The Basin and Range Province of east central Nevada, despite its vast size and over forty years of exploration, produces oil from only Railroad and Pine Valleys. Oil production has exceeded 36 MMBO from nine fields. Four fields produce predominantly from Paleozoic fractured and karsted carbonates subcropping beneath Tertiary valley-fill sequences. Prolific Grant Canyon Field of Railroad Valley has produced over 17 MMBO from Upper Devonian dolostones. The Paleozoic fields may be characterized as hanging-wall structures above high- or low-angle Miocene or younger listric normal faults associated with Basin and Range taphrogeny. Five fields produce predominantly from fractured Oligocene acidic ashflow tuffs or Oligocene sandstones. The largest, Trap Springs Field in Railroad Valley, has produced over 11 MMBO. Tertiary fields fall into two overall trap styles. The most significant style, as typified by Trap Springs and Eagle Springs fields, is preferential reservoir preservation in a downdropped fault block. The second trap style is an updip stratigraphic pinchout draped across a structural nose, and accounts for less than 6 MMBO.

Produced oils have been typed and correlated to either the Mississippian Chainman marine shales or Early Tertiary Sheep Pass lacustrine shales. Due to complex tectonic, stratigraphic and thermal histories, source rock maturity varies rapidly; adjacent areas may have Chainman shale thermal maturities as measured by vitrinite reflectance ranging from less than 0.3% Ro to greater than 2.5% Ro. Consequently, basin modeling should be an integral part of exploration strategies.

All production found to date lies within, or directly beneath the Tertiary, with the possible exception of Blackburn Field in Pine Valley. This may be due to the lack of regional reservoir sealing rocks. The most likely Paleozoic seals are the Chainman and Pilot shales. However, these shales are silica- and carbonate-rich, respectively. This mineralogy may cause each unit to be susceptible to fracturing during severe Basin and Range extension. Formation Microscanner, drill stem test and formation water chemistry data may be used to suggest the Paleozoic section is a regional common aquifer and has no internal seals. Tertiary rocks, on the other hand, must contain sealing lithologies. However, the environments of deposition of these alluvial, lacustrine and volcanic rocks are local in nature and preclude development of regional seal lithologies. Effective reservoir seals may be the most elusive element of successful exploration m eastern Nevada.

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