(Aero-)magnetic anomalies have been reported from several commercial hydrocarbon accumulations. However, the processes responsible for such anomalies are relatively poorly understood. This paper conceptually discusses chemical and microbiological processes involved in generating anomalous magnetization related to hydrocarbon accumulations, including hydrocarbon seepage environments. Based on thermodynamic criteria and microbiologic activity, the formation and destruction of magnetic mineral assemblages can be predicted.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 hydro-carbons may result in 'positive,' 'absent,' or 'negative' magnetic contrasts relative to the total magnetization prior to hydrocarbon invasion, depending upon the amounts of authigenic magnetite and pyrrhotite formed relative to the amounts of hematite destroyed. 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.Magnetic mineral assemblages and the resulting magnetic contrasts, such as those predicted in this paper, have been documented from some hydrocarbon seepage environments. Hence, anomalous magnetization can be used for hydrocarbon exploration in association with other surface exploration methods.