722
generative basins or on structural highs neighboring
deeper generative depressions. Large volumes of hydrocarbon source rocks and long time spans for their
drainage are aprerequisite for giant oil and gas accumulations. Evaporites and fine-grained, water-wet
elayey shales and mudrocks, which must not have
open fractures, act as efficient seals on top of the
porous oil and gas traps. Such seal rocks prevent oil
(lower density than water!) and gas from migrating
upward to the surface over long time periods, if the
petroleum system is not affected by subsequent
strong tectonic activity destroying the traps. Large oil
and gas fields still exist in Paleozoic basins, for example in North America, North Africa, and in the
territory ofthe former Soviet Union.
Oil usually migrates over distances not more than
a few tens of km in the updip drainage area of individual tectonic structures. Most of the producible
hydrocarbons and the largest oil fields occur within
the ranges of generative depressions. These conelusions apply to different types of basins (e.g.,
intracratonic basins, rift basins, passive margin basins). Long-distance oil migration is documented from
some foreland basins. Migration of gas, however, is
frequently more difficult to predict.
14.3.8 Summary (Hydrocarbons)
Petroleum systems consist of source rocks, carrier beds, and reservoirs which must be sealed
by impermeable layers (shales or evaporites).
Insoluble organic matter (kerogen types I-III) is
converted to liquid and gaseous hydrocarbons
at temperatures beyond 60-90°C which are
mostly reached at burial depths of 1500-3000
m.
- Biogenic methane gas, occurring in huge quantities as solid gas hydrates in shallowly buried
hemipelagic sediments of the present-day continental slope and rise, are not considered as part
of a petroleum system.
Kerogen type I has the highest potential (8090%) for generating hydrocarbons, but only a
very small fraction of the prirnary organic production is finally converted to hydrocarbons.
- Prirnary migration of hydrocarbons within the
source rocks mainly occurs in liquid phase and
Chapter 14 Hydrocarbons and Coal
Subsurface mapping of hydrocarbon generative
basins, ineluding kerogen analyses and an evaluation
of the thermal history of the basin, is one of the most
important tasks in petroleum exploration. In addition,
the three-dimensional facies distribution and architecture of the depositional system, reservoir and seal
rocks in the neighborhood of the source rocks, and
the tectonic structure of the basin fill have to be investigated.
New frontiers in hydrocarbon exploration include the lower
slope and continental rise of passive margins as weil as
foredeep sediments buried under thin-skinned thrust belts
(e.g. Picha 1996; Wessely and Liebl 1996). Proven reserves, improvements of the methods for extracting oil and
gas, and undiscovered hydrocarbons allow hydrocarbon
production well into the 22nd century (Schollenberger
1998). A crucial point is sustainable development and use
of these energy reserves in combination with increased
efforts to replace them by other energy sourees. Whether
the huge reservoir of gas hydrates below the ocean (mainly
biogenie and still present in the porous source sediment)
can be used in the future as an important energy source, is
an open question.
is aided by overpressure of the pore fluids and
buoyancy (oil has a lower density than water).
Grain surfaces and tiny pores remain waterwet.
Secondary migration in permeable carrier beds
mainly takes place as two-phase mass flow. In
some cases, hydrocarbons dissolved in water
are transported in significant quantities by meteoric flow. Transport by molecular diffusion
may play some role for light hydrocarbons ineluding natural gas.
- Large petroleum accumulations tend to occur
in reservoir rocks located elose to the center of
the generative basins (oil kitchens). The extent
and volume of source rocks is crucial for giant
hydrocarbon generation, but not necessarily a
high content of organic matter.
generative basins or on structural highs neighboring
deeper generative depressions. Large volumes of hydrocarbon source rocks and long time spans for their
drainage are aprerequisite for giant oil and gas accumulations. Evaporites and fine-grained, water-wet
elayey shales and mudrocks, which must not have
open fractures, act as efficient seals on top of the
porous oil and gas traps. Such seal rocks prevent oil
(lower density than water!) and gas from migrating
upward to the surface over long time periods, if the
petroleum system is not affected by subsequent
strong tectonic activity destroying the traps. Large oil
and gas fields still exist in Paleozoic basins, for example in North America, North Africa, and in the
territory ofthe former Soviet Union.
Oil usually migrates over distances not more than
a few tens of km in the updip drainage area of individual tectonic structures. Most of the producible
hydrocarbons and the largest oil fields occur within
the ranges of generative depressions. These conelusions apply to different types of basins (e.g.,
intracratonic basins, rift basins, passive margin basins). Long-distance oil migration is documented from
some foreland basins. Migration of gas, however, is
frequently more difficult to predict.
14.3.8 Summary (Hydrocarbons)
Petroleum systems consist of source rocks, carrier beds, and reservoirs which must be sealed
by impermeable layers (shales or evaporites).
Insoluble organic matter (kerogen types I-III) is
converted to liquid and gaseous hydrocarbons
at temperatures beyond 60-90°C which are
mostly reached at burial depths of 1500-3000
m.
- Biogenic methane gas, occurring in huge quantities as solid gas hydrates in shallowly buried
hemipelagic sediments of the present-day continental slope and rise, are not considered as part
of a petroleum system.
Kerogen type I has the highest potential (8090%) for generating hydrocarbons, but only a
very small fraction of the prirnary organic production is finally converted to hydrocarbons.
- Prirnary migration of hydrocarbons within the
source rocks mainly occurs in liquid phase and
Chapter 14 Hydrocarbons and Coal
Subsurface mapping of hydrocarbon generative
basins, ineluding kerogen analyses and an evaluation
of the thermal history of the basin, is one of the most
important tasks in petroleum exploration. In addition,
the three-dimensional facies distribution and architecture of the depositional system, reservoir and seal
rocks in the neighborhood of the source rocks, and
the tectonic structure of the basin fill have to be investigated.
New frontiers in hydrocarbon exploration include the lower
slope and continental rise of passive margins as weil as
foredeep sediments buried under thin-skinned thrust belts
(e.g. Picha 1996; Wessely and Liebl 1996). Proven reserves, improvements of the methods for extracting oil and
gas, and undiscovered hydrocarbons allow hydrocarbon
production well into the 22nd century (Schollenberger
1998). A crucial point is sustainable development and use
of these energy reserves in combination with increased
efforts to replace them by other energy sourees. Whether
the huge reservoir of gas hydrates below the ocean (mainly
biogenie and still present in the porous source sediment)
can be used in the future as an important energy source, is
an open question.
is aided by overpressure of the pore fluids and
buoyancy (oil has a lower density than water).
Grain surfaces and tiny pores remain waterwet.
Secondary migration in permeable carrier beds
mainly takes place as two-phase mass flow. In
some cases, hydrocarbons dissolved in water
are transported in significant quantities by meteoric flow. Transport by molecular diffusion
may play some role for light hydrocarbons ineluding natural gas.
- Large petroleum accumulations tend to occur
in reservoir rocks located elose to the center of
the generative basins (oil kitchens). The extent
and volume of source rocks is crucial for giant
hydrocarbon generation, but not necessarily a
high content of organic matter.