therefore it is studied using geophysical data and geological surface samples. Multichannel seismic data show
bright spots above potential hydrocarbon reservoirs in carbonate buildups. Amplitude versus offset analysis indicates the presence of gas, and surface geochemical
prospecting indicates thermal hydrocarbon generation.
Heat flow in the Simeulue Basin ranges between 37 and
74 mW/m
2
, as deduced from 1-D petroleum system
modeling. Two possible source rocks (Eocene and EarlyMiddle Miocene; see Geological Time Scale) were
assigned for 3-D petroleum system modeling in the
Simeulue Basin. Due to a similar pre-Miocene geological
evolution in the present-day back-arc and the forearc, it
can be assumed that the back-arc source rocks also occur
in the forearc.
Modeling based on two heat flow scenarios (40 and
60 mW/m
2
) reveals that oil and gas generation is possible
within and below the main depocenters of the central and
southern Simeulue basin (Lutz et al., 2011). Deep burial
(>6 km) of source rocks can compensate for low heat
flow. This is an example of a prolific forearc basin and
highlights the necessity to carefully evaluate the hydrocarbon potential of each basin individually.
Example: gas hydrates: an unconventional
energy resource
Gas hydrates are drawing attention because of their contribution to geological hazards, to climate change, and
because of their potential as a future energy supply. From
the stability diagrams and the phase boundary of methane
hydrate, it can be deduced that elevated pressure and
reduced temperatures are necessary for the formation of
gas hydrates. Additionally, sufficiently high concentrations of gases are essential. These conditions limit the
occurrence of gas hydrates to marine environments
(sediments of continental slopes, rises, and in the deep
seas) and to permafrost environments (sediments in the
Arctic region and in continental ice sheets). At continental
margins, the formation of gas hydrates is primarily controlled by the sedimentation rate and the organic matter
content. Therefore, the gases involved in the gas hydrates
are mostly of biogenic origin. Gas hydrates in permafrost
regions can also be the result of the migration of gases
from deeper hydrocarbon reservoirs into the Gas Hydrate
Stability Zone (Kvenvolden, 1993).
Most of the marine gas hydrates are detected based on
the occurrence of bottom simulating reflectors (BSRs) in
reflection seismic data. The BSR indicates the base of
the Gas Hydrate Stability Zone and mirrors the seafloor.
A BSR marks the phase boundary between free gas in
the deeper sediments and the gas hydrates of the stability
zone, which is determined by temperature, pressure, gas
composition, and pore water salinity. Therefore, BSRs
are not restricted to layer boundaries and may thus intersect them. In reflection seismic data, a BSR is generated
by the inversion in the P-wave velocity from the hydrates
to unconsolidated sediments. However, although the BSR
is often cited as evidence of gas hydrates, the magnitude of
the BSR amplitudes primarily depends on the amount of
free gas below the BSR. There are examples of BSRs with
no associated gas hydrates, or BSRs within the Gas
Hydrate Stability Zone. Nevertheless, the existence of
gas hydrates in sediments decreases, or even hinders the
further migration of gases, so that in marine sediments,
the highest concentration of the gas hydrates is generally
just above the BSRs (Dillon and Max, 1998).
Much research is still needed to determine the geological and economic feasibility of extracting gas hydrates.
Also the environmental implications of gas hydrate exploration must be considered thoroughly for safe and environmentally friendly exploration. Until now, the exploration
of gas hydrates has not been successful due to the complex
technical requirements for economic extraction, despite
existing offshore and onshore know-how from the oil
and gas industry. This is because, in contrast to conventional gas deposits, gas hydrate reservoirs react largely
unpredictably to changes in physical and chemical environmental parameters. Therefore, better understanding of
the reservoir behavior of gas hydrates is required before
significant commercial exploration could take place.
However, even if only a small fraction of the estimated
gas hydrates could be produced, the resource could be
very important (Collet, 2000).
Potential options for the recovery of gas hydrates are
based on changing the factors that control their phase stability. This can be achieved by three principal methods.
Producing the free gas below the Gas Hydrate Stability
Zone reduces the local pressure so that some of the gas
hydrates at the base of the Gas Hydrate Stability Zone dissociates and can be produced (depressurization). In the
case of thermal stimulation, the dissociation of gas
hydrates is achieved by increasing the temperature either
by heating or by injecting warm surface water into the
Gas Hydrate Stability Zone. Injection of inhibitors such
as methanol into the gas hydrate-containing sediments
changes the chemical composition so that some of the
gas hydrates dissociate and release gas.
Summary
Energy resources from marine environments are of
increasing importance. This is valid for both renewable
and fossil energy resources: renewable energy represents
at present only a minor percentage of global primary
energy consumption but is growing very rapidly. Natural
oil and gas are the most important offshore energy
resources. Information concerning the amount of the producible and usable oil and gas resources depends on the
geological conditions of the deposits, the state of the scientific and technical knowledge, the technological potential of the development, and production as well as on the
economic and political requirements.
