The ARCO 1 Paul Gibbs well was drilled to a depth of 17,774 ft within the Cordilleran fold and thrust belt of northwest Montana from September 1983 to October 1984. This well was a test for subthrust Paleozoic hydrocarbon source and reservoir rocks below metasedimentary rocks of the Proterozoic Belt Supergroup. The well drilled only metasedimentary rocks of the middle Proterozoic Prichard Formation and intrusive diabasic sills within the middle and lower Prichard Formation. The several diabasic sills account for a gravity anomaly over the Purcell anticlinorium where the well was drilled, as well as for the extensive seismic reflectors felt by many to represent Paleozoic rocks.

Where penetrated by the Gibbs well, the Prichard Formation consists of about 14,000 ft of metasiltstones and quartzites intruded by several diabasic sills totaling about 3,900 ft thick. Two of the sills are more than 1,000 ft thick. The well penetrates a 500-ft thick duplex zone of the Pinkham thrust near 17,000 ft, and it appears that the lower portion of the lower Prichard Formation has been thrust over the lower part of the middle Prichard Formation, a throw of about 7,000 ft.

Gas shows were present in the well for considerable thicknesses within the Prichard Formation, particularly where extensive tectonic brecciation created fracture porosity. Source rock studies of the well cuttings indicate that the Prichard Formation contains intervals with sufficient organic carbon to have acted as a hydrocarbon source rock for much of its greater than 1,300 m.y. existence and possibly even since Laramide tectonism and overthrusting. It is believed that the gas shows in the Prichard Formation are indigenous to the Prichard Formation and may well represent the oldest indigenous hydrocarbons drilled to date in the world, in the range from 1,430 to 1,300 m.y. old. The area of the Belt Supergroup is felt to represent a new hydrocarbon province with potential for future di coveries within its Proterozoic metasedimentary rocks.

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