HEDRICK, C. L., and S. L. DOROBEK, Department of Geology, Texas A&M University, College Station, TX; and S. C. RUPPEL, Bureau of Economic Geology, University of Texas, Austin, TX

ABSTRACT: Sequence Stratigraphy, Diagenesis, and Porosity / Permeability Distribution in Permian Carbonate Reservoirs, Monahans Field, West Texas

The Permian Clear Fork Formation (upper Leonardian) at Monahans field, Ward and Winkler Counties, West Texas, consists of highly cyclic, platform interior deposits. The reservoir at Monahans field is in the upper Clear Fork unit and Glorieta Formation, which are located stratigraphically between the underlying Tubb Sandstone and the overlying San Andres Formation. Thus, the upper Clear Fork-Glorieta interval was deposited during a third-order sea level cycle. Two orders of higher frequency cyclicity also are identifiable in this interval and include: (1) meter-scale shallowing upward cycles of shallow subtidal to peritidal facies (fifth order cycles); and (2) 10-20 m thick packages of meter-scale cycles (fourth order cycles).

The basal part of the upper Clear Fork unit contains the most limestone in Monahans field suggesting that the efficacy of dolomitizing solutions increased up-section, during late highstand to lowstand deposition related to third order sea level fluctuations. Geochemical characteristics of replacive dolomite in the upper Clear Fork unit show very subtle stratigraphic trends. Replacive dolomite from the basal upper Clear Fork unit is the most Ca-rich and probably is the least recrystallized in the entire section. Most other replacive dolomite is nearly stoichiometric, has homogeneous dull cathodoluminescence, and has low trace element contents (Mn, Fe, Sr, Na). Carbon and oxygen stable isotopic compositions show consistent stratigraphic covariation within individual fourth order cycles, suggesting similar diagenetic histories for all cores examined during this study. Carbon isotopic values are most negative and oxygen isotopic values are most positive near the interval of maximum flooding in the third order cycle. In this interval, fourth and fifth order cycles are dominated by subtidal facies and accommodation probably was greatest. Isotopic compositions cannot be statistically related to any other parameters like lithofacies, dolomite crystal size, or porosity or permeability.

Statistically, the best interconnected porosity within the field is intercrystalline porosity between dolomite crystals, followed by grain-moldic porosity. Permeability also is enhanced by late stage dissolution of dolomite and anhydrite.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.