Chapter 6
Mudrocks, Shales, Silica Deposits and Evaporites
Knut Bjørlykke
6.1
Mudrocks and Shales
Mudrocks and shales are the most abundant lithologies
in most sedimentary basins. They are important
because shales include source rocks for oil and gas,
and recently large reserves of gas have been found in
shales. Shales may therefore be reservoir rocks
because they may have significant porosity and some
(although small) permeability to flow gas. The rising
production of oil and gas from shales (shale gas and
shale oil) in recent years has led to a new interest in the
primary composition and the properties of mudstones
and shales as functions of the burial depth and temperature. The seismic image for sandstones also depends
on the properties of adjacent shales.
There is however no precise definition for mud or
mudrocks. It is used to describe fine-grained rocks with
a relatively high content of clay-sized particles, mostly
clay minerals but also other minerals. Carbonate mud is
discussed under carbonate sediments. The upper limit
for clay particles is 0.004 mm in the geological literature, but in the engineering literature (soil science)
0.002 mm is commonly used. Because clay minerals
are essentially flat flakes, they have large surface areas,
some minerals like smectite having several hundred m
2
/
g. Kaolinite minerals are much larger (>0.01 mm) and
have much smaller surface area. There is a cohesion
between small particles, and clay minerals also have a
surface charge due to broken bonds in the mineral
structure. This cohesion plays an important role in
sedimentary processes of erosion, transport and deposition since most clastic sediments contain significant
amounts of clay. The properties of clays are not only
controlled by the mechanical strength of the grains but
also by the composition of the pore fluid. This is also
true during sediment compaction. Kaolinite has much
lower surface charge than smectite and illite.
Mudrocks and shales are often treated as one lithology, but they vary greatly as a function of both mineral
composition and grain-size distribution. A relatively
large fraction of grains may be larger than clay size, but
as long as the larger particles are floating in a finer matrix
the properties are dominated by the clay-sized particles.
Here we will discuss siliceous (i.e. non-carbonate)
mudrocks and clay. The clay minerals in mudrocks
may have different origins:
(1) Clay minerals formed by weathering of igneous
and metamorphic rocks.
(2) By erosion of older shales and mudrocks.
(3) From volcanic ash.
(4) By diagenesis on the seafloor and during burial.
The clay mineral assemblage produced by
weathering depends on the composition of the rocks
that are being weathered, and the climate. A humid
climate will favour the formation of kaolinite. In a granite or gneiss, feldspar, mica and most other silicate
minerals will dissolve and the aluminum and silica will
precipitate as kaolinite, and after long periods of
weathering, as gibbsite (Al(OH) 3 . The quartz grains
will be weathered out as sand and their grain size will
reflect the quartz crystal size range in the parent rock (see
Chap. 3). The result is a bimodal distribution of sand and
kaolinitic clays.
In areas with mostly basic rocks like anorthosite,
gabbro and basalts, weathering will only produce clay
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_6, # Springer-Verlag Berlin Heidelberg 2015
217
Mudrocks, Shales, Silica Deposits and Evaporites
Knut Bjørlykke
6.1
Mudrocks and Shales
Mudrocks and shales are the most abundant lithologies
in most sedimentary basins. They are important
because shales include source rocks for oil and gas,
and recently large reserves of gas have been found in
shales. Shales may therefore be reservoir rocks
because they may have significant porosity and some
(although small) permeability to flow gas. The rising
production of oil and gas from shales (shale gas and
shale oil) in recent years has led to a new interest in the
primary composition and the properties of mudstones
and shales as functions of the burial depth and temperature. The seismic image for sandstones also depends
on the properties of adjacent shales.
There is however no precise definition for mud or
mudrocks. It is used to describe fine-grained rocks with
a relatively high content of clay-sized particles, mostly
clay minerals but also other minerals. Carbonate mud is
discussed under carbonate sediments. The upper limit
for clay particles is 0.004 mm in the geological literature, but in the engineering literature (soil science)
0.002 mm is commonly used. Because clay minerals
are essentially flat flakes, they have large surface areas,
some minerals like smectite having several hundred m
2
/
g. Kaolinite minerals are much larger (>0.01 mm) and
have much smaller surface area. There is a cohesion
between small particles, and clay minerals also have a
surface charge due to broken bonds in the mineral
structure. This cohesion plays an important role in
sedimentary processes of erosion, transport and deposition since most clastic sediments contain significant
amounts of clay. The properties of clays are not only
controlled by the mechanical strength of the grains but
also by the composition of the pore fluid. This is also
true during sediment compaction. Kaolinite has much
lower surface charge than smectite and illite.
Mudrocks and shales are often treated as one lithology, but they vary greatly as a function of both mineral
composition and grain-size distribution. A relatively
large fraction of grains may be larger than clay size, but
as long as the larger particles are floating in a finer matrix
the properties are dominated by the clay-sized particles.
Here we will discuss siliceous (i.e. non-carbonate)
mudrocks and clay. The clay minerals in mudrocks
may have different origins:
(1) Clay minerals formed by weathering of igneous
and metamorphic rocks.
(2) By erosion of older shales and mudrocks.
(3) From volcanic ash.
(4) By diagenesis on the seafloor and during burial.
The clay mineral assemblage produced by
weathering depends on the composition of the rocks
that are being weathered, and the climate. A humid
climate will favour the formation of kaolinite. In a granite or gneiss, feldspar, mica and most other silicate
minerals will dissolve and the aluminum and silica will
precipitate as kaolinite, and after long periods of
weathering, as gibbsite (Al(OH) 3 . The quartz grains
will be weathered out as sand and their grain size will
reflect the quartz crystal size range in the parent rock (see
Chap. 3). The result is a bimodal distribution of sand and
kaolinitic clays.
In areas with mostly basic rocks like anorthosite,
gabbro and basalts, weathering will only produce clay
K. Bjørlykke (*)
Department of Geosciences, University of Oslo, Oslo, Norway
e-mail: knut.bjorlykke@geo.uio.no
K. Bjørlykke (ed.), Petroleum Geoscience: From Sedimentary Environments to Rock Physics,
DOI 10.1007/978-3-642-34132-8_6, # Springer-Verlag Berlin Heidelberg 2015
217