720
der eUXlmc conditions in intra-shelf basins, or
shallow-water carbonates and evaporites on the
higher parts ofthe platform (Fig. 14.12b). The hydrocarbon source rocks accumulated from middle Jurassic to late Cretaceous times in the starved shelf basins, especially during transgressive periods. The richest basinal source rocks (thinly laminated limemudstones, kerogen type 11) are of Oxfordian to
Kimmeridgian age. They usually contain between 3
and 5% organic carbon, and their oil yield can be as
high as 2% of the total rock volume. The neighboring
marginal reef mounds, ooidal grainstones and other
shallow-water sands were used as reservoirs.
The porosity of these rocks was improved in pi aces by
subaerialleaching during times of low sea-Ievel stand. The
reservoirs are sealed by shales and supratidal evaporites
(Fig. 14.12c). The temperature gradient from Jurassie to
present time varied only slightly around 3.5°C/lOO m. At
2000 m depth ab out 90°C and at 2700 m approximately
110°C are reached in drillholes. Upper Cretaceous and
Tertiary source rocks remained immature.
During the Upper Cretaceous the outer shelf region was
affected by the onset of orogenie movements creating a
foredeep along the Tethys margin. The deposition of platform sediments continued up to the late Alpine orogeny in
the Miocene, when the northeastem part of this area was
overthrust by the Zagros chains and anticlinal traps formed
in the foredeep now occupied by the Gulf (Fig. 14.12a).
For further information see, e.g., Murris (1985), Stoneley
(1987), Watts and Biome (1990), Beydoun et al. (1992),
Alsharhan and Naim (1997).
Several giant oil fields on the south side of the Gulf
are associated with relatively shallow burial depths
of the older source rocks. The oil has a relatively
high sulfur content indicating the marine euxinic
depositional environment and possibly the influence
of the intercalated sulfate rocks. Cross-strata migration of oil from deeper source beds to higher reservoirs appears to have been facilitated by (oil-filled)
microfractures. Gas fields coincide with those areas
where the source rocks are buried below 5000 m and
have reached the zone of metagenesis.
The exceptionally prolific habitat for oil and gas
of the Gulf region is not a result of extraordinary rich
andJor thick source rocks, but rather originates from
the vast horizontal dimensions of the depositional
platform area (2000 to 3000 km wide and 5000 km
long). In addition, abundant highly porous and permeable reservoirs as weH as efficient seals lie in
elose proximity to the source rocks. Tectonic structures are simple and of very large scale, aHowing
comparatively long-distance migration and a high
degree of reservoir fill. FinaHy, the area was not affected by a major phase of post-depositional erosion
and intensive, deep meteoric water circulation.
Chapter 14 Hydrocarbons and Coal
14.3.5 Hydrocarbons in Other Basin Types
Many petroleum systems evolved in passive continental margin settings, e.g. along both sides of the
Atlantic Ocean and the Gulf of Mexico. A number of
these systems are associated with extensive reef
growth at the shelf edge or salt structures in deeply
buried thick salt deposits (cf. Sects. 6.4.8 and 12.2).
Oil and gas fields have been frequently described
from foreland basins, e.g. those of North America in
front of the Rocky Mountains (summary, e.g., in
Shannon and Naylor 1989). Hydrocarbon habitats of
puH-apart basins, e.g. those of California, are normaHy limited in size. The general principles of generation, migration, and storage of hydrocarbons are
largely the same as briefly outlined above, although
in detail many special features have to be taken into
ac count (see references listed above).
South Caspian basin. A unique hydrocarbon habitat
in some ways is the South Caspian basin ineluding
the Kura depression in Azerbaijan. The evolution of
this backarc basin is associated with the elosure of
one part (the Paratethys) of the former Tethys Ocean
and is not fuHy understood. Its extremely thick Mesozoic and Cenozoic sedimentary sequence (up to
25-30 km?) at least partly rests on a fragment of oceanic crust. The specific features of this basin in terms
of a petroleum system are:
(1) Rapid subsidence and high sedimentation rates,
especiaHy since the Upper Miocene (3000-5000 m in
thickness, sedimentation rates up to 2500 mlMa during the last 2 Ma).
(2) High excess fluid pressure and widespread development of mud diapirism and mud volcaneos.
(3) Very low geothermal gradients (1.0-1.8°C/l00
m).
As a result, the present-day depth of oil generation in
the Caspian Sea is about 9 km and that of gas 14 km;
farther to the west, the oil window occurs at depths
of 6000-12000 m. Oil generation commenced late (at
the end of the Pliocene) and is ongoing today. Structural traps in deltaic silts and sands began to form
during the Middle to Late Pliocene. The peak of tectonic activity was reached in the Quaternary leading
to vertical displacements up to > 1 000 m, tight folds,
and mud diapirs.
The reservoirs consist of sands embedded in a shale sequence and are found mainly at depths of 3-9 km. The
many oil and gas-condensate accumulations in the neighborhood ofthe city ofBaku, mostly found in Pliocene sediments, have been exploited since many years. For further
information see, e.g. Narimanov (1993), Bagirov et al.
