49
Origin and Occurrence
Thermodynamic calculations and experimental studies confirm that n-alkanes (common petroleum
components) do not spontaneously evolve from methane at pressures typically found in sedimentary
basins, and so the theory of an abiogenic origin of hydrocarbons suggests deep generation (below
200 km).
However, it is now generally accepted, but not conclusively proven, that petroleum formation
predominantly arises from the decay of organic matter in the Earth. It is therefore from this scientific aspect that petroleum formation is referenced in this text. Nevertheless, alternative theories
should not be dismissed until it can be conclusively established that petroleum formation is due to
one particular aspect of geochemistry.
From the chemical point of view inorganic theories are interesting because of their historical
importance, but these have not received much attention. Geological and chemical methods have
demonstrated the optical activity of petroleum constituents, the presence of thermally labile organic
compounds, and the almost exclusive occurrence of oil in sedimentary rocks. Other theories
attempted to correlate the occurrence of coal strata and oil in the Earth’s crust with coal being the
precursor to crude oil or both fossil fuels formed from the same precursors at the same time but as
the result of divergent paths. Chemical and geological investigations do not support this concept and
the idea is considered to be obsolete.
3.2.2 BIogenIC orIgIn
Petroleum is a naturally occurring hydrocarbon mixture but hydrocarbons that are synthesized by
living organisms usually account for less than 20% of the petroleum (Hunt, 1996). The remainder
of the hydrocarbons in petroleum is produced by a variety of processes that converts other organic
material to hydrocarbons as part of the maturation processes generally referred to as diagenesis,
catagenesis, and metagenesis. These three processes are a combination of bacteriological action and
low-temperature reactions that convert the source material into petroleum. During these processes,
migration of the liquid products from the source sediment to the reservoir rock may also occur.
Most geologists view crude oil and natural gas as the products of compression and heating
of ancient vegetation over geologic timescales. According to this theory, it is formed from the
decayed remains of prehistoric marine animals and terrestrial plants. Over many centuries this
organic matter, mixed with mud, was buried under thick sedimentary layers of material. The
resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy
material known as kerogen and then into liquid and gaseous hydrocarbons in a process known
as catagenesis. These then migrate through adjacent rock layers until they become trapped
underground in porous rocks called reservoirs, forming an oilfield, from which the liquid can be
extracted by drilling and pumping.
These reactions are thought to be very temperature sensitive: reactions that produce recognizable
oil commence at about 130°C (266°F), and those that continue the breakdown of oil into natural gas
commence at about 180°C (356°F). The range of 130°C–150°C (266°F–302°F) is generally considered the oil window. Though this corresponds to different depths for different locations around the
world, a typical depth for an oil window might be 13,125–16,400 ft (4,000–5,000 m). Three conditions are essential for oil reservoirs to form: a rich source rock, a migration conduit, and a trap (seal)
that forms the reservoir.
The reactions that produce oil and natural gas are often modeled as first-order breakdown reactions, in which kerogen breaks down to oil and natural gas by a large set of parallel reactions, and
oil eventually breaks down to natural gas by another set of reactions.
During the past 600 million years, incompletely decayed plant and animal remains got buried
under thick layers of rock. It is believed that petroleum consists of the remains of these organisms
but it is the small microscopic plankton organism remains that are largely responsible for the relatively high organic carbon content of fine-grained sediments such as shale that are believed to be the
principal source rocks for petroleum.
Origin and Occurrence
Thermodynamic calculations and experimental studies confirm that n-alkanes (common petroleum
components) do not spontaneously evolve from methane at pressures typically found in sedimentary
basins, and so the theory of an abiogenic origin of hydrocarbons suggests deep generation (below
200 km).
However, it is now generally accepted, but not conclusively proven, that petroleum formation
predominantly arises from the decay of organic matter in the Earth. It is therefore from this scientific aspect that petroleum formation is referenced in this text. Nevertheless, alternative theories
should not be dismissed until it can be conclusively established that petroleum formation is due to
one particular aspect of geochemistry.
From the chemical point of view inorganic theories are interesting because of their historical
importance, but these have not received much attention. Geological and chemical methods have
demonstrated the optical activity of petroleum constituents, the presence of thermally labile organic
compounds, and the almost exclusive occurrence of oil in sedimentary rocks. Other theories
attempted to correlate the occurrence of coal strata and oil in the Earth’s crust with coal being the
precursor to crude oil or both fossil fuels formed from the same precursors at the same time but as
the result of divergent paths. Chemical and geological investigations do not support this concept and
the idea is considered to be obsolete.
3.2.2 BIogenIC orIgIn
Petroleum is a naturally occurring hydrocarbon mixture but hydrocarbons that are synthesized by
living organisms usually account for less than 20% of the petroleum (Hunt, 1996). The remainder
of the hydrocarbons in petroleum is produced by a variety of processes that converts other organic
material to hydrocarbons as part of the maturation processes generally referred to as diagenesis,
catagenesis, and metagenesis. These three processes are a combination of bacteriological action and
low-temperature reactions that convert the source material into petroleum. During these processes,
migration of the liquid products from the source sediment to the reservoir rock may also occur.
Most geologists view crude oil and natural gas as the products of compression and heating
of ancient vegetation over geologic timescales. According to this theory, it is formed from the
decayed remains of prehistoric marine animals and terrestrial plants. Over many centuries this
organic matter, mixed with mud, was buried under thick sedimentary layers of material. The
resulting high levels of heat and pressure cause the remains to metamorphose, first into a waxy
material known as kerogen and then into liquid and gaseous hydrocarbons in a process known
as catagenesis. These then migrate through adjacent rock layers until they become trapped
underground in porous rocks called reservoirs, forming an oilfield, from which the liquid can be
extracted by drilling and pumping.
These reactions are thought to be very temperature sensitive: reactions that produce recognizable
oil commence at about 130°C (266°F), and those that continue the breakdown of oil into natural gas
commence at about 180°C (356°F). The range of 130°C–150°C (266°F–302°F) is generally considered the oil window. Though this corresponds to different depths for different locations around the
world, a typical depth for an oil window might be 13,125–16,400 ft (4,000–5,000 m). Three conditions are essential for oil reservoirs to form: a rich source rock, a migration conduit, and a trap (seal)
that forms the reservoir.
The reactions that produce oil and natural gas are often modeled as first-order breakdown reactions, in which kerogen breaks down to oil and natural gas by a large set of parallel reactions, and
oil eventually breaks down to natural gas by another set of reactions.
During the past 600 million years, incompletely decayed plant and animal remains got buried
under thick layers of rock. It is believed that petroleum consists of the remains of these organisms
but it is the small microscopic plankton organism remains that are largely responsible for the relatively high organic carbon content of fine-grained sediments such as shale that are believed to be the
principal source rocks for petroleum.