368
8 ALLOCHTHONOUS SEDIMENTS
0 -
Ground
level
Increasing
depth of
burial
I Tangential contacts
/
/
l].Long contacts ~]
"-z._3J/
@
TTI. Sutured and concavoconvex contacts
0 ~
Decreasing porosity
:55%
5 ---,
Increasing no. of
I. 5 %
contacts per grain
Fig. 8.26. The destruction of porosity in sandstones with increasing depth. Previously interpreted as largely
due to pressure solution, cathodoluminescence shows that diagenetic silica cementation is the main process.
significantly diminish permeability (Wilson and Pittman, 1977; Guvan et aL, 1980). There
are four main types of clay to consider: kaolinite, illite, montmorillonite, and chlorite.
Kaolinite grows as discrete crystals often with a shape like an accordian (Fig. 8.27A).
These crystals are sometimes so extended that they look like a worm (described as "vermicular" by the learned). Kaolinite cement obviously destroys porosity, but it does not
do as much damage to permeability as other authigenic clays, which, though they may
diminish porosity by the same amount, volume for volume, are far more destructive of
permeability.
Illite tends to form fibrous crystals that grow radially from the edges of sand grains
(Fig. 8.27B). These furry illite jackets extend across the throat passages, thus diminishing permeability.
Montmorillonite has the worst effect on permeabilitity because its lattice structure
can absorb water, expand, and destroy permeability (Fig. 8.27C). Thus, for example,
a petroleum reservoir with a montmorillonitic clay may be quite permeable, except
where it has been damaged by mud filtrate around a well bore.
The productivity of a petroleum reservoir may be may be controlled by its clay mineralogy. In the Lower Permian (Rotliegende) gas fields of the North Sea, illitic cemented
8 ALLOCHTHONOUS SEDIMENTS
0 -
Ground
level
Increasing
depth of
burial
I Tangential contacts
/
/
l].Long contacts ~]
"-z._3J/
@
TTI. Sutured and concavoconvex contacts
0 ~
Decreasing porosity
:55%
5 ---,
Increasing no. of
I. 5 %
contacts per grain
Fig. 8.26. The destruction of porosity in sandstones with increasing depth. Previously interpreted as largely
due to pressure solution, cathodoluminescence shows that diagenetic silica cementation is the main process.
significantly diminish permeability (Wilson and Pittman, 1977; Guvan et aL, 1980). There
are four main types of clay to consider: kaolinite, illite, montmorillonite, and chlorite.
Kaolinite grows as discrete crystals often with a shape like an accordian (Fig. 8.27A).
These crystals are sometimes so extended that they look like a worm (described as "vermicular" by the learned). Kaolinite cement obviously destroys porosity, but it does not
do as much damage to permeability as other authigenic clays, which, though they may
diminish porosity by the same amount, volume for volume, are far more destructive of
permeability.
Illite tends to form fibrous crystals that grow radially from the edges of sand grains
(Fig. 8.27B). These furry illite jackets extend across the throat passages, thus diminishing permeability.
Montmorillonite has the worst effect on permeabilitity because its lattice structure
can absorb water, expand, and destroy permeability (Fig. 8.27C). Thus, for example,
a petroleum reservoir with a montmorillonitic clay may be quite permeable, except
where it has been damaged by mud filtrate around a well bore.
The productivity of a petroleum reservoir may be may be controlled by its clay mineralogy. In the Lower Permian (Rotliegende) gas fields of the North Sea, illitic cemented