Zoe K. Shipton

Deformation bands in sandstone: a review

Deformation bands are the most common strain localization feature found in deformed porous sandstones and sediments, including Quaternary deposits, soft gravity slides and tectonically affected sandstones in hydrocarbon reservoirs and aquifers. They occur as various types of tabular deformation zones where grain reorganization occurs by grain sliding, rotation and/or fracture during overall dilation, shearing, and/or compaction. Deformation bands with a component of shear are most common and typically accommodate shear offsets of millimetres to centimetres. They can occur as single structures or cluster zones, and are the main deformation element of fault damage zones in porous rocks. Factors such as porosity, mineralogy, grain size and shape, lithification, state of stress and burial depth control the type of deformation band formed. Of the different types, phyllosilicate bands and most notably cataclastic deformation bands show the largest reduction in permeability, and thus have the greatest potential to influence fluid flow. Disaggregation bands, where non-cataclastic, granular flow is the dominant mechanism, show little influence on fluid flow unless assisted by chemical compaction or cementation.

FAULT TIP DISPLACEMENT GRADIENTS AND PROCESS ZONE DIMENSIONS

Abstract Finite displacement gradients measured at fault tips appear to contradict the predictions of the post-yield fracture mechanics (PYFM) model for fault tip propagation proposed by Cowie, P. A. and Scholz, C. H. (1992) Journal of Structural Geology , 14, 1133–1148. The results of a high resolution survey of a 3.6 km long normal fault in SE Utah are presented as evidence that the contradiction is real and not simply due to problems of limited resolution. A theoretical explanation for finite tip gradients is then proposed which involves a positive stress feedback between sequential fault slip increments. According to this growth model, strength heterogeneities on a fault surface limit the size of individual ruptures so that only a patch of the fault moves at any one time. Each slipping patch produces a stress perturbation which raises the shear stress on adjacent healed portions of the fault as well as the surrounding rock volume. Healing takes place after each slip event, allowing local strength recovery. Using a simple two-dimensional planar fault model, we show that when the size of the slipping patch is much smaller than the dimensions of the fault plane, and strength recovery is geologically instantaneous, the displacement profile follows an approximately linear decrease towards the tip similar to natural examples. A bell-shaped displacement profile, with tip gradients that tend to zero, is predicted only in the special case where the size of each slip patch equals the fault plane dimensions. Our main modification of the earlier model is that the size of the process zone wake, or frictional breakdown zone, scales with the dimensions of the slipping patch as opposed to the entire fault length. Model results show that the stress field at the tips of faults formed by this mechanism decays rapidly, so the range of significant interaction is small compared to the fault dimensions.

A conceptual model for the origin of fault damage zone structures in high-porosity sandstone

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 modifications are based on field observations of deformation mechanisms within the Navajo Sandstone, the throw profiles of the faults, and inferences concerning likely slip-patch 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.

What do you think this is? "Conceptual uncertainty" in geoscience interpretation

Interpretations of seismic images are used to analyze sub-surface geology and form the basis for many exploration and extraction decisions, but the uncertainty that arises from human bias in seismic data interpretation has not previously been quantified. All geological data sets are spatially limited and have limited resolution. Geoscientists who interpret such data sets must, therefore, rely upon their previous experience and apply a limited set of geological concepts. We have documented the range of interpretations to a single data set, and in doing so have quantified the �conceptual uncertainty� inherent in seismic interpretation. In this experiment, 412 interpretations of a synthetic seismic image were analyzed. Only 21% of the participants interpreted the �correct� tectonic setting of the original model, and only 23% highlighted the three main fault strands in the image. These results illustrate that conceptual uncertainty exists, which in turn explains the large range of interpretations that can result from a single data set. We consider the role of prior knowledge in biasing individuals in their interpretation of the synthetic seismic section, and our results demonstrate that conceptual uncertainty has a critical influence on resource exploration and other areas of geoscience. Practices should be developed to minimize the effects of conceptual uncertainty, and it should be accounted for in risk analysis.

