Page 201 - Geochemical Remote Sensing of The Sub-Surface

178 K T. Jones, M.D. Matthews and D.M. Richers

10~ < SAND ~,01t 4(MM~ CARB ~;0%

i:+:i:+:'. "-,'-''+''''+'"-
l ~ o . o o % ~,,,
3~ Content ~ Gas Content >
Ca~RB - O~

T "~ CAM - I~ "

i Gas Content
- O~
Gas Content i 9 . ~ [ D C.,ARB - 0%
I T I I 501,1( SAND '~"0~
<.<,
o.<c....
I'
u.
m 3 I
I Gas ~tent---'--~
+ +- Gas Q~tent >
Fig. 5-26. Scheme for lithological classification of samples prior to interpretation of gases
released by acid extraction (reproduced with permission of Australian Petroleum Production and
Exploration Association from Poll, 1975).

average, or background, gas content is computed. The gas content in each group is
assumed to be distributed according to a Laplace-Gauss law. Each subset is then
assumed to have a uniform efficiency of desorption and its own background and
anomaly threshold. As shown in Fig. 5-26, for calcareous sediments these are very high,
due to the effectiveness of the acid attack. The mean normal standard can be computed
for each set yielding dimensionless values that can be added together for mapping,
regardless of the sediment type. This technique has been applied by Poll (1975) in the
Gippsland Basin and by Devine (1977) and Devine and Sears (1985) in the Cooper Basin
in Australia. Reasonably positive results were reported in all three cases.
The acid-extraction technique relies on the ability of soil and minerals to retain
hydrocarbons that migrate past them through the soil pore system. It is therefore not
subject to the fluctuation involved in the soil-air system but hopefully represents some
averaged or integrated signal over time. As noted above, the samples must be corrected
for lithologic efti~cts by only making comparisons within a given lithology or by
specifically analysing certain minerals. Corrections must always be applied because
adsorption occurs in both the fine-grained fractions and in carbonates, which often
release disproportionately large amounts of hydrocarbons.