The Mississippian Chappel formation at Conley field in Hardeman County, Texas, produces oil and gas from skeletal grainstone buildups, not from Waulsortian-style mud mounds or crinoid banks. Petrographically defined microfacies indicate that these depositional bodies represent current-swept, bioclastic sand waves consisting of grainstones and packstones that contain 24-69% 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 to 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 crinoidal 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 intra-particle in nature, permeability is low--typically 1.0 md and less except where fractures are present.

Although Conley field has a large amount of post-Mississippian structural closure, the reservoir is dependent on the petrographic and stratigraphic properties that resulted from Chappel deposition and early diagenetic processes. Exploration and development techniques in such a field require careful attention to paleostructures and their telltale isopach signatures.

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