Soil Hydrocarbon Alteration: Solid Phase Versus Gas Phase Hydrocarbon Detection

James M. Fausnaugh
Geochemical Surveys, Geotech.org, Littleton, CO

The main assumption that drives surface exploration techniques is that hydrocarbon gases from the reservoir seep to the surface where they are detected and measured by various methods. This investigation puts forth the alternative explanation that the hydrocarbon anomalies being detected at the surface come from soil organics altered by seeping reservoir hydrocarbons rather than by the direct measurement of the reservoir hydrocarbons.

Surface soil hydrocarbons are ubiquitous and occur in an infinite number of configurations. Because of the diversity of compounds present in near surface soils, Humic and Fulvic acids are the primary compounds of interest. Altered carbohydrates and proteins also contribute to the surface geochemical signature. The soluble Fulvic acids appear to be the source of most measured hydrocarbon anomalies while the non-water soluble Humic acids contribute when degraded chemically or physically. Condensation and addition reactions between Fulvic acids and the available free alkanes are the pathways to altered hydrocarbon signatures found over oil and gas reservoirs. The physical and chemical properties of these two types of soil organic matter account for many of the surface geochemical phenomena observed.

The soluble altered Fulvic acids are measured in aqueous phase by UV-Vis Spectroscopy. UV-Vis absorbance spectra of soil hydrocarbons correlate well with thermally desorbed gas phase soil hydrocarbons. GC/MS was used to identify potential sources of measureable solid, aqueous, and gas phase soil hydrocarbons.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas