The San Marcos quadrangle is located on the landward edge of the Texas Gulf Coastal Plain, about 60 miles southeast of the Texas Central Mineral Region. The area of the quadrangle is about 1,060 square miles, or nearly 1/25 of the area of Texas.

The surface formations comprise the upper Comanchean; all of the Upper Cretaceous; and the Midway, Wilcox, and Claiborne of the Eocene Tertiary. All the upper Comanchean and Upper Cretaceous formations exposed at the surface are encountered in the subsurface on the area of Tertiary outcrops. No evidence of major disconformities is revealed by the drill.

The quadrangle embraces a sector of the Balcones fault zone. The regional dip throughout this area is southeasterly. The principal structures are faults roughly pandlehng the strike of the formations. Four distinct fault systems occur within this area: the main Balcones, with the downthrown side on the southeast; and the Staples-Springs, Luling-Burdette Wells-Cibolo, and the Manford (Bethany Church) systems, which have the downthrown side on the northwest.

Two oil fields occur within the area: the Luling and the Lytton Springs fields. The Luling structure is a closed uplift of the faulted monocline type and is located on the lower Wilcox outcrop. The Lytton Springs structure is a domal uplift associated with extrusive and intrusive igneous material along a small fault and is located on the Midway outcrop. In the Luling field the oil is produced from the top of the Edwards limestone. In the Lytton Springs field the oil is produced from a porous mass of serpentine encountered in the lower Taylor formation.

On December 31, 1926, the total production of the Lytton Springs field was 4,643,000 barrels, and the daily production was about 3,000 barrels. On this date, the total production of the Luling field was 31,672,000 barrels, and the daily production was about 18,900 barrels.