Reactive transport and fluid pathways in fracture-controlled flow systems
Creators: name: Stenhouse, Paul
Publisher: name: The Australian National University
Year: 2014

description: This thesis uses mapping and analysis of C/O stable isotopes to explore the distribution and evolution of fracture-controlled fluid flow in two vein-rich, limestone-hosted fault systems: The Dar Al Baydha (DAB) Fault system in northern Oman and a network of low-displacement faults in the Helvetic Alps, Switzerland. The DAB Fault is 25km long and has a maximum throw of &gt;750m. There is also a subordinate, low displacement fault network formed adjacent to the DAB Fault. This study has explored a segment of that network in the Hail Ash Shas area (HAS network). In the Helvetic nappes, work has predominantly focused on a late to post-nappe emplacement fault network. Most faults in this network cannot be traced beyond a single outcrop and typically have throws &lt;10m. Hydrothermal calcite fill in the primary slip zones (PSZs) and arrays of macroscopic extension veins in the adjacent damage zones indicate significant dilation in both systems. Importantly, the association between extension veins and faulting indicates low differential stress and near-lithostatic fluid pressure (Pf) at failure and thus predominantly fluid-driven failure. D18O compositions of vein calcite in the DAB Fault vary from host rock values (&gt;25.5 per mil) to 13.7 per mil. In the Helvetics, d18O compositions of vein calcite vary from host rock values (&gt;19 per mil) to 10.6 per mil. The extent of 18O-depletion in both systems can be caused only by influx of fluids from one or more external reservoirs with d18O compositions that are in disequilibrium with the host rock. Reactive transport modelling indicates that the DAB Fault has a time-integrated-fluid-flux (TIFF) of 100,000mol/cm2, whereas the HAS network has a modelled TIFF of 1,000,000mol/cm2. In comparison, the Solalex network (in the Helvetic nappes) has an estimated TIFF of 500mol/cm2 . While externally-sourced fluids infiltrated the DAB Fault along much of its strike length, the distribution of strongly 18O depleted vein compositions in the DAB Fault system indicates a very heterogeneous distribution of high fluid flux. Structural features such as some segment boundaries and termination zones locally hosted high fluid fluxes. However, this relationship is not ubiquitous and varies with time. This highlights the 4D complexity of fluid pathways in the DAB Fault. The TIFF estimate for the HAS network is an order of magnitude greater than for the DAB Fault. This demonstrates the effectiveness of low displacement fault networks for fluid transport, even when they are proximal to large co-active faults. The high pore fluid factors and complex hydraulic connectivity in this fault network suggests that it may have predominantly evolved as an invasion percolation network. In contrast to the DAB Fault, the network of faults in the Helvetic nappes has much lower modelled TIFF. However, the limited spatial extent of faults and apparent lack of 2D geometric connectivity, combined with the evidence for transport distances &gt;1km again highlights the complexity of 3D hydraulic connectivity and indicates that relatively immature fault systems can still effectively drain overpressured reservoirs.