Abstract: Exploration History of South Texas Lower Cretaceous Carbonate Platform

T. D. Cook

The search for hydrocarbons in Lower Cretaceous reservoirs of south-central Texas has been continuous for more than 60 years. To date, accumulations have been found in significant quantities in only four areas: (1) 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 are the reservoirs for accumulations in the fault trends, and porosity and permeability are reasonably good. Few limestones in the reef trend were very 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 faults trends, and is nonexistent in the reef reservoirs. Proved ultimate yield for the fault trends is about 350 million bbl of oil and 1.5 Tcf of gas. Reserves for the reef trend's dry gas 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 Formation still holds the promise of success but lies at considerable depths across much of the area. Edwards and Glen Rose rocks are more densely explored, but there are ample opportunities for new plays even in those 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.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas