Page 222 - Geochemistry of Oil Field Waters

SEDIMENTARY ROCKS 209

Other exchange reactions occur, whereby montmorillonite transforms to
illite (Burst,‘1969). Ions are adsorbed by negatively charged clays from the
sea water (Krauskopf; 1956). Dissolved salts hydrolyze; e.g., olivine hydrol-
yzes to serpentine as follows:

2Mg2 SiO, + 3H2 0 =+ 3Mg * 2Si02 2H2 0 + Mg(OH)2

Hydration and dehydration reactions occur, such as the gypsum-anhydrite
relation :

CaS0, + 2H20 + CaSO, * 2H20

As depth of burial increases, many mineralogical changes take place in the
sediments. In the Gulf Coast Tertiary, the clay mineral montmorillonite
gradually disappears at depths between 2,500 and 3,000 m (Burst, 1969). It is
replaced by mixed layer and illitic clay minerals. This change involves
chemical alteration and also the release of water of crystallization. This
change appears to be temperature dependent with the reactions starting at
about 100°C.
Other mineral changes occur in the sandstones such as the deposition of
secondary silica overgrowths on the quartz grains causing resultant loss in
porosity. Available data differ on the loss of porosity with burial depth
(Atwater and Miller, 1965; Philipp et al., 1963). Much of the silica may
come from outside the porous sand, such as from shales whose pore water is
traversing the sand as a result of compaction. Authigenic clay minerals such
as kaolinite also form in the pores.
Interstitial water in deeply buried sediments sometimes is at a pressure
close to the weight of the overburden. This pressure may be sufficient to
burst the rock and allow the water to move out through the fissures which it
forms. These fissures are principally vertical and extend upward into zones
of lower temperature. The water forming and subsequently filling the fis-
sures usually is a mixture of salty interstitial sedimentary pore water and
water from the clay minerals released by their recrystallization. If it is hot,
and saturated with silica and other minerals, it could force its way upward
and as it cools deposit quartz, feldspar, calcite, and other minerals. Possibly
many hydrothermal ore veins were formed by interstitial sedimentary waters
rather than by “juvenile” waters.
These hydrothermal veins contain metallic minerals composed of com-
pounds containing copper, zinc, lead, gold, and silver. The process is a
geothermal convection cell and it is able to concentrate and segregate useful
minerals. The process has many points of resemblance to the concentration
and segregation of petroleum - the principal difference being that the geo-
thermal convection cells operate at higher temperatures.