Page 166 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 143

It appears therefore that the direct detection of hydrocarbon gases is not the only
means of identifying areas of active microseepage, but that a myriad of other possible
secondary techniques can be used either as adjuncts, or as solitary techniques in
themselves, to infer the presence of hydrocarbons in the subsurface environment. Most
of these utilise the detection and subsequent analysis of gaseous hydrocarbons, while
other methods employ the detection and analysis of liquid hydrocarbons, non-
hydrocarbon gases, the presence and relative concentration of bacteria, and even the
presence (or absence) of inorganic compounds and elements. For the most part, however,
methods that directly measure the hydrocarbon content of soils or soil atmospheres have
met with the most acceptance.

PHYSICAL BASIS FOR MIGRATION OF HYDROCARBONS TO THE SURFACE

Basic assumptions

The fundamental assumption of near-surface hydrocarbon prospecting techniques is
that thermogenic hydrocarbons generated and trapped at depth leak in varying quantities
towards the surface of the Earth. That these hydrocarbons present in the near-surface
environment represent the products of generation and migration from subsurface points
of origin is a necessary conclusion that is universally accepted with respect to
hydrocarbon macroseepage. Examples abound, such as the Santa Barbara Channel seeps,
the La Brea Tar pits of Los Angeles, the Athabasca Tar Sands, etc. The same
relationship has been equally well established, although less commonly accepted, for
microseepage.
A further assumption is that the pattern and intensity of this leakage also provides
information on preferential pathways that the leakage follows, and as such can be
combined with additional geologic information to predict broad subsurface hydrocarbon
fairways. In fact, in some instances it has been claimed that such data can identify areas
of reservoired hydrocarbons. This last claim is often the subject of heated debate,
however, commonly depending in which camp (for or against geochemistry) the
explorationist resides.
The physical state of the hydrocarbons during transport is not well known; see
Matthews (1996a) and Matthews (1996b) for a full discussion. Nevertheless, most of the
models proposed for the transport of these fluids from source to reservoir (aqueous
transport, micellular, discrete oil-phase transport, gaseous transport, etc.) are applicable
to the continued transport of hydrocarbons from these source beds and/or reservoirs to
the near-surface environment. An additional constraint on land is that the last stage of
transport is generally above the water table. The physics of transport can be subdivided
into two categories, effusion and diffusion.