Fluids and Rocks - Introduction to Carbon Capture and Sequestration - page 384

minerals that bond the grains tightly, giving them strength and resistance
to erosion.
Figure 9.2.2 shows examples of sandstone and shale and in
Table 9.2.2 the properties of these rocks are compared. Sandstones
have a large porosity and high permeability, which makes these rocks
suitable for CO 2 sequestration. Shales, on the other hand, have a lower
porosity and a much lower permeability, which makes shale an ideal seal
or caprock.
Table 9.2.2 shows that the different clastic rocks can also be char-
acterized by differences in their mineral composition [9.1]. Figure 9.2.3
shows their structures and Box 9.2.1 gives some more details on their
structures. These rocks are mixtures of quartz grains, other tectosilicates
(primarily feldspars), phyllosilicates (such as mica, smectite, chlorite, and
kaolinite), and carbonates. Sandstones tend to be dominated by quartz
minerals, but shales by phyllosilicates.

Geological formations

We have discussed the clastic rocks that can be found in geological
formations. Let us now discuss how these different clastic rock regions

(a) (b)

Figure 9.2.2 Sandstone and shale
(a) Sandstone. Image by Dave Waters, Oxford University.
(b) Shale. Reproduced with permission from Paleontological Research Institution ,
Ithaca, New York.