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Gas geochemistry surveys for petroleum 217

microseeps through gas geochemical surveys, detection of these alteration products by
surface geophysical and microbial methods of prospecting is also possible, as, in some
cases, is their detection by satellite remote sensing (Chapter 7). Indeed, gas geochemical
survey data should be integrated with data obtained using these other methods and
interpreted by combining all available geological, geophysical and geochemical data.

Gas migration

Mechanisms of gas migration and the medium through which migration takes place
should be considered simultaneously, although in practice they tend to be discussed
separately. The mechanisms of gas migration can be divided broadly into three
categories, with migration in each category governed by a well-established physical law:
(1) diffusion, governed by Fick's Law; (2) buoyancy, governed by Archimedes' Law; and
(3) mass flow, governed by Darcy's Law. These forces are not discussed in detail here;
only a brief account is given of the contribution they make to the formation of surface
gas anomalies.
Diffusion, by definition, is a spontaneous tendency to eliminate a concentration
gradient. Because it involves no external forces, it is a constant but slow process. On the
whole, however, diffusion is a fringe force in terms of the speed and scale of gas
migration, and other more rapid processes are superimposed upon it.
Buoyancy originates from the density difference between water and oil or gas
bubbles. MacElvain (1969) proposed an ascending-microbubble theory to explain the
presence of microseepages. He noted that "colloidal-size gas bubbles are readily
displaced upward by the surrounding water at rates up to several millimetres per second,
regardless of any sedimentary particles that may intrude in the way of their upward
zig-zag Brownian path. Such exceedingly small bubbles can quickly ascend hundreds
and even thousands of feet in a manner not available to large gas bubbles or to individual
gas molecules. Large bubbles have too large a surface area to be able to demonstrate
kinetic or Brownian oscillation, whilst individual molecules of dissolved gas possess
insufficient difference of density for gravitational displacement". This theory explains
the sharp edges and limited lateral displacement of surface gas anomalies, their quick
response to reservoir pressure changes, and the general absence of hydrocarbons heavier
than C5 in surface anomalies.
Mass flow is a bulk movement of hydrocarbons in monophase or dissolved in water.
It requires an external force (pressure gradient or structural stress) and a well-defined
conduit or plumbing system. The microseepage resulting from mass flow is therefore
characteristically confined to a small area or belt, within which the anomalies are of high
contrast and often contain some high molecular-weight hydrocarbons.