Page 182 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 159

detector which yields a total hydrocarbon signal. This system is claimed to produce
direct vertical anomalies over reservoirs at depth. This technology appears reasonable for
detection of seepage which is large enough to produce free gas bubbles, but for feeble
seepage (i.e., below water solubility levels) the effectiveness would seem to be reduced
by dispersion due to underwater currents.

Soil gas

The hydrocarbon gases migrating through soil pore spaces are not dissipated and
diluted to the same extent as those in the atmosphere. There are, however, problems
posed by the very low levels of hydrocarbon gases and by the diurnal "breathing" of
many near-surface soils. In order to overcome these problems, soil-gas techniques which
integrate the hydrocarbon signal were introduced by Pirson (1946), Horvitz (1950),
Kartsev et al. (1959), Karim (1964), Heemstra et al. (1979), Hickey (1983), Hickey et al.
(1983) and Klusman and Voorhees (1983).
Karim (1964) published data on laboratory adsorption studies for light hydrocarbons
using activated charcoal, molecular sieve (diatomaceous earth) and silica gel. As shown
in Table 5-VII, these substrates greatly increase the concentrations available for analysis,
but selective adsorption severely affects the relative compositions of the individual
gases. The lightest gases are obviously not as effectively trapped by adsorption
techniques as are the heavier, less volatile components. This is particularly true for
methane and ethane. The adsorption capacities of the substrates are also strongly reduced
by moisture content, which may vary from site to site, particularly since the sampling is
conducted in the ground where moisture content varies more rapidly than in the
atmosphere.
Klusman and Voorhees (1983) introduced a variation of this technique which uses
sample collection on charcoal wire over extended collection times, followed by analysis
using a quadrupole mass spectrometer. The advantages cited are lower field expenses,
increased field mobility, improved signal-to-noise ratio and negation of barometric and
other meteorological factors. Major drawbacks are that the most mobile light gases are
not collected by the charcoal wire, so that the samples comprise mainly the intermediate
to heavier molecular-weight components, which include butane through gasoline and
diesel. Multivariate statistical techniques are required to interpret the large number of
mass peaks recorded, which includes both parent and multiple daughters. In some cases
qualitative information based on fragment patterns of the adsorbed compounds is
possible (Fig. 5-17). However, different molecular species and their fragment patterns
overlap; for example, propane and carbon dioxide have identical masses (44) and thus
cannot be separated. The exploration value of these data lies in the demonstrated
presence of reservoir-type hydrocarbons at the surface and the composition noted in the
lighter to heavier fragment patterns.