Page 192 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 169

TABLE 5-XI

Composition ratios of soil-gas hydrocarbons over Pleasant Creek gas storage area

Date C1/Cn CI/C2 (C3/CI)x 1000
May 1975 90 20 19
July 1975 89 18 24
July 1976 89 16 20

a chromatographic effect would be expected for gas that migrated through the Earth. The
fact that the compositions of the soil-gas data from auger holes match the underlying
reservoirs confirms that the major migration mechanism to the near-surface must be via
faults and fractures, rather than by diffusion.
The percent-methane compositions from the auger hole surveys conducted over the
Sacramento and San Joaquin Basins are plotted in Fig. 5-21. There is a decrease from
98% methane in the north of the Sacramento basin to 90% in the south part, whilst the
soil gas over the San Joaquin Basin has 82% methane. These data imply that a soil-gas
grid would have defined local differences regionally. Furthermore these geochemical
data are repeatable (Table 5-XI); the percent-methane values on Fig. 5-21 were all
determined at least two or three times over a three-year period and found to be
repeatable. Compositional data have remained repeatable throughout our experience with
soil-gas surveys.

L-..., f

Fig. 5-21. Percent-methane in soil gas over the Sacramento and San Joaquin Basins, California.