Page 221 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 197

The presence of butanes in the sniffer data clearly separates the southern gas-
producing trend from the area to the north of Block A-252. The grids over both Block
A-152 and Block A-198 and the profile data north of Block A-252 exhibit a clear lack of
propane and butane anomalies. The presence of mainly methane in the northern area
suggests that these anomalies are derived from biogenic gas sources.
Marine compositional crossplots from the anomalies observed in Block A-152 and
A-198, and from the regional profile are compared in Fig. 5-34d. All three areas fall
exactly as expected, based on the known oil and gas producing reservoirs within this
survey area. Blocks A-152 and A-198 are similar to the fairly dry gas type Pleistocene
reservoirs found in West Cameron, in the Louisiana offshore area, and are indicative of
only gas potential. Block A-198 anomalies appear to contain even drier gas data than
those in Block A-152. Data from both of these blocks plot below the cluster associated
with the major Pliocene gas-producing trend that lies to the south of Blocks A-152 and
A-198. The increase in ethane, propane and butanes in this southern gas-producing area
suggests that these gas fields contain Pliocene gas from a more petrogenic source,
whereas the areas to the north appear to be dominated by biogenic gas sources that do
not contain C2+ components. It should be noted that the new field discoveries (A-129, A-
154 and A-200), highlighted in Fig. 5-33a, were made after the sniffer survey was
completed.

Great Basin, Railroad Valley, Nevada

The third example in this chapter is an onshore survey conducted in Railroad Valley,
Nevada. This example is abstracted from an integrated two-year (1984-85) remote-
sensing and surface-geochemical research project which provides an excellent example
of the exploration value of combined remote-sensing and geochemical studies in frontier
basins (Jones et al., 1985; Burtell et al., 1986). The variability of sample spacing used
over this two-year programme, coupled with repeated, even-closer detailed sampling on
grids in 1985, allows a demonstration of the sampling artifacts that can be created by
over-interpreting a low-density regional survey. In addition, the repeatability of soil-gas
surveys is demonstrated. The compositional correlations predicting oil versus gas, as
shown in the two previous examples, are extended to differentiating non-commercial
heavy-oil deposits from their lighter counterparts, neither of which contains any
significant gas production.

Fig. 5-33. High Island geochemical sniffer survey 1988: a) sniffer track map; b) marine
compositional crossplot.