SEISMIC IMAGING OF METHANE IN SMALL GLACIAL DRIFT RESERVOIRS, PIATT COUNTY, ILLINOIS

Drift gas, primarily methane and hydrogen gas, is produced by bacteriological activity related to buried organic matter in glacial drift. Some drift gas deposits in Illinois are large enough to supply heating or cooking fuel for individual homes for several years. More often, gas accumulations have detrimental effects such as poor water quality and degraded pumps and plumbing systems. Because of the high density contrast between water-filled and gas-filled pores, gas accumulations result in large amplitude reflections of compressional seismic waves. Nearly all of the incident seismic energy is reflected back from, and absorbed by, the gas-bearing sand layer. Very little energy passes through the gas, such that deeper horizons produce very small amplitude reflections. Gas zones are identifiable by the “bright spot” reflectors overlying “blank” zones in the profile. On the other hand, seismic shear waves propagate from grain-to-grain contacts and are insensitive to changes in pore fluids. Hence, gas accumulations are transparent to shear waves.

We have collected co-located p-wave and s-wave reflection profiles in an area of central Illinois plagued by “gassy” water wells that have been developed in shallow glacial sand and gravel deposits. Shallow gas accumulations were detected based on large amplitude reflections overlying zones of poor reflectivity in the Quaternary sections of the profiles. Shear wave profiles indicate that these reflections correspond to shallow highly porous channel or depression fills at the interface representing the Sangamon intraglacial period. Test drilling at one of these sites revealed organic material at the zone associated with the gas. This buried organic material is the likely source for biogenic production of the methane.