The San Marcos arch, a broad southeast-plunging structure which separates the Rio Grande and East Texas basins, is the habitat of major, structurally controlled oil and gas accumulations. Current exploratory emphasis is on stratigraphic-trap accumulations in shallow sandstones of the Midway Group (Paleocene-Eocene); however, excellent stratigraphic-trap possibilities may exist in rocks of Late Jurassic and Early Cretaceous age.

The stratigraphic and structural influence of the San Marcos arch can be detected in sediments ranging in age from Jurassic to Miocene, and extending from the Llano uplift to the present shoreline. Salt-controlled structures (salt domes), which are prominent structural types adjacent to the northern and southern flanks of the arch near the coast, are absent over the arch.

Good possibilities exist for encountering stratigraphic traps on top and around the flank of the arch, particularly in the Upper Jurassic Smackover Formation and the Cotton Valley Group, and their lateral equivalents. However, favorable conditions for the stratigraphic entrapment of hydrocarbons probably are present also in younger formations, including Lower Cretaceous rocks.