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.
|Publicly Available Date||May 30, 2023|