NUMERICAL MODEL OF BRINE CIRCULATION DURING THE ACCUMULATION OF A CARBONATE PLATFORM AND IMPLICATIONS FOR REFLUX DOLOMITIZATION – PERMIAN SAN ANDRES FORMATION

A numerical model of the Permian San Andres Formation shows the evolution of refluxing brines during deposition of this carbonate succession. We hypothesize that evaporite-rich supratidal deposits were the source of high-salinity fluids, which circulated through the platform in response to density and hydraulic gradients and caused early dolomitization. Platform geometry, sequence-stratigraphic framework, depositional environments, and distribution of the main lithologies are well constrained from outcrop and subsurface data. This information was used to construct a variable-density fluid flow and solute transport model using the code Basin2. Main controls on fluid circulation in this system are: relative sea level, topography, concentration of the fluid source, size of the fluid source, distribution of porosity and permeability, and duration of the flow regime. The combination of these effects produces complex circulation patterns that evolve over time. The source of dolomitizing fluids migrates across the platform top in response to relative sea-level fluctuations and feeds a brine plume within the underlying sediments. The brine plume also migrates, grows, and shrinks in response to such fluctuations. Salinity distributions across the platform are complex and change both spatially and temporally. Dolomitization is most likely to occur in areas of simultaneous high-salinity and high-flow rate. This finding. These findings help explain the dolostone/limestone distribution observed in San Andres outcrops and subsurface.