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In diagenetic environments containing hydrocarbons, the most important magnetic minerals formed appear to be magnetite and pyrrhotite, whereas the most important magnetic mineral dissolved or replaced is hematite. Hence, hydrocarbon seepage and migration may result in positive, absent, or negative anomalous magnetization (magnetic contrasts) relative to the total magnetization prior to hydrocarbon invasion, depending on the amounts of magnetite and pyrrhotite formed relative to the amounts of hematite destroyed.
Where anomalous magnetization is formed by hydrocarbon seepage, the magnetic contrasts are located in relatively reducing geochemical plumes. Such plumes are located above leaking subsurface hydrocarbon accumulations if buoyancy is the predominant driving force for hydrocarbons. Commonly, however, migration of the escaping hydrocarbons is dominated by advection. Hydrocarbon seepage, therefore, may be vertical or may have any direction other than vertical, depending on the position of the leaking hydrocarbon accumulation in the prevailing flow system and on the distribution of high-permeability pathways such as carrier beds and faults. In these cases, magnetic contrasts are deflected or displaced, and they are not likely to be vertically above the hydrocarbon source.
Anomalous magnetization (magnetic contrasts) has been documented from some hydrocarbon seepage environments and can be used for hydrocarbon exploration in association with other surface exploration methods. Similar magnetic contrasts should also be detectable in core, and could be used for evaluation of subsurface proximity to hydrocarbon traps. The magnitude and spatial distribution of such magnetic contrasts, whether at the surface or in the subsurface, depend on geochemical, microbial, geophysical, sedimentologic, and hydrogeologic processes. These factors must be taken into consideration in aeromagnetic and ground-based prospecting for hydrocarbons. In particular, the use of anomalous magnetization as an exploration tool should always be accompanied by a hydrogeologic study.
The results of this study suggest that magnetic contrasts could also be used in environmental studies, for example, to detect hydrocarbons leaking from storage tanks or waste disposal sites. On the other hand, magnetic contrasts may be generated also by natural and anthropogenic processes that have no genetic relationships to hydrocarbons. Consequently, anomalous magnetization may or may not be genetically related to natural or anthropogenic hydrocarbon accumulations.
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