--> Abstract: Outcrop-based Model of Synsedimentary Dolomitization of the Permian San Andres Fm, by Beatriz Garcia-Fresca, F. Jerry Lucia, Charlie Kerans, and John M. Sharp; #90089 (2009)

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Outcrop-based Model of Synsedimentary Dolomitization of the Permian San Andres Fm

Beatriz Garcia-Fresca, F. Jerry Lucia, Charlie Kerans, and John M. Sharp
Jackson School of Geosciences The University of Texas at Austin

A hydrogeologic model shows that brine reflux is an efficient mechanism for delivering enough magnesium to dolomitize a carbonate platform. Outcrops of the Permian San Andres Formation in the Guadalupe Mountains, Brokeoff Mountains, and Algerita Escarpment provide a comprehensive framework for constraining the synsedimentary circulation of dolomitizing brines. Our robust sequence stratigraphic framework provides fundamental boundary conditions for the modeling: timing of sediment accumulation, platform geometry, relative sea level and shoreline position, location of restricted environments where brines may be sourced, and distribution of facies, lithologies, and petrophysical properties across the platform.

We used Lucia’s reservoir modeling approach to convert outcrop descriptions into a petrophysical model. This process includes assigning values of porosity, permeability, and decompaction coefficients to different facies on the basis of properties of modern carbonate sediments. Then we defined modeling layers on the basis of vertical successions of muddy versus grainy fabrics and major stratigraphic surfaces. Petrophysical properties were upscaled by averaging the data, and six rock types were defined to be input into the hydrogeological modeling.

The model simulates accumulation of the San Andres Platform and calculates fluid flow and solute transport in response to relative sea-level fluctuations. It tracks porosity loss due to compaction and concomitant permeability feedback.

Our results show that thick sections of dolostone can be formed by the cumulative effect of superimposed reflux events. Our petrophysical reconstruction and flow model successfully approximate the dolostone/limestone pattern observed in San Andres Formation outcrops.

AAPG Search and Discovery Article #90089©2009 AAPG Southwest Section Meeting, Midland, Texas, April 26-29, 2009