Thermal effects of fluid flow in steep fault zones

Abstract: Two-dimensional, porous-medium, steady-state, coupled fluid- and heat-transport models are used to investigate some of the hydrogeological and thermal consequences of steeply dipping fault damage zones in a normally pressured basin setting. Simple geometries and conservative petrophysical properties can produce large-scale buoyancy-driven circula-tion, both outside and within the fault zone. An average (homogeneous) ba in permeability of only 7 mD kh (horizontal permeability) and 0.07 mD kv (vertical permeability) results in a free convection cell of this type, with the upflow being localized by a 300 m wide fault zone (50 mD kh, 0.5 mD k,.). Steady-state temperature anomalies as large as 15°C at the top of the fault zone can be produced by this arrangement. Smaller values of basin permeability still result in a similar circulation pattern, but at flow rates which produce t mperature anomalies that are below detection levels. When the basin fill is more heterogeneous (layered), highe r permeabilities can exist in some layers without large-scale convection occurring, because of the dampening effect of other, lower permeability units. In realistic geometrical configura-tions that are similar to the North Sea Central Graben, the fluid flow system is dominated by within-fault-zone convective circulation that produces local (<10 km half-wavelength)