm thick and tens of m long and across which offsets are a few mm; zones of deformation bands constituted of many closely spaced deformation bands across which offsets are a few cm or dm; and slip surfaces , that is, distinct surfaces within zones of deformation bands across which offsets are a few m to a few tens of m. Deformation bands represent highly localized deformation; analogous localization within a field of homogeneous deformation is theoretically possible in inelastic materials with certain ranges of constitutive parameters. Crushing and consolidation of sandstone within a band cause the material there to become stiffer than the surrounding porous sandstone. A zone of deformation bands behaves mesoscopically much as a stiff inclusion in a soft matrix. According to the constitutive model assumed to investigate the formation of deformation bands, an instability can develop, and strain increments within the zone of deformation bands can become boundlessly large when the far-field stresses reach critical values. This instability is here associated with the formation of slip surfaces.

Three-dimensional analyses of slip distributions on normal fault arrays with consequences for fault scaling

Abstract Many fault arrays consist of echelon segments. Field data on ancient and active faults indicate that such segmented geometries have a pronounced effect on the distribution of fault slip. Outcrop measurements of slip on arrays of fault segments show that: (i) the point of maximum fault slip generally is not located at the centre of a fault segment; (ii) displacement gradients steepen towards the adjacent fault for underlapping faults; and (iii) displacement gradients become more gentle near the tips of overlapping faults. Numerical analyses suggest that mechanical interaction between neighbouring faults may cause such asymmetrical slip distributions. This interaction occurs through local perturbation of the stress field, and does not require the faults to be connected. For normal faults, the degree of fault interaction, and hence the degree of asymmetry in the slip distribution, increases with increasing fault height and fault overlap and with decreasing fault spacing. The slip magnitude along a discontinuous fault array can be nearly equal to that of a single larger continuous fault provided the segments overlap with small spacing. Fault interaction increases the ratio between fault slip and fault length, especially for closely spaced, overlapping faults. Slip-to-length ratios also depend on the three-dimensional fault shape. For normal faults, the slip-to-length ratio increases with increasing fault height. The effects of fault interaction and three-dimensional fault shape together can lead to more than one order of magnitude variation in slip-to-length ratio for the simple case of a single slip event in a homogeneous isotropic rock. One should expect greater variation for the more complex conditions found in nature. Two-dimensional fault scaling models can not represent this behaviour.

Surface morphology of columnar joints and its significance to mechanics and direction of joint growth

Columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes. New observations show that each band contains a single plumose structure and thus represents an individual crack, or joint segment, formed during a discrete growth event. Analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing, and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies. Columnar joints form by nucleation and growth of new cracks on the edges of older cracks. Each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face, where thermal stress is concentrated. Inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle-ductile transition of lava; outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava. Cracks often diverge slightly from the planes of previous cracks, probably because of spatial and temporal changes in directions of local principal stresses. Mechanical interaction causes a diverging crack to overlap, curve toward, and usually intersect the previous crack behind its edge, leaving a blind tip that points in the overall growth direction of the columnar joints. This and other directional criteria are applied to determine joint-growth patterns in several lava flows of the western United States. In two-tiered and multi-tiered flows, downward-growing columnar joints usually meet upward-growing joints well below the middle of the flows, which indicates very rapid cooling of upper portions relative to lower portions. This supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows, whereas conduction is probably the dominant cooling mechanism at the bases.

Characteristics of joint propagation across layer interfaces in sedimentary rocks

Abstract Interfaces between dissimilar layers play a fundamental role during joint propagation in layered sedimentary rocks, limiting the vertical extent and physical continuity of joint traces. Joints do, however, communicate across interfaces between dissimilar layers, forming an overall composite joint, which is the collection of vertically aligned but discrete joint segments. Detailed fractographic analysis of the surface features of these segments reveals several characteristics of the incremental propagation of a composite joint in the alternating siltstone and shale turbidite sequence of the Genesee Group of the Appalachian Plateau, central New York. (1) Joint segments confined by interfaces are arranged in-plane with each other in a sequential manner for layers of similar properties. (2) Out-of-plane arrangement is common for propagation across thin or discontinuous inhibiting layers. (3) Thick inhibiting layers do not allow communication among joints occurring in adjacent layers above and below. (4) If the thick inhibiting layers fractured, usually these joints initiate at the tip of a pre-existing joint, in an adjacent layer, and propagate away with a slightly different orientation. An analysis of the maximum principal stress in an unjointed layer, due solely to a joint in an adjacent layer, separated by a thin resistant layer, provides a conceptual basis for understanding the incremental nature of composite joints and their step-like geometry.

Effect of mechanical interaction on the development of strike-slip faults with echelon patterns

Abstract The overlapping geometry of echelon strike-slip faults is well-known. We have quantified this observation by measuring the amount of overlap and separation, and plotting them against each other for over 120 examples. Although there is large scatter, the data show a linear trend suggesting that the overlap increases proportionally with separation, up to a limiting value. We have analyzed this conspicuous relationship in terms of fault interaction by using a numerical model based on displacement discontinuity. The results show that fault interaction is, in fact, an important factor in contributing to the overlapping geometry of echelon strike-slip faults.

Diverse Pliocene-Quaternary tectonics in a transform environment, San Francisco Bay region, California

The San Francisco Bay region occupies part of the diffuse transform boundary zone between the Pacific and North American plates. Although dextral strike-slip faulting is dominant, the plate motion is expressed in a variety of ways. Some strike-slip faults are parallel with the plate boundary, but some are slightly oblique. The major strike-slip faults are zones in which an interplay occurs between strands, some of which are en echelon. This interplay may be responsible for some apparent pull-apart basins and presumed normal faults along the Calaveras fault zone. In extensive areas between strike-slip fault zones, there are many compressional structures—folds and reverse faults—that are Pliocene-Quaternary in age, hence largely coeval with the strike-slip faults. Some of the compressional structures are oblique to, and others are parallel with, the major strike-slip faults. We have divided the San Francisco Bay region into domains on the basis of the types, orientations, and relationships of structures, or, in one case, lack of strong deformation. Each domain has responded in its own way to the regional northwest-southeast dextral shear imposed by the relative plate motion. In an attempt to understand the origin of various structures and the interrelationships among them, we compared established models with our observations. Some observed geometric relationships agree quite well with classical Coulomb-Anderson and simple shear models, but others require further explanation. Models based on fault interaction seem to apply to certain cases. For example, the East Bay Hills domain, which is between the left-stepping Calaveras and Hayward–Rodgers Creek fault zones, is under compression resulting from interaction between the two strike-slip zones.