The transition from brittle faulting to cataclastic flow in porous sandstones: Mechanical deformation (original) (raw)
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Micromechanics of brittle faulting and cataclastic flow in Berea sandstone
Journal of Structural Geology, 1996
The micromechanics of failure in Berea sandstone were investigated by characterizing quantitatively the evolution of damage under the optical and scanning electron microscopes. Three series of triaxial compression experiments were conducted at the fixed pore pressure of 10 MPa and confining pressures of 20, 50 and 260 MPa, respectively, corresponding to three different failure modes: shear localization with positive dilatancy, shear localization with relatively little dilatancy and distributed cataclastic flow. To distinguish the effect of non-hydrostatic stress from that of hydrostatic pressure, a fourth suite of hydrostatically loaded samples was also studied. Using stereological procedures, we characterized quantitatively the following damage parameters: microcrack density and its anisotropy, pore-size distribution, comminuted volume fraction and mineral damage index. In the brittle regime, shear localization did not develop until the post-failure stage, after the peak stress had been attained. The microcrack density data show that very little intragranular cracking occurred before the peak stress was attained. We infer that dilatancy and acoustic emission activity in the prefailure stage are primarily due to intergranular cracking, probably related to the shear rupture of lithified and cemented grain contacts. Near the peak stress, intragranular cracking initiates from grain contacts and this type of Hertzian fracture first develops in isolated clusters, and their subsequent coalescence results in shear localization in the post-failure stage. The very high density of intragranular microcracking and pronounced stress-induced anisotropy in the post-failure samples are the consequence of shear localization and compactive processes operative inside the shear band. In contrast, Hertzian fracture was a primary cause for shear-enhanced compaction and strain hardening throughout the cataclastic flow regime. Grain crushing and pore collapse seem to be most intense in weakly cemented regions. Finite element simulations show that the presence of cement at grain contacts alleviates the tensile stress concentration, thus inhibiting the onset of Hertzian fracture and grain crushing.
Experimental studies of fault zone development in a porous sandstone
1997
This study investigates the processes involved in the formation and evolution of faulting in high porosity sandstone using laboratory triaxial compression testing. Faults in highly porous sandstone significantly affect the porosity and permeability of the rock, and typically occur as anastamosing compound bands of damage. Previously only the individual unit of these deformation band structures had been re-produced in the laboratory, possibly due to limitations on sample size. Now by deforming large specimens, I have not only produced zones of deformation bands, but also observed their hierarchical development as a function of strain for the first time. A series of dry tests were carried out on initially intact 100 mm diameter cores of Locharbriggs sandstone, at a constant confining pressure of 34.5 MPa, a constant axial strain rate of 5xlOE-6 Is and increasing amounts of axial strain. Samples were driven over their failure curves and then subjected to differing amounts of post failu...
Effects of pre-existing faults on compaction localization in porous sandstones
Tectonophysics, 2018
The formation of deformation bands can significantly modify the strength and transport properties of porous sedimentary rocks. Among the different types of deformation bands, compaction bands exhibit porosity reduction with little to no shear displacement. Natural compaction bands have previously been reported and studied in only a few areas. They often coexist with faults and other localized deformation structures. We mapped the geometrical relation between compaction bands, shear bands and faults in Lower Cretaceous porous sandstone at Makhtesh Katan, Israel. To understand the effect of pre-existing faults on the formation of compaction bands, we conducted deformation experiments on pre-faulted Bentheim sandstones. These experiments produced compaction bands consistently intersect the pre-existing fault. To gain better mechanical understanding of the observed band geometry, we also carried out three-dimensional (3D) numerical simulations with the input elastic moduli and yield strength well-constrained from the deformation experiments. We demonstrated that the formation of deformation bands is dictated by stress concentrations associated with the pre-existing fault. Frictional slip along the heterogeneous fault plane can produce a local stress concentration that would be responsible for further localized damage and the development of deformation zones. When fault slip is restricted (a possible result of high confinement), compaction bands initiate at high stress concentration sites resulting from geometrical irregularities of the fault. Finally, using a plane-strain twodimensional (2D) linear-elastic model with the geometry of the faults mapped in the outcrop, we were able to provide a mechanical explanation of the distribution for deformation bands observed at the Makhtesh Katan study area.
