ABSTRACT

The Wilcox Group in west-central Louisiana occurs as a wedge of terriginous clastic sediment of Paleocene age which prograded into the northern margin of the ancestral Gulf of Mexico. In this study, subsurface correlations provide the basis for dividing the Wilcox Group into three primary intervals. Proceeding upsection, these intervals are the lower Wilcox, upper Wilcox, and Carrizo Formation. The lower Wilcox is further subdivided into four regional lithologic units, informally referred to as Intervals 1, 2, 3, and 4.

Lignite occurs as component facies of alluvial valley, deltaic, and lagoonal rocks. Lignite is identified from electric-log response on the basis of an "operational" definition, and environmental interpretations of associated facies are derived from log responses characteristic of deltaic environments. Expected properties of lignites are predicted by analogy with modern peat deposition and comparison with other ancient lignite accumulations.

The Carrizo Formation was deposited as meanderbelt facies within an alluvial plain, but is devoid of any significant lignite accumulation because of destruction of overbank deposits.

The upper Wilcox is a lagoon barrier-bar complex characterized by deposition of fine-grained sediment updip and strike-oriented accumulation of coarse-grained sediment downdip. The interval is basically not lignite bearing. A maximum of 8 seams occur updip of major strike-oriented sandstone. These lignites are expected to be of lagoonal origin and of poor quality.

The lower Wilcox is a typical progradational deltaic complex to the east and marginal delta plain to the west. The lower Wilcox is the major lignite-bearing interval of the Wilcox Group and contains up to 35 lignite seams. The lignites are associated with interdistributary flood-basin deposits and should be of good quality, based on similarity to analogous deposits elsewhere.