Abstract: Geomorphic Exploration Used in Discovery of Trap Spring Oil Field, Nye County, Nevada

Norman H. Foster

Eagle Springs oil field was discovered in 1954 by Shell Oil Co.; it represented the first commercial oil production in Nevada and the second oil field in the entire Great Basin, the first field having been the Rozel Point oil field on the northeast side of Great Salt Lake in Utah. Oil is trapped at Eagle Springs in a combination truncation and fault trap. The Oligocene Garrett Ranch volcanic rocks and the Eocene Sheep Pass lake deposits, both of which are reservoirs, are thinned by erosion or truncated at the basin margins in Railroad Valley. Parts of the trap are formed by complete truncation of Garrett Ranch and Sheep Pass. Other parts of the trap are formed by pure fault closure. The top seal is formed by the Miocene and younger valley-fill sedimentary rocks (Horse Cam Formation) and the bottom seal by nonpermeable underlying Paleozoic units.

A closed dome is present in the valley fill overlying the truncation-fault trap at Eagle Springs field. The shallow four-way dip does not carry with depth and has no bearing on the trap. The presence of this shallow domal feature misled explorers in the region for many years, and numerous wells were drilled on high blocks at basin margins, or on horst features within grabens. These wells normally penetrated Paleozoic rocks below the valley fill because the reservoirs had been stripped from the highs by erosion.

Recognizing why previous exploration had failed and that the highest structural position on the truncation-fault wedge, where oil would be trapped, was preserved in the present structural lows adjacent to the bald highs, led to another startling discovery. Eagle Springs-type traps could be localized by mapping fault patterns on aerial photographs! In the past a fault scarp at the mountain front bordering horst blocks had been mapped. These faults, however, are relatively minor, and normally do not represent the main boundary fault between horst and graben. The main fault is some distance basinward of the mountain front, normally as much as 1 to 7 mi (1.6 to 11 km). The main fault can be mapped by characteristic slope changes, and in some places by scarplets on the bajada surfaces exte ding into the basins. Springs commonly mark the position of the main fault. In addition, highs along the basin margin can be delineated by fault and topographic patterns.

Special care must be exercised in mapping because of the presence of beach ridges formed by previous lakes in many basins. Internal drainage basins appear to be easier to interpret than those with external drainage; however, both types can be mapped. Field checking is a necessity. The present structural lows at the basin margins thus can be delineated and represent prospective areas. Trap Spring field was delineated with this technique. Seismic lines shot over Eagle Springs and Trap Spring confirmed the position of the faulting and truncation.

This technique can be utilized in other Basin-and-Range grabens which are favorable from the standpoint of source rocks, maturity, and reservoirs.

AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah