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Elephant Hunting in Nevada
Abstract: Alan K. Chamberlain
The central Nevada thrust belt provides an opportunity to explore for giant oil and gas fields. Thick, thermally
mature, organic-rich, lacustrine oil shales deposited in the Mississippian Antler basin flood plains are the source
beds for the fifty million barrels of oil already produced in Nevada. Karsted unconformities, stromatoporoid reefs,
impact breccias, and sandstones make Nevada's Devonian reservoir rocks most favorable for giant accumulations.
Late Cretaceous thrusting created the compressional features of the prolific Canadian foothills, Utah/Wyoming
thrust belt and the central Nevada thrust belt.
Typically, oil seeps are associated with oil-bearing thrust belts worldwide. However, a blanket of Tertiary vol-
canics sealed in many of Nevada's oil seeps and concealed Nevada's thrust belt. Some of these seeps, including
Grant Canyon, Blackburn, Trap Spring, and Eagle Springs oil fields, built up enough oil to become commercial.
So far, all of Nevada's crude has been produced from these commercial oil seeps. Little effort has been expended
to identify the source of these commercial oil seeps because of the lack of an accurate geologic map and model. In
contrast to other states, the State of Nevada has never surveyed its mineral potential. The cursory geologic map-
ping by the federal government is not adequate for exploration purposes. Old depositional and deformational mod-
els, based on insufficient data, have been entrenched into the literature, thus impeding exploration.
An old model championed by the United States Geological Survey is the theory that the Mississippian Antler
Basin siliciclastics were deposited as flysch turbidites into a deep foreland basin between the Antler highlands in
central Nevada and the Utah hingeline in central Utah. However, new field data indicates regressive sequences
containing vascular plant roots (Stigmaria) penetrating bedding planes and lacustrine palynomorph assemblages.
This new data dispels the old model and supports a new depositional environment model. The new model shows
that the richest and most oil-prone Mississippian source rocks are lacustrine oil shales. Lacustrine oil shales make
oil exploration in the Antler Basin very attractive. Cumulative thicknesses of these world-class lacustrine oil
source rocks are measured in thousands of feet in outcrops and wells. They are thick enough and rich enough to
generate trillions of barrels of oil.
Until the early 1980's the typical exploration practice in Nevada was to drill just the Tertiary valley fill in syn-
clines. Therefore, most of the eight hundred wells drilled in Nevada penetrate only syncline Tertiary valley fill.
Few wells have penetrated any Paleozoic section. However, two significant fields were found by drilling "too
deep" and penetrating Devonia n rocks below the Tertiary unconformity. Oil flows from Devonian reservoirs in the
Blackburn and Grant Canyon oil fields. One well in Grant Canyon flowed 4000 barrels a day for ten years. It has
now produced more than 15,000,000 barrels of oil since its discovery in 1983. The Grant Canyon reservoir con-
sists of 200 to 400 feet of karst breccia at the top of the Middle Devonian Simonson Formation. This karst interval
is found in wells and measured sections throughout the eastern Great Basin. In addition to the karst interval, stro-
matoporoid reefs, impact breccia, quartz sandstones and other intervals provide world-class reservoir rocks within
the eastern Great Basin Devonian sequences. An isopach of all the Devonian sequences reveals a structurally com-
pressed basin, the Sunnyside Basin, and can be used to predict the spacial distribution of potential Devonian reser-
voir rocks. The Simonson karst breccia interval alone has the capacity to store billions of barrels of oil in certain
structures. A careful analysis of logs from the few wells that penetrated other significant portions of Paleozoic
rocks shows that, contrary to preconceived notions, many intervals provide similar reservoir rocks.
Another deeply entrenched notion that discouraged exploration investment is that the north-south structural
grain of the eastern Great Basin was caused by Tertiary extension which could have compromised seals on older,
compressional structures. However, new mapping is revealing many uncharted compressional features and a lack
of extensional features. The new maps demonstrate that the region underwent much more compression than previ-
ously thought. Furthermore, some of these features show no evidence of being broken by major Tertiary exten-
sional faults. Several unbroken compressional structures in the Timpahute Range, 50 miles south of the prolific
Grant Canyon field, are exposed.
Another example of an intact compressional feature is the Golden Gate fault fold 40 miles south southeast of
the prolific Grant Canyon field and ten miles north of the Timpahute Range. The Golden Gate fault fold is ten
miles long and five miles wide and has more than five thousand feet of closure. It may have trapped billions of
barrels of oil before it was breached by headward erosion of the Colorado River. New mapping reveals that no
Tertiary extensional faults compromise the structure. Similar structures, along strike that have escaped erosion,
likely contain billions of barrels of oil and trillions of cubic feet of gas. Oil seeping from these giant fields is
probably the source for the commercial oil seep fields in Nevada. However, old opinion and theories based on lit-
tle or poor geologic mapping have obscured the true understanding of Nevada geology for at least five decades.
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