Anisotropic pore fabrics in faulted porous sandstones

Thanks to SEM technician Peter Chung at the University of Glasgow, David Wilde and Peter Greatbatch at Keele University for careful thin section preparation, Chris Wibberley and Tom Blenkinsop for input and Kieran Keith from Harlaw Academy, Aberdeen for help collecting petrophysical data. Thanks to Fabrizio Storti, Michael Heap and Toru Takeshita for helping to improve this paper with their constructive reviews. This work forms part of a NERC Standard award for DH (NE/N003063/1), which is gratefully acknowledged.Peer reviewedPublisher PD

Spatial variability in topology, connectivity and permeability of deformation band networks: Insights from the Jurassic Entrada Sandstone, Utah

Networks of deformation bands have the potential to act as baffles to fluid flow, yet little work has been devoted to addressing their network properties, such as topology and connectivity. Motivated by this, we investigate the two-dimensional spatial variability of areal intensity, node and branch topology, and network connectivity within select deformation band networks in the Jurassic Entrada Sandstone (Utah). The results highlight distinct topological signatures for deformation band networks, dominated by Y-nodes, and IC- and CC-branches. Low proportion of isolated II-branches reflects the evolution of deformation bands through bifurcation and abutment (Y-nodes) to form interconnected networks. A key observation is that network connectivity and topology exhibit great spatial variability within one and the same deformation band network. We evaluate effective permeabilities across the deformation band network, incorporating a topological measure of network connectivity into the permeability calculations. Results highlight that deformation band networks with >1.5 connections per branch can significantly reduce effective permeabilities, whereas networks with low connectivity may offer pathways for tortuous fluid flow.publishedVersio

THE GEOMETRY AND TOPOLOGY OF DEFORMATION BAND NETWORKS IN VOLCANICLASTIC ROCKS: A CASE STUDY FROM SHIHTIPING, SOUTH-EASTERN TAIWAN

Deformation bands are tabular strain localization features, common in porous and granular rocks. These structures of millimeter to centimeter thickness can occur as single bands and develop into clusters or networks of bands. Deformation bands have been extensively documented in siliciclastic rocks, whereas fewer studies address deformation bands in porous volcaniclastic rocks. In recent years, volcaniclastic reservoirs have become a hot topic in petroleum- and geothermal exploration, groundwater aquifers and CO2 storage. Deformation bands are generally associated with a permeability reduction from one to three orders of magnitude compared to the host rock and deformation band networks may affect subsurface fluid flow patterns. Knowing the network properties of deformation bands are therefore crucial when predicting their impact on fluid flow. This M.Sc. project quantifies clusters and networks of deformation bands in volcaniclastic rocks from Shihtiping, Eastern Taiwan. A thorough topological analysis of deformation band networks has been carried out, focusing on characterizing the distribution and connectivity of bands within a network. Individual deformation bands were analyzed based on their geometry, including length, density, and intensity, to access the spatial relationship of bands within the networks. The quantitative relation of nodes and branches provides the basis for describing the connectivity in the studied deformation band networks. The deformation band networks are generally dominated by connecting Y-nodes and fully connected (C-C) branches, resulting in high average connectivity. Furthermore, the topological characteristics can be associated with bifurcating, abutting, and splaying bands, and bands are less prone to crosscut one another. The highest connectivity is related to mature deformation band networks and deformation band networks in fully developed faults. This supports the theory that the connectivity of deformation networks develops with time and maturity (strain). The analyses of the deformation bands show that nodal distribution, intensity and connectivity are vulnerable to lithological heterogeneities across the network. This study strengthens our understanding of the development of deformation bands in volcaniclastic rocks and explores the evolution of connectivity in deformation band networks. Quantifying the topological and geometrical characteristics of deformation band networks is essential as it generates parameters used to assess the potential for fluid flow in a reservoir.Masteroppgave i geovitenskapGEOV399MAMN-GEO

