Page 191 - Geochemical Remote Sensing of The Sub-Surface

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

Where seeps contain gases from more than one reservoir, their compositions may not
match those of any of the underlying reservoirs. Mixing of a shallow oil and a deep gas
will generally yield an oily but intermediate-type composition. Without some knowledge
of the reservoir possibilities, this type of signature cannot be recognised. Nevertheless,
the intermediate nature of the seep will indicate some liquid potential at depth. Thus,
dry-gas basins can be distinguished from basins that have at least some liquid oil or
condensate potential. As suggested by Bernard (1982), the presence of fairly large
ethane-propane-butane anomalies strongly suggests an oil-related source.
Pixler (1969) found that the gases observed during drilling could distinguish the type
of production associated with the hydrocarbon show during mud logging and published
the graph shown in fig. 20f. Pixler's data were obtained by monitoring the C j-C5
hydrocarbons collected by steam-still reflux gas sampling during routine mud logging.
Individual ratios of the C2-C5 light hydrocarbons with respect to methane provided
discrete distributions that reflect the true natural variations of formation hydrocarbons
from oil and gas deposits. Ratios below approximately 2 or above 200 indicated to Pixler
that the deposits were non-commercial. The upper range for these ratios for dry-gas
deposits has been enlarged by Verbanac and Dunia (1982), who studied more than 250
wells from 10 oil and gas fields. Their data, shown in fig. 5-20h, suggest the following
upper limits for dry-gas reservoir ratios: C~/C2 <350, C~/C3 <900, CI/C 4 <1,500, C~/C5
<4,500. These ratios clearly aid in defining transition between thermogenic and biogenic
gases. Another empirical rule suggested by Pixler is that the slope of the lines defined by
these ratios must increase to the right; if they do not, the reservoir will be water-wet and
therefore non-productive. Verbanac and Dunia (1982) suggested that a negative slope
connecting individual ratios may result from fractured reservoir zones of limited
permeability.
Auger-hole soil-gas data for the surveys over the three basins described above are
plotted on a Pixler-type diagram of reservoir gases in Fig. 5-20g. Direct comparison of
these two independent data sets is very striking and proves the concept of migration of
reservoired hydrocarbons to the surface. It is important to note that amounts of migrated
gases almost always decrease in the following order: methane > ethane > propane >
butane. Thus, in a Pixler-type diagram, soil gas-data, like reservoir data, generally plot as
line segments of positive slope for the soil gases to represent a typical migrated seep gas.
Exceptions to this order have been noted where surface source rocks were drilled, which
thus far have yielded ratios with lighter gases depleted in relation to heavier gases.
According to Leythaeuser et al. (1980), this would be expected if gases in the boundary
layer very near the surface followed a diffusion model. Thus, compositional changes
related to diffusion might be expected at or very near a boundary layer where the
hydrocarbon gas concentration approaches zero. This behaviour has been observed when
comparing soil gas probe data measured at very shallow depths (0.3-0.6 m, 1-2 feet) with
the corresponding data from 4 metre (13 feet) auger holes. The shallow probe data are
always "oilier", indicating preferential loss of methane and implying diffusion from the
4-m (13-feet) level to the surface. If diffusion were the dominant migration mechanism,