Abstract

Chemical compositions of near-surface hydrocarbon soil gases are largely determined by the hydrocarbons in nearby underlying reservoirs. Compositions and ratios of the light hydrocarbons, methane, ethane, propane, and butane, predict whether oil or gas is more likely to occur in the prospect area. Normalized histograms of near-surface hydrocarbons are strongly correlative with those of reservoir gas and with gas from shows recorded in downhole mud logging. This suggests that the migration of reservoired light hydrocarbons into near-surface soils allows 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 many geochemical anomalies are governed more by the local tectonic structure than by the position and shape of the actual deposit. Thus, geochemical prospecting alone cannot predict whether a soil-gas anomaly is associated with a commercial deposit. It can only verify the presence of petroleum hydrocarbons and predict whether gas or oil is likely to occur. Geochemical prospecting yields excellent regional evaluations of hydrocarbon potential.--Modified journal abstract.

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