Recent advances in surface geochemical prospecting have enabled 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 of near-surface hydrocarbon soil gases, measured by flame ionization gas chromatography, are largely determined by the hydrocarbons in nearby underlying reservoirs. By using compositions and ratios of the light hydrocarbons, methane, ethane, propane, and butane, one may predict whether oil or gas is more likely to be discovered in the prospect area. Near-surface hydrocarbons are best represented by normalized histograms of composition data. These histograms are strongly correlative with those of reservoir gas and with compositions o gas from shows recorded 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 to be utilized for regional hydrocarbon evaluations.

Geochemical profiles over known production areas are shown for the Sacramento and San Joaquin basins in California and for the Utah-Wyoming Overthrust belt. Geochemical predictions were documented by subsequent drilling near the Pineview field in Utah. The data imply that the Pineview field should extend westward into an area containing a dry hole. In addition, a new gas discovery--on the intersection of a Landsat lineament and a large methane anomaly--was made 6.5 km (4 mi) northeast of the Pineview field by Amoco in 1981.

Most of the geochemical examples reported show direct anomalies over known fields. However, seeps can be laterally displaced in certain geologic settings. In addition, geochemical investigations indicate that seep magnitudes depend on tectonic activity to aid gas migration along faults and fractures, which appear to provide the major migration pathways. This fault association suggests that diffusion is 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 many geochemical anomalies are governed more by the local tectonic structure of the region than by the position and shape of the actual deposit. Regional groundwater flow is less significant. Thus, geochemical prospecting, when used alone, cannot predict whether a particular soil-gas anomaly is associated with a commercial deposit. It can only be used to verify the presence of petroleum hydrocarbons and to predict whether gas or oil is likely to occur in a potential structure. Geochemical prospecting yields excellent regional evaluations of hydrocarbon potential.