Page 186 - Geology and Geochemistry of Oil and Gas

HYDROCARBON EXPULSION (‘‘PRIMARY MIGRATION’’) 155
The above discussion shows that during lithogenesis the source rocks reside in an
extremely stressed energy state. Such a system tends to achieve the state of
equilibrium. The energy is redistributed within the system (source rock) and is
transported (along with the fluids) to the adjacent beds with a lower potential energy
level. This results in the leveling of the energy non-uniformity. The process is
unavoidably accompanied by folding and faulting.
Thus, a particle of organic matter, with all the internal energy accumulated within
it, gets into a sedimentary sequence, which is in a highly stressed-energy state. In the
process, the organic matter is altered, first of all by way of losing various mobile
components, including relic hydrocarbons.
The detachment of individual molecules from the organic matter may be initiated
by various energy potentials. An excellent monograph on the primary oil migration
was published by Beletskaya (1990). The study is based on the analysis of data in the
literature and on the laboratory experiments. Beletskaya concluded (p. 277) that the
following factors cause the detachment of microoil from the source organic matter:
(a) desorption of the reservoir water, gas and hydrocarbons; (b) temperature increase
due to the sediment burial or due to endothermic processes in sediments; and (c)
possibly, diffusion processes.
Actually, the number of reasons may be greater than that. Beletskaya (1990) treats
organic matter as a passive object, whereas in fact it is one of the most active, energy-
saturated components of the system (see Chapter 7). To achieve the energy
equilibrium, it can expel the ‘‘extra’’ particles or molecules and can cause exothermic
reactions.
The compaction of sediments with the included organic matter begins at the stage
of diagenesis and continues during catagenesis. The compaction caused by the
increased pressure and changes in the structure of the substance drastically decrease
the surface of particles and, hence, their surface to energy. A decrease in the surface
energy makes it easier for particles (and molecules) to be detached from the
‘‘mother’’ (source) substance. If a water or hydrocarbon film covered the surface of
the solid substance, such film may have moved either to the adjacent particle with
higher surface energy or to the pore space. In the latter case, if hydrocarbons were
present, they may form a free phase (liquid or gas). Obviously, hydrophilic or
hydrophobic properties of the surfaces, or changes thereof, would affect the course
of the process.
Thus, individual organic matter components detaching from the source matter
may be considered as the first stage of expulsion process. The following second stage
would be the displacement (migration) of the released matter. The third stage is the
primary accumulation (i.e., the formation of a substance that is moving in the pores
according to the law of hydrodynamics). The nature and direction of the second and
third stages would depend of the physical state of migrating substance, which will
determine the physical or physicochemical processes providing for the displacement
of substance and the selection of the avenue of migration.
Hydrocarbon molecules detached from the source matter get into the surrounding
water and form a true (molecular) solution. The solubility of gas and liquid
hydrocarbons in water have been extensively studied (e.g., Chekalyuk and Filos,