Modeling reflux dolomitization

Fullmer, Shawn M.
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The process of reflux dolomitization is commonly invoked to explain dolostone occurrence and distribution. The time required for this process to take place is not well understood. Volumetric flow rates are the dominant control on the time requirement. Published platform scale numerical models of reflux require unrealistically high permeability values and reaction efficiencies to explain the mapped dolomite volume. Approaching the problem at the high-frequency-cycle scale allows tighter constraints on the variables and a simpler geologic setting to investigate the permeability and reaction efficiencies required to form a dolostone unit. The high-frequency cycle used as a model in this study is located near the top of the Cretaceous age upper Glen Rose Formation in central Texas. It is an upward shallowing, Albian age cycle that consists of a subtidal mud-dominated packstone overlain by an evaporitic tidal-flat cap. The dolostone extends 1.5 meters down from the cycle top. Permeability values reflecting conditions at the onset of dolomitization were reconstructed by, 1) obtaining current porosity and permeability values from outcrop samples, 2) approximating porosity loss due to burial diagenesis, 3) interpreting pre dolomitization rock fabrics from thin sections, and 4) calculating permeability using the uncompacted porosity and rock-fabric information. The data was entered into a variable density flow model and the results suggest that the time required to form this 1.5 m dolostone unit is between 300 and 1,000 years using a harmonic mean permeability of 260 md (2.6 x 10⁻¹³ m²) and magnesium exchange efficiencies between 10-40%. These more realistic permeability and efficiency values are much lower than the 100 - 10,000 Darcy's (10⁻¹¹ - 10⁻⁹ m²) permeability and 100% reaction efficiency values used in current published reflux models