478
10.5.6 Black Shales
(Hydrocarbon Source Rocks)
The preservation of organic carbon in various
depositional environments is a topic of particular
interest in hydrocarbon exploration. In the evaluation
of petroleum source rocks (Chap. 14), the total organic carbon concentration, Ce. is a crucial factor.
Carbonate source rocks should contain at least 0.3%
organic carbon and clastic source rocks at least 0.5%
(Tissot and WeIte 1984). However, the nature of the
buried organic matter also plays an important role
(Sect. 14.1.2). Oil-prone organic constituents are
better preserved if the residence time of the organic
particles in the zone of intense benthic activity is
limited by high bulk accumulation rates. Therefore, if
two marine source rocks have the same lithology and
organic carbon content, the more rapidly deposited
one usually has a greater potential for generating oil
(Johnson Ibach 1982).
The so-called black shales are one of the most important groups of these source rocks. Black shales
are fine-grained, dark-colored, compacted sediments
which usually display distinct lamination and contain
~0.5%, commonly 1-4%, frequently up to 7%, and
occasionally up to 20% of organic matter. Their dark
gray color is caused by finely dispersed iron sulfides
and organic compounds. Widespread microbial mats
formed at the sediment surface often play a significant part in the generation of lamination (e.g. Gerdes
and Krumbein 1987; Kemp 1996).
However, not all dark shales are true black shales
with sufficiently high organic matter contents, and
not every black shale is a hydrocarbon source rock
(cf. Sect. 14.1). Black shales develop under anoxic,
dysoxic (0.1 <0 2 <1.0 mVl) and, to some degree, under oxic conditions (0 2 )1.0 mlll). The state of oxygenation at the sea floor and in the uppennost sediment is reflected by the presence, scarcity, or absence
of macro-benthos (epifauna) and burrowing organisms (infauna). Many sections of black shales show
repeated changes from anaerobic to dysaerobic and
aerobic biofacies (cf. Sect. 13.3) and a kind of
cyclicity in their organic carbon contents at different
time scales (cf. Sects. 7.8 and 7.9).
This has been demonstrated in some detail by several authors (e.g. Wetze I 1991; van Buchem et al. 1995).Oxygenation events in the organic-rich siliceous Miocene
Monterey Formation, Califomia, can be subdivided into
Chapter 10 Sedimentation Rates and Organic Matter
two groups (Ozalas et al. 1994): (1) re1ated to increased
bottom currents and c1astic input (Glossifungites
ichnofacies on firmground) and (2) climate- and sea levelcontrolled events producing little change in the quiet-water
pelagic sedimentation (soft-ground deposit feeders including Chondrites).
Many authors have pointed out the positive relationship
between organic matter content and sea-level highstand
(e.g. Herbin et al., 1991, in the Kimmeridge Clay in northwestern Europe, which is a major source rock ofthe North
Sea).
Of the large number of publications on black shales all
over the world, only a few can be mentioned here. Sumrnaries have been provided, e.g., by Arthur and Sageman
(1994), Wignall (1994); the specific biofacies of black
shales are described by Savrda et al. (1991) and Oschmann
(1994). The latter as wen as specific geochemical characteristics of black shales, such as the relationship between
OC, iron and sulfur, cannot be discussed in this section.
The most favorable terrestrial and aquatic environments for the formation of hydrocarbon source rocks
are summarized in Fig. 10.11. They include:
- Swamps and the tidal flat zones of marine deltas
(Sect. 3.4.2).
- Estuaries and lagoons with high nutrient supplies
(Sect. 3.2.3).
- Basins with restricted circulation on the continental
shelf and in continental borderlands (e.g. off California).
- Silled adjacent seas (e.g. Black Sea; Sect. 4.3.1).
- Shelves and upper continental slopes under zones
of coastal upwelling (Sect. 5.3.3).
- Widely extended deep-sea regions during periods
of sluggish oceanic circulation.
In most of these environments, intermediate to relatively high sedimentation rates (up to several100's of
m/Ma) prevail. The kerogen types of the organic matter, mentioned in Fig. 10.11, are explained in Sect.
14.1.2. The presence of exinite and liptinite-type
kerogen is typical of aquatic environments with relatively low benthic degradation loss. Inertinite ("dead
carbon") characterizes well oxygenated conditions,
including reworking, in which most of the organic
compounds have been destroyed. The significance of
sediments ± rich in organic matter as source rocks of
hydrocarbons is discussed further in Sect. 14.2.
