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Abstract

Depositional porosity and permeability distribution in Guadalupian-age grainstone-dominated parasequences of the central Guadalupe Mountains have been moderately to strongly modified by diagenesis. Early diagenetic features in the ramp crest and inner shelf crest can be related to inferred paleohydrology that developed during high-frequency (parasequence-scale) sea-level lowstands. In the San Andres example, leaching of carbonate grains and resulting precipitation of intergranular cement in a meteoric lens beneath an emergent bar crest produced a well-defined, highly porous but relatively low-permeability moldic zone in the thickest part of a grain-dominated parasequence. In the Grayburg example, preferential preservation of intergranular porosity and minor leaching along a paleo-water table produced a thin (less than 30 cm) stratiform zone of higher than average permeability. A zone of preferentially preserved intergranular porosity developed within an inferred meteoric lens beneath an emergent bar crest. Diagenetically influenced lateral and vertical changes in permeability distribution are predicted to be typical of intermittently exposed ramp crest and inner shelf crest environments. These examples demonstrate the efficacy of parasequence-scale mapping of depositional and diagenetic facies for improving prediction of permeability distribution in analogous strata.

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