Global Compilation and Analysis of Fault Zone Permeability

Download or read book Global Compilation and Analysis of Fault Zone Permeability written by Jacek Scibek and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: "Fault zones are a ubiquitous part of the of the Earth’s brittle crust, influencing a wide range of processes including hydrothermal circulation, ore deposition, crustal strength, and seismicity. However, despite the large amount of drilling and in situ hydraulic testing worldwide, there are no comprehensive summary statistics and analyses of fault permeability. The objective of my PhD research was to develop an improved understanding of fault zone permeability at the crustal scale through data analysis and field research.The initial statistical analysis was done to investigate the biases and trends in observations in fault zones in various geoscience disciplines. The revised database is 429 datasets (at 364 locations), and it enabled a more complete statistical description of trends and distributions of the permeability with depth, and between different rock types. The permeability ratio (fault zone / protolith) is used as a metric to describe crustal permeability variation. The average of this ratio value varies from +2 to +3 orders of magnitude over the 10 km depth range. The maximum ratio value occurs at shallow depths, between 1 and 3 km. From the bulk permeabilities, mostly from in situ drillhole tests, the depth-distribution of permeability was synthesized. In the upper few kilometres of the crust, fault zones serve as fluid conduits (bulk permeability >10-17 m2 threshold). The distribution of world-wide fault permeability values, when compared to the geothermal-metamorphic curve that represents the trend for time-averaged metamorphic fluid fluxes, is consistent with the trend over depth in fault zone permeability. This finding implies that fault zones control and serve as large-scale fluid conduits through the metamorphic crust in tectonically active regions at least. Another important and new result was a significant and relatively good correlation (r2 = 0.5 to 0.8) between the bulk permeability of the fault zone (mostly the damage zone) and the surrounding protolith in most rock types considered here, but not in coarse siliciclastic rocks. This empirical relationship may be used to predict the permeability of fault zones from the permeability of the protoliths and depth in the brittle crust, in the absence of other data. The thesis also includes original field-based research (Gryphon uranium deposit). A wide (>350 m) fault zone was tested for matrix permeability in drillcore (>700 drillcore samples). The fractured fault damage zone accounts for most of the transmissivity (67%), but the enhanced porosity from rock alteration acts to magnify the conduit of the damage zone. The ability to separate the permeability and transmissivity into components within highly heterogeneous fault zone is a step forward in better understanding the permeability structure of faults, but it requires a large number of tests, such as presented here. My results suggests that present-day permeability of fault zones, where adequately "preserved", offer direct insights into past fluid flow and paleo-permeability"--