Page 245 - Geochemical Remote Sensing of The Sub-Surface

218 T. Ruan and Q. Fei
SURFACE EXPRESSIONS OF HYDROCARBON MIGRATION

Gas anomalies

According to Kartsev et. al. (1959), Sokolov (1970), Horvitz (1986) and Fei and
Ruan (1991), gas microseepage associated with petroleum has three characteristic styles
of surface anomaly: (1) apical; (2) annular; and (3) linear. One or any combination of
these surface expressions may occur over a given oil or gas field.
An apical anomaly comprises either a continuous zone of elevated gas concentrations
or an area with erratic elevated gas concentrations directly over the oil or gas field. An
annular or halo anomaly has the form of a doughnut of continuous or discontinuous high
gas concentrations surrounding a central zone of lower or background values, the latter
overlying the surface projection of the oil or gas field. In a linear or belt anomaly, high
gas concentrations are found continuously or intermittently along a line or confined in a
belt, usually associated with faults, fracture zones, or matured source beds.
Both apical and annular anomalies can be explained by diffusion models (although
diffusion alone is unlikely to be responsible for them), whilst linear anomalies are
readily attributable to mass flow through an elongated conduit, such as a fault or fracture
zone. Some investigators have argued that diffusion can only lead to the formation of an
apical anomaly (Price, 1986; MacElvain, 1969), but this argument is only valid if
diffusion only takes place in a homogeneous medium. This is rarely the case in the
natural environment and through inhomogeneous media diffusivity is anisotropic and
variable up to several orders of magnitude. Indeed with certain combinations of media,
annular anomalies can be obtained by diffusion (Ruan et. al., 1985a, 1985b).
It is vital, but not necessarily easy, to recognise whether an anomaly is apical,
annular or linear, because the interpretation placed upon each of them leads to quite
different courses of action. The model used to explain an apical anomaly favours
drilling the anomaly peak, whilst the model for an annular anomaly favours the area
within the ring of high values. Follow-up of linear anomalies needs to take into account
structural or lithological information.
All types of anomalies can be present over a given oil or gas field depending on the
relative depth of the field, its caprock lithology and structural control, the gas species in
the microseepage, their modes of occurrence, near-surface lithologies and soil types,
climate and even the time of year. This wide range of factors renders gas anomaly
interpretation difficult.

Alteration

Migrating hydrocarbons may also interact with the rock column through which they
pass. These interactions become most pronounced as the ascending hydrocarbons
approach the surface and strive to equilibrate with changing conditions characterised by