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ISAACS, CAROLINE M., MARGARET A. KELLER*, and KENNETH J.
BIRD
U. S. Geological Survey, Menlo Park, CA
Abstract: Potentially Hazardous Trace Elements in the Monterey Formation, California
Marine organic-carbon-rich rocks, intensively studied as sources of petroleum, are commonly enriched in trace elements that are potentially hazardous to aquatic life, human health, and the environment. Compared to World Shale Average (WSA) abundance, the Miocene Monterey Formation, the most widespread petroleum source rock in California, is enriched in numerous trace elements.
Our prior studies have shown that elements highly positively correlated (r2 > 0.75) with organic carbon in these rocks include chromium, copper, nickel, antimony, selenium uranium, vanadium, and zinc; elements significantly correlated (r2 = 0.4-0.75) include arsenic, barium, cadmium, and molybdenum. Of these 12 elements, selenium poses a particular environmental hazard, but arsenic, cadmium, copper, molybdenum, nickel, antimony, uranium, and zinc are also of environmental concern. New chemical analyses of borehole cuttings from 5 wells in the Santa Maria-Santa Barbara areas show similar patterns of trace-element enrichment and indicate that selenium-enriched stratigraphic zones in the Monterey Formation can be reliably identified from the association of selenium with uranium, a natural gamma-ray emitter. Calibration of selenium with uranium content and gamma-ray response allows the bedrock concentration of selenium and other potentially hazardous trace elements to be estimated with the use of gamma-ray logs.
This method is proposed for making regional subsurface maps showing the distribution of potentially hazardous traceelement-enriched bedrock in California and other areas of North America where petroleum source rocks, such as the Cretaceous Pierre Shale of the midcontinent and Great Plains region, may represent similar potential environmental hazards.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California