Shell Oil Company discovered Eagle Springs oil field, Nevada's sole producing area, in 1954. Since late 1963, Texota Oil Company and Western Oil Lands have extended production more than 1 mi. east, completing several prolific wells (up to 1,000 b/d). To date, the productive limits of the field have not been defined entirely. Shell's discovery was drilled on a seismic anomaly reportedly mapped in Miocene valley-fill deposits. Cumulative production to April 1, 1966, is 1,208,530 bbls. of oil, with proved recoverable reserves from 13 productive wells estimated at 10,000,000 bbls.

Most of the production is from carbonates of the Eocene Sheep Pass Formation and from porous zones in Oligocene welded tuffs. A well in the southwest part of the field has produced a small amount of oil from Pennsylvanian(?) carbonate. The Sheep Pass Formation, which is characterized by the absence of volcanic material, was deposited in a local lacustrine basin that covered parts of the present-day Railroad Valley, White River Valley, and adjacent mountain ranges. The Oligocene welded tuffs are part of an extensive ignimbrite sequence that once covered much of Nevada and western Utah. The pay zones in the field are discontinuous, being absent both in some of the Eagle Springs Unit wells and in several of the exploratory tests drilled in the region. However, a significant percentage of wells drilled in east-central Nevada has found hydrocarbon shows in both Tertiary and Paleozoic sediments.

The oil trap at Eagle Springs appears to result from a combination of faulting, folding, truncation, and overlap; impermeable Miocene fanglomerate overlaps truncated Oligocene and Eocene reservoir beds along a northwest-plunging anticlinal nose, as mapped at the base of the Miocene valley fill. Closure on the east is provided in part by a major boundary-fault zone exhibiting 10,000-15,000 ft. of apparent stratigraphic displacement. This fault zone separates the field from the uplifted Grant Range on the east.

Although a fairly complete Paleozoic succession has been mapped in nearby mountain ranges, much of that section is absent in some of the Railroad Valley wells. For example, Mississippian overlies Cambrian in Shell's discovery well, probably because of pre-Eocene faulting. Within the central part of the "Sheep Pass basin," little or no angular discordance separates the Eocene from the upper Paleozoic sediments. However, at the edge of this depositional basin, these same rock units are separated by a distinct angular unconformity. Oligocene pyroclastic rocks disconformably overlie Eocene Sheep Pass sediments both in the field and at most observed outcrops within the "Sheep Pass basin." Basin-and-range normal faulting began during late Oligocene or early Miocene time. Movement along thes faults continued at least until the end of the Tertiary. A quartz monzonite intrusive body which was penetrated in Shell's discovery well is of Miocene age as determined by K-Ar dating methods.

It is logical to assume that additional significant oil accumulations will be found in eastern Nevada. However, in order to discover them, a coordinated exploration program is required.

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