Hydrocarbons and Magnetizations in Sedimentary Rocks

FRUIT, DAVID, R. D. ELMORE, M. ENGEL, S. IMBUS, and M. LEACH, University of Oklahoma, Norman, OK

Hydrocarbons can have variable effects on the magnetic properties of sedimentary rocks. Understanding the nature of these effects has implications for dating hydrocarbon migration and magnetic prospecting. Previous work on hydrocarbon saturated calcite speleothems has established that hydrocarbons can create the chemical conditions that lead to precipitation of magnetite and acquisition of an associated chemical magnetization. The mechanism(s) of magnetite authigenesis, however, is unresolved. Geochemical studies of the speleothems provide some information on the nature of the relationship. For example, there is a positive correlation between the amount of extractable organic material and magnetic intensity, although there is no apparent correlation between percent asphaltenes and mag etic intensity. The level of biodegradation is variable, and samples with high magnetic intensities have, in general, lower apparent biodegradation levels than those with low magnetic intensities. These results suggest that biodegradation is not the only mechanism of magnetite precipitation.

Although hydrocarbons can cause an increase in magnetization due to precipitation of magnetic phases in some rocks, in red beds there is an overall decrease in magnetization due to dissolution of hematite. For example, hydrocarbon migration into the Schoolhouse Member of the Maroon Formation (Pennsylvanian) in northwestern Colorado and the Rush Springs Formation (Permian) in Oklahoma caused dissolution of diagenetic hematite, bleaching, and a reduction in magnetic intensity. Magnetite and pyrrhotite are present in hydrocarbon-bearing sandstone and in some well cemented samples there are stable magnetizations that may be related to hydrocarbon migration.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)