Diagenetic/Stratigraphic Modeling: Wilcox Group, Texas Gulf Coast

Jeffrey A. May, Sharon A. Stonecipher

One of the most serious problems facing exploration or development geologists is determining what depositional system they are working in and where they are within that system. No one technique is perfect for making this determination; each approach has its limitations. For example, it is difficult to extrapolate seismic stratigraphic interpretations down to the scale of 20-ft pay sand. And, even though log and core analyses can be resolved on a very fine scale, results of these analyses are often ambiguous and whole core is often unavailable. Another tool to help resolve these ambiguities, i.e., diagenetic/stratigraphic modeling, is currently being developed.

The basic premise of diagenetic/stratigraphic modeling is similar to that of simple stratigraphic modeling, i.e., facies interpretations are based upon (1) the depositional process recorded and (2) the lateral variation and vertical sequences of these processes. Stratigraphic models depend primarily upon physical processes, whereas diagenetic/stratigraphic modeling requires additional knowledge of chemical processes. For example, depending on the energy and character of the depositional process, each facies will contain a different subset of the original provenance mineralogy, and exhibit a different set of textural characteristics. In addition, chemistry of bottom waters during deposition governs formation of specific early authigenic clay and carbonate cements. Therefore, petrograph cally determined compositions can be used to complement, and test, traditional methods of defining depositional environments. We apply this method to the Wilcox Group of the Texas Gulf Coast.

The Wilcox Group represents the first major, regional clastic wedge built over and beyond the Cretaceous carbonate shelf edges. The Wilcox, however, like many other Gulf Coast reservoirs, is a source of controversy. Were the Wilcox sands deposited entirely by shallow marine processes, or was deep-water sedimentation also involved? Our work involves diagenetic/stratigraphic modeling to resolve this debate.

We have analyzed over 5,000 ft of core from 16 wells to (1) constrain the character of deposition and (2) determine the importance of facies vs. provenance vs. syndepositional tectonics on diagenetic trends. All cores to date are attributed to subenvironments of the delta plain, delta front, and continental shelf. Turbidites are present along delta fronts and are encased in shelf mudstones; however, no submarine fan sequences have yet been found.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.