ABSTRACT

Hydrocarbon microseepage is known to occur from sub- surface accumulations of hydrocarbons. This hydrocarbon microseepage eventually reaches the surface where the introduction of hydrocarbons into the soil substrate leads to significant changes in the soil chemistry over and adjacent to hydrocarbon reservoirs. These changes in the soil chemistry are the basis of a host of different tools that are used to measure these soil changes in the search for hydrocarbon reservoirs. One of the most effective tools is the direct measurement of the hydrocarbons present in the near surface environment. The majority of microseepage from reservoirs can be seen to be occurring in a directly vertical direction. This microseepage produces a "geochemical chimney" over commercial reservoirs. The "geochemical chimney" is a direct manifestation of the hydrocarbons seeping upwards into the soils overlying commercial reservoirs. These seeping hydrocarbons induce major chemical changes in the chimney as a direct result of hydrocarbon consumption by bacteria. These chemical changes can be seen in differences in the Eh, pH and conductivity of the soils overlying commercial reservoirs. Measurements made of the soil hydrocarbons, Eh, pH and conductivity of individual soil samples over an area show irrefutable proof of vertical migration. Examples of soil-gas surveys over Trempealeau and Rose Run seismic prospects in Ohio will be used to substantiate this. Soil-gas samples taken in a grid pattern over known seismic remnants are seen to differentiate regional background values over dry holes from anomalously high hydrocarbon values over productive remnants. Numerous soil-gas surveys conducted by the author over Trempealeau and Rose Run seismic prospects in Ohio have led to an overall successful prediction rate of 80%. Dry holes have been correctly predicted at an 85% rate.