“Geochemical Fingerprint Plots” represent geochemical mixtures that are diagnostic of targeted oil or gas accumulations. They express how well soil gas hydrocarbon components correlate together in separate factors. These plots provide a powerful tool for discriminating between coal gas-, natural gas- and oil-dominated reservoirs.

The fingerprint plots above represent the combination of strongly correlated soil gas hydrocarbon components for both coal bed methane and oil plays in the Powder River Basin of Wyoming and Montana. Although these plays are 50 miles apart, they are underlain by identical stratigraphic successions. The fingerprint plot for the coal bed methane play shows a correlation between methane, ethane, and propane, but the absolute methane concentration dwarfs that of ethane and propane. The fingerprint plot for the Tensleep Oil Field also reveals a strong correlation between the C1-C3 hydrocarbons but in contrast, normal butane and pentane are also dominant in this factor. This example demonstrates how we can differentiate between gas and oil-dominated reservoirs and how we extract a signature diagnostic of oil or gas from a complex geochemical mixture.

The strong association of methane and ethane in adsorbed soil gas reflects the character of natural gas in the 6,600 to 7,700-foot deep Trenton-Black River dolomitized breccia reservoir at Glodes Corner, which is situated in a gas-dominated region.

The correlation of heavy hydrocarbons in adsorbed soil gas reflects the character of heavy oil in a 5,000 to 6,000-foot deep detached dolomite reservoir at Grant Canyon. Natural gas is essentially absent from this reservoir.

Discriminant Function Plots are used to represent the degree of separation between oil, gas and dry reservoirs in multivariate space. These plots are derived from discriminant analysis, which is used to determine those variables that discriminate between two or more naturally occurring groups. In this case the three groups are “Dry, Gas, and Oil” and the variables are thermally desorbed C1-C12 and CO2.

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