Page 184 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 161

Typical Gas Spectrum
o
rYo

,; ' 2' ' :1o , ,;o ' 1~0 ' 1~o ' 2~o ' ~o '
M/Z

Typical 011 Spectrum

,
,
,,ll .... Ih t,,, ,I, .... ,,
i ! -
140 ,~o ' 2~o ' ~0
M/Z
Fig. 5-17. Typical mass spectra of gas and oil.

Although the concept and approach of this technique are excellent, it does not
integrate the flux of hydrocarbons heavier than butanes during the one to two weeks for
which the collectors are left in the soil. Hydrocarbons heavier than butanes are liquids,
and do not migrate more than a few centimetres during the short collection period. It
may be equally effective to place a soil sample in a jar with the collection wire; the
collection efficiency could probably even be increased by heating the sample jar.
Direct sampling of free soil gas requires that a sampling probe be inserted into the
ground to collect a soil gas sample. The deeper the penetration, the more difficult and
expensive the procedure becomes, eventually requiring that analysis be conducted on
drilling fluids or rock samples recovered from a hole. Deeper holes almost always
encounter water, which also influences the collection of free gases, forcing one to
analyse the gas content of some type of recycled water or mud system which is used to
drill the hole.
Although sampling from holes of any depth is possible, for simplicity two free soil-
gas techniques will be discussed and compared (as case studies): shallow probes
(Matthews et al., 1984) which penetrate to 1.2 m (4 feet); and auger holes (Jones and
Drozd, 1983) which are 3.5 m (12 feet) deep. These methods differ mainly in terms of
the resulting soil-gas sample. The shallow-probe samples are influenced more by closer