Page 197 - Geology and Geochemistry of Oil and Gas

166 FORMATION OF HYDROCARBON ACCUMULATIONS

First-order pattern within the fields, grouped together on the basis of common
geotectonic conditions, include the following changes based on the decrease in scale
(Fig. 9.5)
I. Regional, with transition from trap to trap along the anticlinal trend.
II. Local, through the stratigraphic section, with change in depth of a stratigraphic
unit.
III. Interreservoir, dependent on change in the topographic depth of the individual
reservoir. During the process of field development and production, this pattern
has been associated with changes in the properties of oil and water with time.
Second-order patterns, also having a definite regularity, may be qualitatively or
quantitatively depend on several factors including (1) lithology, (2) reservoir-rock
properties, (3) total salinity and ionic composition of the formation water, (4)
migration of the subsurface fluids, and (5) basin hydrodynamics.
A sedimentary basin usually includes several hydrodynamic systems. Each
individual reservoir can have its own hydrodynamic system. The formation of such
systems (either artesian or ‘‘elision’’) occurs independent of each other, and they may
have independent reservoir pressure distributions.
As long as there is a pressure difference, there is a possibility of cross-flow from
one system to the next. This, however, requires avenues of migration. The most
common avenue for migration is a fault plane, although movement can also occur
through ‘‘windows’’ in the seals separating reservoirs. If such avenues of migration
are wide enough and long-lasting, the reservoirs may form a single complex
hydrodynamic system. The cross-flows from one system to the next one with
the possible formation (neoformation) of the hydrocarbon accumulations are

Fig. 9.5. Classification of patterns of changes in the crude oil properties on the basis of crude oil density
(after Buryakovsky, 1974).