Published 1996 as part of Memoir 66
Copyright © 1996 The American Association of Petroleum Geologists.   All Rights Reserved.
 

Seepage of hydrocarbons from traps and migration from source rocks result in hydrocarbon-contaminated plumes in ground and formation waters. Such plumes are characterized mainly by a marked reduction in the redox potential, causing the generation of magnetic ferrous iron oxides and sulfides and the destruction of ferric iron oxides. The resulting magnetic mineral assemblages can be predicted on the basis of thermodynamic criteria and microbiological processes. Moreover, these assemblages may result in positive, absent, or negative magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion. Thermodynamic modeling further suggests that magnetic contrasts are more likely and tend to become more positive with depth and with closer proximity to the hydrocarbon source(s).

Magnetic mineral assemblages and the resulting magnetic contrasts, such as those predicted here, have been documented from several hydrocarbon seepage environments at or near the land surface. Such magnetic contrasts can be used for hydrocarbon exploration in association with other surface exploration methods. Magnetic exploration for hydrocarbons can also be conducted successfully on drill cores. Furthermore, migration pathways can be delineated by magnetic methods because migration from source to reservoir rocks may generate magnetic mineral assemblages similar to those in seepage from traps.