The shallow Miocene stratigraphic gas field discovered near Foley, Alabama, in late 1979, afforded an excellent opportunity to test whether gas from an accumulation seeps upward through overlying "impermeable" beds in adequate concentrations for detection in the near surface. In mid-1980, when this survey was done, several confirmation wells had been drilled, but the field was not yet producing.

Samples were collected from the bottom of 10-ft holes drilled on a rough 0.5 mi grid and were quickly sealed in gas-tight containers for later analysis. At several sites, sampling was done from grass roots down to 15-20 ft. At sites where the deeper samples had anomalous concentrations of gas, there was virtually no gas from the surface down to 6-8 ft. Below this, where anomalous gas concentrations were encountered, they tended to increase gradually downward. This confirmed previous conclusions that sampling for near-surface surveys should generally be done at 10 ft.

Gas contents of survey samples ranged from 3 to 87 parts per million by volume, and it was virtually all methane (i.e., identical with the gas at Foley). Probability plots revealed a background population with a mean of 10 parts per million, with values above 20 ppm being anomalous. The mapped and contoured anomaly has a striking correspondence to the outline of the field determined by later drilling. These results show that near-surface hydrocarbon surveys can sometimes detect microseepage from petroleum reservoirs and that such surveys can be valuable in exploration.

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