Structural heterogeneity and permeability in faulted eolian sandstone: Implications for subsurface modeling of faults

We determined the structure and permeability variations of a 4 km-long normal fault by integrating surface mapping with data from five boreholes drilled through the fault (borehole to tens of meters scale). The Big Hole fault outcrops in the Jurassic Navajo Sandstone, central Utah. A total of 363.2 m of oriented drill core was recovered at two sites where fault displacement is 8 and 3-5 m. The main fault core is a narrow zone of intensely comminuted grains that is a maximum of 30 cm thick and is composed of low-porosity amalgamated deformation bands that have slip surfaces on one or both sides. Probe permeameter measurements showed a permeability decline from greater than 2000 to less than 0.1 md as the fault is approached. Whole-core analyses showed that fault core permeability is less than 1 md and individual deformation band permeability is about 1 md. Using these data, we calculated the bulk permeability of the fault zone. Calculated transverse permeability over length scales of 5-10 m is 30-40 md, approximately 1-4% the value of the host rock. An inverse power mean calculation (representing a fault array with complex geometry) yielded total fault-zone permeabilities of 7-57 md. The bulk fault-zone permeability is most sensitive to variations in fault core thickness, which exhibits the greatest variability of the fault components.

Structural models: Optimizing risk analysis by understanding conceptual uncertainty

Abstract Geoscience may be regarded as an uncertain science, as it is often based on the interpretation of equivocal data. Analysis of multiple interpretations of a single dataset has shown that conceptual uncertainty can result in a wide range of interpretational outcomes. Many geological models based on a wide variety of concepts were developed by different geoscientists for the same dataset. In this paper we suggest methods to improve the effectiveness of interpretation workflows based on understanding of how geoscientists apply concepts to equivocal datasets, the processes they use, the effects of their previous experience, and their use of broader contextual information. We argue that understanding the influence of conceptual uncertainty on interpretation of equivocal data and modification of current workflow practices can improve risk management.

How Thick is a Fault? Fault Displacement‐Thickness Scaling Revisited

Fault zone thickness is an important parameter for many seismological models. We present three new fault thickness datasets from different tectonic settings and host rock types. Individual fault zone components (i.e., principal slip zones, fault core, damage zone) display distinct displacement-thickness scaling relationships. Fault component thickness is dependent on the type of deformation elements (e.g., open fractures, gouge, breccia) that accommodate strain, the host lithology, and the geometry of pre-existing structures. A compilation of published fault displacement-thickness data shows a positive trend over seven orders of magnitude, but with three orders of magnitude scatter at a single displacement value. Rather than applying a single power-law scaling relationship to all fault thickness data, it is more appropriate and useful to seek separate scaling relationships for each fault zone component and to understand the controls on such scaling.

Analysis of CO 2 Leakage through 'Low-Permeability' Faults from Natural Reservoirs in the Colorado Plateau, East-Central Utah

Abstract The numerous CO2 reservoirs in the Colorado Plateau region of the United States are natural analogues for potential geological CO2 sequestration repositories. To understand better the risk of leakage from reservoirs used for long-term underground CO2 storage, we examine evidence for CO2 migration along two normal faults that cut a reservoir in east-central Utah. CO2-charged springs, geysers, and a hydrocarbon seep are localized along these faults. These include natural springs that have been active for long periods of time, and springs that were induced by recent drilling. The CO2-charged spring waters have deposited travertine mounds and carbonate veins. The faults cut siltstones, shales, and sandstones and the fault rocks are fine-grained, clay-rich gouge, generally thought to be barriers to fluid flow. The geological and geochemical data are consistent with these faults being conduits for CO2 moving to the surface. Consequently, the injection of CO2 into faulted geological reservoirs, including faults with clay gouge, must be carefully designed and monitored to avoid slow seepage or fast rupture to the biosphere.

