Absence of magnetic anomalies due to seepage‐induced “magnetoelectric effects” and implications for sulfide self‐potentials

Abstract

The magnetic field due to an azimuthally symmetric distribution of subsurface electric current, which may be associated with vertical hydrocarbon seepage, vanishes above the ground surface. However, geologic conditions in general do not provide perfect isotropic conditions, and, in theory, some magnetic fields can be generated by the magnetoelectric effect. Sedimentary rock sections are relatively isotropic, particularly in the stratigraphic plane. Thus, hydrocarbon seepage‐induced magnetic fields above the ground surface may never be significant, even if strong electric currents are associated with the seepage. All current‐generated magnetic fields, of whatever origin, must obey Ampère’s law; no line integral operation can distinguish a magnetic field having one origin from that of any other origin. Therefore, seepage‐induced magnetic anomalies, if they exist at all, are probably useless for hydrocarbon exploration because it is virtually impossible to distinguish them from other anomalies of unknown origin. Thus, even if hydrocarbon seepage is capable of producing oxidized and reduced zones, the significance of any generated electric currents is very doubtful. Since the seepage‐induced electrochemical model lacks a conducting mass in the columnar reducing zone and/or an electric barrier around the reducing zone, electric potential due to excess electric charges developed between the two zones would tend to dissipate.