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SCHALLA, ROBERT A.
Independent/Consulting Geologist, Billings, MT

Abstract: A Re-Evaluation of the Hydrocarbon Potential of the Southern Montana Disturbed Belt

In the Helena Salient portion of the Disturbed Belt of southwestern Montana, the Lombard Thrust has transported late Proterozoic and younger rocks over a footwall complex of uncertain age and structural complexity. Geological interpretations of the subsurface structure, stratigraphy and hydrocarbon potential of this area have been the focus of recent controversy. Two diametrically opposed interpretations, one developed by petroleum industry personnel and the other by the U.S. Geological Survey, are based on widely differing data sets obtained from drill cuttings from the same well.

The Norcen #1-11 Kimpton Ranch well (Sec.11, T4N,R2W) targeted structures in Phanerozoic strata which were believed to exist in the footwall of the Lombard Thrust. This well was junked and abandoned at a depth of 14,846 feet in early 1991. Industry geologists concluded that Phanerozoic strata were encountered in a complex imbricate stack beneath Proterozoic strata in the hanging wall. Data from the same well were later analyzed by the U.S. Geological Survey. The USGS concluded that no evidence existed to substantiate the existence of rocks younger than Proterozoic.

Surface mapping and limited seismic data allude to the possibility of huge oil and gas reserves in this part of the Montana Disturbed Belt. A recent re-evaluation of this area attempts to reconcile industry and USGS interpretations. The results of this study bear directly on political decisions that are currently being made regarding industry access to what could be one of the largest remaining petroleum provinces in the onshore lower 48 states.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana