Page 76 - Petrophysics

50 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

After leaving the source rock, the hydrocarbons migrate upward
through permeable beds until they reach a sealed hydrocarbon trap
where accumulation occurs forming a hydrocarbon reservoir. This
process has been called the secondary migration, which is governed
principally by buoyancy and hydrodynamic flow [9].

PRIMARY MIGRATION

The geochemical evidence of the generation for petroleum shows
that hydrocarbons do not generally originate in the structural and
stratigraphic traps in which they are found. The petroleum reservoirs
are porous, permeable geologic structures, whereas the source rocks
have been identified as compacted, impermeable shales. Inasmuch as
the source rocks are impermeable, the method of expulsion of oil from
the shales where it is generated is not obvious. Considerable data on the
expulsion of water from shale during compaction show that most of the
pore water is squeezed out during burial before the temperature required
for the generation of petroleum is attained (Figure 2.8).
Compaction of sediments begins as soon as the sediments begin
to accumulate. During original accumulation, the loose, fine-grained
sediments contain more than 50% water. As they are buried deeper,
due to subsidence and continued deposition of sediments on top, the
interstitial water from the deeper sediments is expelled, resulting in
a decrease in porosity and an increase in density. The material acquires
cohesive strength as the grains are pressed together tightly. Chemical
changes occurring in the interstitial fluids produce precipitates that
cement the grains into an even more cohesive formation.
The major oil generation occurs well below the depth at which
compaction of the shale is almost complete. Consequently, the dis-
placement of oil from most source rocks could not have taken place
when the shales were being compacted [6]. Expulsion of oil during
compaction may have taken place in a few isolated cases where rapid
burial resulted in the development of abnormally high pore pressures
or zones of abnormally high temperatures at shallow depths. Barker
contends that petroleum may be expelled from the top and bottom of
source rocks due to the pressure gradient that develops during deep
burial 1161. After expulsion of the pore water, petroleum formed in
the organically rich shale may form a continuous phase and move
along a network of fine, thread-like channels under the applied physical
stress 1131.
Some clay minerals (smectites in general) contain bound water
within the lattice structure of the clay particles. This bound water is
expelled when the smectites are transformed to illite, which begins