Page 160 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 137

TABLE 5-I

Literature review ofbiogenic light hydrocarbon production (C1-C4)
Reference Study
Coleman, 1979 Methane in glacial till gases: ~i3c1; CI/C 2 > 1000
Davis and Squires, 1954 Cellulose, ethanol and sewage fermentation: C2-C-4 < 2 x I0-5%;
Cl/C2 > 105
Voytov et al., 1975 Swamp gases, glacial till gases, subsoil bacterial gases: C1 > 99%;
C2-C4 < 10-4'~'; C1/C2 > 106
Kim and Douglas, 1972 Cellulose fermentation: C~ > 99%; C~/C2 > 500; C2-C4 <10-2%
Stahl, 1974 Bacterial gases: C~ >99%; 813CI <-60%
Bukova, 1959 Anaerobic bacterial decomposition, soil gases, sewage, silt:
C~ > 99%; Ci/C2 >1000

Origin of light hydrocarbon gases in the near-surface

The near-surface occurrence of ethane through butanes is of fundamental importance
to the purpose of this chapter and to the usefulness of these gases as prospective
indicators of buried natural gas and petroleum deposits. An extensive review of the
literature suggests that C2-C 4 hydrocarbons can be generated biogenically; however,
solid proof exists only for methane and ethylene as major products of bacteria (McKenna
and Kallio, 1965). A review of the literature shown in Table 5-I provides conflicting
evidence for the biogenic occurrence of the C2-C 4 hydrocarbons, although most of the
literature suggests an abiogenic, thermocatalytic origin for these gases. Compositionally,
however, these gases display large variations and do not resemble compositions
characteristic of petroleum gases. All these studies are further characterised by
methane:ethane ratios in excess of 1000 and a percent methane composition >99%, and
are quite uncharacteristic of petrogenic gases. Some of the results reported before the
invention of the gas chromatograph must be regarded with suspicion due to limitations
of the analytical methods employed and possible sampling collection at locations
contaminated by mixed biogenic and petrogenic gases. Russian researchers have
illustrated that some of the earlier analytical methods, such as the combustion technique
of Kartsev et al. (1959), can measure gases that are mistaken for hydrocarbons.

Laboratory and field evidence of biogenic C2-C4 hydrocarbons

Studies were conducted at Gulf Research & Development Company by Janezic
(1979) to investigate the anaerobic microbial evolution of C~-C 4 hydrocarbons upon
decomposition of various organic substrates including green plant branches, grass