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ABSTRACT: Thermodynamic and Kinetic Constraints on the Formation and Destruction of Magnetic Minerals in Subsurface Environments Containing Hydrocarbons

BURTON, E. A., and J. Qi, Northern Illinois University, DeKalb, IL, and H. G. MACHEL, University of Alberta, Edmonton, Alberta, Canada

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. Consideration of extant kinetic data however, modifies the mineral assemblages predicted by thermodynamics.

Thermodynamic 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. However, even though pyrite should be the thermodynamically stable phase over a large range of pH conditions, kinetic inhibition results in formation of metastable amorphous FeS, which converts to pyrite at relatively slow rates at pH < 7, and at insignificant rates at pH > 7.

These results constrain the conditions for formation and destruction of magnetic minerals in subsurface environments and have important applications for predicting the locations of hydrocarbon accumulations from anomalous magnetization patterns found in core as well as at the surface.

 

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)