Page 178 - Geology and Geochemistry of Oil and Gas

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Chapter 9

FORMATION OF HYDROCARBON ACCUMULATIONS

9.1. SEDIMENTARY BASINS

Oil and gas fields in any sedimentary basin are not distributed uniformly. They
form non-uniform assemblages, and each one of these assemblages usually has
stratigraphically similar oil- and gas-bearing deposits with similar regional geology
(exceptions to this rule, however, are known). Such assemblages can be called ‘‘oil
and gas accumulation zones’’. They may form (1) around a large field, (2) over a
large arch, (3) satellite fields, and (4) without a large field, thus composing an
‘‘association’’. Such assemblages may include a number of fields more or less of
equal size (depending on the size of traps) along (a) the structural swells, (b) some
pinch-out zones, or (c) stratigraphic unconformities on monoclines. Sometimes, in
the center of a massive plate, it may be quite difficult to identify a structural unit,
which controls association of several fields. It is not necessary for every hydrocarbon
field to be a member of an oil and gas accumulation zone. There could be an
independent ‘‘zone’’ with a single field.
Grouping of hydrocarbon fields associated with large structural unit determines
some common features of the environment of formation of accumulations and fields
within an oil and gas accumulation zone. This is predicated on the similarity of the
formation of sediment sequences, which, in turn, is based on the common tectonic
evolution of the region and on similar conditions of formation of traps of various
types. The identification of oil and gas accumulation zones at different exploration
stages ensures the success of exploration operations and enables one to forecast the
subsurface temperature, pressure, and phase composition of hydrocarbons.
The oil and gas accumulation zone is sometimes called a ‘‘trend’’. The latter,
however, is most commonly used to describe a linear distribution of fields with the
productive deposits of about the same age. It should not be confused with the term
‘‘play’’ that presupposes not so much the structural commonality as the
commonality of oil and gas sequence. Thus, two or more ‘‘plays’’ may be included
in a single oil and gas accumulation zone. As an example, in the northeastern
Caucasus, with its frontal ranges, there are two oil and gas accumulation zones,
which include four plays of Tertiary and Cretaceous age in each zone. On the other
hand, a single play may include the oil and gas deposits associated with different oil
and gas accumulation zones. In the above example, there are two plays (Tertiary and
Cretaceous) for the two oil and gas accumulation zones but each play involves both
zones.
Otis and Schneidermann (1997) evaluated four different concepts of exploration
based on the degree of knowledge about the specific geologic feature:
1. Basin concept. Basin concept involves an assessment of (1) the potential of source
rocks, (2) capacity of reservoirs, (3) capacity of traps, (4) sealing properties, (5)