ocean water varies from 7.9 in zones with upwelling of
deep water to about 8.3 in parts of the North Atlantic
and the Barents Sea where high productivity removes
carbon dioxide from the sea water. Increased CO 2 in
the atmosphere and in the ocean, however, shows large
local variations due to ocean water circulation,
biological activity and reactions with carbonate
minerals.
5.3
Skeletal Components
5.3.1 Introduction
The evolution of organisms precipitating carbonate
skeletons has played an important role in the accumulation of carbonate sediments and their properties. The
skeletal material is widely different in size, with
diameters ranging from a few micrometres in
coccolithophores to more than a metre in some
bivalves and sponges. The range and taxonomic diversity of major groups of skeletal organisms are shown
in Fig. 5.2. However, in this text book we have limited
the descriptions to those groups which are most commonly encountered in carbonate hydrocarbon
reservoirs.
The first condition for obtaining relatively pure
carbonate deposits is that there must be very little or
no supply of terrigenous sediments such as sand or
clay which otherwise would dilute the carbonate content. Many of the carbonate-secreting organisms
require clear water because they filter out nutrients
from the seawater. The presence of mud will kill
organisms like corals, severely reducing carbonate
production. This puts an important limitation on the
occurrence of carbonate sediments in sedimentary
basins.
5.4
Plants
5.4.1 Stromatolites
“Stromatolite” is the term for lamination in carbonate
rocks due to the accumulation or precipitation of carbonate ascribed to bacterial growth. Stromatolites
have an intermediate status between fossils and sedimentary structures. They consist of cyanobacteria
(earlier called blue–green algae) which have a growth
form as unsegmented, micron-sized filaments or
unicells (coccooids). The filaments occur in rows or
strands within a sticky mucilaginous sheath. Only a
few forms produce a biochemically precipitated
skeleton, usually tubiform. Other cyanobacteria may
generate organic films on the sediment surface, which
trap and bind lime mud to form irregular laminae.
C
Coccolithophores
Foraminifera
Foraminifera
Green algae
Bivalves
Gastropods
Corals
Trilobites
Brachiopods
Bryozoans
Crinoids
Echinoids
Coralline
red algae
C
M
M
P
P
Planktic
Dominant
Important
Minor sediment producers
Benthic
Fig. 5.2 Diversity, abundance and relative importance of various calcareous marine organisms as sediment producers. P ¼
Palaeozoic; M ¼ Mesozoic; C ¼ Cenozoic (modified after Wilkinson 1979)
154
N.-M. Hanken et al.
deep water to about 8.3 in parts of the North Atlantic
and the Barents Sea where high productivity removes
carbon dioxide from the sea water. Increased CO 2 in
the atmosphere and in the ocean, however, shows large
local variations due to ocean water circulation,
biological activity and reactions with carbonate
minerals.
5.3
Skeletal Components
5.3.1 Introduction
The evolution of organisms precipitating carbonate
skeletons has played an important role in the accumulation of carbonate sediments and their properties. The
skeletal material is widely different in size, with
diameters ranging from a few micrometres in
coccolithophores to more than a metre in some
bivalves and sponges. The range and taxonomic diversity of major groups of skeletal organisms are shown
in Fig. 5.2. However, in this text book we have limited
the descriptions to those groups which are most commonly encountered in carbonate hydrocarbon
reservoirs.
The first condition for obtaining relatively pure
carbonate deposits is that there must be very little or
no supply of terrigenous sediments such as sand or
clay which otherwise would dilute the carbonate content. Many of the carbonate-secreting organisms
require clear water because they filter out nutrients
from the seawater. The presence of mud will kill
organisms like corals, severely reducing carbonate
production. This puts an important limitation on the
occurrence of carbonate sediments in sedimentary
basins.
5.4
Plants
5.4.1 Stromatolites
“Stromatolite” is the term for lamination in carbonate
rocks due to the accumulation or precipitation of carbonate ascribed to bacterial growth. Stromatolites
have an intermediate status between fossils and sedimentary structures. They consist of cyanobacteria
(earlier called blue–green algae) which have a growth
form as unsegmented, micron-sized filaments or
unicells (coccooids). The filaments occur in rows or
strands within a sticky mucilaginous sheath. Only a
few forms produce a biochemically precipitated
skeleton, usually tubiform. Other cyanobacteria may
generate organic films on the sediment surface, which
trap and bind lime mud to form irregular laminae.
C
Coccolithophores
Foraminifera
Foraminifera
Green algae
Bivalves
Gastropods
Corals
Trilobites
Brachiopods
Bryozoans
Crinoids
Echinoids
Coralline
red algae
C
M
M
P
P
Planktic
Dominant
Important
Minor sediment producers
Benthic
Fig. 5.2 Diversity, abundance and relative importance of various calcareous marine organisms as sediment producers. P ¼
Palaeozoic; M ¼ Mesozoic; C ¼ Cenozoic (modified after Wilkinson 1979)
154
N.-M. Hanken et al.