Part 4 - Berg - Hughes Center

Arkansas and Louisiana. This study showed that the Midway Group ranges in thickness
in southwest Arkansas and northwest Louisiana from about 400 to 600 ft, and that the
basal, calcareous portion ranges in thickness from 40 to about 100 ft. The change from
the calcareous, marly or chalky, shales of the lower Midway to the dark, steel-gray, finely
laminated, silty shales or mudstones of the upper Midway, is apparently transitional
(Alexander, 1935).
Murray (1955) stated that the contact between the Midway and the underlying
Cretaceous strata in southern Arkansas is marked by a bed of white to gray ash, having a
maximum observed thickness of about 6 ft. This bed of volcanic ash has been extensively
used as a marker for the Midway-Cretaceous contact. In wells examined in southwestern
Arkansas and northwestern Louisiana, this bentonite bed was observed to lie either at the
top, or in the uppermost part of the lower, calcareous section of the Midway.
Wilcox Group
Because of prolific hydrocarbon production from numerous reservoirs in Texas,
Louisiana and Mississippi, the Wilcox Group of Paleocene/Eocene age, has been
extensively studied (Echols and Malkin, 1948; Fisher and McGowen, 1967; Galloway,
1968; McCulloh and Eversull, 1986; Sassen et al., 1988; Tye et al., 1988, 1991; Echols,
1991; Echols and Goddard, 1992, 1993; Glawe, et al., 1999). Sourced primarily from the
northeast and northwest, the Wilcox in northern Louisiana reaches a thickness of 1,000 ft;
and where production exists in Winn Parish, it is 2,200 ft thick. In Colgrade field, the
reservoirs are at a depth of 1,500 ft with porosities of 15 to 35% and permeability of 200
to 600 md (Nelson, 1963). In Nanichtoches Parish, the Wilcox reservoirs are at a depth of
5,100 to 5,800 feet. Southeast of the North Louisiana Salt Basin, the Wilcox is oil
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