Single- and two-phase fluid flow properties of cataclastic fault rocks in porous sandstone
Marine and Petroleum Geology, 2012
Understanding the impact of faults on fluid flow in the subsurface is important for the extraction of oil, gas and groundwater as well as the geological storage of waste products. We address two problems present in current industry-standard workflows for fault seal analysis that may lead to fault rocks not being represented adequately in computational fluid flow models. Firstly, fluid flow properties of fault rocks are often measured only for small-scale faults with throws not exceeding a few centimetres. Large seismic-scale faults (throws >20 m) are likely to act as baffles or conduits to flow but they are seldom recovered from subsurface cores and consequently fault rock data for them is sparse. Secondly, experimental two-phase fluid flow data is lacking for fault rocks and, consequently, uncertainties exist when modelling flow across faults in the presence of two or more immiscible phases. We present a data set encompassing both single-and two-phase fluid flow properties of fault and host rocks from the 90-Fathom fault and its damage zone at Cullercoats Bay, NE England. Measurements were made on lowthrow single and zones of deformation bands as well as on slip-surface cataclasites present along the w120 m throw main fault. Samples were analysed using SEM and X-ray tomography prior to petrophysical measurements. We show that single deformation bands, deformation band zones and slipsurface cataclasites exhibit dissimilar single-and two-phase fluid flow properties. This is due to grainsize reduction being more pronounced in slip-surface cataclasites and changes in microstructure being fault-parallel for deformation bands but mostly fault-perpendicular for slip-surface cataclasites. A trend of fault rocks with low absolute permeabilities exhibiting lower relative permeabilities than more permeable rocks at the same capillary pressure is evident.
Experimental Study of Localised Deformation in Porous Sandstones
This PhD thesis presents a laboratory study aiming at a better understanding of the stress-strain response of the Vosges sandstone (porous rock) tested at a range of confining pressures (i.e., 20-190 MPa) and different axial strain levels. Localised deformation was captured at different scales by a combination of full-field experimental methods, including Ultrasonic Tomography (2D), Acoustic Emissions (3D), X-ray Tomography (3D), and 3D volumetric Digital Image Correlation, plus thin section and Scanning Electron Microscope observations (2D). These experimental methods were performed before, during and after a number of triaxial compression tests. The combined use of the experimental techniques, which have different sensitivity and resolution, described the processes of shear band and shear-enhanced compaction band generation, which formed at low to intermediate and relatively high confining pressures, respectively. Pure compaction bands were not identified. The deformation bands were characterised as zones of localised shear and/or volumetric strain and were captured by the experimental methods as features of low ultrasonic velocities, places of inter- and intra-granular cracking and structures of higher density material. The two main grain-scale mechanisms: grain breakage (damage) and porosity reduction (compaction) were identified in both shear band and shear-enhanced compaction band formation, which presented differences in the proportions of the mechanism and their order of occurrence in time.
