Page 199 - Geochemical Remote Sensing of The Sub-Surface

176 V.T. Jones, M.D. Matthews and D.M. Richers

correlation among the gases may reflect a near-equilibrium condition achieved through
time for the adsorption-desorption process in soils. Hence, the signal seen by the
desorption technique may effectively integrate and smooth rapid changes one might
expect to see with a free-gas technique such as auger holes.
At the Gulf Research Facility in Pittsburgh, Pennsylvania, there are two producing
gas wells, and 38 sites were selected to test the ability of the disaggregation technique to
define gassy areas. Not only did the test yield gassier results than those obtained at Rose
Hill, but also the results were again comparable to those obtained using the auger hole
technique. Table 5-XIII is a compilation of these results. Clearly, the two data sets
reflect a more gas-prone area for the Gulf Research Facility than for the Rose Hill area.
Although the data set for the disaggregation technique is only half of the size of the data
set from the auger holes, it still yields useful information regarding composition of the
subsurface reservoirs.

Acid extraction

A technique which measures only the most tightly-bound gas was originally
developed by Horvitz (1939, 1945, 1950, 1954, 1957, 1965, 1969). In this technique the
sample is subjected to acid digestion under vacuum at an elevated temperature of about
80~ Further developments by Debnam (1969) and Horvitz (1972, 1980, 1981) involved
corrections for lithology to reduce the effect of acid-soluble minerals biasing the data.
Debnam (1969) noted that soil samples could be dried, pulverised and sieved without
affecting their hydrocarbon content. He also noted that sieving sand samples to <200
mesh gave analytical values comparable with those produced by clay samples from the
same location. Horvitz developed a wet-sieving technique to concentrate the analysis on
only the clay fraction of the sediment.
McCrossan et al. (1971) evaluated the acid-extraction technique in the western part
of Alberta. This extensive survey of over 4561 samples covering 15 townships
concluded that the distribution of anomalous points was random and was strongly biased
by samples rich in carbonate minerals. Adequate corrections for amounts of acid-soluble
material were not successful and it was concluded that this method could not be used in
areas covered by glacial till.

TABLE 5-XIII

Diagnostic soil-gas ratios at the Gulf Research Facility, Pittsburgh, Pennsylvania

Survey method No. samples % methane CI/C 2 (C3/CI)XI000
Free soil gas 73 89 18 21
Disaggregated gas 38 91 25 22