416
9 AUTOCHTHONOUS SEDIMENTS
sands and reefs, which are highly permeable when first deposited. Lime muds, which
though initially porous are of low permeability, follow different diagenetic pathways.
9.2.5.3 Diagenesis of Lime Muds
The diagenesis and petrophysics of lime muds are far simpler than for calcarenites. The
main reason for this is that though lime muds are often as porous as calcarenites, they
are far less permeable due to the small size of the throat passages. They are thus not
nearly as susceptible to flushing by fluids of diverse chemistry. An important distinction
must be made at the outset between lime muds made of aragonite and those made of
calcite.
From the Pre-Cambrian until the Cretaceous, lime muds were made almost entirely
of aragonite. When buried the aragonite reverts to calcite. As already seen this reaction
results in an 8% increase in volume, and a corresponding decrease in porosity (Note,
this is not an 8% decrease in porosity, but an increase of 8% by volume of rock, i.e., an
aragonite mud with 40% porosity and 60% grains would lose 48% porosity by the aragonite" calcite reversion.) The rearrangement of crystals, coupled with compaction, however, generally leads to a total loss of porosity and permeability. Thus it is a matter of
observation that most lime mudstones are hard, tight splintery rocks of negligible porosity and permeability. They can only become reservoirs if secondary porosity has been
generated by fracturing, dolomitization, or solution.
An exception to this general rule is provided by a particular type of lime mud known
as chalk. Chalk is a fine-grained limestone composed largely of coccoliths, the calcitic
plates of coccospheres (Fig. 9.10). These are skeletal remains of a group of nannoplanktonic golden-brown algae (Black, 1953). Coccolithic limestones first became important
toward the end of the Jurassic Period. By the middle of the Cretaceous Period vast quantities of coccolith muds began to be deposited across the continental shelves of the ocean
basins of the world. These gave rise to the Chalk Group of northwest Europe (Hakansson et al., 1974; Hancock, 1998) and its worldwide equivalents, such as the Austin Chalk
of Texas (Ager, 1993). Chalk is deposited with porosities and permeabilities similar to
those of aragonitic lime muds. Because chalks are of calcitic composition, however, they
do not undergo the early diagenetic recrystallization of aragonite to calcite. Thus chalks
generally remain as "chalky" friable rock, retaining porosities on the order of 20-30%.
They have considerable storage capacity and may serve as aquifers or petroleum reservoirs (Scholle, 1977; Scholle et al., 1983). To actually yield their contained fluids, chalk
must have permeability. This may occur due to fracturing. Alternatively, chalks can still
retain some original intergranular permeability. This is best preserved when compaction has been inhibited by abnormally high pore pressures (relieving the stress at grain
contacts) or petroleum invasion, as was noted earlier when discussing sand reservoirs.
These conditions are responsible for the productivity of the chalk reservoirs of the Ekorisk group of fields in the Norwegian sector of the North Sea (Byrd, 1975; Heur, 1980).
Here the Cretaceous chalk has been fractured over salt domes. Simultaneously, rapid
burial and high heat flow favored overpressuring and hydrocarbon generation. Early
diagenesis of chalks is minimal, but worthy of comment. Horizons which show evidence
of early cementation are widespread stratigraphically, though not volumetrically important. These penecontemporaneously cemented layers are termed "hardgrounds."
9 AUTOCHTHONOUS SEDIMENTS
sands and reefs, which are highly permeable when first deposited. Lime muds, which
though initially porous are of low permeability, follow different diagenetic pathways.
9.2.5.3 Diagenesis of Lime Muds
The diagenesis and petrophysics of lime muds are far simpler than for calcarenites. The
main reason for this is that though lime muds are often as porous as calcarenites, they
are far less permeable due to the small size of the throat passages. They are thus not
nearly as susceptible to flushing by fluids of diverse chemistry. An important distinction
must be made at the outset between lime muds made of aragonite and those made of
calcite.
From the Pre-Cambrian until the Cretaceous, lime muds were made almost entirely
of aragonite. When buried the aragonite reverts to calcite. As already seen this reaction
results in an 8% increase in volume, and a corresponding decrease in porosity (Note,
this is not an 8% decrease in porosity, but an increase of 8% by volume of rock, i.e., an
aragonite mud with 40% porosity and 60% grains would lose 48% porosity by the aragonite" calcite reversion.) The rearrangement of crystals, coupled with compaction, however, generally leads to a total loss of porosity and permeability. Thus it is a matter of
observation that most lime mudstones are hard, tight splintery rocks of negligible porosity and permeability. They can only become reservoirs if secondary porosity has been
generated by fracturing, dolomitization, or solution.
An exception to this general rule is provided by a particular type of lime mud known
as chalk. Chalk is a fine-grained limestone composed largely of coccoliths, the calcitic
plates of coccospheres (Fig. 9.10). These are skeletal remains of a group of nannoplanktonic golden-brown algae (Black, 1953). Coccolithic limestones first became important
toward the end of the Jurassic Period. By the middle of the Cretaceous Period vast quantities of coccolith muds began to be deposited across the continental shelves of the ocean
basins of the world. These gave rise to the Chalk Group of northwest Europe (Hakansson et al., 1974; Hancock, 1998) and its worldwide equivalents, such as the Austin Chalk
of Texas (Ager, 1993). Chalk is deposited with porosities and permeabilities similar to
those of aragonitic lime muds. Because chalks are of calcitic composition, however, they
do not undergo the early diagenetic recrystallization of aragonite to calcite. Thus chalks
generally remain as "chalky" friable rock, retaining porosities on the order of 20-30%.
They have considerable storage capacity and may serve as aquifers or petroleum reservoirs (Scholle, 1977; Scholle et al., 1983). To actually yield their contained fluids, chalk
must have permeability. This may occur due to fracturing. Alternatively, chalks can still
retain some original intergranular permeability. This is best preserved when compaction has been inhibited by abnormally high pore pressures (relieving the stress at grain
contacts) or petroleum invasion, as was noted earlier when discussing sand reservoirs.
These conditions are responsible for the productivity of the chalk reservoirs of the Ekorisk group of fields in the Norwegian sector of the North Sea (Byrd, 1975; Heur, 1980).
Here the Cretaceous chalk has been fractured over salt domes. Simultaneously, rapid
burial and high heat flow favored overpressuring and hydrocarbon generation. Early
diagenesis of chalks is minimal, but worthy of comment. Horizons which show evidence
of early cementation are widespread stratigraphically, though not volumetrically important. These penecontemporaneously cemented layers are termed "hardgrounds."