Modelling dynamic interaction between geothermal and reflux circulation using TOUGHREACT: the importance of temperature in controlling dolomitisation and anhydrite cementation

This study uses TOUGHREACT to investigate interactions between heat, solute and fluid flow driven by geothermal convection and brine reflux, and the resultant diagenesis in a generic carbonate platform. Reflux of brines (85 ‰) rapidly restricts geothermal convection to the margin, eliminating lateral temperature contrasts. Brines penetrate to >2 km depth within 1 M.y., but fluid flux is most rapid at shallow depth due to the initial permeability-depth relationship, specified anisotropy and positive feedbacks between dolomitisation, porosity and permeability. Below the brine pool dolomitization is complete to 150-200 m depth within 1 M.y., and beneath this extends a zone of partial dolomitization. In contrast there is only minor dolomitization of the platform margin by geothermal convection. Although reflux dolomitization significantly enhances reservoir quality at shallow depth, associated anhydrite precipitation occludes porosity beneath the main dolomite body. The predicted anhydrite volume is almost double that suggested by simulations which fail to incorporate heat transport, whilst the zone of partial dolomitisation beneath the main dolomite body is also greater. Increasing geothermal heat flux provides little support for geothermal circulation, but does accelerate rates of reflux diagenesis. Reduction of platform top temperature from 40 to 25oC results in slower reactions and downward displacement of anhydrite diagenetic zone, which may become completely decoupled from the brine source. When brine-generating conditions cease, subsurface brine flow will continue and has been suggested as a drive for continued dolomitisation (a variant of reflux circulation termed latent reflux). TOUGHREACT simulations demonstrate that latent reflux does not form a significant amount of dolomite due to prior mg2+ consumption at shallow depth, although as geothermal circulation becomes re-established platform margin dolomitization rates increase.