Controls from formation of early replacement dolomites and diagenetic anhydrite: Reactive transport modeling of dynamic interactions between geothermal and reflux circulation. 9th Middle East Geosciences Conference, GEO 2010.

Geothermal heating and brine reflux have been invoked to explain early dolomitization of platform carbonates. Reactive transport modeling (RTM) suggests that geothermal convection can form a wedge-shaped dolomite body thickest at the platform margin, while reflux can form a tabular body which thins away from the brine source. In natural systems flow will respond to both drives and vary through time with changes in platform top conditions, for example as brine pools develop and disappear, and this is likely to significantly impact both dolomitization and associated anhydrite precipitation. A model technique (TOUGHREACT) is used to investigate the dynamic interactions of geothermal convection and brine reflux. Reflux of brines (85‰) rapidly restricts geothermal convection to the platform margin where only minor dolomitization occurs. Brines infiltrate to considerable depth, but fluid flux is most rapid at shallow depth due to reducing permeability with depth, permeability anisotropy, and diagenetic modification of permeability. Simulations suggest complete dolomitization to 150-200 m depth within 1 My beneath the brine source. Reflux dolomitization may enhance reservoir quality at shallow depth where associated anhydrite precipitation occludes porosity beneath the main dolomite body. The predicted anhydrite volume is almost twice that suggested by earlier simulations that do not incorporate heat transport. Increasing geothermal heat flux has little effect on geothermal circulation, but does accelerate reflux diagenesis. Cooling the platform top from 40 to 25°C slows reactions and displaces the anhydrite zone downwards so it may become completely decoupled from the brine source. When brine-generating conditions cease, subsurface brines will continue to flow and have been suggested as a drive for continued dolomitization (a variant termed “latent reflux”). Our 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. RTM offers considerable potential for improving our understanding of diagenetic reactions and their impact on reservoir quality in such hybrid flow systems. However, the veracity and utility of predictions depend on the specification of meaningful boundary and initial conditions, and the temperature regime appears to play a critical role in the dolomitization and anhydritization story.