The 20-million-bbl Grant
Canyon oil field and its satellite feature, the 1-million-bbl Bacon Flat
field, are located at the eastern edge of Railroad Valley, in Nye County,
Nevada. An 11 mi2 (28 km2)
3-D
seismic survey was
acquired in 1993 to image the complicated structures in the area.
Interpretation ambiguities
were reduced by calibrating the seismic data with known geology from 21
wells drilled prior to the
3-D
survey and four subsequent wells. Still,
interpretation of this complex area was arduous, hampered by extreme velocity
variations in the overlying basin-fill sediments that degraded seismic
data quality and skewed the imaged time structures.
Integrated seismic and well
data show that the reservoirs of the Grant Canyon and Bacon Flat fields
are remnants of detached Devonian carbonate strata that were emplaced over
younger Paleozoic strata. Following emplacement, the reservoir rocks were
faulted, eroded, and buried under more than 3500 ft (1070 m) of basin-fill
sediments as Railroad Valley developed. Beneath the Devonian reservoirs
in the oil fields, a flexure in the younger Paleozoic strata plunges northwesterly
into the Railroad valley basin. Under the flexure, a flank of the Cretaceous
Troy Pluton dips about 30° northwest. The overlying mass of Paleozoic
rocks has slipped down the surface of the pluton, probably contributing
to much of the faulting and subsequent erosional scouring of the field
structures.
To date, the
3-D
seismic
survey has contributed to the discovery of 206 ft (63 m) of additional
oil column in the Bacon Flat field and a
residual
oil accumulation on the
flank of the Grant Canyon field. Other oil traps within the survey area
may be masked by velocity gradients in the basin fill or by the misinterpretation
of seismic data in areas lacking well control.