Abstract

Greater Natural Buttes gas field (GNB) covers about 400 sq mi (1050 sq km) in T. 8-12 S., R. 18-24 E. Uintah County, Utah. It includes the following gas fields reported individually by the Utah Division of Oil, Gas and Mining: Bitter Creek, Bonanza, Buck Canyon, Devils Playground, Natural Buttes, Oil Springs and Rock House.

The GNB field is on gentle regional northwest dip on the southeast flank of the Uinta Basin. Gas is produced from stratigraphic traps in (1) marginal lacustrine sandstones in the Eocene Green River Formation, (2) fluvial sandstones enclosed in redbeds of the Paleocene and Eocene Wasatch Formation (the main production), and (3) braidplain sandstones interbedded with carbonaceous shales and coal in the Upper Cretaceous Mesaverde Group.

Gas was first discovered in GNB in 1955, and during the next 6 years several widely scattered gas fields were coalesced to form the GNB. Through 1991, over 400 wells had been completed as gas producers in GNB. As of 9/91, the Utah Division of Oil, Gas and Mining reported cumulative production for GNB of 334 billion cubic feet of gas (Bcfg) (9.5 billion cubic meters of gas (Bcmg)) and 1990 annual production of 23 Bcfg (0.65 Bcmg).

The GNB area corresponds to the area of concentration of gilsonite veins in the basin. No faults have been mapped at the surface, but normal faulting with throws of up to 170 ft (58 m) occurred during deposition of the lower part of the Green River Formation and allowed gas from Mesaverde Group rocks to migrate upward into the Wasatch Formation sandstones and possibly into the Green River Formation sandstones. Late Eocene-Oligocene subsidence of the Uinta Basin caused tensional fractures in the Green River and overlying Uinta Formations above the faults and gilsonite was injected into the fractures.

Gas in the Green River sandstones may be a mixture of gases which have migrated from lacustrine source beds deeper in the basin and from the Mesaverde carbonaceous source beds.