10.5.6 Black Shales
(Hydrocarbon Source Rocks)
The preservation of organic carbon in various
depositional environments is a topic of particular
interest in hydrocarbon exploration. In the evaluation
of petroleum source rocks (Chap. 14), the total organic carbon concentration, Ce. is a crucial factor.
Carbonate source rocks should contain at least 0.3%
organic carbon and clastic source rocks at least 0.5%
(Tissot and WeIte 1984). However, the nature of the
buried organic matter also plays an important role
(Sect. 14.1.2). Oil-prone organic constituents are
better preserved if the residence time of the organic
particles in the zone of intense benthic activity is
limited by high bulk accumulation rates. Therefore, if
two marine source rocks have the same lithology and
organic carbon content, the more rapidly deposited
one usually has a greater potential for generating oil
(Johnson Ibach 1982).
The so-called black shales are one of the most important groups of these source rocks. Black shales
are fine-grained, dark-colored, compacted sediments
which usually display distinct lamination and contain
~0.5%, commonly 1-4%, frequently up to 7%, and
occasionally up to 20% of organic matter. Their dark
gray color is caused by finely dispersed iron sulfides
and organic compounds. Widespread microbial mats
formed at the sediment surface often play a significant part in the generation of lamination (e.g. Gerdes
and Krumbein 1987; Kemp 1996).
However, not all dark shales are true black shales
with sufficiently high organic matter contents, and
not every black shale is a hydrocarbon source rock
(cf. Sect. 14.1). Black shales develop under anoxic,
dysoxic (0.1 <0 2 <1.0 mVl) and, to some degree, under oxic conditions (0 2 )1.0 mlll). The state of oxygenation at the sea floor and in the uppennost sediment is reflected by the presence, scarcity, or absence
of macro-benthos (epifauna) and burrowing organisms (infauna). Many sections of black shales show
repeated changes from anaerobic to dysaerobic and
aerobic biofacies (cf. Sect. 13.3) and a kind of
cyclicity in their organic carbon contents at different
time scales (cf. Sects. 7.8 and 7.9).
This has been demonstrated in some detail by several authors (e.g. Wetze I 1991; van Buchem et al. 1995).Oxygenation events in the organic-rich siliceous Miocene
Monterey Formation, Califomia, can be subdivided into
Chapter 10 Sedimentation Rates and Organic Matter
two groups (Ozalas et al. 1994): (1) re1ated to increased
bottom currents and c1astic input (Glossifungites
ichnofacies on firmground) and (2) climate- and sea levelcontrolled events producing little change in the quiet-water
pelagic sedimentation (soft-ground deposit feeders including Chondrites).
Many authors have pointed out the positive relationship
between organic matter content and sea-level highstand
(e.g. Herbin et al., 1991, in the Kimmeridge Clay in northwestern Europe, which is a major source rock ofthe North
Sea).
Of the large number of publications on black shales all
over the world, only a few can be mentioned here. Sumrnaries have been provided, e.g., by Arthur and Sageman
(1994), Wignall (1994); the specific biofacies of black
shales are described by Savrda et al. (1991) and Oschmann
(1994). The latter as wen as specific geochemical characteristics of black shales, such as the relationship between
OC, iron and sulfur, cannot be discussed in this section.
The most favorable terrestrial and aquatic environments for the formation of hydrocarbon source rocks
are summarized in Fig. 10.11. They include:
- Swamps and the tidal flat zones of marine deltas
(Sect. 3.4.2).
- Estuaries and lagoons with high nutrient supplies
(Sect. 3.2.3).
- Basins with restricted circulation on the continental
shelf and in continental borderlands (e.g. off California).
- Silled adjacent seas (e.g. Black Sea; Sect. 4.3.1).
- Shelves and upper continental slopes under zones
of coastal upwelling (Sect. 5.3.3).
- Widely extended deep-sea regions during periods
of sluggish oceanic circulation.
In most of these environments, intermediate to relatively high sedimentation rates (up to several100's of
m/Ma) prevail. The kerogen types of the organic matter, mentioned in Fig. 10.11, are explained in Sect.
14.1.2. The presence of exinite and liptinite-type
kerogen is typical of aquatic environments with relatively low benthic degradation loss. Inertinite ("dead
carbon") characterizes well oxygenated conditions,
including reworking, in which most of the organic
compounds have been destroyed. The significance of
sediments ± rich in organic matter as source rocks of
hydrocarbons is discussed further in Sect. 14.2.