high-angle listric faults related to fault-bounded rift
basins, while the distal margin is characterized by
extremely thinned continental crust that is potentially separated from the oceanic crust by a domain of exhumed
subcontinental mantle. The boundary between the two
domains is at the point where listric faults cut across the
entire crust reaching the mantle (see Lithosphere: Structure and Composition). In the proximal margin, typically
a detachment is interpreted between the brittle upper crust
and the mantle, while at the distal margin, brittle upper
crust and upper mantle are separated by only a thin lower
crustal layer or are juxtaposed (Figure 3). This leads to
coupling and the development of a detachment at the
crust-mantle boundary, allowing the exhumation of the
mantle further seaward.
Volcanic rifted margins are characterized by up to
15 km thick wedges of volcanic flows (see “Intraplate
Magmatism”) (Hinz, 1981; Mutter et al., 1982),
manifested in multichannel seismic reflection data as seaward dipping reflectors (SDRs), and high-velocity
(Vp > 7.3 km/s) lower-crust seaward of the rifted continental margin (Figure 3). Because thinning of the continental crust typically occurs over a comparably short
distance (50–100 km), the continent-ocean transition is
abrupt and located in the vicinity of the SDRs (Figure 3).
Various models place the continent-ocean transition, and
thus the seaward limit of the presence of potential rift
basins, at the seaward edge, in the center of, or at the landward edge of the seaward dipping reflectors. The effusive
magmatism of volcanic margins and the corresponding
lavas represented by the seaward dipping reflections
obscure the seismic images of potential underlying fault
blocks that undoubtedly accompanied the rifting. The
major challenge will be a better imaging of sub-basalt
structures to understand the complexity in the rift development, the heat flow history, and hydrocarbon maturation
and thus to identify prospective plays (Skogseid, 2001).
Forearc basins (see Subduction) are only by way of an
exception in the focus of petroleum companies. Such
basins are located at almost all convergent continental
margins between the deep-sea trench and the magmatic
arc (Figure 3). Active margins stretch for a total length
of some 44,000 km (von Huene and Scholl, 1991), mainly
around the Pacific Ocean, but also along the Indonesian
Island Arc, in the Caribbean, the Mediterranean, and the
Arabian Sea. The widespread occurrence of this basin type
makes it worth reviewing its hydrocarbon potential.
Forearc basins may overlie either continental crust or
accreted rocks that became attached to the continental
plate during the collision of the involved plates. Commonly, forearc basins are bounded seaward by an outerarc high that is part of the accretionary prism. The tectonic
evolution of forearc basins is mainly controlled by the subduction of an oceanic plate. Important factors controlling
the basin dynamics are convergence rate, lateral movements of the bending axis, dip angle of the oceanic plate,
obliquity of convergence, roughness and sedimentary
cover of the oceanic crust, and the sedimentary input of
the landward magmatic arc and continent. Forearc basins
typically show an elongated shape, extending parallel to
the trench-arc system. Their width commonly ranges
between 25 and 125 km, but their length may reach
500 km. Forearc basins are widespread; however, their
hydrocarbon potential is believed to be low. This is due
to generally low heat flow and the complicated structural
history with long-lasting and usually ongoing tectonic
activity affecting the basins, mainly terrigenous sediment
input from the volcanic arc, and thus the questionable
presence of prolific source rocks. Nevertheless,
a tectonic basin classification alone is not sufficient to
evaluate the petroleum potential of a sedimentary basin.
The heat flow values of forearc basins range between
20 and 45 mW/m
2
, with a typical value of 40 mW/m
2
(Dickinson 1995). In contrast, passive margins typically
exhibit a heat flow between 30 and 50 mW/m
2
, or even
more in cases of forced fluid flow. Relatively cold oceanic
crust is subducted at continental margins, and this reduces
the heat flow beneath the forearc basin. Heat flow is typically further reduced by high sediment input from the volcanic arc and the adjacent continent.
Natural gas hydrate is an unconventional energy
resource that is currently unexploited. Gas hydrate is
a potentially vast source of hydrocarbon energy; the dissociation of 1 m
3 of gas hydrate can release 164 m
3 of gas.
Gas hydrates occur under conditions of high pressure
and low temperature and are abundantly found in marine
environments and the permafrost environments of polar
regions. However, the exact volumes are unknown and it
is unclear how the gas may be produced. Some estimation
predict that the carbon bound in gas hydrates is twice as
high as in all other recoverable and non-recoverable
energy resources such as oil, gas, and coal (Kvenvolden,
1993). Not only gas hydrates, but also the free gases
trapped below the hydrates can be seen as potential energy
sources.
Example: Simeulue forearc basin of the
Sunda Arc, Indonesia
The Sumatra-Java area is part of the Sunda Arc (see Subduction) that stretches from the Andaman Sea in the northwest to the Banda Sea in the east. Along the Sunda Arc,
the Indo-Australian Plate subducts under the Eurasian
Plate. The western Indonesian forearc basins extend for
more than 1,800 km from northwest of Aceh to southwest
Java. The width of the basins varies from less than 70 km
to the south of the Sunda Strait to about 120 km to the west
of northern Sumatra. The basins form a strongly subsiding
belt between the elevated Sumatra Paleozoic-Mesozoic
arc massif cropping out along Sumatra and Java and the
rising outer-arc high (Berglar et al., 2010).
