50
The Chemistry and Technology of Petroleum
Chemically, it is generally proposed that petroleum is formed through the progressive chemical
change of materials provided by microscopic aquatic organisms that were incorporated over eons
in marine or near-marine sedimentary rocks. In fact, the details of petroleum genesis (diagenesis,
catagenesis, and metagenesis) have long been a topic of interest. However, the details of this transformation and the mechanism by which petroleum is expelled from the source sediment and accumulates in the reservoir rock are still uncertain.
Transformation of some of this sedimentary material to petroleum probably began soon after
deposition, with bacteria playing a role in the initial stages and clay particles serving as catalysts.
Heat within the strata may have provided energy for the reaction, temperatures increasing more or
less directly with depth. Some evidence indicates that most petroleum has formed at temperatures
not exceeding about 100°C–120°C (210°F–250°F), with the generation of petroleum hydrocarbons
beginning as low as 65°C (150°F).
The formation of petroleum hydrocarbons by long-term thermal reactions was advocated at an
early date. In 1888, Engler demonstrated that pressure distillation of fats yields an oil product having high olefin content. This highly unsaturated material (protopetroleum) was believed to have
been formed from the fatty components of the organic debris in sediments by mild thermal cracking
and polymerization. Paraffin hydrocarbons were assumed to be formed by decarboxylation of fatty
acids and the olefins underwent isomerization to cyclic hydrocarbons.
Early theories of petroleum formation such as this are no longer valid, but there appears to be
no reason that low-temperature decarboxylation, deamination, cyclization, hydrogenation, isomerization, or other types of reactions might not proceed under the conditions known to exist in
nature. Indeed, at a relatively low temperature (200°C–250°C, 390°F–480°F) polyene structures
(i.e., –C=C–C=C–C=C– etc.) can be cyclized to produce aromatic hydrocarbons. For example,
m-xylene has been found in the pyrolysis products of bixin and capsanthin, and other carotenoids
have yielded not only m-xylene but also toluene and 2,6-dimethylnaphthalene.
Following from this, in 1911 Engler was the first author to postulate that an organic substance
other than coal was the source material of petroleum. He invoked the concept of three separate development stages. In the first stage, animal and vegetable deposit accumulate on the bottom of inland
seas (lagoon conditions) and are then decomposed by bacteria; the carbohydrates and the bulk of the
protein are converted into water-soluble material or gases and thus removed from the site. The fats,
waxes, and other fat-soluble and stable materials (rosins, cholesterol, and others) remain.
In the second stage, high temperatures and pressures cause carbon dioxide to evolve from compounds containing a carboxyl group, and water is produced from the hydroxy-acids and alcohols to
leave a bituminous residue. Continued application of the heat and pressure causes light cracking,
producing a liquid product with high olefin content (protopetroleum). Engler also produced experimental evidence that showed that distillation of fats under pressure brought about the formation of
a petroleum type of material, and he assumed that time and high pressure offset the fact that the
temperature in oil source rocks is lower than that used experimentally.
In the third stage, the unsaturated components of the protopetroleum are polymerized under
the influence of contact catalysts and thus the polyolefins are converted into paraffins and/or cycloparaffins (naphthenes). Aromatics were presumed to be formed either directly during cracking, by
cyclization through condensation reactions, or even during the decomposition of protein. It was
also proposed that Grahamite and Gilsonite (see Chapter 1) were formed from petroleum by means
of polymerization and oxidation reactions. The essential elements of this theory have survived.
However, the main objection to a theory of this type is that the high-temperature sequence and the
composition of the end product obtained in these experiments (paraffins and unsaturated hydrocarbons) differ essentially from that of petroleum, which consists chiefly of paraffins, cycloparaffins
(naphthenes), and aromatics.
