A near-surface hydrocarbon soil-gas technique developed by Gulf Research & Development Co. has been shown to be capable of predicting whether oil or gas is more likely to be discovered in the prospect area. These predictions are made by using the percent compositions and ratios of methane to ethane and propane. Typical average values are:

Table

Intermediate values are expected for many hydrocarbon accumulations.

Extensive studies compiled and reported in the literature have clearly shown that reservoired hydrocarbons contain varying amounts of methane and heavier homologs. Frequency histograms of the sum or ratio of methane homologs illustrate that gas from gas deposits is quite distinguishable from gas in oil deposits. Gases from gas-condensate or combined oil and gad provinces plot intermediate between those of gas or oil only provinces as expected.

Light-hydrocarbon ratios have been used successfully to predict the petroleum potential of a formation by monitoring C₁ to C₅ hydrocarbons from a steam-still reflux gas sampling system during routine mud-logging operations. Individual ratios of the C₂ to C₅ light hydrocarbons with respect to methane have been demonstrated to provide discrete distributions which reflect the true natural variations of formation hydrocarbons between oil and gas deposits. Analyses of these same ratios for soil-gas hydrocarbons yield nearly the same limits for delineation of oil and gas potential. This correspondence with the actual formation gases shows that the upward migration of reservoired light hydrocarbons into near-surface soils represents a viable mechan sm, allowing near-surface geochemical exploration techniques to be utilized for prospect evaluation.

Normalized histograms of composition data have been constructed which better represent the actual near-surface hydrocarbon populations.

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