ABSTRACT: Depth- and Lithofacies-Related Diagenesis in the Wilcox Group of Southwest Louisiana: Implications for Porosity Evaluation

Bruce R. Wilcoxon, Ray E. Ferrell, Jr.

Core from the Ragley Lumber D-1 well (Allen Parish, Louisiana) was analyzed to determine the relative importance of depth- and lithofacies-related parameters on the diagenetic history of the Paleocene-Eocene Wilcox Group of southwest Louisiana. Five lithofacies, ranging from clean sand to silty shale, were sampled at three different depth intervals (3.7, 4.3, and 4.8 km). Optical and scanning electron microscope petrography, x-ray diffraction, backscattered electron image analysis, and pyrolysis were utilized to quantify the products of diagenesis.

Pyrite, early chlorite, and mixed-layer illite/smectite precipitated during early diagenesis as depositional pore fluids were expelled from sediments by initial compaction. During intermediate diagenesis, increased water/rock interaction at elevated temperatures and pressures led to K-spar dissolution, quartz overgrowths, kaolinite and calcite precipitation, and the initiation of the smectite-to-illite transition. Late diagenetic effects are observed only in the lower two depth intervals and include the destruction of kaolinite; the precipitation of chlorite, iron carbonates, and fibrous illite; and the apparent increased rate of organic matter decarboxylation.

Factors associated with burial (temperature, compaction, pressure) are responsible for controlling the specific diagenetic reactions and their timing. At each depth, all lithofacies have similar diagenetic mineral assemblages but differ from one another with respect to the abundance of the authigenic phases, e.g., late chlorite is most abundant in the sand-rich lithofacies. Compaction during early diagenesis was especially detrimental to the porosity of lithofacies containing abundant detrital clay. Quartz overgrowths arrested compaction in the clean sand lithofacies. Calcite locally destroys porosity, and there is no evidence of carbonate leaching to form secondary porosity. Leached detrital K-spar produced authigenic kaolinite, resulting in no net gain in porosity. Kaolinite may hav reduced effective permeability. Late authigenic minerals had little effect on porosity but reduced net permeability. Consequently, the clean, quartz-rich sandstones have the greatest reservoir potential of the lithofacies examined.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990