Page 167 - Geochemical Remote Sensing of The Sub-Surface

144 v.T. Jones, M.D. Matthews and D.M. Richers

Physical transportation by effusion

Effusion transport is believed to be the dominant mode of moving hydrocarbons to
the reservoir and to the near-surface environment. The sharp localised nature of many
anomalies associated with microseepage and macroseepage is more consistent with an
effusion model rather than a diffusion model. The experience of the authors in
monitoring leakage from gas storage reservoirs and controlled experiments where
subsurface gas pressures were typical of true reservoirs suggests vertical transport rates
of several metres (tens of feet) per day, clearly greater than the distances of migration
dictated by the diffusion mechanism alone (Jones and Thune, 1982).
The sharp and often linear nature of anomalies suggests that faults and fractures play
an important part in the movement of these gases. Major linear features discernible on
satellite images, as well as other remotely-sensed media, from Patrick Draw, Wyoming,
show such a relationship (Richers et al., 1982). The Lost River, West Virginia, Geosat
study (Matthews et al., 1984) shows anomalously-high soil-gas values in relation to
linear features on imagery. There are anomalously-high gas values along faults in the
San Joaquin Basin and in the Wyoming-Utah Overthrust Belt (Jones and Drozd, 1983).
The Russians have shown that the magnitude of soil-gas values on faults increases
dramatically shortly after an earthquake in which fault movement is involved (Zorkin et
al., 1977). An extensive study, involving 105 observation wells, 3-5 m deep, was set up
over the Mulchto oilfield in northeastern Salchalin. A total of 3,700 samples was
collected and analysed over a four-month period with the most active wells sampled
daily (Table 5-IV). The results from this study provide impressive evidence for the
tectonic relationship of this leakage gas flux (Fig. 5-7). This study leaves no doubt that
faults and fractures provide the main control on the effusion of gases from the
subsurface.

Physical transportation by diffusion

Diffusion, on the other hand, is a slow and widely-dispersive process. Antonov et al.
(1971) measured hydrocarbon diffusion coefficients for a variety of rock types from
several hydrocarbon provinces in the former USSR. They discovered that the
coefficients of diffusion vary over a wide range (10-3-10 -s cm2/s) depending on the
particular lithology and geologic conditions.
The time required for diffusion to occur can sometimes be restrictive. Indeed the time
required not only often exceeds the age of the hydrocarbon accumulation but also quite
often exceeds the age of the host rock. If this were the dominant process for migration,
then the appearance of soil-gas anomalies in the near subsurface would indicate only
very shallow accumulations. If a non-steady state exists, where the hydrocarbon signal
observed represents only 0.001 times the steady-state signal, then diffusion times could
be reduced by a factor of 25 compared to that of the steady-state model. Table 5-V