Chapter 5
Carbonate Sediments
Nils-Martin Hanken, Knut Bjørlykke and Jesper Kresten Nielsen
Carbonate sediments are a part of the carbon cycle
(Fig. 1.14). CO 2 in the atmosphere dissolves in water
and makes carbonic acid (H 2 CO 3 ) which reacts with
Ca
2+ or Mg
2+ to precipitate CaCO 3 or MgCO 3 . This
process is an important sink for CO 2 . The rate of
carbonate sedimentation globally is controlled by the
supply of cations (mostly Ca
2+ and Mg
2+ ) into the
ocean from rivers. This again is a function of the rate
of weathering of Ca-bearing silicate minerals like plagioclase, pyroxene, amphibole and chlorite. Dissolution of basic rock along a spreading ridge may also add
to the supply of calcium to the ocean water.
Weathering of limestones will release CO 2 and
therefore does not contribute as a sink for CO 2 . During
contact metamorphism when limestones are heated (to
550–600
C), CaCO 3 reacts with SiO 2 to form CaSiO 3
(wollastonite) and CO 2 . Large amounts of CO 2 formed
this way are released through volcanoes. When
minerals like wollastonite are uplifted, exposed and
weathered, Ca
2+ is released into rivers and the ocean
again. Another part of the CO 2 in the atmosphere is
reduced by plants (i.e. algae) and stored as carbon or
organic compounds which can generate petroleum.
These may also be oxidised to CO 2 .
Carbonate sediments are for the most part formed
(born) within sedimentary basins even if there may be
a clastic supply of carbonate sediments. The sedimentology of carbonates therefore differs in many respects
from siliceous sand and mud. Carbonates may precipitate chemically from the seawater, but most of the
limestones are composed of calcareous organisms.
The properties of limestones are therefore closely
linked to their biological origin and the mineralogy
of the carbonate skeletons. The primary composition is
very important for their alteration during burial (diagenesis) and consequent reservoir properties (porosity
and permeability), which to a large extent is controlled
by chemical processes. Since both biologically and
chemically precipitated carbonate sediments are composed of minerals we will first examine the mineralogy
and geochemistry of carbonates. This is important for
understanding diagenetic reactions and prediction of
reservoir quality.
5.1
Geochemistry of Carbonate
Minerals
Carbonate minerals consist of CO
2À
3 and one or more
cations. The most common cations in carbonate
minerals together with their mineral names are listed
in Table 5.1. The common rock-forming carbonate
minerals are either rhombohedral (calcite) or orthorhombic (aragonite) in crystal habit. Where cations
with small ionic radii are incorporated into carbonates
a trigonal (rhombohedral) crystal lattice is formed,
while larger cations result in orthorhombic unit cells.
N.-M. Hanken (*)
Department of Geology, UiT - The Arctic University of
Norway, Tromsø, Norway
e-mail: Nils-Martin.Hanken@uit.no
K. Bjørlykke
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
J.K. Nielsen
North Energy ASA, New Area Exploration, Alta, Norway
e-mail: jesper.nielsen@northenergy.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_5, # Springer-Verlag Berlin Heidelberg 2015
151
Carbonate Sediments
Nils-Martin Hanken, Knut Bjørlykke and Jesper Kresten Nielsen
Carbonate sediments are a part of the carbon cycle
(Fig. 1.14). CO 2 in the atmosphere dissolves in water
and makes carbonic acid (H 2 CO 3 ) which reacts with
Ca
2+ or Mg
2+ to precipitate CaCO 3 or MgCO 3 . This
process is an important sink for CO 2 . The rate of
carbonate sedimentation globally is controlled by the
supply of cations (mostly Ca
2+ and Mg
2+ ) into the
ocean from rivers. This again is a function of the rate
of weathering of Ca-bearing silicate minerals like plagioclase, pyroxene, amphibole and chlorite. Dissolution of basic rock along a spreading ridge may also add
to the supply of calcium to the ocean water.
Weathering of limestones will release CO 2 and
therefore does not contribute as a sink for CO 2 . During
contact metamorphism when limestones are heated (to
550–600
C), CaCO 3 reacts with SiO 2 to form CaSiO 3
(wollastonite) and CO 2 . Large amounts of CO 2 formed
this way are released through volcanoes. When
minerals like wollastonite are uplifted, exposed and
weathered, Ca
2+ is released into rivers and the ocean
again. Another part of the CO 2 in the atmosphere is
reduced by plants (i.e. algae) and stored as carbon or
organic compounds which can generate petroleum.
These may also be oxidised to CO 2 .
Carbonate sediments are for the most part formed
(born) within sedimentary basins even if there may be
a clastic supply of carbonate sediments. The sedimentology of carbonates therefore differs in many respects
from siliceous sand and mud. Carbonates may precipitate chemically from the seawater, but most of the
limestones are composed of calcareous organisms.
The properties of limestones are therefore closely
linked to their biological origin and the mineralogy
of the carbonate skeletons. The primary composition is
very important for their alteration during burial (diagenesis) and consequent reservoir properties (porosity
and permeability), which to a large extent is controlled
by chemical processes. Since both biologically and
chemically precipitated carbonate sediments are composed of minerals we will first examine the mineralogy
and geochemistry of carbonates. This is important for
understanding diagenetic reactions and prediction of
reservoir quality.
5.1
Geochemistry of Carbonate
Minerals
Carbonate minerals consist of CO
2À
3 and one or more
cations. The most common cations in carbonate
minerals together with their mineral names are listed
in Table 5.1. The common rock-forming carbonate
minerals are either rhombohedral (calcite) or orthorhombic (aragonite) in crystal habit. Where cations
with small ionic radii are incorporated into carbonates
a trigonal (rhombohedral) crystal lattice is formed,
while larger cations result in orthorhombic unit cells.
N.-M. Hanken (*)
Department of Geology, UiT - The Arctic University of
Norway, Tromsø, Norway
e-mail: Nils-Martin.Hanken@uit.no
K. Bjørlykke
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
J.K. Nielsen
North Energy ASA, New Area Exploration, Alta, Norway
e-mail: jesper.nielsen@northenergy.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_5, # Springer-Verlag Berlin Heidelberg 2015
151