Geology of Eagle Springs Oil Field, Railroad Valley, Nevada
Jerome Hansen and Carl Schaftenaar
Great Basin Exploration Consultants, Inc.
In 1954, Shell drilled a seismically-defined four-way closure and discovered Nevada's first oil field, Eagle Springs. The well spud in Quaternary-Miocene valley fill sediments, and at 3355' (1023 m), a 95' (29 m) thick mass of brecciated Paleozoic carbonate was encountered which was determined to be a landslide deposit. Drilling continued through the valley fill until 6450' (1966 m), when oil was tested from porous, fractured Oligocene welded tuff below Unconformity A. Subsequent drilling established oil production from the lacustrine carbonates of the Eocene Sheep Pass Formation. During the 1960's, Texota drilled infill wells and Pennington extended the field to the east on a higher fault block. In the 1990's, Foreland drilled additional development wells, and discovered a new reservoir in brecciated limestone of the Permian Riepe Spring formation 0.5 miles (0.8 km) south of the main field. The oil is a black, 26-29° API gravity crude, pour point of 65-85°F (18-29°C), sourced from Mississippian Chainman and Tertiary Sheep Pass shales. Oil is trapped where the volcanic and carbonate reservoirs have been truncated by Unconformity A and sealed by clay-rich valley fill on a north-plunging nose. A 3D seismic survey was run over the field in 1993. Interpretation of the seismic is difficult, particularly in the eastern part of the field, due to the lack of sonic logs and the large lateral velocity change in the valley fill from slower velocity lake beds in the west to high-velocity fanglomerate adjacent to the range-bounding fault on the east. The 3D seismic was instrumental in the discovery of Ghost Ranch field, a landslide deposit in the valley fill in 1998. Eagle Springs has produced 5.5 MMBO, while Ghost Ranch has produced 616 MBO.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013