Coccolithophores are abundant (commonly as many as
half a million individuals per litre of water) in the
photic zone of modern oceans. Coccoliths are important constituents of some fairly deep basinal and distal
shelf sediments.
Deep-sea drilling has shown that coccolithophores
were very much more widespread during the Cretaceous and Lower Tertiary than they are today. The
colder climate which began in the Miocene shifted the
northern limits for coccolithophore sedimentation
southwards. Many pelagic species have very specific
temperature requirements and their occurrence
provides an important contribution to the
palaeoecology of marine sediments of various ages.
5.4.3.2 Mineralogy
The plates of the coccolithophores consist of low-Mg
calcite which makes them fairly stable during
diagenesis.
5.4.3.3 Geological Range
Coccolithophores range from the Triassic to Recent,
but were not common before the Jurassic. They were
particularly abundant in the Cretaceous and Lower
Tertiary, forming thick, extensive chalks. They comprise approximately 25% of present-day calcareous
oozes and up to 90% of some Cretaceous and Tertiary
chalks.
5.4.3.4 Significance for the Petroleum
Industry
Chalks made up of coccoliths may be extremely
porous. The porosity is in the form of primary interparticle micropores and may be as high as 40–50%,
even in deeply buried chalks. Although their porosity
may be high, permeability is generally low (less than
a few mD) because of small pore-throat diameters
(generally <1 μm). Hydrocarbon-filled interparticle
pores in chalks are therefore productive only in combination with some other pore type, preferably fracture
pores, which increases the permeability. Fractured
chalks of Maastrichtian (late Cretaceous) to Danian
(early Tertiary) age form major hydrocarbon
reservoirs in the Central Graben of the North Sea
(Ekofisk and associated fields). These are in fact the
world’s only major oilfield in such rocks, but the low
permeability of this fine-grained lithology creates
some production problems.
5.4.4 Calcispheres
Calcispheres are of uncertain biological affinity, but
many consider them to be algae because there is a
great similarity between non-ornamented fossil
calcispheres and reproductive cysts of some living
green algae (dasycladacean algae). They are generally
40–200 μm in diameter and are composed of a thin
(commonly 3–30 μm), well-defined calcite wall,
enclosing a single spherical chamber (Fig. 5.13). The
wall may be singly layered or have several concentric
layers distinguishable by the alternation of uniform
micritic texture with a fabric showing radial elements.
The wall has radial pores, but there is no aperture. The
outer surface may bear spines. The chamber is normally filled with cement or sediment.
5.4.4.1 Ecology
Many fossil calcispheres seem to owe their origin to
dasycladacean algae, and as such, they provide a useful
palaeo-environmental indicator. Devonian and
Permian calcispheres are most common in shallow
subtidal environments, especially in restricted or
back-reef environments. In the Cretaceous,
calcispheres are especially important in pelagic
deposits such as Chalk sediments in the North Sea.
5.4.4.2 Geological Range
Calcispheres are widely known from Upper
Palaeozoic limestones, especially Devonian and Carboniferous, but also occur in older and younger strata.
Calcispheres have no particular significance for
petroleum geology. However, they may be an important component improving the overall potential of a
source rock.
5.4.5 Distribution of Algae in Modern
and Older Carbonate Sediments
If one places a profile extending from a basin centre to
a coast with reef development (Fig. 5.14), we see the
following distribution of algae:
A. The sedimentation basinward consists largely of
planktonic algae (coccolithophores), planktonic
foraminifera and some other calcareous organisms.
B. In the reef facies we find mainly red algae which
build strong, solid structures of carbonate, which in
conjunction with the corals can resist waves which
162
N.-M. Hanken et al.
half a million individuals per litre of water) in the
photic zone of modern oceans. Coccoliths are important constituents of some fairly deep basinal and distal
shelf sediments.
Deep-sea drilling has shown that coccolithophores
were very much more widespread during the Cretaceous and Lower Tertiary than they are today. The
colder climate which began in the Miocene shifted the
northern limits for coccolithophore sedimentation
southwards. Many pelagic species have very specific
temperature requirements and their occurrence
provides an important contribution to the
palaeoecology of marine sediments of various ages.
5.4.3.2 Mineralogy
The plates of the coccolithophores consist of low-Mg
calcite which makes them fairly stable during
diagenesis.
5.4.3.3 Geological Range
Coccolithophores range from the Triassic to Recent,
but were not common before the Jurassic. They were
particularly abundant in the Cretaceous and Lower
Tertiary, forming thick, extensive chalks. They comprise approximately 25% of present-day calcareous
oozes and up to 90% of some Cretaceous and Tertiary
chalks.
5.4.3.4 Significance for the Petroleum
Industry
Chalks made up of coccoliths may be extremely
porous. The porosity is in the form of primary interparticle micropores and may be as high as 40–50%,
even in deeply buried chalks. Although their porosity
may be high, permeability is generally low (less than
a few mD) because of small pore-throat diameters
(generally <1 μm). Hydrocarbon-filled interparticle
pores in chalks are therefore productive only in combination with some other pore type, preferably fracture
pores, which increases the permeability. Fractured
chalks of Maastrichtian (late Cretaceous) to Danian
(early Tertiary) age form major hydrocarbon
reservoirs in the Central Graben of the North Sea
(Ekofisk and associated fields). These are in fact the
world’s only major oilfield in such rocks, but the low
permeability of this fine-grained lithology creates
some production problems.
5.4.4 Calcispheres
Calcispheres are of uncertain biological affinity, but
many consider them to be algae because there is a
great similarity between non-ornamented fossil
calcispheres and reproductive cysts of some living
green algae (dasycladacean algae). They are generally
40–200 μm in diameter and are composed of a thin
(commonly 3–30 μm), well-defined calcite wall,
enclosing a single spherical chamber (Fig. 5.13). The
wall may be singly layered or have several concentric
layers distinguishable by the alternation of uniform
micritic texture with a fabric showing radial elements.
The wall has radial pores, but there is no aperture. The
outer surface may bear spines. The chamber is normally filled with cement or sediment.
5.4.4.1 Ecology
Many fossil calcispheres seem to owe their origin to
dasycladacean algae, and as such, they provide a useful
palaeo-environmental indicator. Devonian and
Permian calcispheres are most common in shallow
subtidal environments, especially in restricted or
back-reef environments. In the Cretaceous,
calcispheres are especially important in pelagic
deposits such as Chalk sediments in the North Sea.
5.4.4.2 Geological Range
Calcispheres are widely known from Upper
Palaeozoic limestones, especially Devonian and Carboniferous, but also occur in older and younger strata.
Calcispheres have no particular significance for
petroleum geology. However, they may be an important component improving the overall potential of a
source rock.
5.4.5 Distribution of Algae in Modern
and Older Carbonate Sediments
If one places a profile extending from a basin centre to
a coast with reef development (Fig. 5.14), we see the
following distribution of algae:
A. The sedimentation basinward consists largely of
planktonic algae (coccolithophores), planktonic
foraminifera and some other calcareous organisms.
B. In the reef facies we find mainly red algae which
build strong, solid structures of carbonate, which in
conjunction with the corals can resist waves which
162
N.-M. Hanken et al.