Page 248 - Geochemical Remote Sensing of The Sub-Surface

Gas geochemistry surveys for petroleum 221

conditions must be avoided. As the nature of adsorption onto and desorption from soil is
poorly understood, the procedures are basically empirical.
Polarised molecules such as higher molecular-weight hydrocarbons can be adsorbed
via electrostatic forces. Organic or inorganic colloids (humic matter and clay minerals)
are electrically charged and possess high adsorption capacities. Hydrocarbon gas
molecules adsorbed in this way can be released by the addition of more competitive
molecules (e.g., dichloromethane) to substitute in the occupied sites.

Microbubbles

Gases occur as microbubbles in soil moisture (MacElvain, 1969). Gases occur in this
mode only in humid climates; where soils contain no moisture, as in arid climates,
microbubbles are also absent. The concentration of gases in this mode of occurrence is
much higher than that in the free molecule and adsorbed modes, and the gas composition
is more representative of the sources. Provided soil samples are kept moist,
microbubbles can be driven out by a stream of more active molecules and their gas
composition determined. Again, absolute concentrations are undefined, and the
determinations are relative.

Mineral constituents

Gases occur as inclusions in secondary minerals, especially carbonates. Being
protected by the host minerals from dissipation and microbial attack, hydrocarbons in
this form are very stable. As a result, sampling and sample handling are simplified and
the shelf life of samples is long, which is convenient for large surveys extending over
long periods. The analytical procedures, which usually involve gas release by acid
treatment, are easily standardised and data obtained at different times can be plotted on
the same map (Horvitz, 1986). Also the gas composition may have altered considerably,
usually becoming enriched in heavier hydrocarbons, which can lead to over-optimistic
interpretations.
Furthermore, the existence of carbonates of different origins may complicate the
data. Soils derived from matured source rocks, particularly carbonates, always contain
very high concentrations of hydrocarbons with compositions of matured oily sources. If
the parent rock of the soil was transported by a surface agency such as water, wind or
ice, gas anomalies reflect the source area of the sediments rather than the oil or gas
potential of the underlying strata. The western part of the Bayinhaote Basin of west
China is characterised by Archaean metamorphic rocks covered by thousands of metres
of Cenozoic red clastic sediments with no oil or gas potential. Acid treatment of soils
revealed very high C~-C5 concentrations (up to 600 ~tl/kg of C~ and 70 ~tl/kg C2).
However, it was found that the anomalous soils were derived from carbonate fragments