Page 250 - Geochemical Remote Sensing of The Sub-Surface

Gas geochemistry surveys for petroleum 223

evolved in the laboratory by heating to high temperatures and their concentrations are
determined instrumentally.

Soil

Soil samples are usually collected at the surface proper or at depths of up to several
metres below the surface. As far as possible, samples are kept in their original condition
until they are prepared for analysis in the laboratory. The analytical procedures are
invariably partial extraction techniques. This approach has been widely used in other
branches of exploration geochemistry, but assumes particular importance in gas
geochemical surveys for petroleum.
For gases adsorbed on soils, the essential requirement is to regulate desorption
parameters, so that variations in these parameters do not introduce variations into the
concentrations desorbed. In order to prevent production of new gas species during the
thermal desorption, the heating temperature is usually kept under 200~ or applied only
for milliseconds (Kiusman et al., 1986). Stable thermal desorption conditions can be
attained by using a metal heating block maintained at a fixed temperature, into which a
tube containing the soil sample is placed for a fixed time. The gases released are then
condensed in a loop held in liquid N2, which subsequently acts as the input to the gas
chromatograph (Fig. 6-4). If the soil samples are in sealed containers, simple vibration at
elevated temperature produces an equilibrium concentration of gases in the container
headspace that can be taken as a measurement proportional to the concentration of
adsorbed hydrocarbons.
For the extraction of hydrocarbons in carbonate minerals and microbubbles, the
method widely used is acid treatment under vacuum, for which most laboratories in
China use the apparatus shown in Fig. 6-5. The soil sample is heated to 40~ in tile flask,
then 1:6 HCI is added until its reaction with the warm sample ceases. The mixture of
gases evolved is filtered through NaOH solution to remove CO2 (which makes up the
bulk of tile evolved gases) and the remaining gases are injected into a gas
chromatograph.
A selective method to flush out microbubbles is hydrogen stripping. This was first
proposed by Schaefer et ai. (1978) and has been developed by Ruan and Cheng (1991).
Soil samples are kept in a natural wet state and an aliquot of saturated NaCI solution is
added to make a slurry, through which H2 is passed. The gases stripped from the sample
are condensed in a liquid N2 cold trap, which is then used as the gas sample loop in the
carrier gas line to a gas chromatograph (Fig. 6-6).