FRUIT, DAVID J., JOHN DELAUGHTER, and R. DOUGLAS ELMORE, School of Geology and Geophysics, University of Oklahoma, Norman, OK; and M. STUPAVASKY, Sapphire Instruments, Ruthven, Ontario

ABSTRACT: Effects of Hydrocarbon Microseepage on Magnetic Susceptibility of Soils

The hypothesis that hydrocarbon microseepage can cause anomalous increases insoil magnetic susceptibility was tested by sampling transects across oil and gas accumulations. Numerous sites with multiple samples per site were used to distinguish possible anomalies from background noise.

Transects over several fields indicate anomalies with statistically significant increases over off-field background levels. A thirteen mile transect in southern Oklahoma shows magnetic anomalies linked with underlying color changes in redbeds above hydrocarbon traps. Sampling of vertical soil profiles indicates that the uppermost horizons contain the anomalies. Mineral identification by Curie balance indicates magnetite and possibly pyhrrotite as the magnetic carriers. Sampling at a new field discovery in northeastern Oklahoma suggests a ring anomaly and accurately reflects the fields limits eventually delineated by drilling.

The results suggest that microseeping hydrocarbons can cause anomalies in soil magnetic susceptibility. The process is complex and identification of anomalies is controlled by a number of variables. Soil type and variations in parent material can mask the effect. The anomalies may be positive or negative and the dynamic nature of the anomalies is not well understood. Collection and analysis techniques (e.g., use of both mass and volume units) are critical to obtaining meaningful data. The formation of the magnetite is also open to question although it could involve inorganic processes and/or bacterial activity. Despite the complexities, measuring soil magnetic susceptibility is quick and inexpensive and may be an effective direct hydrocarbon indicator under certain conditions.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.