414
9 AUTOCHTHONOUS SEDIMENTS
Particular attention has been directed toward finding the source of the calcium carbonate for this deep connate type of cement. Two major ones have been proposed: internal and external. The internal source is provided by pressure solution, the external
by migrating connate fluids. These are discussed in turn. There is evidence for considerable intrastratal solution in deeply buried carbonates. The evidence is both microscopic
and macroscopic. Pressure solution can often be seen to have taken place in some deeply
buried skeletal limestones. Stylolites provide evidence of solution on a much larger scale
(Carozzi and von Bergen, 1987). A stylolite is a sutured boundary between two rocks
(Fig. 9.9A). A microstylolite is one between two grains. Stylolites are commonly seen
in limestones and orthoquartzites. Microstylolites are seen at quartz: quartz and calcite:
calcite grain contacts. Stylolites are usually parallel or subparallel to bedding. Microstylolites occur at burial depths as shallow as 90 m (Shlanger, 1964), stylolites as shallow
as 60 m (Dunnington, 1967). It is axiomatic that stylolites are caused by solution. Estimates of the amount of solution can be made from both the maximum thickness of a
stylolite or from the offset of planar surfaces, such as fractures or clay laminae (Fig. 9.9B).
Stylolite surfaces are commonly marked by clay and/or organic matter left behind by the
solution process. Stylolites normally occur in nonporous impermeable formations, so
there are problems in establishing not only how the material is dissolved, but also how it
is transported. Studies of the Dukhan field of Qatar, and the Murban field of Abu Dhabi,
showed that as much as 30-40% of the original rock volume was lost by stylolitization
of limestone (Dunnington, 1967). Stylolites are absent in the porous oil-bearing part of
the reservoirs, but are abundant in the water-bearing intervals. Porosity drops off markedly below the oil:water contacts of the fields (these are two of many cases that demonstrate how petroleum preserves porosity).
Stylolitization seems to be restricted to monomineralic rocks with less than 10%
clay. When this figure is exceeded there is still evidence of extensive solution. This is
seen by the gradual development of wispy argillaceous laminae that pass gradationally
into nodular limestones. These look superficially like conglomerates of limestone clasts
in clay matrix (Robin, 1978). This process, together with stylolitization, provides evidence for extensive solution of carbonate during deep burial. This must contribute to
the large amount of carbonate cement seen in many limestones.
The second source for the calcium carbonate needed for deep calcite cement could
be connate fluids. The ions may have been derived from the solution of shallower limestones, as previously discussed, or from fluids squeezed from compacting clays. There is
a problem with invoking an external source for the cement. It is possible to calculate how
much calcium carbonate is required, and the necessary concentrations of calcium and
carbonate ions in pore fluids. These calculations show that whole oceans of water must
pass through a carbonate sediment before it is completely cemented (Weyl, 1958).
This discussion of the source of calcite cement is important. Because a modern reef
may have up to 80% porosity and a skeletal sand some 60%, it follows that over half
of a completely nonporous limestone is made of cement. It appears most likely that in
deeply buried lime sands porosity is lost very largely by intrastratal solution and reprecipitation. As burial develops permeability diminishes, and the role of migrating fluids
as agents of cementation declines.
The foregoing account of diagenesis and porosity evolution applies only to carbonate
9 AUTOCHTHONOUS SEDIMENTS
Particular attention has been directed toward finding the source of the calcium carbonate for this deep connate type of cement. Two major ones have been proposed: internal and external. The internal source is provided by pressure solution, the external
by migrating connate fluids. These are discussed in turn. There is evidence for considerable intrastratal solution in deeply buried carbonates. The evidence is both microscopic
and macroscopic. Pressure solution can often be seen to have taken place in some deeply
buried skeletal limestones. Stylolites provide evidence of solution on a much larger scale
(Carozzi and von Bergen, 1987). A stylolite is a sutured boundary between two rocks
(Fig. 9.9A). A microstylolite is one between two grains. Stylolites are commonly seen
in limestones and orthoquartzites. Microstylolites are seen at quartz: quartz and calcite:
calcite grain contacts. Stylolites are usually parallel or subparallel to bedding. Microstylolites occur at burial depths as shallow as 90 m (Shlanger, 1964), stylolites as shallow
as 60 m (Dunnington, 1967). It is axiomatic that stylolites are caused by solution. Estimates of the amount of solution can be made from both the maximum thickness of a
stylolite or from the offset of planar surfaces, such as fractures or clay laminae (Fig. 9.9B).
Stylolite surfaces are commonly marked by clay and/or organic matter left behind by the
solution process. Stylolites normally occur in nonporous impermeable formations, so
there are problems in establishing not only how the material is dissolved, but also how it
is transported. Studies of the Dukhan field of Qatar, and the Murban field of Abu Dhabi,
showed that as much as 30-40% of the original rock volume was lost by stylolitization
of limestone (Dunnington, 1967). Stylolites are absent in the porous oil-bearing part of
the reservoirs, but are abundant in the water-bearing intervals. Porosity drops off markedly below the oil:water contacts of the fields (these are two of many cases that demonstrate how petroleum preserves porosity).
Stylolitization seems to be restricted to monomineralic rocks with less than 10%
clay. When this figure is exceeded there is still evidence of extensive solution. This is
seen by the gradual development of wispy argillaceous laminae that pass gradationally
into nodular limestones. These look superficially like conglomerates of limestone clasts
in clay matrix (Robin, 1978). This process, together with stylolitization, provides evidence for extensive solution of carbonate during deep burial. This must contribute to
the large amount of carbonate cement seen in many limestones.
The second source for the calcium carbonate needed for deep calcite cement could
be connate fluids. The ions may have been derived from the solution of shallower limestones, as previously discussed, or from fluids squeezed from compacting clays. There is
a problem with invoking an external source for the cement. It is possible to calculate how
much calcium carbonate is required, and the necessary concentrations of calcium and
carbonate ions in pore fluids. These calculations show that whole oceans of water must
pass through a carbonate sediment before it is completely cemented (Weyl, 1958).
This discussion of the source of calcite cement is important. Because a modern reef
may have up to 80% porosity and a skeletal sand some 60%, it follows that over half
of a completely nonporous limestone is made of cement. It appears most likely that in
deeply buried lime sands porosity is lost very largely by intrastratal solution and reprecipitation. As burial develops permeability diminishes, and the role of migrating fluids
as agents of cementation declines.
The foregoing account of diagenesis and porosity evolution applies only to carbonate