Page 205 - Geology and Geochemistry of Oil and Gas

174 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

shape, lateral extent, and height in relation with the positions of possible hydro-
carbon cross-flows, and
type of the barrier restricting the migration, which at the same time defines the
accumulation boundaries.
Any trap classification reflecting these conditions can be considered practical.
The accumulation of hydrocarbons, filling the trap, is an elementary accumula-
tion of hydrocarbons, the major parameters of which are their quality and quantity
and the accumulated energy. Besides, additional parameters must provide necessary
information for a successful appraisal and subsequent development of the accumu-
lation. Such parameters include the degree to which the traps are filled and the
nature of the gas–oil–water contacts. The former depends on the environment of
trap formation and the tectonic evolution of the basin. In many cases, certain pat-
terns in the degree to which the traps are filled can be identified in a basin for a
particular stratigraphic interval. This may be successfully used in exploration and
estimation of resources of a particular stratigraphic sequence.
The locations of the gas–oil–water contacts are important during the exploration
and development phases. The knowledge of the tilt of oil–water contact, if present,
may prevent drilling of unnecessary wells. The task gets more complicated if there
are no definite contacts, replaced by transitional zones. Besides, the contour must be
closed (also against a barrier).
If the amount of hydrocarbons in an accumulation is sufficient to conduct pro-
duction, the accumulation is called commercial. This concept is uncertain and de-
pends on many factors, such as the geographic location, available infrastructure,
depth, quality of hydrocarbons, and the presence of beneficial or harmful admix-
tures.
Most common classifications of the size of oil and gas accumulations in the US
and Russia are presented in Table 10.1. This table shows that in the US greater effort
is devoted to the development of small accumulations than in Russia. The reasons
for that include the following:
1. A more advanced oil and gas-producing equipment in the US enables to produce
economically from the smaller accumulations.
2. Smaller accumulations are neglected in Russia, and only larger accumulations are
developed because of economics and lower demand of oil than in US.
The classification of the accumulations by the average rate of production is also
quantitative. Kontorovich et al. (1975), for example, proposed the classification
presented in Table 10.2.
The class boundaries are tentative and may be changed to fit a specific situation
for a particular petroliferous province. It is important to keep in mind the fact
that the flow rates reflect not only the natural conditions of the accumulation
(lithology, fluid properties, energy of accumulation, etc.), but also the technology
(development techniques, well facilities, etc.). Besides, the boundary between the
low-rate and non-commercial classes is determined by the economical–technical
reasons. The price of the oil and gas determines the position of such boundary, and
the political situation (e.g., embargo) may impose the development of even very
small accumulations.