207
Chemical Composition
boiling below 343°C (650°F) (Hirsch et al., 1974). Pyrrole derivatives and indole derivatives account
for about two-thirds of the nitrogen while the remainder is found in the basic alkylated pyridine and
alkylated quinoline compounds.
The saturate constituents contribute less to the VGO than the aromatic constituents but more
than the polars that are now present at percentage rather than trace levels. VGO is occasionally
used as a heating oil but most commonly it is processed by catalytic cracking to produce naphtha or
extraction to yield lubricating oil.
Within the VGO, saturates, distribution of paraffins, iso-paraffins, and naphthenes is highly
dependent upon the petroleum source. Generally, the naphthene constituents account for approximately two-thirds or 60% of the saturate constituents, but the overall range of variation is from
<20% to >80%. In most samples, the n-paraffins from C 20 –C 44 are still present in sufficient
quantity to be detected as distinct peaks in gas chromatographic analysis. Some (but not all)
crude oils show a preference for odd-number alkanes. Both the distribution and the selectivity
toward odd-numbered hydrocarbons are considered to reflect differences in the petrogenesis of
the crude oil.
The bulk of the saturated constituents in VGO consists of iso-paraffins and especially naphthene species although isoprenoid compounds, such as squalane (C 30 ) and lycopane (C 40 ), have been
detected. Analytical techniques show that the naphthenes contain from one to more than six fused
rings accompanied by alkyl substitution. For mono-and diaromatics, the alkyl substitution typically
involves several methyl and ethyl substituents. Hopanes and steranes have also been identified and
are also used as internal markers for estimating biodegradation of crude oils during bioremediation
processes.
The aromatics in VGO may contain one to six fused aromatic rings that may bear additional
naphthene rings and alkyl substituents in keeping with their boiling range. Mono- and diaromatics account for about 50% of the aromatics in petroleum VGO samples. Analytical data show the
presence of up to four fused naphthenic rings on some aromatic compounds. This is consistent with
the suggestion that these species originate from the aromatization of steroids. Although present at
lower concentration, alkyl benzenes and naphthalenes show one long side chain and multiple short
side chains.
The fused ring aromatic compounds (having three or more rings) in petroleum include phenanthrene, chrysene, and picene as well as fluoranthene, pyrene, benzo(a)pyrene, and benzo(ghi)
perylene.
The most abundant reported individual phenanthrene compounds appear to be the 3-derivatives.
In addition, phenanthrene derivatives outnumber anthracene derivatives by as much as 100:1.
In addition, chrysene derivatives are favored over pyrene derivative.
Heterocyclic constituents are significant contributors to the VGO fraction. In terms of sulfur
compounds, thiophene and thiacyclane sulfur predominate over sulfide sulfur. Some molecules
even contain more than one sulfur atom. The benzothiophenes and dibenzothiophenes are the prevalent thiophene forms of sulfur.
In the VGO range, the nitrogen-containing compounds include higher molecular weight pyridines, quinolines, benzoquinoline derivatives, amides, indoles, carbazole, and molecules with
two nitrogen atoms (diaza compounds) with three and four aromatic rings are especially prevalent (Green et al., 1989). Typically, about one-third of the compounds are basic, that is, pyridine
and its benzologs while the remainder are present as neutral species (amides and carbazoles).
Although benzo- and dibenzo-quinolines found in petroleum are rich in sterically hindered structures, hindered and unhindered structures have been found to be present at equivalent concentrations in source rocks. This has been rationalized as geo-chromatography in which the less polar
(hindered) structures moved more readily to the reservoir.
Oxygen levels in the VGO parallel the nitrogen content. Thus, the most commonly identified oxygen compounds are the carboxylic acids and phenols, collectively called naphthenic acids (Seifert
and Teeter, 1970).
Chemical Composition
boiling below 343°C (650°F) (Hirsch et al., 1974). Pyrrole derivatives and indole derivatives account
for about two-thirds of the nitrogen while the remainder is found in the basic alkylated pyridine and
alkylated quinoline compounds.
The saturate constituents contribute less to the VGO than the aromatic constituents but more
than the polars that are now present at percentage rather than trace levels. VGO is occasionally
used as a heating oil but most commonly it is processed by catalytic cracking to produce naphtha or
extraction to yield lubricating oil.
Within the VGO, saturates, distribution of paraffins, iso-paraffins, and naphthenes is highly
dependent upon the petroleum source. Generally, the naphthene constituents account for approximately two-thirds or 60% of the saturate constituents, but the overall range of variation is from
<20% to >80%. In most samples, the n-paraffins from C 20 –C 44 are still present in sufficient
quantity to be detected as distinct peaks in gas chromatographic analysis. Some (but not all)
crude oils show a preference for odd-number alkanes. Both the distribution and the selectivity
toward odd-numbered hydrocarbons are considered to reflect differences in the petrogenesis of
the crude oil.
The bulk of the saturated constituents in VGO consists of iso-paraffins and especially naphthene species although isoprenoid compounds, such as squalane (C 30 ) and lycopane (C 40 ), have been
detected. Analytical techniques show that the naphthenes contain from one to more than six fused
rings accompanied by alkyl substitution. For mono-and diaromatics, the alkyl substitution typically
involves several methyl and ethyl substituents. Hopanes and steranes have also been identified and
are also used as internal markers for estimating biodegradation of crude oils during bioremediation
processes.
The aromatics in VGO may contain one to six fused aromatic rings that may bear additional
naphthene rings and alkyl substituents in keeping with their boiling range. Mono- and diaromatics account for about 50% of the aromatics in petroleum VGO samples. Analytical data show the
presence of up to four fused naphthenic rings on some aromatic compounds. This is consistent with
the suggestion that these species originate from the aromatization of steroids. Although present at
lower concentration, alkyl benzenes and naphthalenes show one long side chain and multiple short
side chains.
The fused ring aromatic compounds (having three or more rings) in petroleum include phenanthrene, chrysene, and picene as well as fluoranthene, pyrene, benzo(a)pyrene, and benzo(ghi)
perylene.
The most abundant reported individual phenanthrene compounds appear to be the 3-derivatives.
In addition, phenanthrene derivatives outnumber anthracene derivatives by as much as 100:1.
In addition, chrysene derivatives are favored over pyrene derivative.
Heterocyclic constituents are significant contributors to the VGO fraction. In terms of sulfur
compounds, thiophene and thiacyclane sulfur predominate over sulfide sulfur. Some molecules
even contain more than one sulfur atom. The benzothiophenes and dibenzothiophenes are the prevalent thiophene forms of sulfur.
In the VGO range, the nitrogen-containing compounds include higher molecular weight pyridines, quinolines, benzoquinoline derivatives, amides, indoles, carbazole, and molecules with
two nitrogen atoms (diaza compounds) with three and four aromatic rings are especially prevalent (Green et al., 1989). Typically, about one-third of the compounds are basic, that is, pyridine
and its benzologs while the remainder are present as neutral species (amides and carbazoles).
Although benzo- and dibenzo-quinolines found in petroleum are rich in sterically hindered structures, hindered and unhindered structures have been found to be present at equivalent concentrations in source rocks. This has been rationalized as geo-chromatography in which the less polar
(hindered) structures moved more readily to the reservoir.
Oxygen levels in the VGO parallel the nitrogen content. Thus, the most commonly identified oxygen compounds are the carboxylic acids and phenols, collectively called naphthenic acids (Seifert
and Teeter, 1970).