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Abstract

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.

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