(1997), Inan et al. (1997).
der eUXlmc conditions in intra-shelf basins, or
shallow-water carbonates and evaporites on the
higher parts ofthe platform (Fig. 14.12b). The hydrocarbon source rocks accumulated from middle Jurassic to late Cretaceous times in the starved shelf basins, especially during transgressive periods. The richest basinal source rocks (thinly laminated limemudstones, kerogen type 11) are of Oxfordian to
Kimmeridgian age. They usually contain between 3
and 5% organic carbon, and their oil yield can be as
high as 2% of the total rock volume. The neighboring
marginal reef mounds, ooidal grainstones and other
shallow-water sands were used as reservoirs.
The porosity of these rocks was improved in pi aces by
subaerialleaching during times of low sea-Ievel stand. The
reservoirs are sealed by shales and supratidal evaporites
(Fig. 14.12c). The temperature gradient from Jurassie to
present time varied only slightly around 3.5°C/lOO m. At
2000 m depth ab out 90°C and at 2700 m approximately
110°C are reached in drillholes. Upper Cretaceous and
Tertiary source rocks remained immature.
During the Upper Cretaceous the outer shelf region was
affected by the onset of orogenie movements creating a
foredeep along the Tethys margin. The deposition of platform sediments continued up to the late Alpine orogeny in
the Miocene, when the northeastem part of this area was
overthrust by the Zagros chains and anticlinal traps formed
in the foredeep now occupied by the Gulf (Fig. 14.12a).
For further information see, e.g., Murris (1985), Stoneley
(1987), Watts and Biome (1990), Beydoun et al. (1992),
Alsharhan and Naim (1997).
Several giant oil fields on the south side of the Gulf
are associated with relatively shallow burial depths
of the older source rocks. The oil has a relatively
high sulfur content indicating the marine euxinic
depositional environment and possibly the influence
of the intercalated sulfate rocks. Cross-strata migration of oil from deeper source beds to higher reservoirs appears to have been facilitated by (oil-filled)
microfractures. Gas fields coincide with those areas
where the source rocks are buried below 5000 m and
have reached the zone of metagenesis.
The exceptionally prolific habitat for oil and gas
of the Gulf region is not a result of extraordinary rich
andJor thick source rocks, but rather originates from
the vast horizontal dimensions of the depositional
platform area (2000 to 3000 km wide and 5000 km
long). In addition, abundant highly porous and permeable reservoirs as weH as efficient seals lie in
elose proximity to the source rocks. Tectonic structures are simple and of very large scale, aHowing
comparatively long-distance migration and a high
degree of reservoir fill. FinaHy, the area was not affected by a major phase of post-depositional erosion
and intensive, deep meteoric water circulation.
Chapter 14 Hydrocarbons and Coal
14.3.5 Hydrocarbons in Other Basin Types
Many petroleum systems evolved in passive continental margin settings, e.g. along both sides of the
Atlantic Ocean and the Gulf of Mexico. A number of
these systems are associated with extensive reef
growth at the shelf edge or salt structures in deeply
buried thick salt deposits (cf. Sects. 6.4.8 and 12.2).
Oil and gas fields have been frequently described
from foreland basins, e.g. those of North America in
front of the Rocky Mountains (summary, e.g., in
Shannon and Naylor 1989). Hydrocarbon habitats of
puH-apart basins, e.g. those of California, are normaHy limited in size. The general principles of generation, migration, and storage of hydrocarbons are
largely the same as briefly outlined above, although
in detail many special features have to be taken into
ac count (see references listed above).
South Caspian basin. A unique hydrocarbon habitat
in some ways is the South Caspian basin ineluding
the Kura depression in Azerbaijan. The evolution of
this backarc basin is associated with the elosure of
one part (the Paratethys) of the former Tethys Ocean
and is not fuHy understood. Its extremely thick Mesozoic and Cenozoic sedimentary sequence (up to
25-30 km?) at least partly rests on a fragment of oceanic crust. The specific features of this basin in terms
of a petroleum system are:
(1) Rapid subsidence and high sedimentation rates,
especiaHy since the Upper Miocene (3000-5000 m in
thickness, sedimentation rates up to 2500 mlMa during the last 2 Ma).
(2) High excess fluid pressure and widespread development of mud diapirism and mud volcaneos.
(3) Very low geothermal gradients (1.0-1.8°C/l00
m).
As a result, the present-day depth of oil generation in
the Caspian Sea is about 9 km and that of gas 14 km;
farther to the west, the oil window occurs at depths
of 6000-12000 m. Oil generation commenced late (at
the end of the Pliocene) and is ongoing today. Structural traps in deltaic silts and sands began to form
during the Middle to Late Pliocene. The peak of tectonic activity was reached in the Quaternary leading
to vertical displacements up to > 1 000 m, tight folds,
and mud diapirs.
The reservoirs consist of sands embedded in a shale sequence and are found mainly at depths of 3-9 km. The
many oil and gas-condensate accumulations in the neighborhood ofthe city ofBaku, mostly found in Pliocene sediments, have been exploited since many years. For further
information see, e.g. Narimanov (1993), Bagirov et al.
(1997), Inan et al. (1997).