Page 208 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 185

intersecting fractures). Further from the centre of the fracture zone, the maximum values
fall until they merge with those typical for the background of the area. This distribution
of free soil-gas magnitudes as a function of distance from the centre of the fracture zone
is shown in Fig. 5-29C (Richers et al., 1986). Disaggregation data from Patrick Draw
exhibit a similar pattern, although the increase near the centre of the fracture zone is not
as great; acid extraction data from this example show no obvious relationship, clearly
suggesting that different analysis techniques are extracting gases from different sources.
The following examples illustrate the means of interpreting what are often referred to
as direct anomalies using preferential pathway models. These direct anomalies may be
either vertically over their subsurface source, or laterally displaced by varying amounts
(Sokolov, 1971b; Pirson, 1969; Laubmeyer, 1933). What is generally not realised is that
most areas contain microfractures to the extent that they allow gases to escape vertically.
Using a coal-bum experiment in the central Wyoming coal region, Jones and Thune
(1982) showed that a definite vertical-migration component could be identified. In that
experiment, gases formed during combustion appeared both in soil gases directly above
the retort and up-clip along the bedding planes of the strata involved in the burn. Thus,
vertical signals from a known subsurface origin were shown to exhibit cross-
stratigraphic migration, presumably due to the presence of fractures in the system. A
second horizontally-displaced component also migrated along the bedding planes at the
same time.
An example of the use of direct anomalies and the preferential pathway model is
shown in Fig. 5-30, which shows an idealised subsurface cross-section through the Lost
River field in West Virginia along with a propane profile (Matthews et al., 1984). From
this profile and with some knowledge of the geology, it can be seen that a large anomaly
is probably caused by updip leakage of the fractured Devonian Oriskany reservoir at
depth. This outcrop anomaly is due to updip leakage along the bedding plane of the
reservoir facies. A smaller but significant anomaly is related to leakage from a fault that
strikes along and to the east of the crest of the producing anticline. Blind drilling on the
outcrop anomaly would have resulted in a dry hole, whereas drilling just west of the fault
anomaly would have encountered the producing structure. Appropriate geological
modelling identifies the location at which to drill.
An alternative to the direct anomaly interpretation method relies on identifying one
of two types of halo: (1) local lows, source background areas surrounded by highs; or (2)
extremely low areas, surrounded by areas of moderate concentration. These halos are
consistent with the initial results obtained with soil-gas analysis techniques (Rosaire,
1938; Horvitz, 1939, 1945, 1954, 1985; McDermott, 1940; Rosaire, et al., 1940), which
indicated that adsorbed and occluded hydrocarbons occur in greater quantities around the
edges of production, whereas relatively lower values are found directly above
production. Halo anomalies have been recognised in many regions of the former USSR
(Kartsev et al., 1959). Horvitz (1969, 1980) has emphasised that although other
hydrocarbon distribution patterns are recognised, including direct anomalies, the halo