Page 56 - Geology and Geochemistry of Oil and Gas

CAPROCKS 33
Eq. 2.19 shows that the hydrodynamic environment may improve or lower the
sealing capability of caprocks. In an extreme case, the water potential in the reservoir
may exceed the water potential of the bed overlying the caprock by the value of
capillary pressure (P c ). In such a situation, the caprock will be open for the vertical
flow of hydrocarbons, and the trap will not exist even when potential distribution in
the reservoir bed is favorable.
Type III caprocks are typical for rocks with a rigid matrix and intense fracturing.
Such caprocks are mainly developed over the old platforms in regions of low tectonic
mobility, with no detectable hydrodynamic breakdown of the section. Formation
water potential in such regions is practically equal throughout the section and cor-
responds to the calculated hydrostatic potential. The breakthrough potential for
such caprocks is determined solely by the capillary pressure:

P b ¼ P wr þ P c (2.20)
The possible height of accumulation column is

P c
H ¼ (2.21)
Dg G o
The equation above indicates that even at a low capillary pressure the accumu-
lation column height may be significant when G o is close to Dg (the gradient of
Archimedes buoyancy force). If these gradients are equal, the accumulation may be
preserved even in the absence of a definitive lithologically identifiable caprock. P c in
the equations above may be calculated using the Laplace equation.
Major conclusions from the studies of correlation between clay mineralogy and
their sealing properties have been summarized by Klubova (1984) as follows:
The permanency in the composition of the silicate layer is a characteristic of the kaolinite
group minerals. As a result, replacements within the lattice are very rare and the charges
within a layer are compensated. The connection between silicate layers in the C-axis di-
rection is implemented through hydrogen atoms, which prevents the lattice from expanding,
ruling out the penetration of water and polar organic liquids.
The silicate layer in the montmorillonite mineral group is variable due to a common iso-
morphic replacement in octahedral and narrower tetrahedral sheets. This replacement results
in the disruption of the lattice neutrality. Extra charge that occurs with such replacements is
compensated by exchange ions. Ion properties that maintain lattice neutrality in montmo-
rillonite minerals (valence, size of the ion radius, polarization, etc.) define the capability of the
lattice to expand along the C-axis. As a result, water and polar organic liquids can penetrate
the interlayer spaces. This, in turn, leads to an increase in the volume, which drastically lowers
permeability and some other properties, but at the same time improves sealing capabilities.
The silicate layer of the illite mineral group is similar to the montmorillonite one. However,
the excessive negative charge of the lattice is due mainly to the isomorphic replacements
within tetrahedral sheets. The proximity between the source of negative charge and basal
surfaces causes a stronger connection between the silicate layers of illite group compared to
montmorillonite’s.
Admixture of sand and silt degrades the sealing properties of clays. Especially
important are the textural changes due to this admixture.
Not only the mineral composition of a rock and organic matter content, but also
the pore water are important in forming the major sealing properties of clays, such as