3.2 POROSITY AND PERMEABILITY
79
GRAIN SIZE:
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SET 21
EROSIVE BOUNDARY
SET 22
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Fig. 3.29. Sketch of cross-bedded sandstone core showing vertical variations in permeability. Vertical scale is
centimeters. Horizontal scales on graphs show permeability in millidarcies. (From Hurst and Rosvoll, 1991.
Copyright 9 1991 Academic Press.)
Vertical permeability variations are not too significant in thin cross-beds, but they
become important in cross-beds several meters thick. This is seen, for example, in the
Rotliegendes gas reservoirs of the southern North Sea, where eolian cross-beds up to
5 m thick show large vertical permeability variations (Van Veen, 1975). Pryor (1973)
and Weber (1982) have addressed the problem of three-dimensional variation in crossbedded sands. They both conclude that permeability is highest in the deepest and central part of trough cross-beds. This is partly due to the grain-size variations already discussed, but also because the set boundaries are commonly defined by shale laminae
that act as permeability barriers.
On a still larger scale there tend to be significant vertical variations in different types
of sand bodies. Channels, for reasons described in Section 6.3.2.2.3, tend to show a
vertical upward decline in grain size and, therefore, may possess a vertical decline in
permeability. Barrier bar and mouth bar sands, however, commonly possess upwardcoarsening grain-size profiles (for reasons discussed in Sections 6.3.2.6 and 6.3.2.5.1, respectively). Thus they commonly show an upward increase in permeability.
In a simple world channels trend down the palee 9
whereas barrier bars are
aligned parallel to it (Fig. 3.31). Therefore, most sedimentary deposits have complex
regional permeability variations. These variations are economically important for a
79
GRAIN SIZE:
%
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0 10 20 30 40 50 60 0 10 20 30 40 50 60
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SET 21
EROSIVE BOUNDARY
SET 22
.
3O
50
90110
2600
2O
4o
60
8O
loo
""""~~
120
,
,
,
"~
' adoo 9 a oo' 3 oo
9oo
2aoo 2 oo ' a 'oo
Fig. 3.29. Sketch of cross-bedded sandstone core showing vertical variations in permeability. Vertical scale is
centimeters. Horizontal scales on graphs show permeability in millidarcies. (From Hurst and Rosvoll, 1991.
Copyright 9 1991 Academic Press.)
Vertical permeability variations are not too significant in thin cross-beds, but they
become important in cross-beds several meters thick. This is seen, for example, in the
Rotliegendes gas reservoirs of the southern North Sea, where eolian cross-beds up to
5 m thick show large vertical permeability variations (Van Veen, 1975). Pryor (1973)
and Weber (1982) have addressed the problem of three-dimensional variation in crossbedded sands. They both conclude that permeability is highest in the deepest and central part of trough cross-beds. This is partly due to the grain-size variations already discussed, but also because the set boundaries are commonly defined by shale laminae
that act as permeability barriers.
On a still larger scale there tend to be significant vertical variations in different types
of sand bodies. Channels, for reasons described in Section 6.3.2.2.3, tend to show a
vertical upward decline in grain size and, therefore, may possess a vertical decline in
permeability. Barrier bar and mouth bar sands, however, commonly possess upwardcoarsening grain-size profiles (for reasons discussed in Sections 6.3.2.6 and 6.3.2.5.1, respectively). Thus they commonly show an upward increase in permeability.
In a simple world channels trend down the palee 9
whereas barrier bars are
aligned parallel to it (Fig. 3.31). Therefore, most sedimentary deposits have complex
regional permeability variations. These variations are economically important for a