Dennis D. Coleman¹

(1) Isotech Laboratories, Inc, Champaign, IL

ABSTRACT: -70^o/oo Methane, is it Biogenic or Thermogenic?

Carbon isotope analysis has been used for the past 30 years to differentiate biogenic methane from thermogenic methane. But can we really tell the difference based only on isotopic analysis? Biogenic methane formed by fermentation frequently has a carbon isotopic composition similar to that of thermogenic methane. Biogenic methane formed by CO2 reduction on the other hand, is generally depleted in 13C. Methane samples with d13C values between about -55 and -70o/oo are generally dry gases and are usually interpreted as being of mixed biogenic-thermogenic origin. As part of a study to differentiate shallow gas from deep gas in the trona mining area of southwestern Wyoming, a series of samples were collected from the Green River Formation at depths of 1,000 to 1,500 feet. These gases contain all of the normal methane homologs, but at low concentrations. Ethane and propane concentrations are up to 1.1% and 0.6% respectively, higher than can be explained by bacterial activity. The methane d13C values range from -59 to -69o/oo. Traditional thinking would interpret this as mixed gas, but the ethane and propane are also unusually depleted in 13C with d13C values from -39 to -49o/oo and -36 to -44o/oo respectively. The direct correlation between methane, ethane, and propane isotopic values indicates that all three components are predominately of thermogenic origin. This data, along with similar data recently reported from other areas, suggests that some methane with d13C values even in the -70s could be thermogenic in origin, probably representing the very first stages of thermogenic gas generation.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado