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Natural gas samples collected throughout the western half of the Illinois basin were analyzed chemically and isotopically to determine the origin and history of the gases. The samples were obtained from a variety of sources including groundwaters (fresh water and brines), cores, oil wells, and gas wells. Samples from shallow (50 to 500 ft; 15 to 150 m) groundwaters contain methane of bacterial origin; the ^dgrC13 values for methane from these waters are generally in the range of -65 to -90 parts per thousand. Samples from relatively deep (1,500 to 3,000 ft; 450 to 900 m) gas and oil accumulations contain thermogenic methane having ^dgrC13 values in the range of -46 to -56 parts per thousand. Gas accumulations that have isotopic compositions intermedi te between those of the deep thermogenic gases and the shallow bacterial gases may be the result of mixing of gases from these two sources. There is also a general trend of increasing ^dgrC13 values southward, however, presumably reflecting increasing maturity of the source rocks in that direction as indicated by increasing coal rank. Therefore, some of the gases with ^dgrC13 values in the range of -56 to -65 parts per thousand may be "low grade" thermogenic gases. Additional C13/C12 analyses and some D/H analyses are in progress to help resolve this question.
Most of the gas wells are on the margins of the basin. Gases from these wells are isotopically similar to solution gases from oil wells in the center of the basin, but often are depleted in the heavier hydrocarbons, some consisting almost entirely of methane. This chemical difference is believed to be at least partly a result of fractionation during migration of the gas toward the basin margins. Other available data are being assimilated to test this hypothesis.
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