ABSTRACT

Sixshooter Dome is a surface anticline in T3N, R18-19E, along the Lake Basin fault zone, Sweet Grass County, Montana. Originally mapped by the U.S. Geological Survey in 1916, Sixshooter Dome has been the site of over 70 years of exploratory drilling activity.

In 1988, Balcron Oil began an extensive field mapping and wrench fault modeling program in the Sixshooter Dome area to optimize seismic data acquisition. Surface leads resulted in seismic delineation of three separate en echelon folds, two of which were breached at depth by antithetic and synthetic faulting. The middle, intact structure was determined to have approximately 130 ft (39.6 m) of closure at the Dakota reflector.

The Sohio No. 1 Linder, drilled in 1954, was the first well to test the entire Cretaceous section high on the unbreached structure. The mud logger described a significant show of gas in the Lakota Formation. However, a drill-stem test of the interval recovered only 90 ft (27.4 m) of drilling mud with no gas reported. Surface and seismic data, log analysis and mud log shows indicated the Sohio No. 1 Linder was a by-passed producer. Balcron Oil subsequently drilled the No. 14-13 Stimpson in 1991,3000 ft (914.4 m) northeast of the No. 1 Linder. The Stimpson well reached a total depth of 4496 ft (1510.7 m) in the Amsden Formation and was completed in the Lakota Formation flowing 450 mcfpd. Completion of two additional Lakota wells has established significant gas reserves. Calculated absolute open flows for the three Lakota wells range from 1-2 to 11.5 mmcfpd. Four additional wells have been completed in Upper Cretaceous pays and are currently being evaluated.