Effects of Provenance on Clay Diagenesis and Porosity in Upper Wilcox Formation

Michael E. Strickler, Ray E. Ferrell, Jr.

The provenance of sediments in the lower Eocene upper Wilcox Group in southeastern Louisiana was studied in 16 conventional cores from the Lockhart Crossing field by using optical, x-ray diffraction, and electron microprobe methods. Detrital clays were a mixture of highly illitic degraded micas, degraded chlorites, and minor kaolinite derived from the weathered metamorphic, plutonic, and sedimentary rocks of the region. The abundance of gneissic, granitic, and metasedimentary rock fragments, detrital IIb chlorites, 1M and 2M micas, and the iron, magnesium, and aluminum ratios of chlorites and muscovites also support the provenance interpretation.

Clay diagenesis in the southeast Louisiana Wilcox differs from the typical smectite-illite sequence reported from the western Gulf of Mexico. The montmorillonite (derived from volcanics) deposited in the Wilcox of southwestern Texas was very unstable at greater burial depths and temperatures and reacted to form illite. This reaction in turn provided cations for cements and authigenic clays that usually have detrimental effects on permeability. Early carbonate cementation and later dissolution of these cements are the most important events affecting porosity and permeability in the Wilcox of the Lockhart Crossing field. Diagenetic kaolinite, which formed at the expense of feldspars, rock fragments, and detrital clays, has had little effect on permeability unless redistributed. Iron-ric glauconites and chlorites could potentially be detrimental to porosity and permeability during acidization. In general, clay diagenesis has had only minor harmful effects on the porosity and permeabilities of the Wilcox in the eastern Gulf of Mexico basin, and provenance rather than diagenesis has been more important in determining the present clay mineral assemblage.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.