Numerical Model of Reflux Circulation During the Deposition of the Permian San Andres Formation, Guadalupe Mountains and Algerita Escarpment

Beatriz Garcia-Fresca¹, Jerry Lucia¹, and Charlie Kerans^2
1Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas, Austin, TX
²Department of Geological Sciences, Jackson School of Geosciences, The University of Texas, Austin, TX

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 define the geometry, distribution of hydraulic properties, relative sea-level position, and timing of hydrologic simulations. Main controls on fluid circulation in this setting are (1) relative sea level, (2) topography, (3) concentration of the fluid source, (4) size of the fluid source, (5) distribution of porosity and permeability, and (6) 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 over time as well. Dolomitization is most likely to occur in areas of simultaneous high- salinity and high-flow rate. This finding may explain the complex dolostone/limestone relationship observed in San Andres outcrops and subsurface.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas