Abstract: Exploration History of the South Texas Lower Cretaceous Carbonate Platform

By

T. D. (Ted) Cook

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."

Reservoirs which contain the hydrocarbons were deposited in a myriad of environments, all related to a broad carbonate shelf covered by an extremely shallow sea. The sea deepened dramatically at the shelf margin parallel with the reef trend. Dolomites contain the accumulations in the fault trends, and porosity and permeability are reasonably good. Few limestones in the reef trend were extremely porous initially, and late cementation diminished porosity further.

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 recovery for the fault trends is about 350 million bbl of oil and 1.5 Tcf of gas. Reserves for dry gas in the reef-trend reservoirs are difficult to estimate because of highly variable reservoir conditions, but should fall between 1 and 1.5 Tcf.

Intensity of exploration decreases from late Early Cretaceous to older 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 beds. 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

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