Oil Potential of Tertiary Basins in Southwest Montana and Eastern Nevada

Eric H. Johnson

An analogy is made between Tertiary basins in southwest Montana and eastern Nevada. Seismic data show basins in both areas to be northsouth trending grabens with large, basin-bounding normal faults most pronounced along the eastern perimeters of the basins. At least two stages of basin development are evident: (1) early volcanism and basinwide small-scale normal faulting covered by hundreds of feet of valley fill, and (2) subsequent development of basin-bounding faults and the syndeposition of sediment wedges thousands of feet thick.

Underlying the Tertiary valley fill are organic-rich Paleozoic shales such as the Devonian Pilot and Mississippian Chainman Shales of eastern Nevada, and the Devonian Sappington, Mississippian Big Snowy, and Permian Phosphoria shales of southwest Montana. Considerable volumes of these rich source rocks appear to have been thermally immature prior to burial and are presently subjected to optimum temperatures for hydrocarbon generation (180°-220°F) beneath 7,000-10,000 ft of valley fill.

Oil is currently believed to be generated in the basin centers and moves to structural traps at the basin perimeters along complex migration paths. Perimeter anticlines may project deep into a basin, providing conduits that aid in the collection and movement of oil into traps. Also, any trap in the center of a basin may be saturated with oil. Specific comparisons are made between producing fields in Railroad Valley, Nevada, and similar structures in several Montana Tertiary basins.

AAPG Search and Discovery Article #91040©1987 AAPG Rocky Mountain Section Meeting, Boise, Idaho, September 13-16, 1987.