Geology, Distribution, and Original Gas in Place of 1,282 Large Gas Reservoirs, Onshore Texas Gulf Coast and East Texas Basins

E. C. Kosters, N. J. Banta, J. Luna-Melo, R. J. Finley

Information on more than 1,800 onshore Texas gas reservoirs with a cumulative production of at least 10 billion ft³ each has been compiled for the first comprehensive atlas of Texas gas reservoirs. Of these, 1,282 reservoirs are in the onshore Texas Gulf Coast and east Texas basins. Geology, production statistics, and reservoir-size distribution of important gas plays are analyzed. Hydrocarbon plays are defined by producing stratigraphic unit, depositional system, lithology, and trapping mechanism.

Thirty-two plays containing 24 subplays, ranging in age from Cretaceous to late Tertiary (Miocene), occur in the onshore Texas Gulf Coast basin. Ten plays occur in the Frio Formation, which has produced 56% of all gas in the Texas Gulf Coast basin. The three largest Gulf Coast plays are in the Frio Formation. Other significant plays are in the Paleocene-Eocene upper Wilcox Group of south Texas and in the lower Miocene series in Refugio County. Eleven plays with five subplays are defined in the East Texas basin. The largest of these is in Lower Cretaceous shallow marine limestones (Glen Rose, Rodessa, and Pettet Formations), where they occur over the Sabine uplift. This play represents 41% of total production in this region. The second-largest play produces gas from Smackover Formation shallow marine carbonates trapped along the Mexia-Talco fault zone.

Gas production from all 1,282 onshore Texas Gulf Coast and east Texas reservoirs with a minimum cumulative production of 10 billion ft³ each accounts for 80% to 90% of the cumulative production in both areas. Thus, this selection of reservoirs yields significant play outlines. About 84% of the cumulative production from these 1,282 reservoirs is from reservoirs with production of at least 30 billion ft³. Commonly, however, as much as 20-30% of the total production from a single play has been produced by only one or two anomalously large reservoirs. Thus, a calculated mean reservoir size may be misleading.

Original gas in place was calculated using a material balance approach for 450 reservoirs with individual cumulative production of more than 30 billion ft³. Results show that cumulative production for most reservoirs is 70-90% of gas in pressure communication with existing wells.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.