Page 230 - Geology and Geochemistry of Oil and Gas

VERTICAL ZONATION OF HYDROCARBON ACCUMULATIONS 199
monocline, or overthrusts and underthrusts. Therefore, ‘‘field’’ is not just a geologic
term but rather a geologic–technologic one. (See Kontorovich et al., 1975.)
An oil and/or gas field should be characterized first of all by the geological
structure of the area it is located in and by the types of traps. Usually, some ad-
ditional information to characterize the field is provided including:
location of the area relative to larger regional structures of the Earth’s crust;
the presence of different structural stages within the field;
the number of accumulations;
the amount of reserves and their density over the area;
hydrocarbon phase state in the accumulations;
vertical zonation of hydrocarbon distribution;
subsurface temperature and pressure; and
the number of target reservoirs.
There are numerous classifications of the local tectonic structures, including some
specifically petroliferous. The following are the most important parameters of the
classifications:
(1) Almost all classifications are based on general tectonic characteristics commonly
accepted as of the date of classification. They include the location of structures:
(1) the oceanic, continental or transitional crust; (2) the platform or geosyncline;
(3) the center or margin of a tectonic plate; and (4) the divergence and con-
vergence zones.
(2) In all classifications, the formation of crustal element associated with hydro-
carbon accumulation is treated in the context of evolution of the Earth’s crust as
part of the lithosphere (sometimes, astenosphere and rarely, mantle).
(3) Some structures of ‘‘cosmic’’ origin (meteorite impact structures) have been
found to be petroliferous. Donofrio (1981) stated that there are at least 3000
impact craters greater than 10 km in diameter on the Earth’s surface. King
(1979) described an impact structure as follows: An external circular swell is
comprised of loose rocks, which can form a good reservoir rock upon burial.
Bedrock underneath the swell is fractured. The internal depression is usually
filled up with breccia. In some cases, the depression may have a central high or a
ring structure surrounded by a circular depression. The rocks underneath the
crater are usually compacted and form a graben.
Donofrio (1981) considered the Newfield accumulation (USA) an impact structure
in the Carboniferous sediments. The diameter of swell is 5–6 km. An oil accumu-
lation (with about 17 million barrels of recoverable reserves) is found in the breccia
of the crater. The Red Wing Creek Field accumulation of Ordovician age (40–70
million barrels of recoverable reserves) resides in the breccia of central high, 6.5 km
in diameter.
A significant number of ring structures were discovered recently as a result of
surveys from space. Their origin is not clear. Some of them may be impact structures,
whereas most of them are probably of deep-seated origin. Attempts by some ge-
ologists to relate the distribution of hydrocarbon fields to rings are premature. The
assumption that the hydrocarbons migrated from the great depth in the Earth are
speculative.