The Eocene producing trend of the Upper Gulf Coast of Texas is in a nearly 40-mi-wide band roughly paralleling the coastline. The downdip limit of production is approximately 50 mi inland. The most important gas and oil reservoirs are sandstone of the Wilcox and Yegua Formations, of Eocene age, although some gas and oil have been produced from thin sandstone beds in the Queen City, Sparta, and McElroy formations.

The first large Eocene gas discovery was the Katy field, in 1935. It still ranks first in gas reserves among the Eocene fields of the Upper Gulf Coast. Subsequently, large gas reserves were found in the Wilcox in several other areas, and by 1945 the Eocene had been established as a major source of gas. The Eocene producing trend now is probably more important for gas than for oil.

Porosity and permeability values of Wilcox sandstone are low in comparison to those of other producing sandstone of the Upper Gulf Coast. Approximately 80 per cent of the section is of non-marine origin; downdip, interfingering marine and nonmarine sediments grade into a predominantly marine sequence composed primarily of marine shale. Sandstone bodies are lenticular regionally. Permeability and porosity decrease downdip. Known gas accumulations are much more common in the upper Wilcox, perhaps because few test wells have been drilled to the middle and lower parts of the section.

The Yegua Formation is predominantly marine throughout the producing trend and grades into marine shale downdip. Porosity generally is good, but permeability is greatly varied; in general, reservoir characteristics of the Yegua are better than those of the Wilcox but not as good as those of the Frio and Miocene. Yegua sandstone bodies generally are fairly persistent and can be correlated over wide areas.

Regional structure consists of a southeast-dipping monocline with dips ranging from 100 to 300 ft/mi. Normal regional dip is interrupted by shallow and deep-seated salt domes and by regional normal faults, generally downthrown toward the coast.

Gas is found in both the Wilcox and the Yegua in several types of traps, including domal, faulted, and stratigraphic. No large anticlines have been found since 1945, but future discovery of significant gas reserves in the Wilcox is probable because the entire section has not been tested by many exploratory wells. Many exploratory wells have tested the shallow depths, and the possibility of future gas discoveries in the Yegua is poor.