342
8 ALLOCHTHONOUS SEDIMENTS
Bacterial fermentation of organic matter also takes place during shallow burial. These
reactions generate water, carbon dioxide, and biogenic methane. This increases the pH
of the pore fluids, permitting carbonate precipitation. Early carbonate cements include
both calcite and siderite. These cements are often patchily developed as concretions
that may occur intermittently along beds. The early age of their formation can often be
proved because the concretions contain whole fossil shells, while in adjacent laminae
the little shells have been flattened and shattered. Sometimes, the nodules have undergone subsequent dehydration, with carbonate cement growth in the shrinkage cracks
(Fig. 8.9). Fluid inclusion and stable isotope analyses can give much information on the
physical and chemical environment in which this cementation took place.
As temperature and pressure increase with burial depth, the organic fraction of the
mud evolves into kerogen, and bacterial processes diminish in importance. Diagenesis
merges into catagenesis toward about 2 km as the temperature exceeds 50~ In this
zone important clay mineral reactions take place. Recall that montmorillonitic clays
contain structured water within their atomic lattice. At temperatures of about 100110~ the montmorillonite lattice collapses, to expel a large volume of pore water, and
hence increase the pore pressure. Montmorillonite and kaolinite change into illite.
These reactions take place at the depths and temperatures in which oil generation is
known to occur.
Space does not permit the problem of primary oil migration to be discussed. This is
one of the last great geological mysteries: How does petroleum emigrate from an impermeable clay? At these depths shale porosity is down to some 10-20%, and simple
squeezing cannot explain the mechanism of oil emigration. For reviews of the many
theories, see North (1985, pp. 225-240), Hunt (1996, pp. 238-267), and Selley (1996,
pp. 214-225). There must obviously be a close and complex relationship between clay
mineral diagenesis, kerogen maturation, and petroleum emigration. As kerogen matures it undergoes decarboxylation, giving off carbon dioxide. This goes into solution in
the water as carbonic acid. These acid solutions are thus expelled from the clays ahead
of petroleum. This is a process of great importance to sandstone diagenesis (see Section 8.5.3.4.2). With increasing temperature kerogen generates oil at temperatures of
60-120~ (Tissot and Welte, 1984; Hunt, 1996). As burial continues and temperatures
approach 200~ catagenesis phases out into the third phase, termed "metagenesis." During this phase oil generation from kerogen ceases, to be replaced by dry gas formation.
Finally, metagenesis merges into metamorphism. Kerogen has now given up all its petroleum, and is nothing but carbon (graphite). Clay minerals begin to recrystallize into
recognizable crystals of mica and chlorite.
8.3.2. 7 Clays and Clay Diagenesis: Summary
The compaction and consolidation of clays at shallow depths is of significance to the
engineering geologist. The changes that occur in clays as they change to claystones at
greater depths is of interest to the petroleum geologist because this may be relevant to
theories of petroleum genesis and migration. Similarly, mining geologists have considered that low-temperature ore bodies may have been derived from the residual fluids
of compacting clays aided by brines acting as transporting media (e.g., Davidson, 1965;
Amstutz and Bubinicek, 1967).
8 ALLOCHTHONOUS SEDIMENTS
Bacterial fermentation of organic matter also takes place during shallow burial. These
reactions generate water, carbon dioxide, and biogenic methane. This increases the pH
of the pore fluids, permitting carbonate precipitation. Early carbonate cements include
both calcite and siderite. These cements are often patchily developed as concretions
that may occur intermittently along beds. The early age of their formation can often be
proved because the concretions contain whole fossil shells, while in adjacent laminae
the little shells have been flattened and shattered. Sometimes, the nodules have undergone subsequent dehydration, with carbonate cement growth in the shrinkage cracks
(Fig. 8.9). Fluid inclusion and stable isotope analyses can give much information on the
physical and chemical environment in which this cementation took place.
As temperature and pressure increase with burial depth, the organic fraction of the
mud evolves into kerogen, and bacterial processes diminish in importance. Diagenesis
merges into catagenesis toward about 2 km as the temperature exceeds 50~ In this
zone important clay mineral reactions take place. Recall that montmorillonitic clays
contain structured water within their atomic lattice. At temperatures of about 100110~ the montmorillonite lattice collapses, to expel a large volume of pore water, and
hence increase the pore pressure. Montmorillonite and kaolinite change into illite.
These reactions take place at the depths and temperatures in which oil generation is
known to occur.
Space does not permit the problem of primary oil migration to be discussed. This is
one of the last great geological mysteries: How does petroleum emigrate from an impermeable clay? At these depths shale porosity is down to some 10-20%, and simple
squeezing cannot explain the mechanism of oil emigration. For reviews of the many
theories, see North (1985, pp. 225-240), Hunt (1996, pp. 238-267), and Selley (1996,
pp. 214-225). There must obviously be a close and complex relationship between clay
mineral diagenesis, kerogen maturation, and petroleum emigration. As kerogen matures it undergoes decarboxylation, giving off carbon dioxide. This goes into solution in
the water as carbonic acid. These acid solutions are thus expelled from the clays ahead
of petroleum. This is a process of great importance to sandstone diagenesis (see Section 8.5.3.4.2). With increasing temperature kerogen generates oil at temperatures of
60-120~ (Tissot and Welte, 1984; Hunt, 1996). As burial continues and temperatures
approach 200~ catagenesis phases out into the third phase, termed "metagenesis." During this phase oil generation from kerogen ceases, to be replaced by dry gas formation.
Finally, metagenesis merges into metamorphism. Kerogen has now given up all its petroleum, and is nothing but carbon (graphite). Clay minerals begin to recrystallize into
recognizable crystals of mica and chlorite.
8.3.2. 7 Clays and Clay Diagenesis: Summary
The compaction and consolidation of clays at shallow depths is of significance to the
engineering geologist. The changes that occur in clays as they change to claystones at
greater depths is of interest to the petroleum geologist because this may be relevant to
theories of petroleum genesis and migration. Similarly, mining geologists have considered that low-temperature ore bodies may have been derived from the residual fluids
of compacting clays aided by brines acting as transporting media (e.g., Davidson, 1965;
Amstutz and Bubinicek, 1967).