Abstract - Faults can profoundly influence the flow of groundwater and hydrocarbons. The geometry and hydraulic properties of fault zones are investigated for Mesozoic greywacke basement and Miocene sandstone from ca 37 km of tunnels in the southern Taupo Rift, New Zealand. Localised ground water inflows occur almost exclusively (>90%) within, and immediately adjacent to, fault zones. Fault zones in the contrasting lithologies comprise fault rock, small-scale faults, and fractures with thicknesses of 0.01 to 110 m approximating power-law distributions and bulk permeabilities of 10-9-10-12m2. Variability in fault zone structure results in a highly heterogeneous distribution of flow rates. Within basement about 80% of the flow rate occurs from fault zones >10 m wide, with ca 30% of the total localised flow rate originating from a single fault zone (i.e. consistent with the golden fracture concept). No simple relationships are found between flow rates and either fault strike or hydraulic head, with <50% of fault zones in any given orientation flowing. A general positive relationship does however exist between fault zone thickness and maximum flow rate. Higher flow rates from larger fault zones may arise because these structures have greater dimensions and are more likely (than smaller faults) to be connected to other faults in the system and the ground surface. Given the heterogeneous nature of fault zone structure, size and length, predicting the precise location of interconnected high permeability rock within fault zones will be challenging.
Abstract of talk given to:
Geosciences New Zealand Annual Conference, Geological Society of New Zealand, Christchurch, November 2013.