Page 205 - Geochemical Remote Sensing of The Sub-Surface

182 V.T. Jones, M.D. Matthews and D.M. Richers
acid extraction technique on glacial till or acid soils, which normally yield low results.
Without regard to the particular constraints on such data, one could easily overlook a
favourable area. In this case another technique such as free gas would be more
representative.

DATA INTERPRETATION

There are many ways to analyse hydrocarbon gas data with no one particular method
being correct or incorrect. Common sense and a deterministic approach to sound
geologic models are the best guidelines. Integration with other data such as structure,
lithology, soil types and hydrogeology, to name a few, can be most fruitful.

Preferential pathway model

The lack of a model explaining the mechanisms and constraints of hydrocarbon
leakage is often an obstacle to the acceptance of surface geochemical prospecting
(although a similar lack of understanding of the migration of hydrocarbons from source
beds to reservoir has not precluded the acceptance that migration occurs). Assimilation
of the data, however, suggests that much can be explained by a relatively simple model.
The conclusion that effusion is the dominant mode of migration enables us to use the
visual patterns associated with macroseeps as a basis for our microseepage model.
Link (1952) and Levorsen (1967) have summarised the geologic conditions and
controls on macroseepage. There is no reason to expect that these controls should not
apply as well to microseepage; the only real difference should be a matter of scale. In
addition to seepage directly from exposed source beds, controls on surface seepage
include: (1) the surface exposure of reservoir beds or porous carrier facies; (2) porosity
associated with unconformities; and (3) surface expressions of faults and fracture
systems that are pervasive to depth. These controls may be summarised as the focusing
of migration along preferred permeability pathways. Horizontal migration along the
pathway is dominated by grain or bed permeability (including old erosion surfaces and
other unconformities), whilst vertical migration is controlled by cross-stratigraphic
discontinuities.
Horizontal pathways deflect the surface location of the anomaly laterally away from
its subsurface origin. Thus if an anomaly is associated with the surface expression of a
porous formation, one should suspect a down-dip source (or down-groundwater gradient
source). The same conclusions can be inferred for anomalies associated with
unconformities, low angle faults and listric faults.
Vertical pathways are dominated by the intersection of high angle faults and fractures
with reservoir and carrier beds. In this case the surface expression of the source of the
hydrocarbons will lie directly above, or only slightly displaced from the source. The