Abstract: Distribution and Character of Known Deep Natural Gas Resources Based on Data from Large Fields and Reservoirs

T.S. Dyman

Known deep natural gas accumulations occur in many U.S. basins in widely different geologic settings. Three-hundred seventy-seven significant oil and gas reservoirs (reservoirs having known recoverable production of at least 6 Bcf of gas or 1 MMB of oil) produce from depths greater than 14,000 ft, and 256 reservoirs produce from depths below 15,000 ft out of more than 15,000 significant reservoirs in the U.S. according to NRG Associates (1990). Nearly 75 percent of all reservoirs below 14,000 ft produce natural gas and occur in the Gulf Coast, Permian, Anadarko, Williston, San Joaquin, Ventura, Rocky Mountains, and Cook Inlet basins.

Thirteen states contain all of the deep significant oil and gas reservoirs below 14,000 ft in the U.S. Texas has the largest number (121), which are in the Anadarko, Permian, and Gulf Coast basins, The 1970s was the most prolific decade for deep discoveries in the U.S. The Gulf Coast basin led the U.S. with 72 new deep significant oil and gas fields during the decade. Most fields containing deep significant reservoirs (203 of 329 below 14,000 ft) are classified as gas producers (62 percent) although data are incomplete for the Anadarko basin. An additional 25 reservoirs are classified as oil and gas producers. Gas and oil and gas reservoirs outnumber oil reservoirs in all states except Alabama, Florida, and California.

Sixty-seven percent of all significant reservoirs below 14,000 ft (253 of 377) are classified as having structural or combination traps. Stratigraphic traps outnumber structural traps only in the Anadarko and California basins. Sixty percent of all deep significant reservoirs below 14,000 ft (227 of 377) produce from clastic rocks. Clastic reservoir rocks are most abundant in Rocky Mountain basins, and in the Anadarko, Gulf Coast, California, and Alaska basins whereas carbonate reservoirs are most abundant in the Permian and Williston basins. The number of reservoirs decreases with increasing depth, but 26 percent of the total significant reservoirs occur below 17,000 ft.

Of the total cumulative natural gas production in the U.S. (698 Tcf; U.S. Geological Survey), reservoirs deeper than 15,000 ft account for 7 percent (50 Tcf) of the total, and deep significant reservoirs (NRG reservoirs) account for nearly half (22.4 Tcf) of the deep reservoir total. More than half of the gas from deep significant reservoirs (12.4 Tcf) was produced from the Permian basin. Significant reservoirs below 14,000 ft have a known recoverable production of 36.4 Tcf of gas. Although the Gulf Coast basin has only produced 6.2 Tcf of gas from these reservoirs, an additional 6.6 Tcf of gas exists as proven reserves. In the entire U.S., deep natural gas reservoirs account for only a small, but important, portion of the total natural gas production.

Of the total U.S. natural gas resources (nearly 1,300 Tcf), 519 Tcf is considered unconventional including coalbed methane, gas in low-permeability shale and sandstone reservoirs, and deep-basin gas accumulations. The need for new geologic research dealing with all aspects of natural gas exploration and production is obvious.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994