ABSTRACT

The sediments comprising the Wilcox group of Early Eocene age were recognized and studied from the outcrop and shallow wells as early as 1848. It was not until 1906, however, that these deposits between the marine silty clays of the Midway group below and the marine glauconitic, fossiliferous sands of the Claiborne above were named the Wilcox by A. F. Crider.

The Wilcox at its outcrop and in the subsurface is composed of a heterogeneous series, several hundred feet thick, of sandy lignitiferous littoral clays; crossbedded river sands; compact, noncalcareous, lacustrine or lagoonal clays; lignite lentils and stratified deltaic silts. These deposits were laid down along flood plains and inlakes and swamps on a wide, flat coastal plain traversed by shifting streams. The Wilcox epoch ended with an advancing sea which spread deeper water deposits over the wide coastal plain from which their sediments were partially derived. The abundance of subaqueous plant detritus is suggestive of a humid climate with a heavy rainfall.

A structural contour map (Figure 1) on top of the Wilcox group and an isopachous map (Figure 2) of the group are presented as an interpretation of regional structure and thickness affecting the Wilcox group. Although the upper and lower boundaries of the group are reasonably distinct, the prevalence of non-marine sediments and the lack of widespread marine zones renders lateral correlation of individual members virtually impossible.

Although many wells have penetrated through the Wilcox, relatively few of these wells were drilled as Wilcox tests; thus, the oil potentialities of the group have not been fully explored. Also, the results of the meager exploratory work which has been done in search of Wilcox production have been rather discouraging. To the writer's knowledge, only one well in the East Texas basin has been completed as a commercial Wilcox producer, and this well was abandoned after producing only 3,269 barrels of oil. Considerable non-commercial shows of oil have been encountered in the Wilcox group in wells in the southern portion of the East Texas basin. The updip extent of marine conditions, although difficult to evaluate, appear to be the limiting factor in the oil production from the Wilcox group in East Texas. The Wilcox group of East Texas is far more important for its soils, lignite beds, and clays used in the manufacture of brick and pottery than for its oil potentialities.