ABSTRACT: Calcrete Profiles and Porosity Development in the Wagon Wheel (Pennsylvanian) Field, Ward County, Texas
F. E. Abegg, G. A. Grover
The Cisco and Canyon formations in the Wagon Wheel field, located on the western edge of the Central Basin platform, contain 10-15 previously unrecognized calcrete profiles, providing excellent evidence of repeated Late Pennsylvanian subaerial exposure. Evidence for calcretes includes (1) rhizoliths, (2) alveolar texture, (3) circumgranular cracking, (4) tangential needle fibers, (5) calcrete glaebules, (6) light shifts in ^dgr13C profiles, (7) autobrecciation, and (8) laminated crusts. Extreme lateral variability of calcrete profiles makes correlation of subaerial exposure surfaces difficult. Porosity within calcrete profiles is occluded, providing a seal for underlying reservoir facies.
Porosity in the Wagon Wheel field is strongly facies dependent. Porous intervals consist of lenticular skeletal grainstone-packstone facies, typically capped by calcrete profiles. Repeated meteoric phreatic lenses established during Late Pennsylvanian exposure events are interpreted to have formed moldic porosity through selective dissolution in strata containing mineralogically metastable allochems. Secondary porosity development, however, is often balanced by precipitation of eogenetic calcite cement.
Therefore, meteoric diagenesis associated with Wagon Wheel calcrete development commonly occludes and only rarely enhances primary porosity.
Two stages of calcite cementation are recognized: (1) an early pore-rimming nonferroan, nonluminescent calcite cement with thin, moderate to brightly luminescent microzones, and (2) a later ferroan, dully luminescent calcite cement with broad, indistinct zones. Truncated cements in Cisco- and Canyon-derived lithoclasts indicate nonluminescent cement was precipitated from oxidizing meteoric phreatic waters. Microzones were precipitated during brief periods of stagnation in the phreatic lenses. Dully luminescent cement precipitated from groundwaters of a regional aquifer recharged from tectonic highlands to the east.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990