Fault Structure Control on Fault Slip and Ground Motion During the 1999 Rupture of the Chelungpu Fault, Taiwan

The Chelungpu fault, Taiwan, ruptured in a Mw 7.6 earthquake on 21 September 1999, producing a 90-km-long surface rupture. Analysis of core from two holes drilled through the fault zone, combined with geologic mapping and detailed investigation from three outcrops, define the fault geometry and physical properties of the Chelungpu fault in its northern and southern regions. In the northern region the fault dips 45-60 east, parallel to bedding in both the hanging wall and footwall, and consists of a narrow (1-20 cm) core of dark gray, sheared clay gouge. The gouge is located at the base of a 30- to 50-m zone of increased fracture density confined asymmetrically to the hanging wall. Microstructural analysis of the fault gouge in- dicates the presence of extremely narrow clay zones (50-300 lm thick) that are interpreted as the fault rupture surfaces. Few shear indicators are observed outside of the fault gouge, implying that slip was localized within the gouge zone. Slip localization along a bed-parallel surface resulted in a narrow gouge zone that pro- duced less high-frequency ground motion and larger displacements (average 8 m) during the earthquake than in the southern region. Displacement in the southern region averaged only 2 m, but ground shaking consisted of large amounts of high- frequency ground motion. The fault in the southern region dips 20-30 at the surface and consists of a wide (20-70 m thick) zone of sheared, foliated shale with numerous gouge zones. These data demonstrate a potential correlation between fault structure (i.e., gouge width, geometry) and earthquake characteristics such as displacement and ground motion (i.e., acceleration).

Man-made versus natural CO2 leakage: a 400 k.y. history of an analogue for engineered geological storage of CO2

To evaluate sites for long-term geological storage of CO2 and optimize techniques for monitoring the fate of injected CO2, it is crucial to investigate potential CO2 migration pathways out of a reservoir and surface leakage magnitudes. For the first time, we calculate CO2 leakage rates and volumes from ancient fault-related travertines and from an abandoned borehole. U-Th–dated travertine along two faults near Green River, Utah (western United States), shows that leakage has occurred in this area for over 400 k.y. and has switched location repeatedly over kilometer-scale distances. One individual travertine was active for at least 11 k.y. Modern leakage is predominantly through the active Crystal Geyser, which erupts from an abandoned exploration well. Using age data and travertine volume, we calculate magnitudes and rates of CO2 emission. Fault-focused leakage volume is twice as great than diffuse leakage through unconfined aquifers. The leakage rate from a poorly completed borehole is 13 times greater than the long-term time averaged fault-focused leakage. Although magnitudes and rates of any leakage from future storage sites will be highly dependent on local geology and pressure regime, our results highlight that leakage from abandoned wells is likely to be more significant than through faults. INTRODUCTION Carbon dioxide could potentially migrate from underground storage sites through boreholes, poor cap rocks, or faults and fractures. Storage formation integrity, and effects of leaking CO2 on the surface environment, is commonly investigated (Stevens et al., 2001; Kirk, 2011); however, the overburden between the storage formation and the surface is a poorly studied part of the CO2 storage system (Gaus, 2010). Understanding flow through potential migration pathways in the overburden, such as faults or high-permeability strata is crucial for evaluating long-term storage security. The integrity of well bores and their long-term ability to retain CO2 has also been recognized as a significant risk to the long-term security of geological storage (IEAGHG, 2005). Storage operators will be legally required to monitor the fate of injected CO2 (European Commission, 2009). Effective monitoring and engineered remediation depends on the likely nature of migration pathways through the overburden, and on the locations and likely fluxes of CO2 shallow leakage pathways (Cortis et al., 2008; Benson and Hepple, 2005). We investigate a unique location in the Paradox Basin, Utah, (western United States) where fossil travertine mounds allow us to compare ancient CO2 flux via fault-associated fracturing and diffuse leakage through an aquifer, with modern leakage from abandoned wells. The Paradox Basin hosts at least nine natural CO2 accumulations, most of which have contained CO2 for millennia (Gilfillan et al., 2008). CO2-charged springs along the Little Grand Wash (LGW) and Salt Wash Graben (SWG) normal faults (Fig. 1) demonstrate that CO2 is leaking to the surface through fault zones and abandoned wells (Doelling, 1994). The springs are fed from a shallow aquifer, overlain by low-permeability siltstones and shales and charged by CO2 from depth—making this setting analogous to heterogeneous and variably permeable overburden above a geological CO2 storage site. We use U-Th dating of travertine formed by out-gassing of CO2 from springwater to determine flow history along the faults. Combining travertine age and volume allows us to estimate volumes and rates of CO2 emission to the surface through time and compare fault-focused, diffuse and borehole leakage, in a single hydrogeological setting.