Page 196 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 173

In order to maintain reproducibility it is important to measure all volumes accurately.
In a typical operation using 500 ml (one pint) cans, the procedure is to place 300 ml of
degassed salt-water brine into the 500 ml can and add sediment until the can is filled to
the brim, giving 200 ml of sediment and 300 ml of brine. The can is sealed and then
zero-grade nitrogen is injected through a prepared septum to displace 100 ml of brine
and leaving the can with a 2:2:1 mixture of 200 ml brine, 200 ml sediment, and 100 ml
headspace.
Experiments have shown that a fairly long time is required for the adsorbed sediment
gases to completely equilibrate with the headspace. This equilibrium time is shortened
by heating and shaking the cans before analysis. A generally accepted procedure is to
heat the cans for about 12 hours at 60-70~ followed by shaking in a paint mixer for
five minutes. After heating and shaking, the cans are allowed to stand for at least five
further minutes to ensure that dissolved gases return to the headspace.
One of the drawbacks of using this technique is the need to freeze the canned
samples if they cannot be analysed within one or two weeks of their collection. Failure to
follow this procedure can create problems because of the generation of biogenic gas in
the cans or the bacterial oxidation of the hydrocarbon gases to carbon dioxide.
Hydrocarbon concentration values are reported in terms of ppm by volume in the
nitrogen headspace or as ppm or ppb by weight, normalised to the weight of sediment.
Gases concentrations reported by weight are not truly representative of the actual gas
migrating from depth because some of the free gas has been allowed to escape during
collection and sample preparation. Furthermore, the sorbed gas is never completely
extracted into the headspace, and may not always reflect the true gas content of the soil.
The headspace sampling technique can yield useful results if sufficient numbers of
samples can be collected to use statistical populations to suggest anomalous areas. One
should always exercise caution, however, with respect to characterisation of gas
composition, since evaporation during the collection stage always occurs, resulting in the
relative depletion of the lighter gases.

Disaggregation

Extensive soil gas sampling programmes carried out by the petroleum exploration
industry have demonstrated that the crushing and/or disaggregation of soils (including
the action performed in drilling auger holes) is an important component part of the
extraction of gas from the soil. This suggests that it would be advantageous to employ a
soil core disaggregation technique which would closely mirror the effect of auger hole
drilling. A device developed at Citco and commonly used in both industry and academia
for analysing well cuttings appears suitable for accomplishing this objective (Whelan,
1979; Hunt and Whelan, 1979; Whelan et al., 1980). In fact, Richers (1984) has
demonstrated successfully that in some instances, such as at Rose Hill, Virginia, and in