Dynamic Interaction between Geothermal Convection and Reflux Dolomitisation
Density-driven fluid flow occurs in carbonate platforms due to geothermal heating and brine reflux, and both of these systems have been invoked to explain early dolomitization. Previous reaction transport modelling suggested that within 10-30 M.y. geothermal convection can form a wedge-shaped dolomite body thickest in the platform margin, while isothermal simulations indicate that in <1 M.y reflux can form a tabular body which thins away from the fluid source. However, conduction of geothermal heat and advective cooling by geothermal convection and brine reflux are likely to significantly impact both dolomitization and associated anhydrite precipitation. We use TOUGHREACT to investigate interactions between heat, solute and fluid flow driven by geothermal convection and reflux, and the resultant diagenesis in a generic carbonate platform.
Geothermal convection rapidly becomes restricted to the margin, eliminating lateral temperature contrasts across the platform. Reflux brines (85 ‰) penetrate to >2 km depth in 1 M.y., but fluid flux is most rapid at shallow depth due to the specified initial anisotropy and permeability-depth relationship, consumption of Mg2+ by dolomitization and associated increase in poro-perm. Dolomitization is complete in the upper 100-150 m within 1 M.y., and beneath this is a zone of partial dolomitization, whilst there is only minor dolomitization of the margin driven by geothermal convection. Although reflux dolomitization significantly enhances reservoir quality at shallow depth, associated anhydrite precipitation gives net porosity reduction beneath the main dolomite body. The volume of anhydrite cements predicted is almost double that suggested by simulations which fail to incorporate heat transport, whilst dolomite abundance 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. Specification of a lower platform top temperature (25oC rather than 40oC) results in slower reactions and downward displacement of diagenetic zones, which may become completely decoupled from the brines source.
Preliminary simulations demonstrate this approach has considerable potential for improving our understanding of dolomitization and reservoir quality driven by such hybrid flow systems. Future RTM simulations will enable us to evaluate the role and fate of these brines after brine generation at the platform top ceases.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009