Page 174 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 151

gases appear to be dominated by the migratory gases, unless samples are taken within an
outcropping source rock. In addition, the free gases also contain any biological gases
which, because of their recent generation, also occur in the free state. If source rocks or
recycled source-rock materials are present near surface, then the "adsorbed" gases can
obtain a major contribution from these sources. Exclusions are often provided by
sampling in areas where calcite concretions have been deposited from carbon dioxide
generated by biological oxidation of seepage hydrocarbons. This is one reason why
adsorbed gas has been successful in marine offshore environments. A good example is
provided by studies of the Green Canyon macroseeps (Anderson et al., 1983; Pirkle,
1985).
If one can assure that only migratory gas is measured, then the type of gas measured
is unimportant. Including indigenous (syngenetic) gas results in misleading
measurements. This is believed by the authors to be one of the primary causes of failure
in the application of surface geochemical prospecting. Failure to collect a properly-
distributed data set can be equally misleading and result in an incorrect interpretation,
since interpretations will always be the educated guesses of an explorationist.
Any measurement on a real-world sample is always a combination of the free and
bound gas sample types. This is because the process of taking the gas sample generally
requires that the sediment or rock system is disturbed by some mechanical means which
creates the mixing of these sample types. Because of this unavoidable interaction, we
have recognised the need to consider an intermediate sample-collection technique that
measures the more loosely-bound gases liberated into a container containing the core
sample.
Sampling gases that accumulate within the gas-filled "headspace" of a core-sample
container is potentially flawed because of the obvious losses encountered in transferring
a sample to a container. This is further compounded by the difficulty in achieving a rapid
and total equilibration of the core gases into the headspace. An alternative technique for
measuring the loosely-sorbed gas has been proposed by Hunt and Whelan (1979), in
which the headspace equilibrium is obtained mainly by mechanical disaggregation and
heat. In our opinion, this disaggregated gas should more properly be called "adsorbed"
gas. The truly "free" gas is always lost (or at least greatly diminished in volume) from
any sample of core that is brought to the surface for collection and handled before being
put into a sample container (Sokolov, 1971 b). Typical losses are shown in Table 5-VI.
This mechanically-disaggregated gas has been usefully applied as a bridge to relating
the free and bound gas (Richers et al., 1986). Simple mechanical disaggregation always
liberates a considerable volume of gas which, if handled properly, has a predictably
oilier composition than the associated free gas. This change in composition, created by
fractionation of the lighter components, is demonstrated later in examples under case
studies.