A Conceptual Model for the Origin of Fault Damage Zone Structures In High-Porosity Sandstone
Journal of Structural Geology, 2003
We present a conceptual model to explain the development of damage zones around faults in high-porosity sandstones. Damage zone deformation has been particularly well constrained for two 4-km-long normal faults formed in the Navajo Sandstone of central Utah, USA. For these faults the width of the damage zone increases with fault throw (for throws ranging from 0 to 30 m) but the maximum deformation density within the damage zone is independent of throw. To explain these data we modify a previously published theoretical model for fault growth in which displacement accumulates by repeated slip events on patches of the fault plane. The modi®cations are based on ®eld observations of deformation mechanisms within the Navajo Sandstone, the throw pro®les of the faults, and inferences concerning likely slippatch dimensions. Zones of enhanced stress are generated around the tips of each slipping patch, raising the shear stress on adjacent portions of the fault as well as potentially causing off-fault damage. A key ingredient in our model for off-fault damage accumulation is the transition from strain hardening associated with deformation band development, to localised strain softening as a slip-surface develops. This transition occurs at a critical value of deformation density. Once a new slip-surface develops at some distance from the main fault plane and it starts to accumulate throw it can, in turn, generate its own damage zone, thus increasing the overall damage zone width. Our approach can be applied to interpret damage zone development around any fault as long as the host-rock lithology, porosity and deformation mechanisms are taken into consideration. q
Journal of the Geological Society, 2005
In porous geological materials such as sandstone or limestone, fault-related damage zones form arrays of deformation bands, which are planar discontinuities characterized by localized shear and porosity change. We show that the geometry and intensity of fault-related deformation band damage zones is systematic and predictable using standard strain energy density-based criteria. These criteria are used to successfully predict the tendencies for the nucleation and for the propagation of deformation bands as observed in a classic outcrop of fault-related damage zones within the brittly deformed Jurassic Wingate Sandstone exposed in the Laramide-aged Uncompahgre fold, in western Colorado, USA. The separate distributions of volumetric and distortional strain energy density are calculated for the interpreted geometry and stress state of the causative Laramide-aged thrust fault displacements from boundary element calculations of the attendant slip-induced local stresses. Volumetric strain energy density predicts the tendency for deformation band nucleation, the growth stage at which the deformation bands are defined by pore space dilatancy or collapse. Deformation band propagation, where shear occurs along the band, is predicted by distortional strain energy density. Further deformation bands at the Uncompahgre are predicted and observed to be characterized by shear-enhanced dilation.
Geophysical Research Letters, 2011
In order to investigate the role of drainage conditions in deformation and fracture behaviors of porous rocks, the authors carried out a series of rock fracture tests under triaxial compression in the laboratory. The detailed spacetime distribution of acoustic emission due to microcracking was used to examine pre-failure damage and failure behavior in Berea sandstone, which has a porosity of 20% and a permeability of 100 mD. The pore pressures or flow rates at the ends of the test sample were precisely controlled to simulate different drainage conditions. Experimental results indicate that drainage conditions play a governing role in deformation and fracture. The well-established dilatancyhardening effect can be greatly suppressed by dilatancydriven fluid flowing under good drainage conditions. Fast diffusion of pore pressure leads to a significant reduction in rock strength and stabilization of the dynamic rupture process. Furthermore, good drainage conditions have the potential to enlarge the nucleation dimension and duration, thereby improving the predictability of the final catastrophic failure. In addition, compaction bands, which were observed in porous rocks under higher confining pressure, were also observed at low confining pressure (corresponding to a depth of 1 km) in undrained tests. These results are particularly important for research fields in which fluid migration or pore pressure diffusion is expected to play a role, such as hydrocarbon reservoirs, enhanced geothermal systems, geological storage of CO 2 .
Microseismic properties of a homogeneous sandstone during fault nucleation and frictional sliding
Geophysical Journal International, 1994
The formation and evolution of faulting in three initially intact, oil-saturated specimens of Clashach sandstone is examined under conditions of constant strain rate loading at three different confining pressures, simulating the effect of tectonic loading at different depths in the Earth's upper crust. After a fault is formed the specimens are slid for a time, and then the initial confining pressure is increased to simulate the long-term recovery of strength expected in the Earth. The differential stress u and natural acoustic emissions (AE) are measured during the three separate phases of fault nucleation, sliding and strengthening. At the end of each individual phase the fluid permeability is measured by a pulse-decay technique at constant stress. The A E are interpreted using a mean field theory for damage evolution which calculates a mean crack length (c) from the seismic event rate N and the b-value, and a mean energy release rate ( G ) from u and (c).