Anomalous magnetization associated with hydrocarbon seepage: Hydrogeological, geochemical, and geophysical modeling

An integrated quantitative approach is developed to simulate the hydrogeological, geochemical, and geophysical processes that generate anomalous magnetization associated with hydrocarbon seepage. This approach is intended to study how hydrocarbon plumes develop, what chemical reactions occur inside the plume, what minerals dissolve or precipitate before and after hydrocarbon invasion, and what magnitude of magnetic anomaly could be generated by such a plume. The capability of magnetic methods to locate hydrocarbon traps is evaluated based on the results of this study.;Thermodynamic stability diagrams are constructed to qualitatively evaluate the stability fields of selected iron minerals common to diagenetic aqueous species are obtained from SUPCRT92. These diagrams indicate that there is a trend with increasing burial toward enhanced formation and preservation of strongly magnetic minerals.;SOLMINEQ88 is used to quantitatively evaluate the dissolution/precipitation of the magnetic minerals. These simulations indicate that: (1) siderite is the only stable magnetic mineral under the given nonplume conditions; and (2) siderite dissolved and pyrrhotite and pyrite are formed under the given plume conditions. Mass transfer calculations show that 5,000 mg/L of aqueous iron are needed to precipitate 0.09 moles/L of pyrrhotite.;SAKI is used to calculate the magnetic contrasts caused by the hydrocarbon plume. The size, orientation, and magnetic susceptibility of the magnetic bodies are determined based on the hydrogeological and geochemical simulations. The magnitude of the simulated magnetic anomalies is approximately 10 nT.;MIGRATE is used to simulate a hydrocarbon plume and to define plume geometry. The mass transport processes of diffusion and adsorption are modeled. The results demonstrate that the plume has complex shapes and its geometry depends on source size, bedding, diffusion coefficients, retardation coefficients, effective porosity, and density and that the resulting concentration gradients caused by the oil pool can give rise to significant mass redistribution within the reservoir caprocks.;Coupling magnetic surveys with the studies of hydrogeology, petrology, and geochemistry would enhance the success rates in hydrocarbon exploration. Such studies should include hydrogeologic investigations, determination of the relative quantities, types, and origins of iron minerals, and analyses of formation water chemistry.