Eh, pH, and Conductivity

The study of oxidation-reduction processes in soils began in the early 1900’s and has since been applied to biological, limnological, and geochemical systems.  Early petroleum related studies concerned the redox characteristics of sediments, the preservation of organic material, and the ultimate generation of petroleum.  Ground water investigations showed that water with hydrocarbon gases are distinguished by low redox potentials.  Other subsurface studies proposed the use of a redox logging tool.  Early near surface oil exploration studies started with the Hilbig Oil Field and have led to the development of electrode arrays for the in-situ analysis of near surface redox potentials.

Two models have been proposed which would explain the presence of redox phenomena in near surface soils over oil fields. The first model promotes the mineralogical oxidation of the vertically seeping hydrocarbons are the primary cause of redox lows detected in the near surface. Early theories suggest that natural zeolites in the reservoir seal initiate the cracking of large aliphatic hydrocarbons in smaller molecules which results in a net negative charge within the area of hydrocarbon microseepage. The adjacent oxidizing areas establish a path of electrical flow from the electron generating oil reservoir.  The current theory relies on hydrocarbon, rock, and water interactions that produce organic acids and acid anions (OAA’s). For instance, iron oxides react with hydrocarbons to produce pyrite, oxygenated organic compounds, and CO₂.

The second model states that the redox anomalies associated with oil fields are related to microbial oxidation of the vertically seeping hydrocarbons, and not to the hydrocarbons themselves. Low molecular weight hydrocarbons, which are most often associated with near surface direct hydrocarbon techniques, have a very low reactivity caused by their saturated bonding. These bonds can only be broken under strenuously reactive conditions.  By oxidizing the hydrocarbons, the microbes mediate changes in available reactive chemical species that can take up liberated electrons, thereby changing the mineralogy above hydrocarbon deposits.

GRDC provides the Eh and pH methods as a suite free of charge with the hydrocarbon analysis or as a quick reconnaissance exploration tool using near surface soil samples.