Page 246 - Geochemical Remote Sensing of The Sub-Surface

Gas geochemistry surveys for petroleum 219

free oxygen and abundant bacteria. The chemical reactions that take place in this zone
have been discussed by Rosaire (1938), Horvitz (1950), Kartsev et al. (1959) and Price
(1986), who consider that the most probable reactions are:

9 oxidation of hydrocarbons (catalysed by bio-enzymes or clay minerals), accompanied
by the production of CO2 and consumption of oxygen, which leads to a reducing
environment and the formation of new minerals, often manifested by changes in
physical properties and colour;
9 release of organic acids which may attack the common rock-forming minerals
leading to increased concentrations of Ca 2+ and Mg 2+ in solution; and
9 precipitation of carbonates in which some of the migrating gases may be occluded,
sometimes with the result that the rock becomes undulating.

All these reactions are interrelated and evolving in time. Once they reach an
advanced state, an alteration "chimney" is formed. This chimney, although formed
through the microseepage of hydrocarbon gases, may itself obstruct their further
migration. Any apical gas anomalies are subsequently lost. The microseeping gases find
new pathways around the margins of the chimney and surface anomalies then develop
above these new pathways. These anomalies are annular in outline. Thus, since the shape
of a surface anomaly recorded in a gas geochemical survey may depend upon the
presence or extent of an alteration chimney, integrated interpretation of gas survey data
with geological observations and geophysical measurements is particularly important.

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The hydrocarbon gases found at surface may have different origins and migration
histories, and hence different exploration implications. The origins and migration
histories of gases can often be deduced from their modes of occurrence at surface. In
particular, it is important to distinguish epigenetic concentrations from syngenetic
concentrations, and this can often be achieved by careful research into the modes of
occurrence of hydrocarbon gases in soils. In practice, the methodology for a gas
geochemical survey is underlain by a rational selection of the appropriate mode of
occurrence.
It is possible to recognise four distinctly different ways (some with sub-divisions) in
which gases occur at surface: (1) free molecules; (2) adsorbed molecules; (3)
microbubbles; and (4) mineral constituents (Fig. 6-3).