Eagle Springs Oil Field, Railroad Valley, Nye County, Nevada

Abstract

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 bbl a day). To date, the productive limits of the field have not been defined. Shell's discovery was drilled on a seismic anomaly reportedly mapped in Miocene valley-fill deposits. Cumulative production to February 1, 1967, is 1,514,626 bbl of oil, with recoverable reserves from 13 productive wells estimated at 10,000,000 bbl. Most of the production is from carbonate rock of the Eocene Pass Formation and from porous zones in Oligocene tuff. A well in the southwest part of the field has produced a small amount of oil from Pennsylvanian carbonate strata. The 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 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 in some of the Eagle Springs Unit wells as well as in several of the exploratory tests drilled in the region. The oil trap at Eagle Springs appears to have resulted from a combination of folding, faulting, truncation, and overlap; impermeable Miocene valley fill overlaps truncated Oligocene and Eocene reservoir beds along a northwest-plunging anticlinal nose at the base of the Miocene. 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. 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 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 these faults continued at least until the end of the Tertiary. The writers believe that additional significant oil accumulations will be found in eastern Nevada. However, to discover them requires a coordinated exploration program designed to overcome such past problems as (1) inability of geophysical surveys to map accurately the subsurface structure and (2) difficulty in predicting distribution of exploratory objectives in the alluviated valleys. Detailed field mapping and stratigraphic studies of all potential reservoir formations exposed in the mountain ranges must be accurately correlated with combined seismic, gravity, and magnetic surveys in the adjacent valleys.