An integrated analysis of facies control on deformation bands in mixed aeolian-fluvial sandstone reservoirs

Deformation bands are the primary structural element of fault damage zones within porous granular rocks. They are sub-seismic structures that act to modify the petrophysical properties of the host lithology, and as such are an area of focused research to understand their impact on fluid flow in subsurface reservoirs. Deformation bands have been shown to negatively impact fluid flow in reservoirs, with reduced porosity and permeability, and therefore pose a problem for many subsurface energy resources including hydrocarbon exploration and production, groundwater aquifer management, geothermal energies, and carbon sequestration. Deformation bands require a diverse methodological approach in order to fully understand the mechanisms of their formation and their impacts on rock properties. Current understanding of deformation bands has been drawn primarily from field outcrops, subsurface sampling, as well as insights provided by experimental rock mechanics. It is suggested that the formation of these structures and their properties is strongly related to host lithological properties, such as porosity, grain size, sorting and mineralogy, as well as the stress conditions at which they form. Their prevalence within high porosity, coarse-grained lithologies of aeolian origin has shown that grain size and porosity are the primary controls on their formation. However, the prevalence of this facies in the literature presents sampling bias and therefore bias in interpretation of the controls on their formation, with other lithological variables such as grain sorting and bed thickness, relatively understudied. This thesis presents results of an integrated approach to understand the controls on deformation band formation, and the controls on their petrophysical properties. Mixed aeolian-fluvial reservoirs of the United Kingdom, the Triassic Sherwood Sandstone Group and the Devonian Old Red Sandstone Group are host to pervasive deformation band networks associated with faulting, and also offer the unique opportunity to examine the role of extreme lithological variability on their formation. Field sampling of fault damage zones and deformation bands is combined with petrographic and microstructural analysis and complemented with experimental rock mechanics to investigate the link between sedimentary facies and deformation band formation and petrophysical impact. Results of petrographic and microstructural analysis show that the type of deformation band structure formed is function of the porosity and grain size of the host lithology. Grain size distribution analysis reveals fractal grain size relationships which reflect the deformation mechanisms of band formation that is strongly influenced by grain sorting. Deformation bands within aeolian lithofacies display higher porosity and cataclasis than those within fluvial lithofacies, resulting in greater reduction in permeability. This field observation is also supported by triaxial deformation experiments on unconsolidated quartz aggregates in which the porosity, grain size, and mineralogy remain fixed, and sorting is varied. Sorting is found to influence the micromechanics of deformation, and thus the grain textures produced by cataclasis, resulting in greater permeability reduction within well-sorted materials. Measurements of deformation band intensity using two dimensional window sampling also reveal a strong influence of grain sorting on both the intensity of fault damage zones, and the width of deformation bands. Intensity is highest within well-sorted lithologies, where permeability reduction is also highest, however, the average width and width variability of deformation bands is greatest within fluvial facies. It may be proposed that the increased intensity of bands within aeolian facies, may be balanced by the increased width within fluvial facies, and therefore these structures may act to maintain any inherent fluid flow heterogeneity. These observations provide crucial insight into the effects of facies and ultimately lithological properties on deformation bands in mixed aeolian-fluvial reservoirs

The Effect of Authigenic Clays on Fault Zone Permeability

Leverhulme Trust (GrantNumber(s): ECF-2020-560) Natural Environment Research Council (GrantNumber(s): NE/N003063/1) Open access via Wiley agreementPeer reviewedPublisher PD

Anisotropy of permeability in faulted porous sandstones

Thank you to Total E & P UK for funding the project, and especially Chris Wibberley, Claude Gout and Stephane Vignau for input. The author would also like to thank Zoe Shipton and Graham Yielding for their constructive reviews of the manuscript. Thanks also to Manuel Prieto for sharing his MSc pilot study written at the University of Aberdeen, Professor Martin Lee and Peter Chung at the University of Glasgow for SEM use and lastly thank you to Gavin Tennent for access to the Clashach Quarry and for samples.Peer reviewedPublisher PD

