Abstract: Seismic Exploration Leading to Discovery of Trap Spring Oil Field, Nevada

Bert H. Berrong, John H. Vreeland

Seismic exploration for Tertiary objectives in the Basin-and-Range environments requires recognition and appropriate field techniques for the geologic-geophysical problems. Near-surface geologic and topographic problems as well as three-dimensional aspects of trap resolution are involved. Normal seismic exploration using 6- or 12-fold CDP does not have the resolving power needed to delineate traps in these complex valleys. Steep dips, complex faulting patterns, velocity anomalies, and basalt flows in the Tertiary valley fill necessitate the use of 24-fold CDP. Wide-line, side-by-side geophone arrays are sometimes required to minimize spread length without diminishing subsurface redundancy or to attenuate side energy. Two-dimensional "x" geophone arrays were used consisten ly to combat side-energy problems. A dynamite crew is required for seismic bandwidth and because large vehicles would be immobilized in much of the area. Once meaningful seismic data of high quality have been obtained in the field, similar acumen is required for the processing. The data processor must contend successfully with steep dips, complex faulting, and enormous velocity gradients in two dimensions.

Interpretational problems, such as thinning of the volcanic package, fault placement, fault placement, fault orientation, rock-package determination, and shallow basalt flows, are involved. Both Trap Spring field and Eagle Springs field are related in that both exhibit thinning of the ignimbrite package near the basin margin with some fault closure. Proper interpretation requires in-depth geologic knowledge applied during data processing. Velocities determined from seismic data, for example, are almost useless to determine rock types because of extreme variability and an overlapping of rock velocities. Faulting in Railroad Valley is extremely complex, with many small faults, some of which are related to flexures in the volcanic rocks. Seismic determination of sealing versus nonsealing faults is impossible. Most of the faults mapped can be related to surface lineations, thus strongly tying seismic investigations to geomorphology.

AAPG Search and Discovery Article #90964©1978 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah