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The Wilcox Group in west-central Louisiana is a wedge of terrigenous clastic sediments which prograded into the northern margin of the ancestral Gulf of Mexico. Subsurface correlations provide the basis for dividing the Wilcox Group into three primary intervals--lower Wilcox, upper Wilcox, and Carrizo Formation. Furthermore, lower Wilcox is subdivided into four regional lithologic units, informally referred to as intervals 1, 2, 3, and 4.
Lignites occur as component facies of fluvial, deltaic, and lagoonal rocks. Lignites are identified from electric log response based on "operational" definition. Associated environmental interpretations are derived from log responses characteristic of deltaic environments. Expected properties of lignites are predicted from their geologic setting analogous to modern peat deposition and other ancient lignite accumulations.
The Carrizo Formation represents a meander-belt facies within an alluvial plain. The Carrizo Formation is devoid of any significant lignite accumulation because of the destruction of overbank deposits.
The upper Wilcox is a lagoon-barrier bar complex characterized by fine-grained deposition updip and strike-oriented accumulation of coarse sediment downdip. The interval is basically not lignitiferous. A maximum of 8 seams occur updip of major strike-oriented sand accumulation. These lignites are expected to be lagoonal 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 lignitiferous interval of the Wilcox Group. Thirty-five lignite seams are found within the interval, are associated with interdistributary flood-basin deposits, and should be of good quality, based on similarity to deposits elsewhere.
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