The fluid flow properties of fault damage zones

• Main Author: Mitchell, Thomas Matthew
• Published: University of Liverpool 2007
• Subjects: 552
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.485852

Quantification of the fluid flow properties of the Earth's crust is an essential precursor to the understanding of a wide range of geological processes, including earthquake generation and crustal strength, and the recovery of natural resources. Faults playa key role in the migration of fluids around the ;Earth's crust, and therefore the fluid flow properties of fractured rocks and how these properties evolve with time are of major importance. This thesis aims to improve our understanding of the hydraulic transport properties of large fault zones by presenting a large dataset of detailed field and microstructural observations and results from a suite of laboratory experiments to provide a basis for studying the distribution, and fluid flow properties, of damage surrounding large natural fault zones. Damage surrounding the core of faults is represented by both microfracturing of the rock matrix and by macroscopic fracture networks. Microfracture and macrofracture densities and orientations have been analysed on strike slip faults with displacements ranging over 3 orders of magnitude (~O.l2 m - 5000 m). These faults cut crystalline rock within the excellently exposed Atacama Fault Zone, Northern Chile. All faults consist of a fault core and associated damage zone. Damage zone width as defined by macrofractures and microfractures scale with displacement and fault length. Both microfractures (specifically fluid inclusion planes) and macrofractures within the damage zone show a log-linear .decrease in fracture density with perpendicular distance from the fault core. An empirical equation for microfracture density distribution based on the evolution of displacement has been derived for these faults. Preferred microfracture orientations in the damage zone suggest that this damage may predominantly be due to early processes related to enhanced stress at fault tips, in addition to cumulative wear processes from the juxtaposition of geometrical irregularities on the fault plane and damage from dynamic rupture. Fault core widths scale with displacement, with the largest displacement fault showing a wide multiple core zone. Detailed experimental studies of the development of permeability of crustal rock during deformation are essential in helping to understand fault mechanics and constrain larger scale models that predict bulk fluid flow within the crust. The strength, permeability and pore fluid volume evolution of initially intact crystalline rock under increasing differential load leading to macroscopic failure has been determined at water pore pressures of 50 MPa and varying effective pressures from 10 to 50 MPa. Permeability is seen to increase by, up to, and over two orders of magnitude prior to macroscopic failure, with the greatest increase seen at lowest effective pressures. Post-failure permeability is shown to be over three orders of magnitude higher than initial intact permeabilities and approaches the lower the limit of measurements of in situ bulk crustal permeabilities. Increasing amplitude cyclic loading tests show permeabilitystress hysteresis with high permeabilities maintained as differential stress is reduced and the greatest permeability increases are seen between 90-99% of the failure stress. Under hydrothermal conditions without further loading, it is suggested that much of this permeability can be recovered by healing and sealing, and pre-macroscopic failure fracture damage may heal relatively faster than post-failure macroscopic fractures. Pre-failure permeabilities are nearly seven to nine orders of magnitude lower than that predicted by some high pressure diffusive models suggesting that microfracture matrix flow cannot dominate, and agrees with inferences that bulk fluid flow and dilatancy must be dominated by larger scale structures, such as macrofractures. It is suggested that the permeability of a highly stressed fault tip process zone in low-permeability crystalline rocks could increase by more than 2 orders of magnitude, while stress drops related to fracture propagation close damage zone cracks, and some permeability is maintained due to hysteresis from permanent microfracture damage. Future work should aim to quantify experimentally-induced microfractures and. associated permeability measurements, and by relating the fracture densities surrounding natural fault zones with densities seen in experimental deformed samples with known permeabilities, modelling techniques can then be applied to gain estimates of bulk fluid flow of the fracture networks. This will provide a basis for predicting the influence of pore fluid pressures on important geological issues, such as crustal strength.