ABSTRACT: Magnetic Minerals as Indicators of the Presence and Proximity of Hydrocarbons in Subsurface Environments

BURTON, ELIZABETH A., and J. QI, Northern Illinois University, DeKalb, IL, and HANS G. MACHEL, University of Alberta, Edmonton, Alberta, Canada

Currently, anomalous magnetization is not used routinely in hydrocarbon exploration, in part because the causes of anomalous magnetization are not well constrained. However, magnetic exploration may be an effective and relatively low-cost method for detecting the presence and proximity of hydrocarbons. A thermodynamic and kinetic model has been developed to predict the formation and dissolution of magnetic minerals under the temperature and pressure conditions of near-surface to deep burial environments affected by hydrocarbon seepage.

Thermodynamic stability calculations have been used to predict the geochemical conditions for precipitation and dissolution of magnetic minerals at temperatures from 25 to 200 degrees C and under correlative hydrostatic pressures. Stability fields were calculated for hematite, magnetite, pyrrhotite, siderite, pyrite, and aqueous Fe2+ over the ranges for total inorganic carbon, total sulfur, pH, and Eh conditions appropriate to many subsurface environments. Results indicate several general trends. With increasing temperature and pressure, the range of conditions under which siderite is stable relative to pyrite and magnetite increases substantially, pyrrhotite becomes stable at lower pH values, and magnetite is stable relative to hematite at higher Eh conditions. Consideration of extan kinetic data (30 and 65 degrees C), however, modifies the mineral assemblages predicted by thermodynamics. For example, reaction of aqueous Fe2+ and polysulfides results in formation of amorphous FeS that converts to pyrite at relatively slow rates at pH <7, and at insignificant rates at pH >7.

This model can be applied to interpret assemblages of iron-bearing minerals and anomalous magnetization patterns. When coupled with hydrogeological surveys, this information can provide significant constraints on the locations of hydrocarbon accumulations and, thus, can be an important part of hydrocarbon exploration.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)