Page 206 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 183

presence of multiple, stacked porous zones also often results in a surface geochemical
expression that is approximately vertically above its subsurface origin.
The role of faults and fractures is particularly important for microseepage and some
further comment is in order. The close association of near-surface geochemical
anomalies with faults and fractures has been pointed out by, amongst others, Horvitz
(1939), Sokolov (1971b), Richers et al. (1982), Jones and Drozd (1983) and Matthews et
al. (1984). McCrossan et al. (1971) point to the close association of high concentrations
of hydrocarbons in the surface environment with photolineaments. McDermott (1940)
suggests that the permeability of shale is dominated by microfractures and that these
fractures are preferentially normal to the bedding plane. This potentially important role
of microfractures is emphasised by Rosaire (1938), who correctly points out that the
failure to observe displacement does not eliminate the existence of a fault or fracture.
The high permeability of fractures causes them to preferentially focus fluid flow. The
effectiveness of fractures as mass transport systems for fluids is evident from a casual
examination of mineralisation in fractured rocks and leakage of groundwater at fracture
outcrops. Similarly, these fractures act as preferential hydrocarbon pathways, focusing
their flow from source beds to surface.
Faults and interconnected fracture systems have a significant effect on the magnitude
and, less commonly, composition of the near-surface gases. The effect on magnitude is
generally to increase concentrations in fractured areas, whilst the effect on composition
theoretically should be preferential loss of lighter gases compared to heavier gases. In
practice, gas compositions on faults are often lighter or heavier than those at
neighbouring sites. This is believed to be controlled primarily by the depth of the fault
and the composition of the subsurface gases it conducts. Thus deep, basement-related
faults are often gassy because they tap deep over-mature sediments. Shallower faults are
often oily because large molecules migrate more easily than the lighter compounds.
The increase in magnitude in fracture systems can often be abrupt and localised. It
commonly spans several orders of magnitude, going from nil to macroseep levels in the
extreme cases. In an area where there is no significant source of subsurface
hydrocarbons, there are no high-magnitude soil-gas signals, even on faults and fractures.
In a hydrocarbon-bearing environment, however, overall high variance in the data is
more often the case, but the anomaly-to-background ratio is smaller in non-producing
areas than in producing areas. Some of these anomalous zones are associated with
preferential leakage directly from a source bed, while others are from reservoirs. Since
some faults and fractures are sealed locally along their lengths, high-magnitude signals
do not occur everywhere along their length. Thus, we often observe "hydrocarbon
spots", similar to the "helium spots" discussed by Wakita (1978). Naturally, those faults
penetrating only source beds will show a signal that reflects the source beds, whereas
those penetrating a reservoir or both reservoir and source beds will exhibit a larger
anomalous signal. It is not known, however, if one can truly distinguish between the two
types in all instances, although extremely high magnitudes are felt to be more diagnostic