Page 212 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 189

Gleezen (1985) showed that there is promise in using the olef'm contents of soil gases
as a scaling factor to separate seep signals from ambient signals. He was able to define
areas with signatures similar to those of the reservoired gases. It would appear that in
some cases the presence of olefins may merely represent the breakdown of saturated
hydrocarbons by some yet-undetermined process during the migration of gases to the
surface and/or some activity such as biogenic degradation of the saturates in the near-
surface environment (Telegina and Cherkinskaya, 1971).
Compositional information in soil gases has been related to subsurface accumulations
through the application of specific ratios (Jones and Drozd, 1983). Methane-dependent
ratios (Table 5-VIII) are reliable unless multiple sources of gas are present in the area.
An independent methane-rich source biases an oilier composition toward a drier gas
composition. This can sometimes be overcome by plotting histograms of the
compositional data and noting multiple populations in the data. Another set of diagnostic
ratios that are not methane dependent has also been defined and further aid in properly
defining the true potential of an area (Drozd et al., 1981; Williams et al., 1981). In
general, the agreement between the surface compositions with reservoir compositions is
the strongest evidence that surface prospecting can accurately define the potential of an
area.
In addition to compositional information, soil-gas data can yield useful information
according to the presence or absence of anomalously-high magnitudes. To understand
the concept of anomalously-high magnitudes, one must understand the general
distribution of gases in nature. Basically these can be reduced to three main populations
for any given region.

1) An ambient background population (which represents a detectable level of non-
significant hydrocarbon concentrations). This includes mantle-derived hydrocarbons,
contamination, instrumental noise, sampling error, etc.

2) A source background population representing hydrocarbons derived from the
presence of organic-rich source beds in a region. These are generally areal in extent, and
they may or may not be relatively consistent throughout the area depending on local
geologic variations, regional trends or multiple sources.

3) An anomalous population of higher-than-normal concentrations of
hydrocarbons that represent the subsurface presence of concentrated hydrocarbons such
as those found in reservoirs.

Ambient levels, by their very nature, are encountered everywhere, and are always a
component of the total soil-gas signal regardless of the overall hydrocarbon potential of
an area. Their presence may be due to natural catagenesis of organically-poor rocks
during the processes of diagenesis and lithification, and can be thought of as being
syngenetic. Another source is the biogenic alteration of organic matter in the near-