Anomalous Magnetization by Chemical and Microbial Processes in Hydrocarbon Seepage Environments

MACHEL, HANS G., University of Alberta, Edmonton, Alberta, Canada, and ELIZABETH A. BURTON,* Northern Illinois University, De Kalb, IL

Anomalous magnetization has been reported from several commercial hydrocarbon accumulations. Although the processes responsible for such anomalous magnetization are relatively poorly understood, the formation and destruction of magnetic mineral assemblages can be predicted from thermodynamic criteria and microbiologic activity.

Under the influence of hydrocarbons, magnetite and pyrrhotite are the most important magnetic minerals formed, and the most abundant magnetic mineral destroyed is hematite. Hence, the invasion of hydrocarbons may result in anomalous magnetization relative to the total magnetization prior to hydrocarbon invasion, depending on the amounts of authigenic magnetite and pyrrhotite formed relative to the amounts of hematite destroyed. Anomalous magnetization may be present vertically above the location of hydrocarbon seepage or may be laterally displaced where the migration of the escaping hydrocarbons is influenced by forces in addition to buoyancy. This is likely where local or regional groundwater flow has a significant lateral component, or where hydrocarbon seepage is funnelled by incli ed beds and faults of high permeability.

Magnetism may be generated also by natural and anthropogenic processes that have no relationships to an underlying or adjacent hydrocarbon accumulation. Consequently, anomalous magnetization, even if associated with a hydrocarbon accumulation, may or may not be genetically related to it.

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