This report addresses the geology and evolution of the Neogene basin of Railroad Valley in east-central Nevada, which hosts the largest known oil fields in Nevada. Surface geologic data were integrated with large amounts of subsurface data, gleaned from geophysical surveys and hundreds of bore holes, to produce comprehensive conceptual models for the basin and the associated oil fields.

In Chapter 1, detailed geologic maps and a county geologic map are combined with subsurface geological information from 300+ boreholes to create bedrock geology and subsurface structure maps of the Railroad Valley area. Supplemental data include geophysical and soil-gas surveys. Paleozoic and Cenozoic stratigraphy is summarized in three areas: outcrops in the Pancake and Grant ranges to the west and east respectively, and subsurface. Paleozoic stratigraphy of the map area is composed of regionally extensive carbonate and arenaceous formations interspersed with disconformities. These strata are overlain by Paleogene continental and volcanogenic formations and Neogene syntectonic deposits that include some widely distributed megabreccias. Intrusions are generally granitic and include several stocks and scattered dikes and sills that range in age from Jurassic to late Paleogene. The earliest important tectonic event was the development of a foredeep associated with the middle Paleozoic Antler orogeny. The foredeep included the area of Railroad Valley and was the site of deposition of organic-rich Mississippian sediments. Contraction during the late Mesozoic Sevier orogeny is manifest in the map area by folds and thrust faults of the central Nevada thrust belt. This was followed by deposition of organic-rich lacustrine strata in Paleogene basins. A caldera formed on the west side of Railroad Valley in late Paleogene time as part of a regional episode of volcanic activity. Low-angle normal faults developed as volcanism waned in the area. Neogene Basin-Range tectonism characterized by high-angle normal faults followed. This latest episode caused the development of syntectonic deposits that include megabreccias and resulted in the present topography. A paleogeologic map constructed using outcrop and well control shows the distribution of Mississippian and early Paleogene source-rock formations, and temperature information from well control identifies the present thermal regime. Hydrocarbon generation sites are present where Mississippian and Paleogene organic-rich beds have reached adequate thermal conditions. The resulting accumulations have produced nearly 50 million barrels of oil. Important reservoirs include Neogene megabreccias, welded tuffs, and lacustrine carbonate beds. Production has been prolific; most fields have wells that produced 1,000+ barrels per day, and some tested rates exceeded 10,000 barrels per day. Complex structure resulting from the multiple tectonic events affecting the area has impaired exploration and development. Continuous oil-phase potential has not been evaluated. The combination of surface and subsurface geology provides insight into the structural development and thermal regime of the area. The basic geologic elements are replicated in numerous places in eastern Nevada, and Railroad Valley serves as an analog to guide investigations of other areas.

Chapter 2 provides a detailed account of an important stratigraphic marker in the upper part of the Neogene basin in Railroad Valley. A subsurface Pliocene basalt (shown as unnamed basalt, QTb, on plate 2) has been identified using wireline logs in 52 Railroad Valley (RRV) wells at depths that range from 335 to 640 m (1,100 to 2,100 ft) below the surface. The basalt averages 35 m (114 ft) in thickness and is up to 103 m (338 ft) thick. The structure map on top of the basalt shows a synclinal trend oriented northeast-southwest. An isochore map indicates widespread basalt thickness of 48 m (150 ft) that may represent multiple lava flows. Two areas with thickness of 76 to 91+ m (250 to 300+ ft) represent approximate original relief of volcanic features and are similar to height of lower cinder cones in the Lunar Crater volcanic field (LCVF). The subsurface basalt is limited to the southeast by a normal fault. Elsewhere, it is limited by deposition and postdeposition erosion. Three stratigraphic cross sections show the variability of thickness of the basalt section, and that it is composed of multiple flow units, each displaying marked decrease in resistivity near its top. A core of the basalt from Makoil #21-21R Dry Lake (redrill) has phenocrysts of feldspar and pyroxene, a rounded gneissic xenolith, and fractures and vesicles filled with crystalline material and clays. An X-ray diffractogram of basalt cuttings shows major amounts of sodium anorthite and augite, as well as trace amounts of smectite and mixed-layered illite/smectite clays. 40Ar/39Ar dating of the basalt indicates an age of 4.57±0.03 Ma, which correlates to Episode 2 of the eruption history at LCVF. The subsurface basalt is older than an outcrop to the west, where US Highway 6 exits Railroad Valley, that has been dated as 1.09 Ma. Samples from the Makoil #21-21R Dry Lake (redrill) well, two older basalts in the LCVF, and a basalt outcrop at Mud Spring Basin on the east side of Railroad Valley plot within the basalt field of a Total Alkali Silica graph.

