Abstract: Eh-pH Relationships in Near Surface Hydrocarbon Geochemical Anomalies

FAUSNAUGH, JAMES M.

The existence of low redox, or Eh, potentials over oil fields has been well documented. Two models have been proposed which explain the phenomena. The first model is similar to a battery in which current flows between an anode and cathode. Hydrocarbons seeping from the reservoir are naturally "cracked" resulting in, an electron rich, highly reducing, negative pole. The oxidizing areas, adjacent to the seepage anomaly, represent a positively charged, oxygen rich cathode. The second model states that redox lows are related to microbial oxidation of vertically seeping hydrocarbons. Chemical reactions are generally slow at low temperatures, but when catalyzed by microbial interactions, the reactions can proceed at a much faster rate. Geochemical alteration of near surface soils, due to hydrocarbon seepage, involves carbonate, sulfate, nitrogen, and iron reactions. The carbonate is pH dependent, while the sulfate, nitrogen, and iron reactions are Eh dependent.

By superimposing Eh-pH diagrams over Eh and pH measurements of near surface soil samples, many of the procedures and techniques used for surface geochemical exploration can be explained.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky