Page 157 - Geochemical Remote Sensing of The Sub-Surface

134 V.T. Jones, M.D. Matthews and D.M. Richers

Basically, surface geochemical prospecting is a source-rock tool applied at the
surface. The magnitude of a microseep from a reservoir is related to the permeability of
the migration pathway (and not to the economic worth of the reservoir). A surface
geochemical survey is not currently, and perhaps never will be, a stand-alone prospect
tool. However, with judicious use, this technology can provide information on the
maturity of source beds in a basin and the composition of subsurface hydrocarbons. In
addition, detection of surface microseepage allows mapping the surface expression of the
migration patterns created by the expulsion of fluids as a basin compacts and matures.
When used in conjunction with geophysical and geological information, geochemical
data can refine subsurface models of hydrocarbon trapping and migration configurations.
It is only through careful analysis and integration with other exploration tools that one
can achieve the optimum benefits from this technology.
Near-surface hydrocarbon detection techniques have been shown in both the former
USSR and the United States to be capable of distinguishing basins (or large portions of
basins) that are unproductive from those that are productive, and of distinguishing the
type of production (oil, gas, or mixed oil and gas). This ability has been independently
recognised by Jones and Drozd (1979), Mousseau and Williams (1979), Janezic (1979),
Weismann (1980), Drozd et al. (1981), Jones and Drozd (1983), Richers (1984),
McCrossan et al. (1971), Richers et al. (1982, 1986), Horvitz (1985) and Klusman and
Voorhees (1983). Surface geochemical techniques can select which of several frontier
basins has the greatest chance of containing reservoired hydrocarbons, and the expected
composition (gas, oil, mixed), in addition to high-grading portions of these basins that
have the highest potential. The premise that microseeps occur and that they provide
useful information for exploration is no longer questionable.

ORIGINS OF LIGHT HYDROCARBON GASES

Origin of petroleum

The formation of petroleum and natural gas from organic matter through increasing
depth of burial and temperature has been very well established by many geochemical
studies (Tissot and Welte, 1978; Hunt, 1979). As shown in Fig. 5-1, the generation of the
light hydrocarbon gases, methane (C~), ethane (C2), propane (C3) and the butanes (C4),
occurs in three main stages: diagenesis (<50~ catagenesis (50-200~ and
metamorphism (>200~ in which only dry gas and ultimately graphite are formed.
During the first stage bacteria acting under reducing conditions on organic substrates in
sediments form predominantly methane. According to Hunt (1979), about 82% of the
methane and practically all the heavier hydrocarbon gases are formed in the next,
catagenic stage. Ethane, propane and the butanes are formed in the temperature range
from 70 - 150~ with peak generation occurring around 120~ As shown in Fig. 5-1, a
very large thermal methane peak occurs near 150~