Chapter 3 describes the Cenozoic ash-flow tuff stratigraphy and geochronology of Railroad Valley and adjacent ranges, as these strata commonly serve as reservoir rocks for the hydrocarbons. Discovery and production of oil from Cenozoic ash-flow tuffs of Railroad Valley focused attention on the correlation of the subsurface units to outcrops and the locations of corresponding centers of eruption. Improved knowledge of the Cenozoic volcanic and thermal history of the area also refines understanding of the early development of the basin of Railroad Valley. Cenozoic ash-flow tuffs have produced about 18 million barrels of oil, or about 37% of total production from fields in Railroad Valley.

Correlation of the productive units to outcrops has been speculative or imprecise because of the limitations of subsurface samples. To resolve the volcanic stratigraphy, 40Ar/39Ar ages of selected subsurface samples were determined. Wireline logs and visual inspection were used to identify ash-flow tuffs and to select sample horizons. Dates have been determined for 23 cuttings and 8 core sample from 11 wells in Railroad Valley, and three cuttings and one core from two wells in nearby White River Valley. Three dates are on biotite, and one is on a basalt matrix; the rest are on sanidine phenocrysts. Most cuttings are contaminated with cavings from uphole, but these were isolated sufficiently to date the in-place rock. Cavings helped, in turn, to identify stratigraphically higher ash-flow tuffs. Identified ash-flow tuffs range in age from 18.6 to 36.3 Ma and correlate with 13 regional ash-flow tuffs and one local ash-flow tuff.

Published work interprets a major episode of low-angle normal faulting during the late Oligocene to early Miocene. However, significant faulting seems incompatible with the absence of coeval syntectonic deposits or fanning of dips either in outcrop in the Pancake and Grant ranges or the subsurface. A basalt sample from a marker horizon in upper valley fill is 4.6 Ma, which corresponds to Episode 2 of the nearby late Miocene-Pliocene Lunar Crater volcanic field.

Sanidine dates of three samples from producing horizons in two wells at the Trap Spring field indicate the main producing horizon is the Windous Butte Formation. Dating of a sample from a well in the area of the Eagle Springs field indicates production is from the Stone Cabin Formation. Dated cuttings from Sans Spring field show that production is from the upper tuff of Mount Jefferson.

Sample dates and subsurface information show that several wells straddle the northeast side of the Monotony Valley caldera in the southwestern part of Railroad Valley. However, well and outcrop data do not support a postulated Stone Cabin caldera under the northern part of Railroad Valley. Instead, new work documents the Stone Cabin caldera in the White Pine Range northeast of Railroad Valley.

Biotite from samples subjacent to ash-flow tuffs in two wells 1.8 km apart in the northeastern part of Railroad Valley is dated at 17–20 Ma. These dates are younger than dates on overlying ash-flow tuffs. An intrusion at total depth in one well is a possible heat source, or reheating from an unknown origin is also possible. Alternatively, the biotite date may be part of valley fill and records early deposition of young volcanic rock followed by emplacement of megabreccias of older rock. This alternative seems less likely because it would require that individual blocks be as much as 378 m (1,239 ft) thick.