Page 163 - Geochemical Remote Sensing of The Sub-Surface

140 V.T. Jones, M.D. Matthews and D.M. Richers

Distinguishing petrogenic and biogenic hydrocarbons

The results of the studies by Janezic (1979) and Coleman (1979) strongly suggest
that C2-C4 hydrocarbons are not generated biogenically. Most of the previous studies
cited appear to be compromised because they were conducted in natural environments in
which migrated petrogenic gases might have also been present.
Even assuming that small quantities of C2-C4 gases are generated in biological
environments, a methane:ethane ratio greater than 500 appears sufficient to delineate
anaerobic gas production from thermocatalytic gases, since such ratios do not occur in
petrogenic natural-gas deposits. As shown in the test-tube experiments (Table 5-11 and
Fig. 5-5), this value is achieved within two to three days for all substrates studied and
exceeds 100,000 after seven days of incubation. Similar values are cited in the literature
(Frank et al., 1970; Swinnerton and Lamontagne, 1974; Bernard et al., 1976; Sackett,
1977; Reitsema et al., 1978) as the biogenic threshold in marine geochemical
prospecting (Table 5-11I).

HISTORY

The first attempt to relate soil-gas hydrocarbon concentrations to oil and gas deposits
was made in 1929 in Germany by Laubmeyer (1933). Surveying a known oil deposit, he
collected samples of soil gas from systematically-located boreholes 1-2 m deep, after
sealing them from the atmosphere for 24-48 hour periods. Using portable analytical
equipment, he demonstrated that the samples over the deposits were enriched in
methane. Soil-gas investigations were initiated shortly after this time in the then Soviet
Union by Sokolov (1933), who verified Laubmeyer's results (Kartsev et al., 1959), but
measured both methane and heavier hydrocarbons.
Research in the area of surface prospecting was also carried out in the United States
during the 1930s beginning with Teplitz and Rodgers (1935), Rosaire (1938) and
Horvitz (1939). These investigations entailed the collection and analysis of the soils

TABLE 5-III

Literature review of methane/ethane ratios diagnostic of biogenic origin
Reference Diagnostic of biogenic origin
Davis and Squires, 1954 C1/C2 > 500
Reitsema et al., 1978 C~/C2 > 500
Frank et al., 1970 C1/(C2+C3) > 1000
Bernard et al., 1976 CI/(C2+C3) > 1000; 8C 13 -50%
Sackett, 1977 C1/(C2+C3) > 1000; 8C 13 -50%