203
Chemical Composition
Porphyrins also have well-known trivial names or acronyms that are often in more common usage
than the formal system of nomenclature.
When one or two double bonds of a porphyrin are hydrogenated, a chlorin or a phlorin is the result.
Chlorins are components of chlorophylls and possess an isocyclic ring formed by two methylene groups
bridging a pyrrolic carbon to a methine carbon. Geological porphyrins that contain this structural feature
are assumed to be derived from chlorophylls. Etioporphyrins are also commonly found in geological
materials and have no substituents (other than hydrogen) on the methine carbons. Benzoporphyrins
and tetrahydrobenzoporphyrins also have been identified in geological materials. These compounds
have either a benzene ring or a hydrogenated benzene ring fused onto a pyrrole unit.
Almost all crude oil, heavy oil, and bitumen contain detectable amounts of vanadyl and nickel
porphyrins. More mature, lighter crude oils usually contain only small amounts of these compounds.
Heavy oils may contain large amounts of vanadyl and nickel porphyrins. Vanadium concentrations
of over 1000 ppm are known for some crude oil, and a substantial amount of the vanadium in these
crude oils is chelated with porphyrins. In high-sulfur crude oil of marine origin, vanadyl porphyrins are more abundant than nickel porphyrins. Low-sulfur crude oils of lacustrine origin usually
contain more nickel porphyrins than vanadyl porphyrins.
Of all the metals in the periodic table, only vanadium and nickel have been proven definitely
to exist as chelates in significant amounts in a large number of crude oils and tar sand bitumen.
Geochemical reasons for the absence of substantial quantities of porphyrins chelated with metals other than nickel and vanadium in most crude oils and tar sand bitumen have been advanced
(Hodgson et al., 1967; Baker and Palmer, 1978; Quirke, 1987).
If the vanadium and nickel contents of crude oils are measured and compared with porphyrin
concentrations, it is usually found that not all the metal content can be accounted for as porphyrin
constituents (Reynolds, 1998). In some crude oils, as little as 10% w/w of total metals appears to
be chelated with porphyrins. Only rarely can all measured nickel and vanadium in a crude oil be
accounted for as porphyrin-type. Currently, some investigators believe that part of the vanadium
and nickel in crude oils is chelated with ligands that are not porphyrins. These metal chelates are
referred to as nonporphyrin metal chelates or complexes.
Finally, during the fractionation of petroleum (Chapter 9), the metallic constituents (metalloporphyrins and nonporphyrin metal chelates) are concentrated in the asphaltene fraction. The deasphaltened oils (petrolenes and maltenes) (Chapter 1) contain smaller concentrations of porphyrins
than the parent materials and usually very small concentrations of non-porphyrin metals.
8.4 CHEMICAL COMPOSITION BY DISTILLATION
Although distillation is presented in more detail elsewhere (Chapters 9 and 17), it is appropriate to
mention distillation here insofar as it is a method by which the constituents of petroleum can be
separated and identified.
Distillation is a means of separating chemical compounds (usually liquids) through differences
in the respective vapor pressures (Chapter 17). In the mixture, the components evaporate such that
the vapor has a composition determined by the chemical properties of the mixture. Distillation of a
given component is possible, if the vapor has a higher proportion of the given component than the
mixture. This is caused by the given component having a higher vapor pressure and a lower boiling
point than the other components.
By the nature of the process, it is theoretically impossible to completely separate and purify
the individual components of petroleum when the possible number of isomers is considered for
the individual carbon numbers that occur within the paraffin family (Table 8.4). When other
types of compounds are included, such as the aromatic derivatives and heteroatom derivatives,
even though the maturation process might limit the possible number of isomeric permutations
(Tissot and Welte, 1978), the potential number of compounds in petroleum is still (in a sense)
astronomical.
Chemical Composition
Porphyrins also have well-known trivial names or acronyms that are often in more common usage
than the formal system of nomenclature.
When one or two double bonds of a porphyrin are hydrogenated, a chlorin or a phlorin is the result.
Chlorins are components of chlorophylls and possess an isocyclic ring formed by two methylene groups
bridging a pyrrolic carbon to a methine carbon. Geological porphyrins that contain this structural feature
are assumed to be derived from chlorophylls. Etioporphyrins are also commonly found in geological
materials and have no substituents (other than hydrogen) on the methine carbons. Benzoporphyrins
and tetrahydrobenzoporphyrins also have been identified in geological materials. These compounds
have either a benzene ring or a hydrogenated benzene ring fused onto a pyrrole unit.
Almost all crude oil, heavy oil, and bitumen contain detectable amounts of vanadyl and nickel
porphyrins. More mature, lighter crude oils usually contain only small amounts of these compounds.
Heavy oils may contain large amounts of vanadyl and nickel porphyrins. Vanadium concentrations
of over 1000 ppm are known for some crude oil, and a substantial amount of the vanadium in these
crude oils is chelated with porphyrins. In high-sulfur crude oil of marine origin, vanadyl porphyrins are more abundant than nickel porphyrins. Low-sulfur crude oils of lacustrine origin usually
contain more nickel porphyrins than vanadyl porphyrins.
Of all the metals in the periodic table, only vanadium and nickel have been proven definitely
to exist as chelates in significant amounts in a large number of crude oils and tar sand bitumen.
Geochemical reasons for the absence of substantial quantities of porphyrins chelated with metals other than nickel and vanadium in most crude oils and tar sand bitumen have been advanced
(Hodgson et al., 1967; Baker and Palmer, 1978; Quirke, 1987).
If the vanadium and nickel contents of crude oils are measured and compared with porphyrin
concentrations, it is usually found that not all the metal content can be accounted for as porphyrin
constituents (Reynolds, 1998). In some crude oils, as little as 10% w/w of total metals appears to
be chelated with porphyrins. Only rarely can all measured nickel and vanadium in a crude oil be
accounted for as porphyrin-type. Currently, some investigators believe that part of the vanadium
and nickel in crude oils is chelated with ligands that are not porphyrins. These metal chelates are
referred to as nonporphyrin metal chelates or complexes.
Finally, during the fractionation of petroleum (Chapter 9), the metallic constituents (metalloporphyrins and nonporphyrin metal chelates) are concentrated in the asphaltene fraction. The deasphaltened oils (petrolenes and maltenes) (Chapter 1) contain smaller concentrations of porphyrins
than the parent materials and usually very small concentrations of non-porphyrin metals.
8.4 CHEMICAL COMPOSITION BY DISTILLATION
Although distillation is presented in more detail elsewhere (Chapters 9 and 17), it is appropriate to
mention distillation here insofar as it is a method by which the constituents of petroleum can be
separated and identified.
Distillation is a means of separating chemical compounds (usually liquids) through differences
in the respective vapor pressures (Chapter 17). In the mixture, the components evaporate such that
the vapor has a composition determined by the chemical properties of the mixture. Distillation of a
given component is possible, if the vapor has a higher proportion of the given component than the
mixture. This is caused by the given component having a higher vapor pressure and a lower boiling
point than the other components.
By the nature of the process, it is theoretically impossible to completely separate and purify
the individual components of petroleum when the possible number of isomers is considered for
the individual carbon numbers that occur within the paraffin family (Table 8.4). When other
types of compounds are included, such as the aromatic derivatives and heteroatom derivatives,
even though the maturation process might limit the possible number of isomeric permutations
(Tissot and Welte, 1978), the potential number of compounds in petroleum is still (in a sense)
astronomical.