Page 200 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 177

As early as 1940, Sanderson had discussed a number of factors that affected
adsorption of hydrocarbon gases by soils. He noted that the ability of the soil to adsorb
any gas depends upon the type of gas, the characteristics of the soil and the conditions
under which the soil is exposed to the gas. Adsorption will depend upon the type and
surface area of particles and their chemical composition. The surface reactivity will be
modified considerably by the presence of previously-adsorbed molecules, such as carbon
dioxide, water and mineral ions. The condition of adsorption is complicated by
temperature and pressure and length of exposure time in addition to concentrations and
species of gases present. Adsorbed-gas data can, at best, be only approximations of the
original mixture of migrated gases. Another possible problem lies in the quantitative
desorption of the gases from the mineral components of the soil.
Sanderson (1940) observed up to six-fold differences in the ability of soils to adsorb
hydrocarbons in his laboratory. He also noted that the adsorptive characteristics of the
colloidal soil systems would vary slowly with moisture content, time and season. Of
particular significance was his observation that the adsorptive capacity for hydrocarbons
on wet soil was only a small fraction of that for dry soil. A further complication is
created by near-surface biological activity that creates wide variations in the content of
carbon dioxide, nitrous oxide and other biological gases. Overcoming all these problems
is probably impossible; however, it will suffice if the gases are liberated in proportion to
the amounts present so that the analytical results bear some relationship to one another,
and allow identification of potentially prospective areas.
Various other approaches have been devised in attempts to overcome this problem.
Bays (US patent no. 2,165,440) suggested correcting for the sorptive power of the soils
and McDermott (US patent no. 3,120,428) suggested correcting for the surface area. An
alternative technique proposed by Thompson (1971) used ethylenediaminetetracetic acid
(EDTA) at about pH 7 and slightly heated in order to decompose the carbonate minerals
under conditions that do not release such large quantities of carbon dioxide. Thompson
reports that a comparison on duplicate samples shows that the EDTA technique
consistently releases from 94-99.5% of the hydrocarbon gases released by the standard
strong-acid treatment. A further refinement of this method by Thompson et al. (1974)
separates a critical carbonate mineral before analysis. This critical mineral was almost
always found to be dolomite, but occasionally is other carbonate minerals, such as iron
or calcium carbonate. The ratio of hydrocarbons per unit of critical mineral is then
plotted to form a geochemical prospecting map. This technique was reported to highlight
a salt dome in the Gulf of Mexico on which a major oil discovery was made after the
survey was conducted.
Poll (1975) addressed this problem of lithologic corrections by dividing data
according to desorption efficiencies based on their physicochemical properties. The first
step is to prepare a detailed lithological description of the samples. This involves a
differentiation on sediment lithology, sample coherence, structure, cementation and
mineral types, including carbonate and sand percentages. This information is used as
shown in Fig. 5-26 to classify the samples into homogeneous sets for each of which the