Page 210 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 187

and isotopic compositions of carbonate-cemented surface rocks over oil and gas fields
(Donovan, 1974; Donovan and Dalziel, 1977). Stroganov (1969) has confirmed that the
deeper distribution of hydrocarbons only rarely yields a halo pattern, suggesting the
halos have a near-surface origin. Matthews (1985) suggested that diagenetic blockage
related to hydrocarbon emplacement may originate at intermediate depths and then be
exhumed by erosional processes.
Although direct anomalies and halos have conflicting explanations, both appear to be
valid. Indeed, the controversy is significant only if it is assumed that lateral displacement
has not occurred during subsurface leakage. This is certainly a valid assumption in some,
but definitely not all, cases. If the halo pattern is interpreted as a subset of several
preferential pathways, one can assume that at least one major flowpath could become
blocked by diagenetic cement, resulting in a bias of leakage, with a false halo forming as
the gases are diverted around this blockage in an area that previously yielded a direct
anomaly. In one study the occurrence of halos was suggested by adsorbed soil-gas
samples, whilst direct anomalies were observed using free soil-gas samples (Richers et
al., 1986). One must speculate that these techniques measure different aspects of the
leakage phenomena. For this reason, it is felt prudent to always collect both types of
samples whenever economically feasible. In addition one would be well advised to
incorporate geological and geophysical data into the model.
A significant portion of near-surface hydrocarbon survey results appear to be
compatible with the mechanisms of macroseepage, particularly leakage occurring along
preferential pathways. Those anomalies seemingly not coincident with known faults,
fractures, unconformities, bedding planes or other obvious pathways may lie on
pathways unrecognised due to limited or incorrect mapping. Alternatively, some
occurrences may represent processes not completely understood, or processes not validly
extrapolated from macroseepage to microseepage.
The preferential pathway model summarises the movement of hydrocarbon fluids
through the subsurface to their final destination as a surface seep, either directly or by
way of an intermediate trap. It is certainly not definitive nor complete, but illustrates
some of the challenges confronting the petroleum geologist in his quest for new
resources.

Geochemical populations

An altemative to modelling hydrocarbon gas migration as a basis for data
interpretation is to decompose data into geochemical populations. On this basis surface
geochemical data can be interpreted with respect to both composition and magnitude.
The goal of compositional analyses is to be able to characterise the type or types of
subsurface accumulations present and to be able to predict the location at which they
occur. This can be achieved through using ratios of the various hydrocarbon constituents
that are detected in the soil-gas sample. In general, gas reservoirs are commonly