Page 227 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 203

have a much oilier signature, suggesting a relative depletion of volatiles from the sources
of measured soil gases.
A 400-site detailed grid geochemical survey on 300-metre (1000-feet) centres was
carried out the second study year (1985) over this section of the Currant lineament and
further supplemented by aerial-photographic studies in an attempt to characterise the
local expression of this regional lineament. The location of this second-year study with
respect to the 1984 regional survey is shown in Fig. 5-37 (inset). High- and low-altitude
aerial photographic studies reveal that, although the regional feature is well expressed in
TM data, it does not dominate the length, azimuth, or density of the small-scale linear
features. The lineament appears only as a minor group of parallel and subparallel linear
features, which are easily lost in the clusters of more local fracture zones (Fig. 5-38).
The Currant lineament is, however, expressed in gravity data as a trough, suggesting that
it is, in fact, a deep-sourced feature of regional significance, which may influence
subsurface fluid and gas migration along or across its strike.
Light hydrocarbon soil-gas data from the Currant grid area show some alignment of
anomalous values along the strike of the lineament, but it is apparent that the lineament
does not control hydrocarbon magnitudes in this area (Fig. 5-39). Hydrocarbon
magnitudes appear to be controlled, to a greater degree, by the north-south and east-west
small-scale linear features (shown in Fig. 5-38), which probably reflect, to some extent,
the location of subsurface structural faults and fault-related fracture systems. This
relationship is quite important because structures identified by lineament zones are
generally not the sole controlling factor for light hydrocarbon seepage; rather they
simply provide enhanced pathways of migration for gases and fluids. The local geologic
framework and source potential are the most important factors for interpreting the
relationship of hydrocarbon seeps and lineaments.
Compositional data from the Currant grid area adds great insight into the effect a
regional lineament can have on the sources of migrating gases, even though the
lineament is not directly mappable at the local scale. Regional data from the 1984
Railroad Valley programme indicated a compositional shift across the lineament zone.
A methane-ethane crossplot of the 1985 Currant detailed grid data shows two distinct
populations (Fig. 5-40c). Pixler ratio plots of anomalous sites support this subdivision
and actually show two distinct populations, with the vast majority of the gassier sites
plotting to the northwest of the Currant lineament (Fig. 5-40a), on the basin side where
deeper sources exist. These two compositional subpopulations are clearly shown by the
yellow-to-green colour change in the crossplot in Fig. 5-40b. This was first noted in the
1984 survey data and confirmed by the 1985 survey data. It should be kept in mind that
the colour code used for both the dot maps (Fig. 5-40a) and the crossplot (Fig. 5-40b) is
determined by plotting the raw soil-gas data on the Pixler diagram. Thus the colour
compositions are selected to be similar to analyses of samples from actual producing
reservoirs. The combination of both spatial and compositional clustering clearly
demonstrates the stability, and repeatability of light soil-gas data that are properly
collected and analysed.