Page 179 - Geology and Geochemistry of Oil and Gas

148 FORMATION OF HYDROCARBON ACCUMULATIONS

proper timing of trap formation, and (6) proper timing of migration. This
assessment is a screening device only, and does not include commercial
considerations.
2. Petroleum system concept. Petroleum system concept involves a volume of
sedimentary rocks containing hydrocarbons, charged by a single source rock.
Application of this concept requires the presence of the manifestations of
hydrocarbons (seeps, shows, or a producing well); it is applicable in many
frontier basins only by analogy. Recognition of an active petroleum system also
serves as a screening device because it carries no volumetric and, therefore,
cannot estimate a commercial value of HC resources.
3. Play concept. The play is an elemental part of a petroleum system, and is
recognized as having one or more hydrocarbon accumulations identified by a
common (1) geological character of the reservoir, trap, and seal, (2) timing of
migration, (3) preservation, (4) location and environment, (5) fluids, and (6) flow
properties. Individual plays have unique geological and engineering features,
which can be used as a basis for commercial assessment.
4. Prospect concept. The prospect is an individual, potential accumulation. Each
prospect is perceived as belonging to an individual play, characterized by risk
components and a probabilistic range distribution of potential hydrocarbon
volumes within its trap confines.
Based on the position of an oil and gas accumulation zone, play, or prospect
within a larger structure (oil-gas basin) may help forecast some parameters of
hydrocarbon accumulations.
Many publications are devoted to the classification of sedimentary petroliferous
basins (see e.g., Eremenko and Chilingarian, 1991). Common features of all
definitions and classifications are (a) stressing the processes of oil and gas generation,
and (b) more rarely, the processes causing the formation and destruction of
hydrocarbon accumulations.
The formation and destruction of accumulation is affected not only by broad
events in the basin or its major portions (e.g., folding), but also by the local events
within the adjacent areas, such as the erosion of the crest of anticline or appearance
of a conducting fault. The authors do not believe that the ‘‘oil and gas basin’’ should
be identified as a distinct category. The identification of sedimentary basin would be
quite sufficient (Vassoyevich, 1975). A sedimentary basin may be petroliferous if it
contains commercial oil and gas accumulations. If the presence of commercial oil
and gas accumulations is only assumed, such basin is called the prospective basin. At
the same time, this basin may be coal-, gold-, uranium-bearing, etc.
Numerous attempts have been undertaken to find the pattern in the distribution
of accumulations in relation to the plate tectonics concept. The oil and gas
accumulations are discovered in the central and marginal parts of the tectonic plates,
with the greatest number of accumulations present at the margins.
No qualitative or quantitative patterns in the distribution of accumulation have
been discerned in the passive versus active margins. The passive margins appear to
have somewhat more reserves. The hypothesis of the hydrocarbon involvement in
the subduction zones with the subsequent transportation along the fault zones to the