HENRY, W. E., and W. D. TOMLINSON, Phillips Petroleum Company, Bartlesville, OK
General agreement between magnetic susceptibility and light hydrocarbon adsorbed gas content from exploration surveys run on three different continents under diverse climatological conditions supports the hypothesis that long-term microseepage of hydrocarbons may generate magnetic anomalies in near-surface soils and rock overlying buried hydrocarbon accumulations. The magnetic susceptibility of shallow auger and seismic shot-hole samples correlates with the general location and trend of light hydrocarbon adsorbed gas anomalies in the areas studied, but the correlation is not perfect. The locations of specific lows and highs are commonly offset somewhat, and the relative magnitudes of light hydrocarbon and susceptibility anomalies differ locally. Better correlation between light hydroc rbon soil gas content, which is a measure of recently active hydrocarbon microseepage, and magnetic susceptibility data from hand-held auger samples and surface loop measurements, suggests that magnetic anomalies associated with seeping hydrocarabons may be relatively short-term geological phenomena that form quickly in response to seeping hydrocarbons. If hydrocarbon seepage terminates or if the locus of maximum seepage shifts to a new location, the magnetic anomalies may be quickly destroyed by near-surface oxidation. This could explain some of the differences in detail between susceptibility and light hydrocarbon adsorbed gas surveys. Thus near-surface magnetic susceptibility surveys could be used as rapid, inexpensive alternatives to soil gas surveys for identifying areas of active r cent hydrocarbon seepage. Applications would include hydrocarbon exploration and environmental monitoring. Additional work is needed to determine the rate at which hydrocarbon related magnetic anomalies are generated and destroyed under varied near-surface conditions.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)