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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Petroleum Potential of
East Central Nevada
By
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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