Thus, it is generally believed that the generation of petroleum is associated with the deposition of
organic detritus. The detritus deposition occurs during the development of fine-grained sedimentary
rocks that occur in marine, near-marine, or even nonmarine environments (Hood et al., 1975; Tissot
The Chemistry and Technology of Petroleum
Chemically, it is generally proposed that petroleum is formed through the progressive chemical
change of materials provided by microscopic aquatic organisms that were incorporated over eons
in marine or near-marine sedimentary rocks. In fact, the details of petroleum genesis (diagenesis,
catagenesis, and metagenesis) have long been a topic of interest. However, the details of this transformation and the mechanism by which petroleum is expelled from the source sediment and accumulates in the reservoir rock are still uncertain.
Transformation of some of this sedimentary material to petroleum probably began soon after
deposition, with bacteria playing a role in the initial stages and clay particles serving as catalysts.
Heat within the strata may have provided energy for the reaction, temperatures increasing more or
less directly with depth. Some evidence indicates that most petroleum has formed at temperatures
not exceeding about 100°C–120°C (210°F–250°F), with the generation of petroleum hydrocarbons
beginning as low as 65°C (150°F).
The formation of petroleum hydrocarbons by long-term thermal reactions was advocated at an
early date. In 1888, Engler demonstrated that pressure distillation of fats yields an oil product having high olefin content. This highly unsaturated material (protopetroleum) was believed to have
been formed from the fatty components of the organic debris in sediments by mild thermal cracking
and polymerization. Paraffin hydrocarbons were assumed to be formed by decarboxylation of fatty
acids and the olefins underwent isomerization to cyclic hydrocarbons.
Early theories of petroleum formation such as this are no longer valid, but there appears to be
no reason that low-temperature decarboxylation, deamination, cyclization, hydrogenation, isomerization, or other types of reactions might not proceed under the conditions known to exist in
nature. Indeed, at a relatively low temperature (200°C–250°C, 390°F–480°F) polyene structures
(i.e., –C=C–C=C–C=C– etc.) can be cyclized to produce aromatic hydrocarbons. For example,
m-xylene has been found in the pyrolysis products of bixin and capsanthin, and other carotenoids
have yielded not only m-xylene but also toluene and 2,6-dimethylnaphthalene.
Following from this, in 1911 Engler was the first author to postulate that an organic substance
other than coal was the source material of petroleum. He invoked the concept of three separate development stages. In the first stage, animal and vegetable deposit accumulate on the bottom of inland
seas (lagoon conditions) and are then decomposed by bacteria; the carbohydrates and the bulk of the
protein are converted into water-soluble material or gases and thus removed from the site. The fats,
waxes, and other fat-soluble and stable materials (rosins, cholesterol, and others) remain.
In the second stage, high temperatures and pressures cause carbon dioxide to evolve from compounds containing a carboxyl group, and water is produced from the hydroxy-acids and alcohols to
leave a bituminous residue. Continued application of the heat and pressure causes light cracking,
producing a liquid product with high olefin content (protopetroleum). Engler also produced experimental evidence that showed that distillation of fats under pressure brought about the formation of
a petroleum type of material, and he assumed that time and high pressure offset the fact that the
temperature in oil source rocks is lower than that used experimentally.
In the third stage, the unsaturated components of the protopetroleum are polymerized under
the influence of contact catalysts and thus the polyolefins are converted into paraffins and/or cycloparaffins (naphthenes). Aromatics were presumed to be formed either directly during cracking, by
cyclization through condensation reactions, or even during the decomposition of protein. It was
also proposed that Grahamite and Gilsonite (see Chapter 1) were formed from petroleum by means
of polymerization and oxidation reactions. The essential elements of this theory have survived.
However, the main objection to a theory of this type is that the high-temperature sequence and the
composition of the end product obtained in these experiments (paraffins and unsaturated hydrocarbons) differ essentially from that of petroleum, which consists chiefly of paraffins, cycloparaffins
(naphthenes), and aromatics.
Thus, it is generally believed that the generation of petroleum is associated with the deposition of
organic detritus. The detritus deposition occurs during the development of fine-grained sedimentary
rocks that occur in marine, near-marine, or even nonmarine environments (Hood et al., 1975; Tissot