Offshore and deepwater petroleum exploration and
exploration in the Arctic are ongoing since decades. At
present drilling activity has particularly increased in the
ENERGY RESOURCES
225
bright spots above potential hydrocarbon reservoirs in carbonate buildups. Amplitude versus offset analysis indicates the presence of gas, and surface geochemical
prospecting indicates thermal hydrocarbon generation.
Heat flow in the Simeulue Basin ranges between 37 and
74 mW/m
2
, as deduced from 1-D petroleum system
modeling. Two possible source rocks (Eocene and EarlyMiddle Miocene; see Geological Time Scale) were
assigned for 3-D petroleum system modeling in the
Simeulue Basin. Due to a similar pre-Miocene geological
evolution in the present-day back-arc and the forearc, it
can be assumed that the back-arc source rocks also occur
in the forearc.
Modeling based on two heat flow scenarios (40 and
60 mW/m
2
) reveals that oil and gas generation is possible
within and below the main depocenters of the central and
southern Simeulue basin (Lutz et al., 2011). Deep burial
(>6 km) of source rocks can compensate for low heat
flow. This is an example of a prolific forearc basin and
highlights the necessity to carefully evaluate the hydrocarbon potential of each basin individually.
Example: gas hydrates: an unconventional
energy resource
Gas hydrates are drawing attention because of their contribution to geological hazards, to climate change, and
because of their potential as a future energy supply. From
the stability diagrams and the phase boundary of methane
hydrate, it can be deduced that elevated pressure and
reduced temperatures are necessary for the formation of
gas hydrates. Additionally, sufficiently high concentrations of gases are essential. These conditions limit the
occurrence of gas hydrates to marine environments
(sediments of continental slopes, rises, and in the deep
seas) and to permafrost environments (sediments in the
Arctic region and in continental ice sheets). At continental
margins, the formation of gas hydrates is primarily controlled by the sedimentation rate and the organic matter
content. Therefore, the gases involved in the gas hydrates
are mostly of biogenic origin. Gas hydrates in permafrost
regions can also be the result of the migration of gases
from deeper hydrocarbon reservoirs into the Gas Hydrate
Stability Zone (Kvenvolden, 1993).
Most of the marine gas hydrates are detected based on
the occurrence of bottom simulating reflectors (BSRs) in
reflection seismic data. The BSR indicates the base of
the Gas Hydrate Stability Zone and mirrors the seafloor.
A BSR marks the phase boundary between free gas in
the deeper sediments and the gas hydrates of the stability
zone, which is determined by temperature, pressure, gas
composition, and pore water salinity. Therefore, BSRs
are not restricted to layer boundaries and may thus intersect them. In reflection seismic data, a BSR is generated
by the inversion in the P-wave velocity from the hydrates
to unconsolidated sediments. However, although the BSR
is often cited as evidence of gas hydrates, the magnitude of
the BSR amplitudes primarily depends on the amount of
free gas below the BSR. There are examples of BSRs with
no associated gas hydrates, or BSRs within the Gas
Hydrate Stability Zone. Nevertheless, the existence of
gas hydrates in sediments decreases, or even hinders the
further migration of gases, so that in marine sediments,
the highest concentration of the gas hydrates is generally
just above the BSRs (Dillon and Max, 1998).
Much research is still needed to determine the geological and economic feasibility of extracting gas hydrates.
Also the environmental implications of gas hydrate exploration must be considered thoroughly for safe and environmentally friendly exploration. Until now, the exploration
of gas hydrates has not been successful due to the complex
technical requirements for economic extraction, despite
existing offshore and onshore know-how from the oil
and gas industry. This is because, in contrast to conventional gas deposits, gas hydrate reservoirs react largely
unpredictably to changes in physical and chemical environmental parameters. Therefore, better understanding of
the reservoir behavior of gas hydrates is required before
significant commercial exploration could take place.
However, even if only a small fraction of the estimated
gas hydrates could be produced, the resource could be
very important (Collet, 2000).
Potential options for the recovery of gas hydrates are
based on changing the factors that control their phase stability. This can be achieved by three principal methods.
Producing the free gas below the Gas Hydrate Stability
Zone reduces the local pressure so that some of the gas
hydrates at the base of the Gas Hydrate Stability Zone dissociates and can be produced (depressurization). In the
case of thermal stimulation, the dissociation of gas
hydrates is achieved by increasing the temperature either
by heating or by injecting warm surface water into the
Gas Hydrate Stability Zone. Injection of inhibitors such
as methanol into the gas hydrate-containing sediments
changes the chemical composition so that some of the
gas hydrates dissociate and release gas.
Summary
Energy resources from marine environments are of
increasing importance. This is valid for both renewable
and fossil energy resources: renewable energy represents
at present only a minor percentage of global primary
energy consumption but is growing very rapidly. Natural
oil and gas are the most important offshore energy
resources. Information concerning the amount of the producible and usable oil and gas resources depends on the
geological conditions of the deposits, the state of the scientific and technical knowledge, the technological potential of the development, and production as well as on the
economic and political requirements.
Offshore and deepwater petroleum exploration and
exploration in the Arctic are ongoing since decades. At
present drilling activity has particularly increased in the
ENERGY RESOURCES
225