| Correlaton
of Selenium, Uranium, and Gamma-Ray Log Signatures
Geophysical logging is very
important for petroleum exploration because these logs allow geologists
to construct reliable regional distribution maps of strata. Log signatures
are correlated with one another to provide a three-dimensional view of
either reservoir rocks or petroleum source rocks. Gamma-ray logs measure
the natural radioactivity of rocks; other logs measure fluid and electrostatic
properties of rocks. Together, suites of logs provide information about
rocks in the subsurface that otherwise would never be known.
The Monterey Formation, for
example, is exposed in the Coast Ranges of California, but also is present
in the subsurface of many basins, covering some 10,000 square miles (~26,000
square kilometers ). It comprises several thousand cubic miles of rock
deposited over a period of about 12 million years. Composition, clay versus
sand or gravel content, varies significantly, both areally and stratigraphically.
Organic carbon varies from <1% to >15% on the scale of a few centimeters
stratigraphically (and Se varies from <1 ppm to >50 ppm), so analyses
of subsurface cuttings or strip samples and geophysical logging tools
are essential for understanding broader-scale distributions (Isaacs et
al., 1986, 1990; Isaacs, 1987, 1992).
Correlations between uranium,
organic carbon, and natural gamma response are used in the petroleum industry
to estimate organic carbon content of petroleum source rocks in the subsurface
(e.g., Bohacs, 1990). Analyses of uranium and selenium in subsurface cuttings
compare very well with natural gamma ray response in the Arco Bixby #1
well (Isaacs, 1999, unpublished data). This indicates that high-uranium
zones, and therefore high selenium zones, may be reliably identified in
the Monterey Formation (Figure 7). Oil
and gas wells have been widely drilled in California, and geophysical
logs from most of these wells are available. From these logs, high-uranium/high-selenium
horizons can be projected into the surface and near-surface groundwater
zones with the aid of regional geologic maps (Figure
8). In addition to large areas in the San Joaquin Valley, the
Coast Ranges south of San Francisco have sporadic high concentrations
of selenium in wells used for drinking water as well as irrigation (Oster
et al., 1988). In this region, a selenium-enriched (>10 ppm) stratigraphic
zone up to 500 ft thick is widespread. Bedrock hazard maps would help
pinpoint areas of high potential hazard for selenium, as well as many
other environmentally sensitive trace elements.
Figure
7. Relationship of uranium and selenium enrichment in the Monterey
Formation in Arco Bixby #1 Well. Note that the gamma-ray log character
mirrors uranium content.
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Figure 8. Gamma-ray
logs or profiles from rock outcrops may provide reliable means for
estimating selenium content.
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In the arid midcontinent and
Great Plains regions (Figure 9), seleniferous
soils also derive from petroleum source rocks–particularly the Cretaceous
Pierre and Niobrara Shales and Permian Phosphoria Formation (Tourtelot
and others, 1960; Lakin, 1961; Vine and Tourtelot, 1970; Herring, 1991;
Piper and Medrano, 1994; Presser and others, 1994). Gamma-ray logs might
also be used effectively in this area to map the hazard from potentially
high selenium concentrations and other trace elements.
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Figure 9.
Annual mean precipitation in the continental U.S. Note that regions
of the arid and semi-arid West are principal areas of concern for
selenium contamination.
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http://geology.wr.usgs.gov/wreg/env/gamma.html
Contact: Margaret A. Keller (mkeller@usgs.gov)
Western Region Energy Group–Environmental Studies
Modified: July 31, 2002
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