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image of title graphic: hazardous trace elements in petroleum source rocks, monterey formation

The major petroleum source rock of California, the Miocene Monterey Formation is widely regarded as one of the world's classic petroleum source rocks and the origin of most of the oil in California's giant oil fields.

Trace Elements in the Monterey Formation:
Correlation of Selenium and Organic Carbon Content and
Correlation of Uranium and Selenium

The Miocene Monterey Formation is broadly distributed (Figure 2) in the southern San Joaquin Valley, San Francisco Bay area, central Coast Ranges including the Santa Maria Basin as shown below (Figure 4), and Los Angeles Basin. Compared to World Shale Average (WSA) abundance, the Miocene Monterey Formation is enriched in numerous trace elements (Figure 5).

Figure 4. Stratigraphy of the Monterey Formation in the Santa Maria Basin, central coast of California, showing the age and relationship to other Tertiary strata.
(click for larger image)

link to image of diagram showing stratigraphic relationship of the monterey formation and associated units

Figure 5. Trace element analyses of organic-carbon-rich intervals in the Monterey Formation show its relative enrichment compared to the world shale average (WSA).
(click here to view larger table)


Element Average Shale (WSA) Organic-carbon-rich Monterey Formation Relative Enrichment Factor
Silver 0.07 2.1 30
Bromine 4 75 19
Cadmium 0.3 16.5 55
Chromium 90 310 3
Copper 45 103 2
Molybdenum 2.6 38.5 15
Nickel 68 267 4
Antimony 1.5 10.3 7
Selenium 0.6 30.5 51
Uranium 3.7 20.8 6
Vanadium 130 564 4
Zinc 95 335 4
Sources: Turekian & Wedepohl (1961), Piper & Issacs (1995a)

Elements that are highly positively correlated (r2> 0.75) with organic carbon in these rocks include chromium, copper, nickel, antimony, selenium (Figure 6), uranium, vanadium, and zinc; elements significantly correlated (r2= 0.4-0.75) include arsenic, barium, beryllium, cadmium, lutecium, molybdenum, and ytterbium. Selenium (Se) poses a particular environmental hazard, but arsenic, cadmium, copper, molybdenum, nickel, antimony, uranium, and zinc are also of environmental concern. In the Santa Maria-Santa Barbara area of California, Monterey Formation rocks contain as much as 70 parts per million (ppm) Se, and a selenium-enriched (>10 ppm) stratigraphic zone up to 500 ft thick is widespread.

link to image of diagram that shows the relationship of organic carbon content and selenium concentration Figure 6. Selenium is correlated with organic carbon in the Monterey Formation.
(click for larger image)

New study indicates that selenium-enriched stratigraphic zones in the Monterey Formation can be reliably identified from the association of selenium with carbon-rich intervals and uranium (U), a natural gamma-ray emitter (Figure 7; Isaacs, et al., 1999). Establishment of correlations with U-concentration would allow bedrock concentration of selenium and other potentially hazardous trace elements to be estimated with the use of a gamma-ray log, a type of geophysical record widely available from petroleum exploration wells drilled in oil-producing areas of California. This technique may be useful for making regional subsurface maps showing the distribution of potentially hazardous trace-element-enriched bedrock in California and other areas of North America where petroleum source rocks, such as the Cretaceous Pierre Shale (known to contain high selenium and uranium [e.g. Tourtelot, 1956; Landis, 1959]) of the mid-continent and Great Plains region, may represent similar potential environmental hazards.

Figure 7. Similarity of uranium and selenium contents to Gamma-Ray log pattern in the Monterey Formation of the Arco Bixby 1 Well.
(click for larger image)
link to image of bixby 1 well gamma-ray log and uranium and selenium content from sample cuttings

Selenium and other trace elements are thought to be enriched in organic-carbon-rich marine mudstones because of their environment of deposition. Learn more about a depositional model for trace element enrichment.

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Contact: Margaret A. Keller (
Western Region Energy Group–Environmental Studies

Modified: July 31, 2002

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