Hans G. Machel
Oil and gas accumulations that leak may be detectable via magnetic surveys because the escaping hydrocarbons commonly induce the formation of magnetic contrasts, inorganically or microbially. In diagenetic settings at depths of zero to about 5 km, magnetite and pyrrhotite are the most important magnetic minerals formed, whereas hematite is the most abundant magnetic mineral destroyed. Pyrite and siderite are also important in hydrocarbon seepage environments because they may form at the expense of magnetic minerals. In some cases, other magnetic minerals may also be important, including metastable minerals such as greigite.
Seepage of hydrocarbons may result in 'positive', 'absent', or 'negative' magnetic contrasts relative to the total magnetization [or susceptibility] 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 hydrocarbon pools. However, magnetic contrasts may be generated also by natural and anthropogenic processes that have no relationships to an underlying or adjacent hydrocarbon accumulation.
Magnetic mineral assemblages and contrasts that were caused by oil or gas seepage have been documented from several leaking hydrocarbon traps. Hence, magnetic surveys, from the air, on soil or on core samples, can be used for hydrocarbon exploration in association with other surface exploration methods.
AAPG Search and Discovery Article #90959©1995 AAPG Rocky Mountain Section Meeting, Reno, Nevada