Exploration Vignettes in the Rocky Mountain Region: Adon Field, Powder River Basin, Wyoming

Title: Exploration Vignettes in the Rocky Mountain Region: Adon Field, Powder River Basin, Wyoming

Author: John M. (Jack) Parker

Publication: The Outcrop, September 2001, p. 7

Editors’ Note: The Outcrop is starting an “occasional series” of short essays gathered from the experiences of oil-finders in the Rockies. The following essay on Adon field, by Jack Parker, is the first in the series. Among his many activities over a distinguished career, Jack is a long-time RMAG member and past RMAG and AAPG president.

A Boom Goes Bust

On January 25, 1948, the Texas Company (Texaco) completed their Adon #1 well, in Campbell County, Wyoming, for an IP of 244 BOPD on pump. A Minnelusa (Pennsylvanian-Permian) discovery, the well was perforated between 8990 and 9007 feet; perfs were treated with 25 quarts of nitroglycerin.

The word went around the world instantly! Not only was the discovery located near the geographic center of the Powder River Basin where nobody had ever drilled before, but the completion was in a little-known interval. Limited Minnelusa-Tensleep production occurred near the Salt Creek anticline on the edge of the basin. Lower Pennsylvanian rocks produced on the southeast edge of the Basin in the Lance Creek area. The surface geology at the discovery site consisted of regionally west-dipping Paleocene Fort Union Formation. All we knew about the subsurface was that a regional unconformity existed between Paleocene and Upper Cretaceous (Lance Formation) rocks. Subsequently, we learned via privileged information that the Texas Company had shot out two very large structures on a north-south trend, each with about 150-200 feet of east dip against the regional west dip. Within weeks, several dozen seismic crews were operating in the central Powder River Basin from Douglas on the south to Miles City on the north.

What we didn’t know was that the Texaco discovery was from a stratigraphic trap with limited reservoir on a very small west-plunging nose with no closure at the Minnelusa level. The discovery well was plugged in 1952 after producing 28,138 BO. Texaco followed up their discovery with three dry holes in the immediate vicinity; one was on a “separate structure” 3.5 miles NNW of the #1 discovery. A map prepared by Texaco for the WGA 1957 Oil and Gas Field Symposium shows 175 feet ± of east dip on the two large structures of a north-south trend.

The boom fizzled about three years after the Adon #1 strike. Between 1948 and 1951 industry mapped hundreds of spurious seismic structures caused by shallow and deep velocity problems and misinterpretation of poor reflections at the Minnelusa level. Many dry holes were drilled on such structures. A huge blizzard hit the Basin in January 1949 and stopped all drilling and seismic activity by closing roads until early February. Bad weather coupled with disappointing results shut down activity in the area for nearly 30 years. It was not until 1983-1984 that additional Minnelusa production was found in the township where the Adon #1 was drilled. The “new” production was discovered with detailed seismic data that relied on amplitude anomalies instead of structure to delineate high porosity zones characteristic of stratigraphic traps in the Minnelusa Formation.