ABSTRACT

The Mississippian Chappel Formation at Conely Field in Hardeman County, Texas produces oil and gas from skeletal grainstone buildups, not from 'Waulsortian-style" mud mounds or "crinoid banks" as previously thought. Petrographically-defined microfacies indicate that these depositional bodies represent current-swept bioclastic sand waves consisting of grainstones and packstones that contain 25% to 70% fragmented and sorted crinoidal and bryozoan debris.

These carbonate sands are flanked by packstones and wackestones that consist of 20-60% crinoidal debris and up 70% mud. Spiculiferous, silty wackestones to mudstones occupy the platform areas farther away from the sand buildups. Presumably, those muds represent the ambient shelf depositional mode and the sand banks represent conditions that required a special combination of hydrodynamics, topography, and biology. Perhaps subtle, antecedent topography on the Ellenburger surface helped to anchor the sand waves and perpetuate the buildup.

Reservoir porosity occurs only in the crinoid-bryozoan sands where it is linked with the intraparticle spaces in fenestrate bryozoan fragments. To a lesser extent, interparticle pore spaces exist, along with solution-enhanced interparticle porosity. The chief porosity destroyers have been calcite cementation and compaction. Because the porosity is mainly intraparticle in nature, permeability is low (typically less than 1.0 md except where fractures are present).

Even though there is a large amount of post-Mississippian structural closure at Conley Field, the reservoir is dependent on the petrographic and stratigraphic properties that are the results of Chappel-aged depositional and early diagenetic processes. Exploration and development techniques in such a field require careful attention to paleostructures and their "telltale" isopach signatures.