Page 183 - Geology and Geochemistry of Oil and Gas

152 FORMATION OF HYDROCARBON ACCUMULATIONS

traps, accumulate in them through physicochemical interactions with one another,
and form a complex chemical mixture called oil. Unfortunately, this idea does not
answer the above question of the energy source and the mechanism of its action.
The concept of primary migration of oil (expulsion) as a result of the compaction
was widely accepted for a long time. Gubkin (1932) suggested that oil left the source
rock together with the last portions of the squeezed-out water. Thus, he suggested
that oil was generated in the source rocks. Early, in the course of lithogenesis, while
the organic matter has not yet been converted into hydrocarbons, mostly water was
squeezed out. At the initial stage of compaction, the squeezed-out water moved
mostly up, toward the lower pressure zone.
Koichi et al. (1985), in order to investigate the primary migration of oil, have
compacted the Na-montmorillonite clay (mixed with seawater and crude oil) for 25
o
2
days under a pressure of 1000 kg/cm and a temperature of 60 C. The amount of
expelled liquid from the clay was large at the early stage of compaction and
decreased gradually as compaction progressed. The proportion of oil in the expelled
liquid gradually increased with increasing time of compaction. Porosity of the
compacted sample decreased from 81% of original clay to E26%.
As the sediments compact and become rocks, the direction of the squeezed-out
fluid flow changes. Inasmuch as the permeability is usually higher in a lateral
direction, the main fluid flow also occurs in that direction. Thus, the main fluid flow
in a basin is directed toward the basin flanks. Upon compaction, because of low
permeability, the flow of fluids is hindered in clays with formation of abnormally
high formation pressure (AHFP).
Compaction of sediments was studied by many scientists, e.g., Rieke and
Chilingarian (1974), Chilingarian and Wolf (1975, 1976), Buryakovsky (1985a,b),
Beletskaya (1990), and Buryakovsky et al. (1991). It was established that the clay
structure changes upon losing water, which is especially noticeable in the case of
montmorillonite. During diagenesis, the montmorillonite is transformed into illite
(Powers, 1967). Its structure drastically changes and it loses up to 50% of its mineral
mass. As a result, earlier bonded interlayer water is released as free water resulting in
additional fracture porosity. Thus, the avenues for the primary migration are
created. In addition, the additional water for expelling organic matter transforma-
tion products becomes available.
The compaction and consolidation of carbonate oozes are accompanied by
crystallization and, often, with appearance of numerous microfractures. Upon
lithification, the mobile substances within the deposit in part become a component of
the rocks, and in part acquire the ability to move. Upon dolomitization
(Mg 2+ +2CaCO 3 -MgCa(CO 3 ) 2 +Ca 2+ ), up to 13.1% porosity (mainly rhombo-
hedral with some microfractures) can form, creating further avenues for migration
(see Chilingarian et al., 1992, 1996).
Transformation of the montmorillonite to illite and the compaction of the other
clays are controlled by the temperature, composition of the interstitial fluids (e.g.,
presence or absence of potassium ion), and permeability changes in the section.
Temperature increases with the subsidence of rocks (whether carbonate or clastic).
Coefficient of temperature expansion is different for different rocks, water, oil, and