Page 261 - Geochemical Remote Sensing of The Sub-Surface

234 H. Yang, F.D. Van der Meer and J. Zhang

surface. The most persuasive evidence for microseepage is the measurement, sometimes
over many years, of statistically-significant anomalous amounts of light hydrocarbons in
soil gases and soils directly over oil and gas reservoirs (Price, 1986). In these cases the
hydrocarbons in the soil gas or soil have very similar carbon isotope ratios to those in the
underlying reservoirs, whereas the hydrocarbons of near-surface biogenic origin have
different carbon isotope ratios. There is also good compositional correlation between the
hydrocarbons of a microseepage and those in the underlying reservoir (Saunders et al.,
1991).
The occurrence of hydrocarbon microseepage directly above reservoirs points to
vertical migration of hydrocarbons, despite the fact that groundwater movement might
be expected to militate against this. Indeed, the cross-sectional shape of the hydrocarbon
leakage pattern has been termed a "chimney", and most chimneys are nearly vertical
(Tedosco, 1995). Vertical migration through the strata has been attributed to at least four
mechanisms: effusion; diffusion; solution; and gas bubbles.
Effusion as free hydrocarbon gases is thought to be the principal mechanism leading
to macroseepage. It arises as a result of the very large pressure differential that exists
across a petroleum reservoir. Diffusion of hydrocarbon gases that are usually dissolved
in groundwater has been observed through seemingly impermeable barriers (Rosaire et
al., 1940). This form of migration is thought to contribute to microseeps. Also dissolved
low molecular weight hydrocarbons in groundwater migrate through capping shales as a
result of hydrodynamic or chemical potential drive (Duchscherer, 1980). Vertical ascent
of ultra-small (colloidal size) gas bubbles through a network of inter-connected,
groundwater-filled microfractures is advocated by Price (1986). Buoyant colloidal gas
bubbles are readily displaced upward at rates of up to several millimetres per second.
This fast ascent explains the rapid development of light hydrocarbon anomalies in soil
gas over newly-filled gas storage reservoirs, and their rapid disappearance after a
reservoir is depleted.
Although microseeps (and macroseeps) represent leakage from a temporarily
stationary source of petroleum, they do not necessarily indicate the presence of
economically-recoverable hydrocarbons at depth. The economic viability of the
underlying reservoir can only be established by further exploration.

Induced surface manifestations of microseepage

The surface manifestation of hydrocarbon microseepage in not necessarily confined
to the presence of trace quantities of hydrocarbons. Schumacher (1996) made a thorough
review of the major hydrocarbon-induced changes affecting soils and sediments and
their implications for surface exploration.
Schumacher (1996) contended that long-term leakage of hydrocarbons can establish
locally-anomalous redox zones that favour the development of a diverse array of
chemical and mineralogical changes. The bacterial oxidation of light hydrocarbons can