Page 180 - Geology and Geochemistry of Oil and Gas

SEDIMENTARY BASINS 149
sedimentary basins, where they form oil and gas accumulations, is not supported by
field data. Many oil and gas basins do not have even a hypothetical connection with
the subduction zones (e.g., West Siberian, Michigan, and Paris basins).
Based on many oil and gas origin hypotheses, it may appear that the active
margins should have an advantage in terms of the amounts of hydrocarbons
generated (and their lighter, more alkane composition) because of (a) amplitude of
tectonic movements, (b) high thermal stress, and (c) convective supply of the matter
(and heat) from the mantle. No qualitative distinctions are recorded, however, and
the quantitative advantage belongs to the passive margins. The oil and gas
generation is occurring in both the active and passive margins, and the difference in
its intensity is insufficient to cause any apparent varieties.
The taphrogene is the major oil- and gas-producing complex of the passive
margins, whereas for the active margins, it is the riftogene complex (wherever the
rifts are established). Their similarity is apparently due to the way both complexes
form: accumulation occurred after the pronounced tectonic movements. The
advantage of the plate margins becomes obvious for the larger taxons, i.e.,
continents and oceans. The major hydrocarbon reserves are associated with the
continental margins and shelves. The second in importance are the central
continental areas. The central oceanic depressions have low potential.
Brod (1947) introduced a theory that petroliferous basins are associated
with stable subsidence. Tectonics determines the structure of basin with accumu
lations forming within the structural traps. The tectonic processes (oscillating
movements and lateral stresses) also control the facies of the sedimentary sequences
and the location of stratigraphic and lithologic traps. Until now, not much atten
tion was devoted to the uplifting movement occurring against the background of
general subsidence and its accompanying oscillations. Indeed, the uplifting
movement constitutes a force that creates positive structures and gives rise to
the changes in the pressure and temperature, which are major motivators for the
fluid migration.
The origin of a sedimentary basin, its depositional environments, and its tectonic
evolution affects its oil and gas potential.
As far the continents are concerned, petroleum geologists are interested in the
sedimentary basins of (a) internal and marginal platform (plate) areas, (b) foredeeps,
and (c) intermontane troughs. The folded zones are of interest wherever they are
thrust over the platform (plate) margins and wherever the oil and gas accumulations
are discovered underneath them (e.g., Rocky Mountains and Carpathians), or
assumed to be beneath them (e.g., Urals, Trans-Caucasus and Himalayas); also in
intermontane troughs.
Common feature of the large sedimentary basins underneath the World Ocean is
the emergence and evolution over the oceanic or transitional type crust. This affected
the tectonic evolution of the basins, tectonic movements, and the sediment cover, the
very nature of which is due to tectonogenesis. Another characteristic of such basins is
the formation of their sediment fill in deep basins. On the shelf and in the marginal
sea basins the depths are within 1–2.5 km. Over the rest of the World Ocean the
depth is, on average, 4–4.5 km, which rarely decreases to 1–2 km.