ORIGIN AND OCCURRENCE OF OIL 35
2.1 The Formation of Oil
There are two theories on the origin of petroleum: the abiogenic
theory and the biogenic theory (Kenney et al, 2001,2002). Both theories have been intensely debated since the 1860s, shortly after the
discovery of widespread occurrence of petroleum.
The abiogenic theory of petroleum formation postulates that
inorganic substances were the source material. In fact, there are
several recent theories related to the formation of petroleum from
non-biogenic sources in the earth (Gold, 1984,1985; Gold and Soter,
1980,1982,1986; Osborne, 1986; Szatmari, 1989).
Thus, the idea of abiogenic petroleum origin proposes that
large amounts of carbon exist naturally in the planet, some in the
form of hydrocarbons. Hydrocarbons are less dense than aqueous
pore fluids, and migrate upward through deep fracture networks.
Thermophilic, rock-dwelling microbial life forms are in part responsible for the biomarkers found in petroleum. However, their role in
the formation, alteration, or contamination of the various hydrocarbon deposits is not yet understood. 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, most geologists and geochemists support the biogenic
theory of petroleum formation and consider crude oil and natural
gas as the products of compression and heating of ancient vegetation over geological time, although the chemistry of the transformation of the organic matter into petroleum is not well understood
and is largely speculative (Tissot and Weite, 1978; Snowdon and
Powell, 1982; Brooks and Weite, 1984; Speight, 2007a). Over millennia, this organic matter — the decayed remains of prehistoric
marine animals and terrestrial plants — mixed with mud and was
buried under thick sedimentary layers of material. While the minimum and maximum temperatures have never been fully defined
or conclusively proven, heat and pressure caused the remains to
metamorphose, first into a material known as protopetroleum, and
then into liquid and gaseous hydrocarbons by means of a maturation process (catagenesis) (Califet and Oudin, 1966; Barker and
Wang, 1988; Speight, 2007a). The liquid and gaseous hydrocarbons
2.1 The Formation of Oil
There are two theories on the origin of petroleum: the abiogenic
theory and the biogenic theory (Kenney et al, 2001,2002). Both theories have been intensely debated since the 1860s, shortly after the
discovery of widespread occurrence of petroleum.
The abiogenic theory of petroleum formation postulates that
inorganic substances were the source material. In fact, there are
several recent theories related to the formation of petroleum from
non-biogenic sources in the earth (Gold, 1984,1985; Gold and Soter,
1980,1982,1986; Osborne, 1986; Szatmari, 1989).
Thus, the idea of abiogenic petroleum origin proposes that
large amounts of carbon exist naturally in the planet, some in the
form of hydrocarbons. Hydrocarbons are less dense than aqueous
pore fluids, and migrate upward through deep fracture networks.
Thermophilic, rock-dwelling microbial life forms are in part responsible for the biomarkers found in petroleum. However, their role in
the formation, alteration, or contamination of the various hydrocarbon deposits is not yet understood. 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, most geologists and geochemists support the biogenic
theory of petroleum formation and consider crude oil and natural
gas as the products of compression and heating of ancient vegetation over geological time, although the chemistry of the transformation of the organic matter into petroleum is not well understood
and is largely speculative (Tissot and Weite, 1978; Snowdon and
Powell, 1982; Brooks and Weite, 1984; Speight, 2007a). Over millennia, this organic matter — the decayed remains of prehistoric
marine animals and terrestrial plants — mixed with mud and was
buried under thick sedimentary layers of material. While the minimum and maximum temperatures have never been fully defined
or conclusively proven, heat and pressure caused the remains to
metamorphose, first into a material known as protopetroleum, and
then into liquid and gaseous hydrocarbons by means of a maturation process (catagenesis) (Califet and Oudin, 1966; Barker and
Wang, 1988; Speight, 2007a). The liquid and gaseous hydrocarbons