Calcitization and Silicification of Evaporites in Guadalupian Back-Reef Carbonates of the Delaware Basin, West Texas and New Mexico
ULMER, DANA S., and PETER A. SCHOLLE, Southern Methodist University, Dallas, TX
Outcrop of the Seven Rivers, Yates, and Tansill formations contain numerous examples of evaporites that have been replaced by both quartz and calcite. The original evaporites consisted of discrete horizons, scattered nodules, enterolithic layers, and individual crystal laths of gypsum and/or anhydrite within a predominantly dolomitic matrix.
Based on field and petrographic observations, evaporite replacement proceeded from the exterior to the interior of the nodules. The earliest replacement was by euhedral, black megaquartz containing abundant hydrocarbon inclusions. Calcite replacement followed silicification and consists of coarse, equant, blocky spar. Isotopic analyses of these calcites form two distinct groups: the first group ranges from -10.9 to -20.10 (average -16.40) C(13) and -6.4 to -13.80 (average -10.90) O(18); the second group ranges from +1.4 to -5.80 (average -2.40) C(13) and -6.2 to -14.10 (average -9.20) O(18) (all relative to PDB).
Evaporite silicification was coeval with hydrocarbon migration as indicated by the inclusion data. Calcitization, however, was associated with mid-Tertiary block faulting that uplifted the area causing deep groundwater circulation. The isotopically very light calcites resulted from the mixing of meteoric fluids and hydrocarbon-rich pore fluids, probably during early uplift while these strata were still at significant depth. The calcites with heavier isotopic values were produced somewhat later by meteoric fluids that had little or no contact with hydrocarbons. Evaporite diagenesis in the Delaware basin is an ongoing process that started during hydrocarbon migration, continued over millions of years, and has the potential to significantly change the porosity of these units.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)