Page 231 - Geology and Geochemistry of Oil and Gas

200 CLASSIFICATIONS OF OIL AND GAS ACCUMULATIONS

Significant features of an oil–gas field include characterization of the rock se-
quence (see Chapter 2), distinct expression of the structure, and complexity of the
structure. Obviously, this characterization depends on the geologic evolution of a
specific region. The latter, in turn, is defined by the tectonic processes.
A structural element can be defined by its height and the steepness of its flanks.
The height is the difference in depth from the highest point of structure to the spill
point. The structural height controls the accumulation column, the possible gravity
differentiation of fluids in the accumulations, maximum values of the surplus pres-
sure, and the possibility of accumulation being ‘‘squeezed-out’’ from the trap
(Chapter 3). The steepness of flanks first of all affects the gravity buoyancy force ( f )
updip the reservoir:
f ¼ gDr sin a (10.2)
where g is the gravitational acceleration, Dr is the difference in the density of fluids,
and a is the dip angle.
Difference between the density of hydrocarbons (oil and/or gas) and that of water
in a reservoir can create an overpressure (Fig. 10.13). Obviously, in the case of gas,
the overpressure will be more pronounced.
The structural complexity of the area, where the hydrocarbon field is located, very
significantly affects the natural and technical conditions of the field. The natural
conditions are characterized by the formation and existence (within a specific area of
the Earth’s crust) of different hydrocarbon accumulations (different in terms of type,
shape, and fluid content) and their possible interrelations. The technical conditions
encompass the exploration, appraisal, testing, and production techniques. They also
depend on the geologic structure, including the characteristics of the rocks enclosing
the hydrocarbon accumulation, as discussed in Chapter 2.
One of the primary characteristics of the field is the number and type of deposits.
The number of deposits and their grouping among the separate structural elements
of the field (if they are clearly distinguishable) define the basis for the appraisal and
development of productive formations. The grouping of latter is necessary for plan-
ning commingled production and possible recompletions. The hydrocarbon reserves
and their commercial category must be determined for each field. Frequently, the
2
‘‘specific density of reserves’’ (tons of oil or cubic meters of gas per 1 km of the field
area) is used to characterize hydrocarbon fields. The higher the specific density, the
more efficient the field development (field facilities and the efficiency of their uti-
lization).
Several zones may be identified in the vertical section of each field, on the basis
of the oil and gas occurrence. The authors of this book propose to distinguish
five vertical zones on the basis of temperature, pressure, and phase relationships
(Table 10.4).
The depth boundaries in Table 10.4 are very tentative, because they depend on the
quality of fluids and the nature of heat flow. Temperatures are not specified, because
they will change for the selected depth and pressure intervals, depending on the heat
flow. Zone 3 is within the critical pressure range for water. In the case of a
‘‘stretched’’ heat flow, the temperature even at a depth of 4.5–5 km may be just