ABSTRACT

The lower Wilcox Group (Eocene) of Louisiana, Mississippi, and Alabama consists of deposits of four principal depositional systems; the Holly Springs Delta System which is volumetrically the largest system, the Pendleton Bay-Lagoon System which extends into eastern Texas, a restricted shelf system which lies to the east of the delta system, and an unnamed fluvial system which crops out along the flanks of the northward trending Mississippi trough. Sand isolith maps outline the geometry of the delta mass and show at least three lobe complexes separated by mud-rich interdeltaic-subembayments.

Detailed facies mapping using information derived primarily from electric logs allows recognition of seven principal component facies of the delta system, including the (1) bar-finger sand facies, (2) interdistributary bay mud-silt 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 relationships, 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 both the Holly Springs Delta System, and Rockdale Delta System, which comprises 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 muds occurred.