Abstract

The Madden anticline is a large, Laramide-age structure in the northeastern corner of the Wind River basin in Fremont and Natrona Counties, Wyoming. Since 1968, when gas in significant quantities was discovered on this structure, approximately 275 bcf (7.8x10⁹ m³) of gas have been produced from fluvial sandstones in the Paleocene Lower Fort Union and Upper Cretaceous Lance Formations and from marginal marine sandstones in the Upper Cretaceous Mesaverde Formation and Cody Shale. Most of the reservoirs are overpressured and production rates are enhanced by natural fracturing on the crest of the structure.

The No. 1-5 Bighorn well, spudded in October 1983, penetrated previously undrilled sub-Cody rocks on the anticline and reached a total depth of 24,877 ft (7582 m) in Precambrian rocks. Numerous potentially productive intervals were penetrated below the Cody including sandstones of the Upper Cretaceous Frontier, the Lower Cretaceous Muddy and Cloverly, the Jurassic Morrison, the Triassic Crow Mountain, and the Pennsylvanian Tensleep Formations and carbonates of the Permian Park City Formation and the Mississippian Madisons Limestone. In August 1985, the well was reported as completed through perforations from 23,758 to 23,902 ft (7240 to 7285 m) in the undifferentiated Mississippian Madison Limestone/Ordovician Bighorn Dolomite and flowed gas at the rate of 20 mmcf per day (5.66x10⁵ m³/d) after mild acid stimulation.

All of the Mesozoic strata were overpressured requiring 17 lb / gal (2.037x10³ kg/m) mud to maintain balance. A more normal pressure gradient was encountered near the top of the Paleozoic interval.

The Madden anticline is a foreland structure which developed during the late Paleocene through early Eocene. A large buried thrust fault subparallel to the Owl Creek Mountain thrust system controls the position of the structure, exhibiting up to 4,700 ft (1432 m) of displacement at the Madison and basement levels. Hydrocarbons appear to have been generated from Tertiary and Cretaceous gas-prone source rocks as well as from oil-prone source rocks in the Permian Park City Formation. Gas reservoirs in Paleozoic strata were probably oil accumulations earlier in the development of the structure. With continued burial and structural growth, these reservoirs passed through the "oil window" producing large quantities of thermogenic gas.

Plans for development of the Madden anticline indicate that as many as 20 additional wells may be required to fully develop the sub-Cody interval. Based on gross recoverable reserve estimates of 5 to 10 tcf (1.4 to 2.8x10¹¹ m³) of gas for all zones, Madden anticline will be an important producer for many years to come.