Page 188 - Geology and Geochemistry of Oil and Gas

HYDROCARBON EXPULSION (‘‘PRIMARY MIGRATION’’) 157
A. M. Bloch (in: Eremenko and Chilingar, 1996) and Simonenko (1988), such water
in limited pore space may create superconcentrated solutions (similar to melts) that
become unstable upon transition to larger pore space. When these solutions enter
such pore space with weak surface forces, hydrocarbons and other dissolved
components are ‘‘salted-out’’. The presence of elevated ore concentrations at the
contact of clays and sands may serve as an indirect confirmation of this
phenomenon.
The above explanation, which needs an experimental confirmation, may explain
the primary migration and accumulation of oil. Tsarev (1978) suggested that film of
microoil forms at the boundary of residual (irreducible) and ‘‘submelted’’ water. The
‘‘submelted’’ water layers, together with the microoil, have high mobility and will be
squeezed out of the rock first.
Kruglikov (1976) proposed a somewhat different mechanism of the film-type
migration of hydrocarbons. According to him, the forming gas bubbles, which are
enshrouded with an oil film, can migrate. The mobility of such an emulsion within
the rock may be quite difficult because of the similar size of gas bubbles (with an oil
film) and narrow canals.

9.2.6. Dissolution in Compressed Gases (See Retrograde Dissolution in Chapter 6)

A compressed free hydrocarbon gas, especially in the presence of methane
homologs and CO 2 , is capable of extracting hydrocarbons and other oil-like
substances from the organic matter (see e.g., Zhuze, 1986; Beletskaya, 1990).
According to Beletskaya, at a depth of about 1 km, a cubic meter of gas (a mixture of
methane+methane homologs+CO 2 corrected to standard conditions) in a near-
3
static environment can dissolve 0.0012 m of oil. This shows that the primary
migration of oil is quite possible in the presence of free gas.
Material balance calculations by Neruchev (1979) showed that the gas, released as
a result of decomposition of organic matter, can dissolve up to 40% of the generated
bitumen. Taking into account the phase composition of fluids migrating toward an
accumulation, the quantitative ratios and the solubility of released gases in water,
Neruchev concluded that only when the organic matter content in rocks exceeds 2%,
can some CO 2 and methane form an independent phase and transport liquid
hydrocarbons. Therefore, this primary migration mechanism is possible when the
organic matter content in a rock exceeds 2%, but is an unlikely scenario for all oil
accumulations.

9.2.7. Diffusion

Dissipation of energy and matter is a common phenomenon in the Earth’s
crust. This phenomenon definitely occurs in the process of primary migration of
oil, but cannot in principle explain the primary and, even more so, the secondary
hydrocarbon accumulation because of its positive entropic nature (concentration