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Abstract

Thermodynamic stability diagrams have been constructed for selected iron minerals common to diagenetic settings ranging from near surface to about 6,000 m of depth. These diagrams cover geochemical conditions from 25°C and I bar to 200°C and 600 bars, aqueous sulfur species activities (HS, H2S, or SO42) from 10−2 to 10−14 bicarbonate activities of 10−1 to 10−4, pH values from 5 to 9, Eh values from -0.1 to −1.5 V, and divalent iron activities of 10−4 to 10−6.

The diagrams presented allow predictions of the geochemical conditions favoring generation of anomalous magnetization in hydrocarbon seepage environments. According to these predictions, there are several striking trends in magnetic mineral assemblages and resultant magnetic properties of the rocks: (1) increasing temperature and pressure (i.e., increasing burial) will enhance formation and preservation of magnetic minerals; (2) decreases in magnetic susceptibility will be dominant in surface and shallow subsurface environments, whereas increases in susceptibility will become more common with increasing depth; (3) magnetic susceptibilities in cores taken close to hydrocarbon traps will generally decrease, but at greater distances from traps are more likely to increase.

These predictions may significantly enhance success rates in hydrocarbon exploration because trends in magnetic susceptibilities and/or remanence can indicate the proximity as well as direction of hydrocarbon traps. These trends also suggest that magnetic surveys are more likely to be successful when applied to subsurface cores than when used in surface exploration. In areas targeted for exploration, the utility of predicted trends in magnetic properties is maximized through coupling magnetic surveys with other geological studies, particularly determination of the relative quantities, types and origins of iron- and sulfur-bearing minerals, and at least limited analyses of formation water chemistry.

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