Free and Adsorbed Gas Relationships within Macroseeps

A variety of methods have been proposed over the past 30 years by the petroleum industry for surface geochemical prospecting. The most direct methods focus on the analyses of dispersed oil and gas components in the form of hydrocarbon gases or bitumen. Measurement of the light C1-C4 gases often involves measuring both the free and adsorbed gases. Macroseeps by definition consist of free phase hydrocarbons, including the light gases. Free soil gas data, if properly collected and analyzed, exhibits remarkable repeatability and stability, and has been shown to have the ability to predict the oil versus gas potential of an unknown basin through the compositions of the light gases that have migrated to the surface (Jones and Drozd, 1983).

An extensively accepted misconception is that all the gases contained in recent sediments must have migrated from depth, and that all of the gas from the free, loosely bound and tightly bound states should be added together to obtain the total gas from depth. Well-documented case studies where both the free and adsorbed gases associated with a macroseep were measured will demonstrate the fallacy of this concept. Three specific case studies will show that there are no adsorbed gases associated with the free gases that define these macroseeps. Interpretation of the adsorbed gas data indicates that the adsorbed gases measured in these three cases are dominated by non-migrated (syngenetic) gases rather than migrated (epigenetic) gases. It is obvious that in these three cases that the adsorbed gases have not migrated from depth and are not useful for surface geochemical exploration.

The following adsorbed gas methods will be discussed: Horvitz acid-extraction adsorbed gas method, disaggregation (blender and ball-mill methods), and contrasted with the interstitial free gases.

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