Page 190 - Geology and Geochemistry of Oil and Gas

PRIMARY ACCUMULATION AND FREE PHASE MIGRATION 159
Magara (1981, 1982) estimated that the oil saturation should be at least 14.5% in
order for a continuous stream of oil to form in a rock. Cartmill (1979) proposed that
the minute oil droplets flocculate into clusters coated with water. If this migration
mechanism was operative, the oil might not have left any traces in the rock. Whether
or not the oil is able to move as such emulsions has not yet been studied. It is not
clear if it would be possible for such ‘‘clusters’’ to get through the pore throats and
canals. The stability of such emulsions under reservoir conditions is also unknown
(this is important for the primary accumulation).
Dickey (1975) proposed that a substantial volume of oil could migrate through a
hydrophobic rock. According to him, in such a case, the concentration barrier for
the phase migration of the oil may be as low as 1%. Actually, the rock does not have
to be hydrophobic. It must contain conductive channels lined with hydrophobic
organic substances (such as, residual asphaltenes) or minerals (sulfur, graphite,
talcum, and/or sulfides).
The concentration barrier for the oil movement may actually be lowered from
25% (at uniform saturation) to 1% (at non-uniform saturation). Eremenko (1985)
proposed a non-uniform spreading of the organic matter transformation within
rocks and non-uniform distribution of the products of this transformation. Within
each limited volume of rock containing organic matter, energy redistribution occurs
in the rock–water–organic matter system due to external and internal factors. High-
energy compounds formed as a result begin to move toward the areas with lower
energy or toward the adjacent reservoir rocks. This movement occurs most likely
according to the laws of film-type migration. The process develops with an increase
in the volume of organic matter (due to external energy). This results in the
appearance, movement and superposition of molecular films one over the other. As
the number of film layers increases, their molecular connections with the rock and
the source organic matter decreases. Eventually, they blend together and form a
single substance. Probably, Klubova (1975) observed such phenomena under the
microscope.
Other mechanisms of filling of fractures with fluids are possible. For instance, a
substance formed in pores may be squeezed out into a fracture. The process is non-
uniform in time and space. The degree of organic matter transformation at different
parts of the same sequence, especially if it is heterogeneous, differs. Non-uniformity
of primary accumulation process results in a sporadic distribution of primary
accumulations (streams) of hydrocarbons obeying the laws of mechanics.
The Shklovsky’s model (in: Eremenko and Chilingar, 1996) cannot be applied in
this case because there is no indefinite cluster and there is a directional change in
time. The saturation needed for the movement of a continuous phase (a stream) may
be reached in just a few channels (canals). The average saturation in rock may be
actually very low, just a fraction of one percent. In the proposed concept, the non-
uniformity of the medium, the processes occurring within the medium, and the
energy stress of the rock sequence are essential factors of the primary hydrocarbon
migration and accumulation.
From this position, a uniform organic matter accumulation within the uniform
sequences (such as the Maicopian and Bazhenov formations in Russia and the Green