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Controls on Convection Along Fault Systems and Implications for Hydrothermal Dolomitization in a Cenozoic Rift System

Abstract

Hydrothermal dolomites and associated leached limestones can form significant hydrocarbon reservoirs. However, conditions under which hydrothermal systems can deliver adequate volumes of fluids of appropriate chemistry remain poorly understood. The key role of permeable fault zones as pathways for migration of diagenetic fluids is recognized in both at both long (orogenic/rift event) and short (seismic event) time scales. Geothermal convection associated with rifting can involve large volumes of diagenetic fluid over prolonged periods. Using a suite of TOUGH2 simulations we explore controls on the temperature and volumetric fluxes of geothermal convection through a complex sedimentary sequence cut by high angle faults. We compare patterns of fluid flow and temperature with that of both stratbound and massive dolomite geobdoies within the Eocene Thebes Formation associated with the Hammam-Faraun fault (HFF), Egypt. With only shallow burial, the timing, source (modified seawater) and temperature (60-120°C), of the dolomitizing fluids are relatively well constrained. 2D simulations indicate that first order controls on fluid flux are fault zone transmissivity (a product of fault zone width and effective permeability) and fault geometry (depth and spacing between faults). Closely spaced (<2km in Thebes) faults with a high transmissivity result in sustained single pass (open) convection. When both faults extend to the surface, circulation is largely constrained within the damage zone. Only when one fault tips out at depth can fluids discharge though the Thebes. The temperature of these fluids accords with those observed in the stratabound dolomites, though fluxes are too low to form laterally extensive dolomite bodies. Flux is less sensitive to whether faults are antithetic or synthetic, and to the geothermal gradient though at high gradients flux fluids up to 100°C may have discharged from the incipient HFF into the Thebes. Preliminary 3D simulations suggest much more vigorous convection occurs along the plane of the fault, resulting in localised zones of upwelling and downwelling and high contrasts in flux and temperature along-strike. Reducing the problem to 2D, representing faults as 1D pipes, whilst computationally parsimonious, can thus be misleading. To understand fault-related fluid and diagenesis it is critical to consider processes occurring within the fault zone as well as interaction between faults and permeable country rock.