Page 243 - Geochemical Remote Sensing of The Sub-Surface

216 T. Ruan and Q. Fei

Indicator gases

Microseeps comprise a number of different gases that are evolved as a product or
by-product of the generation, migration, accumulation, dispersion and destruction of
petroleum, along with other gases that follow the same migration route. Methane (C~) is
by far the most abundant indicator gas in microseeps by virtue of its correspondingly
high relative abundance in petroleum, its low molecular weight and its chemical
stability. However, as an indicator of oil and gas it suffers interference from methane of
near-surface biological origin (Philp and Crisp, 1982). Thus care must be exercised when
interpreting the C~ patterns of gas geochemical surveys. Discrimination between
thermogenic methane (from oil and gas) and near-surface biogenic methane is possible
by means of carbon isotope determinations, but the method is relatively expensive.
Ethane (C2), propane (C3) butane (C4) and pentane (C5) are useful for characterising the
thermogenic origin of microseeps because they are less likely to form in the near surface
by biological processes. Some higher molecular-weight hydrocarbons (C5 up to C22) are
found in microseeps either because they possess appreciable vapour pressures or because
they occur in the form of an aerosol, which essentially behaves as a gas. Data for a
combination of the foregoing gases provide a fingerprint from which the origin and
significance of surface anomalies can be deduced with greater confidence than is
possible using only data for a single gas (Klusman and Voorhees, 1983).
In addition to hydrocarbons, a number of inorganic gases that follow similar
migration pathways are useful in gas geochemical surveys for petroleum, ltelium
produced by the radiodecay of U and Th in rocks may accumulate in gas traps to
thousands of times of its average concentration of 5.24 ppm in atmospheric air, and
being light and inert, may escape to the surface. Mercury is absorbed from water by
phytoplankton, the very raw material of petroleum" therefore Hg is intrinsically related
to the generation of oil and gas. In remote sedimentary basins where Hg of igneous and
anthropogenic origins is rare, Hg detection can prove useful in gas geochemistry surveys
for petroleum exploration. Since uranium seems to concentrate in the low Eh oil-water
contact zone in oil fields, radon and its daughters find applications in surface prospecting
tbr oil fields. Some natural gas contains H2S and SO2, which are produced by bacteria
attacking sulphates or by thermal alteration of amines (Hunt, 1979). These sulphur gases
are therefore possible surface indicators of natural gas fields, but their ready solubility in
groundwater leads to weak and unstable anomalies. Nitrogen is evolved during thermal
maturation of organic matter, but N2 anomalies are difficult to detect in the near-surface
because of the high background concentration of N2 in the atmosphere.
Thus oil and gas fields are associated with many types of gases. During migration
many of these gases, especially the hydrocarbons, react with their surroundings to alter
the environment, usually by consuming free oxygen and producing more reducing
conditions. The formation of pyrite, magnetite, siderite and other carbonates, as well as
the proliferation of certain bacteria, are amongst the near-surface consequences of
hydrocarbon gas migration. Whilst this chapter is concerned with the direct detection of