The lower Wilcox Group (Eocene) of Louisiana,

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Mississippi, and Alabama consists of deposits of four principal depositional systems: (1) the Holly Springs delta system which is volumetrically the largest system; (2) the Pendleton bay-lagoon system which extends into eastern Texas; (3) a restricted shelf system east of the delta system; and (4) an unnamed fluvial system which crops out along the flanks of the northward-trending Mississippi trough. Sandstone isolith maps outline the geometry of the delta mass and show at least three lobe complexes separated by mud-rich interdeltaic subembayments.

Detailed facies maps, on which information derived primarily from electric logs is used, allows recognition of seven principal component facies of the delta system: (1) bar-finger sand facies; (2) interdistributary bay mud-salt facies; (3) distributary channel sand facies; (4) prodelta mud facies; (5) distributary mouth bar-delta front sand facies; (6) interdistributary deltaic plain sand-mud-lignite facies; and (7) destructional phase sand-mud-lignite facies. Two principal types of delta lobes differentiated by their areal geometry, internal facies relations, and distributary channel development, can be recognized in the Holly Springs delta system. Bird-foot lobes were constructed where distributaries prograded over thick prodelta mud sequences; thinner, more lobate shoal-water delta lobes formed on shallow, sandy shelves or on foundered plains of older deltas.

A distinct correlation between depositional environment and the production of oil exists in the Holly Springs and Rockdale delta systems, which comprise the lower Wilcox of southeast Texas. Sand units associated with facies of the distal margins of individual delta lobes or with the destructional units are the most prolific reservoirs, and production is therefore centered along the flanks of the major lobe complexes where maximum delta destruction and interfingering with marine mud occurred.

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