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Abstract: Hydrocarbon Microseepage and Magnetic Susceptibility of Soils: Field Tests and Magnetic Mineralogy

David Fruit, R. K. Downing, R. D. Elmore, Bruce Moskowitz

Field and rock magnetic studies were undertaken to test the hypothesis that hydrocarbon microseepage can cause anomalous increases in soil magnetic susceptibility and to investigate the nature of the magnetic phases involved. A soil magnetic survey in southern Michigan in the vicinity of the Albion-Scipio and Stoney Point fields indicates a persistent elongate ring anomaly extending for several miles. This anomaly was drilled in 1993 without encountering adequate reservoir rock although gas shows were noted. The soils over a gas storage field in Michigan contain a marked positive anomaly which closely follows the edge of the storage field in many areas. Gas storage or other pressure maintained reservoirs may provide better analogies for undiscovered production than older artially depleted fields which may show a muted seepage profile. Rock magnetic studies indicate that magnetite and possibly pyhrrotite are the magnetic carriers for some soils with susceptibility anomalies. Hysteresis loops indicate that most soil samples contain very fine grained superparamagnetic phases. The results of these studies as well as other previously completed investigations indicate that microseeping hydrocarbons can cause soil susceptibility anomalies that reside in magnetite. The mechanism of magnetite formation is still open to question and could involve inorganic processes and/or bacterial activity.

AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada