Page 172 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 149

hydrocarbons into the near-surface environment above the water table must involve
transport through both water-filled and air-filled pores. Sampling these pore gases is
obviously one of the most fundamental concepts. However, gases can be bound in the
sediment matrix. This latter possibility leads to the development of some disaggregation
and desorption extraction techniques.
Discussion of sampling techniques must involve both "flee" and "bound" gases. To
facilitate this discussion the collection, measurement and analysis of light (C1-C4)
hydrocarbons will be broken into two main categories each with two subcategories: (1)
free gas, which can be vapour or dissolved gas; and (2) bound gas, which can be
adsorbed gas or chemi-adsorbed gas.

Free gas

Gases in the free pore space can be found either in the vapour state or dissolved in
water. Extensive research at Gulf Research and Development Company has
demonstrated that the "free" and "dissolved" gas seeps yield comparable compositional
results, both to one another and to their associated reservoirs when they are properly
collected and analysed (Teplitz and Rodgers, 1935; Jones, 1979; Janezic, 1979;
Mousseau and Williams, 1979; Weismann, 1980; Drozd et al., 1981; Williams et al.,
1981; Jones and Drozd, 1983; Richers, 1984; Price and Heatherington, 1984; Matthews
et al., 1984; Jones et al., 1984). This documentation even extends to numerous
observations over artificial underground gas generation and storage reservoirs (Jones and
Thune, 1982; Jones, 1983; Pirkle and Drozd, 1984).
Sampling of vapour can be extended to any depth above the water table by analysing
the exhaust air from an air-drilled well. Complications occur because of dilution effects
by the air injected for drilling and by the additional fact that the drill bit disaggregates
and liberates rock or matrix gas in the process of drilling the hole.
Dissolved gases must be extracted from the aqueous system before analysis. This is
usually accomplished by a simple gas-water partition into a vapour phase followed by
standard headspace measurement techniques (McAuliffe, 1966). Alternatively a so-
called "stripper" continuously partitions the dissolved gases into a carrier gas which is
then sent to a gas chromatograph for analysis (Mousseau and Williams, 1979; Aldridge
and Jones, 1987). These separations are aided by the very low solubility of the light
hydrocarbon gases.
Standard mud gas logging is one variant of dissolved gas analysis conducted on
deeper drill holes. A gas trap is deployed in the return mud system for extracting the
dissolved and free gases. Compositional information obtained from mud logging gas is
useful for predicting the composition of a potential reservoir (Pixler, 1969). These same
ratios have been found to be indicative of oil versus gas potential from surface seeps
observed from 4 m (12 feet) deep soil-gas measurements or from analysis of gases
dissolved in the shallow groundwater (Jones and Drozd, 1983).