Insights to controls on dolomitization by means of reactive transport models applied to the Benicassim case study (Maestrat Basin, eastern Spain)

Partially dolomitized carbonate rocks of the Middle East and North America host large hydrocarbon reserves. The origin of some of these dolomites has been attributed to a hydrothermal mechanism. The Benicassim area (Maestrat Basin, eastern Spain) constitutes an excellent field analogue for fault-controlled stratabound hydrothermal dolomitization: dolostone geobodies are well exposed and extend over several kilometres away from seismic-scale faults. This work investigates the main controls on the formation of stratabound versus massive dolomitization in carbonate sequences by means of two-dimensional (2D) reactive transport models applied to the Benicassim case study. Simulation results suggest that the dolomitization capacity of Mg-rich fluids reaches a maximum at temperatures around 100 degrees C and a minimum at 25 degrees C (studied temperature range: 25-150 degrees C). It takes of the order of hundreds of thousands to millions of years to completely dolomitize kilometre-long limestone sections, with solutions flowing laterally through strata at velocities of metres per year (m/a). Permeability differences of two orders of magnitude between layers are required to form stratabound dolomitization. The kilometrelong stratabound dolostone geobodies of Benicassim must have formed under a regime of lateral flux greater than metres per year over about a million years. As long-term dolomitization tends to produce massive dolostone bodies not seen at Benicassim, the dolomitizing process there must have been limited by the availability of fluid volume or the flow-driving mechanism. Reactive transport simulations have proven a useful tool to quantify aspects of the Benicassim genetic model of hydrothermal dolomitization.