The search for hydrocarbons in reservoirs of the Lower Cretaceous of south-central Texas has been continuous for more than 60 years. Accumulations have been found in significant quantities in only four areas: (1) the very shallow fault traps high on the San Marcos arch in Caldwell and Guadalupe Counties, (2) a fault trend stretching across central Atascosa County, (3) a fault trend extending from southeastern Atascosa County to southern Gonzales County, and (4) a narrow, elongate band extending across the entire area known as the Stuart City reef trend. Additional minor discoveries are widely scattered in the Maverick basin and San Marcos arch.
Reservoirs which contain the hydrocarbons were deposited in many environments, all related to a broad carbonate shelf covered by an extremely shallow sea. The sea deepened dramatically at the shelf margin parallel with a reef trend. Dolomites contain the best accumulations in the fault trends, and porosity and permeability are reasonably good. Generally, limestones in the reef trend were not extremely porous initially, and late cementation has diminished even that porosity making the reservoir of lesser quality.
Oil is the dominant hydrocarbon in the shallow fields, is less dominant in the other fault trends, and is nonexistent in the reef reservoirs. Proved ultimate hydrocarbons for the platform are about 363 million bbl of oil and 1.7 Tcf of gas. Reserves for the reef trend’s dry gas reservoirs are difficult to estimate because of highly variable reservoir conditions, but they should fall between 1 and 1.5 Tcf.
Exploration has been most intense for the uppermost Lower Cretaceous rocks. The Sligo limestone still holds the promise of success, but lies at considerable depths over much of the area. Edwards and Glen Rose rocks are more densely explored, but there are ample opportunities for new plays even in these formations. Geologists who examine cores and cuttings, determine depositional patterns, understand modern carbonate sedimentology, and study patterns of diagenesis will have an advantage in developing new concepts for exploration.