Commercial oil in Cambrian beds, Lost Soldier field, Carbon and Sweetwater counties, Wyoming

The possibility of finding oil in the basal sands of the Cambrian of Wyoming has long been a matter of speculation. The Cambrian, where well developed, consists of marine shales and limestones in the upper part and a rather thick series of coarse-grained sandstones near the base. Several comparatively shallow tests had been drilled to test the Cambrian on large, well closed structures prior to 1930 but the results were either negative or the reported showings were in doubt. A deep test to granite on the South Oregon Basin field in 1945 found definite indications of oil saturation in basal sands of the Cambrian but due to mechanical difficulties no free oil was recovered from the well. A test to granite in the Lost Soldier field, Sweetwater County, Wyoming, in 1948 resulted in the first commercial well in the Cambrian in the Rocky Mountain region. During the year two additional wells were drilled through the Cambrian in the Lost Soldier field. One of these was completed as a commercial flowing well but the other found the formation too dense to produce. In the same year a producing well was completed in the Cambrian on the adjoining Wertz dome. The Cambrian, which is represented by the Deadwood formation in this area, is ordinarily hard and quartzitic but where oil-bearing it is softer and more friable. The total thickness of the Cambrian is as much as 700 feet but the oil-bearing zone is near the base, almost in contact with the basement complex. It is the writer's belief that the oil produced at Lost Soldier and Wertz from the Cambrian is not indigenous to that formation but has migrated laterally from overlying oil-bearing reservoirs after accumulation in the structure. The writer believes that a well could be drilled at the crest of the Lost Soldier dome, which would yield oil from the basement complex if that formation were penetrated above the oil-water contact in the Tensleep sand.