ABSTRACT

Deposition of the late Paleocene-early Eocene Wilcox Group is controversial. Are Wilcox reservoirs entirely of shallow-marine origin, or are basinal turbidites also present? We analyzed over 5000 feet of core from 15 wells along the Texas Gulf Coast to constrain the depositional environments. We attribute all cores examined to date to 12 subenvironments of delta-plain through continental-shelf settings. These include distributary channel, lake, marine bay, crevasse-splay delta, shoreface, lagoon, tidal flat, tidal channel, distributary-mouth bar, distal bar, prodelta, and shelf. Criteria for facies delineation in core include grain-size variations, physical sedimentary structures, trace fossils, mineralogy, bedding styles, and vertical sequences, all resulting from the interplay of specific physical, biological, and chemical processes operative in each subenvironment. We did not identify any submarine-fan deposits.

We also attempted to determine the importance of depositional facies and provenance on diagenetic trends. Early diagenetic patterns appear to be related to factors such as sediment texture, detrital composition, organic content, and original water chemistry, which were, in turn, controlled directly or indirectly by depositional environment. Rapid lateral and vertical changes in depositional environments produced markedly different early diagenetic patterns in sand units only a few feet or even inches apart. Thus, diagenetic facies defined on the basis of texture, composition, and cements can be used to complement, and test, our interpretations of depositional environments based solely on traditional sedimentologic and genetic-sequence criteria.