Page 164 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 141

themselves for hydrocarbon gases. The use of adsorbed gas on soils was regarded as an
important improvement upon soil gas, as short-term diurnal variations in soil-gas flux
could be avoided by the assumption that soil would have a tendency to establish over
time a metastable equilibrium with the regional flux.

Basic concepts

In the years following these early studies, the basic concepts have remained largely
the same, except that detection limits have been improved with technological advances.
Recent work has focused on compositional ratios or signatures of the light hydrocarbon
gases and their relationship to known hydrocarbon products in the investigated area
(Weismann 1980; Jones and Drozd, 1983).
Emphasis has also been placed on the fundamental principles of surface seepage,
and the interpretation of the data. It is the opinion of the authors that the overall
acceptance of microseep technology in the West has been hindered not only by the
emphasis and success of seismic methods but also because of the lack of a
comprehensive and public surface geochemistry database. There are, by comparison,
more publications on geochemical survey data and basic concepts in the Soviet and
Russian literature. As a consequence, many of our discussions rely on experience gained
in the private sector in the West, supplemented by literature published in the East.
Although the Soviet/Russian literature is clearly positive about surface microseep
technology, the Western literature is strongly divided. Debnam (1969) has reviewed
several cases crediting geochemical prospecting with petroleum discoveries. Overall
success rates are in the range 25-75%. Duchscherer (1980) reports a success rate of 25%,
slightly over the industry average, of which 58% are stratigraphic traps. Sealey (1974a,
1974b) reported a success rate of 80% in Texas using a microbiological technique.

Methods of geochemical prospecting

Geochemical methods of prospecting are classified as direct or indirect. The direct
methods involve detecting the presence of dispersed oil components in the form of
hydrocarbon gases or bitumens in the soils, waters or rocks in the vicinity of oil and gas
accumulations. The indirect methods involve detecting any chemical, physical, or
microbiological changes in the soils, waters, rocks or vegetation spatially associated with
the oil and gas deposits. Figure 5-6 is a schematic diagram outlining most of the direct
and indirect methods currently in use (Kartsev et al., 1959).
Identifying secondary responses generated by leakage of hydrocarbons at the surface
has merit and has been reported by many investigators. These include the use of (1) soil
microbes (Soli, 1954, 1957; Kartsev et al., 1959; Sealey, 1974a; Sealey, 1974b); (2)
reduction effects (Pirson et al., 1969; Donovan, 1974; Ferguson, 1975); (3) carbon and