Fluid flow through deformation band

Cataclastic deformation bands, which are common in porous sandstone, have the potential to restrict fluid flow. Geological studies have shown that permeability of deformation band shear zones can be one to five orders of magnitude less than for the sandstone host rock. However, recent studies based on simplified analytical estimates have shown that fluid flow in jointed deformation bands may not be retarded since joints play an important role in conducting fluids. In this study, 2 dimensional finite element analysis (FEA) is used to simulate the total discharge flow rate through jointed deformations. Variations of single planar and conjugate jointed deformation bands are considered. The study includes a sensitivity analysis of joint aperture, joint and deformation band orientation, joint spacing, and deformation band thickness in order to evaluate the influence of these parameters on the total discharge flow rate via jointed deformation bands. This study also considers the influence of spatial distribution of deformation band, deformation band orientations, and deformation band continuity on fluid flow and provides the comparison with jointed deformation band to investigate whether joints still play a significant role --Abstract, page iii

The geometry and thickness of deformation-band fault core and its influence on sealing characteristics of deformation-band fault zones

Deformation-band faults in high-porosity reservoir sandstones commonly contain a fault core of intensely crushed rock surrounding the main slip surfaces. The fault core has a substantially reduced porosity and permeability with respect to both the host rock and individual deformation bands. Although fault core thickness is a large uncertainty in calculations of transmissibility multipliers used to represent faults in single-phase reservoir flow models, few data exist on fault core thickness in deformation-band fault zones. To provide accurate estimates of deformation-band fault petrophysical properties, we measured fault core thickness at six sites (each 4–15 m [13–49 ft] along strike) along the Big Hole fault in the Navajo Sandstone, central Utah. These data show that the thickness is highly variable and does not correlate with either the amount of slip or the number of slip surfaces. The thickness of the fault core is likely to be dependent on local growth processes, specifically the linkage of fault segments. This suggests that correlations of fault permeability with throw may not apply to deformation-band faults. Simple calculations of two-phase flowproperties based on measured porosity and permeability values suggest that deformation-band faults containing fault core are likely barriers to two-phase flow.More data on the variability of fault core thickness and its petrophysical properties need to be collected to characterize population statistics for models of deformation-band fault fluid-flow properties

Impact of faults on fluid flow in carbonates

To fully characterise the behaviour of carbonate rocks in the subsurface it is important to understand their textural heterogeneity, and how their textures may be modified by faulting. A number of fault zones were investigated in detail, firstly analysing the microstructural, petrophysical as well as mechanical properties of the host rocks. Secondly, describing the fault zone architectures by mapping fault rock distributions and fracture patterns. Lastly, correlating the deformation mechanisms forming the faults to the initial rock properties and the stress conditions during faulting. Moreover, triaxial laboratory deformation was performed on a large number of host rock samples covering all carbonate rock types, as well as the whole range of porosities (10%, and may be either increased or decreased for lower porosity samples. Higher porosity (>10%) carbonates fail due to distributed or localized cataclastic flow or focused damage around the macropores, resulting in porosity reduction. Lower porosity (<10%) carbonates fail in a brittle manner due to brecciation and transitional- or brittle-shearing, leading to porosity increase. Significant reduction in permeability, however, may only be produced by diagenetic processes, such as recrystallization and cementation, or very high-strains, which are able to create vi fine-grained cataclasites. However, even though these fault rocks gain very low permeability, they become prone to brittle deformation. Therefore, these potentially sealing fault rocks may be cut by open fractures if were subjected to further faulting or uplift, and hence, while creating permeability anisotropy in the reservoir, they may not form good seals. Nevertheless, several fault examples in this study showed fracture blunting at the surface of the fault rocks suggesting that fault sealing is possible both in highly-porous and very tight carbonates