Page 192 - Geology and Geochemistry of Oil and Gas

PRIMARY ACCUMULATION AND FREE PHASE MIGRATION 161
Buoyancy is more effective in the case of continuous oil and gas body (a stream)
rather than in the form of discrete droplets. Two questions arise here:
(1) What is the possible size of stream?
(2) What distance can the secondary migration overcome?
The length of the stream (l) depends on the density of oil and water (r and r ),
o w
the inclination angle of the barrier surface (a), and the capillary pressure at the start
of movement. Under the hydrostatic conditions (see Chapters 4 and 5),
ðr r Þg sin a þ Dp=l ¼ 2sð1=R 1 1=R 2 Þ (9.2)
o
w
Levorsen’s (1954) calculations indicate a pronounced effect of buoyancy at a
length of 1 and 10 m of the continuous phase.
As far as Question 2 is concerned, three conditions must be met for the secondary
migration to occur:
(1) Enough fluid must be available along the entire length of movement (migration).
(2) Avenues of migration for this fluid to move must be available.
(3) Availability of forces that could move the fluid along.
Most petroleum geologists believe that the main (maybe even the only) force that
causes the movement of oil and gas is buoyancy (gravitational force). The capillary
forces are considered to constitute the main obstacle to the movement (P. Allen and
J. Allen, 1993, p. 340).
According to the writers of this book, there are many natural forces that may
cause migration of oil and gas. Buoyancy is one of these forces and, quite often, is a
dominant one. One has to keep in mind, however, that buoyancy force decreases
with increasing depth, as physical properties of oil, gas, and water become closer.
Some hydrocarbon accumulations reach tens and even hundreds of kilometers in
length. Thus, the lateral migration (i.e., along the reservoir rock) might have
occurred at least along these distances. Mountainous and piedmont areas of the
folded regions encountered all over the world contain copious oil and gas shows. The
causes for that are twofold: (1) deformed areas include numerous avenues for oil and
gas migration (fractures and faults); and (2) these areas are subject to powerful
erosion, which exposes oil- and gas-saturated beds at the surface. In such cases, the
interreservoir oil and gas migration may be visually estimated to reach at least a few
kilometers. Thus, the knowledge of a specific geological scenario is needed to
establish the length of secondary migration.
The lateral migration in most cases is limited by the position of the axis of the
adjacent depression. The limitations may be associated with obstructions to flow
(often, lithological barriers). Vertical migration in most cases is limited by the
thickness of oil-saturated formations and by the length of faults. Migration is
observed even in such tectonically quiescent regions as West Siberia. According to
Kontorovich et al. (1975, p. 589), the vertical migration could have played a certain
role in the formation of oil and gas accumulations on the Siberian Plate. Examples
include Myldzhinskoye and South Cheremshanskoye fields.
Combination of vertical and lateral migrations is quite possible and the fluids
select the most energy-efficient route regardless of its direction. It appears that most
energy-efficient avenues are linear. Having once selected a route, the fluids tend to