The Simeulue forearc basin extends between Simeulue
Island and northern Sumatra. It matches all features of
a typical forearc basin (e.g., low heat flow, tectonic setting
controlled by subduction). It is a frontier shallow shelf
area with few wells and no wells in the basin center:
224
ENERGY RESOURCES
basins, while the distal margin is characterized by
extremely thinned continental crust that is potentially separated from the oceanic crust by a domain of exhumed
subcontinental mantle. The boundary between the two
domains is at the point where listric faults cut across the
entire crust reaching the mantle (see Lithosphere: Structure and Composition). In the proximal margin, typically
a detachment is interpreted between the brittle upper crust
and the mantle, while at the distal margin, brittle upper
crust and upper mantle are separated by only a thin lower
crustal layer or are juxtaposed (Figure 3). This leads to
coupling and the development of a detachment at the
crust-mantle boundary, allowing the exhumation of the
mantle further seaward.
Volcanic rifted margins are characterized by up to
15 km thick wedges of volcanic flows (see “Intraplate
Magmatism”) (Hinz, 1981; Mutter et al., 1982),
manifested in multichannel seismic reflection data as seaward dipping reflectors (SDRs), and high-velocity
(Vp > 7.3 km/s) lower-crust seaward of the rifted continental margin (Figure 3). Because thinning of the continental crust typically occurs over a comparably short
distance (50–100 km), the continent-ocean transition is
abrupt and located in the vicinity of the SDRs (Figure 3).
Various models place the continent-ocean transition, and
thus the seaward limit of the presence of potential rift
basins, at the seaward edge, in the center of, or at the landward edge of the seaward dipping reflectors. The effusive
magmatism of volcanic margins and the corresponding
lavas represented by the seaward dipping reflections
obscure the seismic images of potential underlying fault
blocks that undoubtedly accompanied the rifting. The
major challenge will be a better imaging of sub-basalt
structures to understand the complexity in the rift development, the heat flow history, and hydrocarbon maturation
and thus to identify prospective plays (Skogseid, 2001).
Forearc basins (see Subduction) are only by way of an
exception in the focus of petroleum companies. Such
basins are located at almost all convergent continental
margins between the deep-sea trench and the magmatic
arc (Figure 3). Active margins stretch for a total length
of some 44,000 km (von Huene and Scholl, 1991), mainly
around the Pacific Ocean, but also along the Indonesian
Island Arc, in the Caribbean, the Mediterranean, and the
Arabian Sea. The widespread occurrence of this basin type
makes it worth reviewing its hydrocarbon potential.
Forearc basins may overlie either continental crust or
accreted rocks that became attached to the continental
plate during the collision of the involved plates. Commonly, forearc basins are bounded seaward by an outerarc high that is part of the accretionary prism. The tectonic
evolution of forearc basins is mainly controlled by the subduction of an oceanic plate. Important factors controlling
the basin dynamics are convergence rate, lateral movements of the bending axis, dip angle of the oceanic plate,
obliquity of convergence, roughness and sedimentary
cover of the oceanic crust, and the sedimentary input of
the landward magmatic arc and continent. Forearc basins
typically show an elongated shape, extending parallel to
the trench-arc system. Their width commonly ranges
between 25 and 125 km, but their length may reach
500 km. Forearc basins are widespread; however, their
hydrocarbon potential is believed to be low. This is due
to generally low heat flow and the complicated structural
history with long-lasting and usually ongoing tectonic
activity affecting the basins, mainly terrigenous sediment
input from the volcanic arc, and thus the questionable
presence of prolific source rocks. Nevertheless,
a tectonic basin classification alone is not sufficient to
evaluate the petroleum potential of a sedimentary basin.
The heat flow values of forearc basins range between
20 and 45 mW/m
2
, with a typical value of 40 mW/m
2
(Dickinson 1995). In contrast, passive margins typically
exhibit a heat flow between 30 and 50 mW/m
2
, or even
more in cases of forced fluid flow. Relatively cold oceanic
crust is subducted at continental margins, and this reduces
the heat flow beneath the forearc basin. Heat flow is typically further reduced by high sediment input from the volcanic arc and the adjacent continent.
Natural gas hydrate is an unconventional energy
resource that is currently unexploited. Gas hydrate is
a potentially vast source of hydrocarbon energy; the dissociation of 1 m
3 of gas hydrate can release 164 m
3 of gas.
Gas hydrates occur under conditions of high pressure
and low temperature and are abundantly found in marine
environments and the permafrost environments of polar
regions. However, the exact volumes are unknown and it
is unclear how the gas may be produced. Some estimation
predict that the carbon bound in gas hydrates is twice as
high as in all other recoverable and non-recoverable
energy resources such as oil, gas, and coal (Kvenvolden,
1993). Not only gas hydrates, but also the free gases
trapped below the hydrates can be seen as potential energy
sources.
Example: Simeulue forearc basin of the
Sunda Arc, Indonesia
The Sumatra-Java area is part of the Sunda Arc (see Subduction) that stretches from the Andaman Sea in the northwest to the Banda Sea in the east. Along the Sunda Arc,
the Indo-Australian Plate subducts under the Eurasian
Plate. The western Indonesian forearc basins extend for
more than 1,800 km from northwest of Aceh to southwest
Java. The width of the basins varies from less than 70 km
to the south of the Sunda Strait to about 120 km to the west
of northern Sumatra. The basins form a strongly subsiding
belt between the elevated Sumatra Paleozoic-Mesozoic
arc massif cropping out along Sumatra and Java and the
rising outer-arc high (Berglar et al., 2010).
The Simeulue forearc basin extends between Simeulue
Island and northern Sumatra. It matches all features of
a typical forearc basin (e.g., low heat flow, tectonic setting
controlled by subduction). It is a frontier shallow shelf
area with few wells and no wells in the basin center:
224
ENERGY RESOURCES