Page 211 - Geochemical Remote Sensing of The Sub-Surface

188 V.T. Jones, M.D. Matthews and D.M. Richers
TABLE 5-XlV

Concentrations of unsaturated hydrocarbons (10 -4 vol. %) generated during oxidation of gaseous
hydrocarbons by a culture of Myc. Flavum incubated at 30-32~ (Telegina and Cherkinskaya,
1971)

Day Experimental conditions Aerobic (21.2% O2) Anaerobic (1.4% 02)
C2 = C3-- Ca-- C2-- C3= Ca =
0 Myc. Flavum present 0 0 0 0 0
Control no bacteria 0 0 0 0 0 0
8-10 Myc. Flavum present 0.004 0 0 0.227 0.062 0
Control no bacteria 0 0 0 0.158 0 0
30 Myc. Flavum present 0 0.003 0 0.064 0.114 0.003
Control no bacteria 0 0 0 0.500 0 0

dominated by the presence of methane, whereas oil reservoirs usually contain additional
quantities of hydrocarbon gases heavier than methane (Nikonov, 1971).
There are three potential origins for gases detected in the near-surface environment:
biogenic, thermogenic (or katogenic) and igneous (including mantle degassing); and
irrespective of the origin, the gases tend to migrate towards the surface due to pressure
and buoyancy effects. Gases from several sources may mix or undergo other
compositional changes such as chromatographic separation, during this migration. Thus
the measured compositions may not always reflect the original subsurface composition.
In most areas mixing presents little problem because gases of thermogenic origin are by
far the most abundant. Furthermore, the tendency for gases of biogenic and igneous
origin to be extremely dry and of a different isotopic composition from thermogenic
gases enables recognition of their presence. Extreme chromatographic separation may
only be recognised by careful isotopic analysis and through the close comparison of
near-surface gas with known reservoir gas in the region. The presence of gas of igneous
origin generally indicates the occurrence of deep, pervasive faulting, and/or the presence
of igneous activity in the area. This association, as well as the extremely methane-rich
character of such gases, allows for the facile distinction between gases from thermogenic
and igneous sources.
Telegina and Cherkinskaya (1971) found that the olefin content of soil gases
decreased relative to saturated hydrocarbons until depths of about 300 m.
Experimentally, as illustrated in Table 5-XIV, olefins can be formed from saturated
compounds in areas of low oxygen content (0.5-3.2 %). The presence of these olefins
may be biogenic (Smith and Ellis, 1963), although Starobinetz (1976) showed a linear
relationship between the concentrations of saturated and unsaturated gases derived from
the thermogenic alteration of organic matter. Sokolov (1971 b), among others, suggested
a relationship between the generation of unsaturated compounds and drilling activity.