Leaking hydrocarbon, in association with a microbial component, can initiate diagenetic Eh/pH controlled reactions in the rocks and soils overlying hydrocarbon accumulations. These chemical reactions cause the mobilization, or precipitation of minerals in the stratigraphic column above the leaking reservoir. Eh (oxidation-reduction potential or ORP) is measured in millivolts with decreasing, or negative values, indicating the potential presence of hydrocarbons and positive values indicating the absence of hydrocarbons. The signature varies with the degree of processing the soil sample is subjected to. Bulk samples with no preparation sometimes yield an apical anomaly. Apical signatures are located over the seepage. Sieved samples produce a halo signature where the Eh lows indicate the edge of the reservoir at depth.
pH is the measure of soil acidity or alkalinity and is inversely related to the ORP measurements. pH anomalies are present due to the precipitation of carbonate related to the oxidation of near surface hydrocarbons. Very low values usually indicate the absence of hydrocarbons. High values towards the edge indicate the halo produced by seeping hydrocarbons while moderate background values populate the apical position.
Conductivity is measured in micro-mhos, or the inverse of ohms, which is the measure of resistivity. Conductivity values are directly related to the amount of inorganic / organic salts present in near surface soils. These salts can be due to the enrichment of sulfur, nitrates, phosphates, halides, and other trace elements associated with the alteration phenomena of hydrocarbon microseepage.