ABSTRACT: Regional Distribution and Chemical Characterization of Permian Capitan Fore-Reef Slope Dolomite: Implications for Paleohydrology

Leslie A. Melim

Dolomitized fore-reef slope facies recently have become significant targets for petroleum exploration in the Permian basin and elsewhere. Despite this, very little is known about the dolomitization process that largely controls porosity distribution in this facies. An integrated field, petrographic, and geochemical study has been conducted in the fore-reef slope strata of the Permian Capitan Formation to provide insight into this problem.

Dolomitization of the Capitan fore-reef slope facies ranges from 0 to 100%. Regionally, the variation is related to stratigraphic age, with older forereef slope facies more dolomitized than younger facies. This generalization is consistent throughout the Guadalupe Mountains and appears to continue into the subsurface. On a local scale, more permeable beds were the first dolomitized, and within these, dolomitization decreases downslope. In the Capitan reef facies, dolomitization is restricted to vertical karst breccia pipes and other zones of high primary porosity.

The majority of Capitan dolomite is finely crystalline (5-30 µ) with average ^dgr¹⁸O = 0.9 and ^dgr¹³C = 5.9. A second, coarser (130-µ) dolomite generation is also present but is not as volumetrically important. The relative positive (for the Late Permian) ^dgr¹⁸O values indicates an evaporitively concentrated dolomitizing fluid.

The most likely paleohydrologic model for the main phase of fore-reef slope dolomitization calls for descending hypersaline brines passing through the reef facies in vertical karst breccia pipes and then following permeable beds down the foreslope. Several sources are possible for these brines, with the most likely being the hypersaline back-reef lagoonal environment.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990