Pleistocene Volcanic Ash Layers in Kern River Oil Field

M. D. McGuire, R. J. Cullip, C. A. Huggins

At least three volcanic ash layers occur within the producing horizons of the Kern River Formation in Kern River field, Kern County, California. The youngest layer is a partially devitrified vitric ash tuff composed of 37% glass, 61% smectite, 1% quartz, and 1% chlorite. Electron microprobe analysis suggests that it is equivalent to the Friant Pumice member of the Bishop Ash Bed group (0.74 Ma). Stratigraphic relationships suggest that the older ash layers may be correlative to the Glass Mountain-D (0.8-0.9 Ma) and Glass Mountain-G (0.97-1.2 Ma) ash beds.

The ash layers appear as low density/high porosity "spikes" on density-neutron logs due to the presence of glass and expandable clays. Isopach maps of the three ash layers indicate that the ash falls are preserved in isolated patches, probably in interchannel facies of the Kern River braided fan system. Ash falls are preserved only where subsequent channeling has not eroded the interchannel siltstones and mudstones. Several producing horizons (informally named K, K1, and K2) contain a relatively high percentage of these fine-grain interchannel deposits. The other producing horizons tend to exhibit coarse grain channel sands and conglomerates.

If age correlations are correct, the K, K1, and K2 zones are deposits of the Kansas glacial stage. During a glacial stage, channel switching should occur less often due to decreased meltwater runoff and fewer catastrophic floods. This may explain the occurrence of the numerous thick siltstone intervals in the K, K1, and K2 zones.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.