Recent development in surface geochemical prospecting have enabled this age-old seep-detection technology to be used to determine the gas versus oil character of a potential fairway. Extensive field work has demonstrated that the chemical compositions exhibited by near-surface hydrocarbon soil gases are strongly coupled to the chemical compositions known to exist in the nearby underlying reservoirs. By using the compositions and ratios of the light hydrocarbons (methane, ethane, propane, and butane), it is possible to predict whether oil or gas is more likely to be discovered in the prospect area. Histograms which represent average soil gas compositions are observed to be strongly correlative with reservoir gas analysis histograms and with compositions from gas shows reco ded in downhole mud logging. This correspondence with the actual formation gases suggests that the upward migration of reservoired light hydrocarbons into near-surface soils represents a viable mechanism, allowing surface geochemical exploration techniques to be utilized for regional hydrocarbon evaluations.

Geochemical investigations indicate that seep magnitudes depend on tectonic activity to aid migration along the fault and fracture avenues which appear to provide the major migration pathways. This fault association suggest the diffusion process to be of secondary importance. Geochemical prospecting must be used with caution, and only in conjunction with geologic and geophysical tools, because the location and shape of geochemical anomalies are commonly governed more by the local tectonic structure of the region rather than by the actual physical shape of the deposit. Thus geochemical prospecting, when used alone, cannot predict whether or not a particular soil gas anomaly is associated with a commercial deposit. It can only be used to verify the existence of petroleum hydrocarbons an to predict whether a potential structure is likely to contain gas or oil. Geochemical prospecting yields excellent regional evaluations of hydrocarbon potential.

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