Page 84 - Petrophysics

58 PETROPHYSICS: RESERVOIR ROCK PROPERTIES

high pressures found in the Gulf Coast Basin of the United States.
Undercompaction of the sediments can occur during rapid sedimentation
and burial of sediments containing a large quantity of clay minerals. The
complete expulsion of water does not occur, leaving the sediments
as a loosely bound system of swollen clay particles with interlayer
water. Continued sedimentary deposition caused a shear zone to develop
by overloading the undercompacted shale. Expulsion of the water
was accompanied by subsidence of blocks of sediments. Thus, the
contemporaneous fault zone of the Gulf Basin is characterized by the
cycle of deposition, temperature increase, expulsion of water, and sub-
sidence of blocks of sediments. As the depth of burial continued, the
increase in temperature induced dehydration of the clays within the
buried zone and contributed to the shearing stresses. The transformation
of montmorillonite to illite during diagenesis and catagenesis occurs
between 150” and 250”F, releasing an amount of water equal to about half
of the original volume, leading to undercompaction in the geopressured
zone. When the fluid pressure exceeds the total overburden pressure,
the faults act as “valves” for discharge of water upward into the
hydropressured aquifers overlying the zone. As the pressure declines, the
“valves” close until the pressure once more exceeds the total overburden
pressure [22, 231.
Another contributor to the fluid overpressure is the temperature
increase that occurs within the geopressured zone. The overlying,
normally pressured sediments that are well compacted possess a lower
thermal conductivity and act as a “blanket,” decreasing the transfer
of heat from the mantle. The heat trapped by the blanket above the
geopressured zone produces an abnormally high temperature in the
formation, which contributes another incremental pressure increase to
the fluid [24].
Geopressured zones along the Gulf Coast generally occur at depths
below 8,000 feet and require careful and expensive drilling technology
whenever the zones are penetrated. The zones usually contain about
3.6 cm3 of methane per m2 of brine (20 SCFhbl).

OILFIELD WATERS

The genesis of petroleum is intimately associated with shallow marine
environments; hence, it is not surprising that water found associated
with oil generally contains dissolved salts, especially sodium and calcium
chlorides. Petroleum source rocks originally formed in lakes or streams,
and the porous sediments that became today’s petroleum reservoirs
could have acquired saline waters by later exposure to marine waters.
Thus, the original waters present in the sediments when they were