8.5 SANDSTONES
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its underlying cemented zone, may be noted on geophysical well logs (Selley, 1992a).
This boundary may also have sufficient velocity contrast to be imaged as a reflecting
horizon on seismic sections, cross-cutting stratigraphic reflectors, as in the Murchison
field of the North Sea (Ashcroft and Ridgway, 1996).
8.5.3.4.2 Kerogen decarboxylation
An alternative process to epidiagenesis for the development of secondary porosity was
suggested by Rowsell and De Swardt (1974, 1976). They postulated that the acidic fluid
necessary for the leaching may have been produced by the decarboxylation of kerogen
dispersed in shales. The earlier account of clay diagenesis showed that maturing kerogen emits carbon dioxide prior to petroleum generation. The carbon dioxide goes into
solution in the connate water as carbonic acid (see Fig. 8.8). This water is expelled from
the shale into permeable formations, leaching out unstable grains and cement, enhancing porosity, and preparing reservoirs to receive the petroleum. Unequivocal evidence
that this process works has come from the Gulf Coast basin of the USA. Here solution
porosity occurs in deeply buried sediments. Because this basin has subsided steadily
from the end of the Cretaceous Period, there is no likelihood that the secondary porosity is due to epidiagenesis (Loucks et al., 1984).
8.5.3.5 Sandstone Diagenesis and Porosity Evolution: Summary
The preceding pages reviewed the factors that control the petrophysical characteristics
of sandstones. The evolution of porosity in sandstones is much simpler than in carbonates because of the greater chemical stability of silica. The porosity of a sand is a reflection of its texture, mode of deposition, and extent of diagenesis. The grain size, grain
shape, sorting, and packing of a sediment play an important role in determining primary intergranular porosity (see Section 3.2.3). Pryor (1973) has shown how these vary
for different environments and has documented the spatial variation of porosity and
the vectorial variation of permeability in different types of sand body.
Studies of the Mackenzie delta of Arctic Canada and of the U.S. Gulf Coast basins
have led to attempts to explain diagenesis in a series of regular stages as sandstones are
progressively buried (e.g., Overton, 1973; Van Elsberg, 1978; Surdam et al., 1984 Surdam and Crossey, 1987). The following account attempts to synthesize these ideas. When
a sand has been deposited it first goes through the essentially physical processes of compaction and dewatering. The early chemical diagenetic changes are dominated by reduction or oxidation, hence this is termed the "redoxomorphic" phase. These reactions
primarily concern oxygen, naturally, iron, sulfur, and organic matter. They are largely
the consequence of bacterial action. Essentially, a sand that has high permeability and
is deposited above the water table will be subjected to oxidizing reactions. This is because the pore system will be subjected both to free air and oxygenated groundwater.
Organic matter is oxidized, and sulfur compounds are oxidized and carried off as soluble sulfate ions. Iron tends to be preserved as ferric oxide. This is red in color and impregnates the clay pellicles of detrital grains. This is why the majority of (but not all)
red sandstones are of continental origin, both eolian and fluvial.
By contrast, low-permeability argillaceous sands, and those deposited below the water table, undergo early diagenesis in a reducing alkaline pore fluid. This is because of
375
its underlying cemented zone, may be noted on geophysical well logs (Selley, 1992a).
This boundary may also have sufficient velocity contrast to be imaged as a reflecting
horizon on seismic sections, cross-cutting stratigraphic reflectors, as in the Murchison
field of the North Sea (Ashcroft and Ridgway, 1996).
8.5.3.4.2 Kerogen decarboxylation
An alternative process to epidiagenesis for the development of secondary porosity was
suggested by Rowsell and De Swardt (1974, 1976). They postulated that the acidic fluid
necessary for the leaching may have been produced by the decarboxylation of kerogen
dispersed in shales. The earlier account of clay diagenesis showed that maturing kerogen emits carbon dioxide prior to petroleum generation. The carbon dioxide goes into
solution in the connate water as carbonic acid (see Fig. 8.8). This water is expelled from
the shale into permeable formations, leaching out unstable grains and cement, enhancing porosity, and preparing reservoirs to receive the petroleum. Unequivocal evidence
that this process works has come from the Gulf Coast basin of the USA. Here solution
porosity occurs in deeply buried sediments. Because this basin has subsided steadily
from the end of the Cretaceous Period, there is no likelihood that the secondary porosity is due to epidiagenesis (Loucks et al., 1984).
8.5.3.5 Sandstone Diagenesis and Porosity Evolution: Summary
The preceding pages reviewed the factors that control the petrophysical characteristics
of sandstones. The evolution of porosity in sandstones is much simpler than in carbonates because of the greater chemical stability of silica. The porosity of a sand is a reflection of its texture, mode of deposition, and extent of diagenesis. The grain size, grain
shape, sorting, and packing of a sediment play an important role in determining primary intergranular porosity (see Section 3.2.3). Pryor (1973) has shown how these vary
for different environments and has documented the spatial variation of porosity and
the vectorial variation of permeability in different types of sand body.
Studies of the Mackenzie delta of Arctic Canada and of the U.S. Gulf Coast basins
have led to attempts to explain diagenesis in a series of regular stages as sandstones are
progressively buried (e.g., Overton, 1973; Van Elsberg, 1978; Surdam et al., 1984 Surdam and Crossey, 1987). The following account attempts to synthesize these ideas. When
a sand has been deposited it first goes through the essentially physical processes of compaction and dewatering. The early chemical diagenetic changes are dominated by reduction or oxidation, hence this is termed the "redoxomorphic" phase. These reactions
primarily concern oxygen, naturally, iron, sulfur, and organic matter. They are largely
the consequence of bacterial action. Essentially, a sand that has high permeability and
is deposited above the water table will be subjected to oxidizing reactions. This is because the pore system will be subjected both to free air and oxygenated groundwater.
Organic matter is oxidized, and sulfur compounds are oxidized and carried off as soluble sulfate ions. Iron tends to be preserved as ferric oxide. This is red in color and impregnates the clay pellicles of detrital grains. This is why the majority of (but not all)
red sandstones are of continental origin, both eolian and fluvial.
By contrast, low-permeability argillaceous sands, and those deposited below the water table, undergo early diagenesis in a reducing alkaline pore fluid. This is because of