54
The Chemistry and Technology of Petroleum
Native protein is rare outside the living cell owing to the ease with which it loses structural
organization (denaturation). In many instances solubility is greatly diminished after denaturation,
and incorporation in the sediment is distinctly favored. Studies on the long-term thermal stability
of amino acids indicate that these substances should survive under the anticipated low-temperature
Earth conditions for long periods of time. Indeed, amino acids in recent sediments, as well as in
shales, are estimated to be some 30 million years old.
Lignin is a mixture of complex, high molecular weight, amorphous substances and forms the cell
wall structure of plants, particularly those of woody type (Sarkanen and Ludwig, 1971). Despite the
extensive work on the lignin of the higher land plants, its structure has not been defined except that
it is probably built up of phenylpropane units containing methoxyl and hydroxyl groups. In sediments it is frequently found as a complex with protein or carbohydrate, but so little is known about
the lignin of marine plants that the ligneous substances in sediments cannot definitely be typed as
to source.
Sterols, which are based on a saturated, condensed ring structure are widely distributed in animals and plants and are also believed to be source material for petroleum although very little is
known of their natural degradation reactions. Additional classes of compounds that also make up
the source material are lipids and terpenoids and, possibly, humic substances, specifically humic
acids and, to a lesser extent, fulvic acids (Gaffney et al., 1996).
Briefly, humic acids are those organic compounds found in the environment that cannot be classified as any other chemical class of compounds (e.g., polysaccharides, proteins, etc.). They are traditionally defined according to solubility. Fulvic acids are those organic materials that are insoluble
in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values
(pH < 2) but are soluble at higher pH values. Humin is the fraction of natural organic materials that
is insoluble in water at all pH values. These definitions reflect the traditional methods for separating
the different fractions from the original mixture.
The humic content of soils varies from 0 to almost 10% w/w. In surface waters, the content
of humic material, expressed as dissolved organic carbon varies from 0.1 to 50 ppm in darkwater swamps. In ocean waters, the dissolved organic carbon varies from 0.5 to 1.2 ppm at the
surface, and the dissolved organic carbon samples from deep groundwater vary from 0.1 to
10 ppm. In addition, about 10% of the dissolved organic carbon in surface waters is found in
suspended matter, either as organic or organically coated inorganic particulate matter (Gaffney
et al., 1996).
Humic materials have a wide range of molecular weights and sizes, ranging from a few hundred
to as much as several hundred thousand atomic mass units. In general, fulvic acids are of lower
molecular weight than humic acids, and soil-derived materials are larger than aquatic materials
(Stevenson, 1989). Humic materials vary in composition depending on their source, location, and
method of extraction; however, their similarities are more pronounced than their differences. The
range of the elemental composition of humic materials is relatively narrow, being approximately
40%–60% carbon, 30%–50% oxygen, 4%–5% hydrogen, 1%–4% nitrogen, 1%–2% sulfur, and
0%–0.3% phosphorus (MacCarthy and Suffet, 1989). Humic acids contain more hydrogen, carbon,
nitrogen, and sulfur and less oxygen than fulvic acids. Studies on humins have shown that they
are similar to humic acids except that they are strongly bound to metals and clays, rendering them
insoluble (Schnitzer and Kahn, 1972).
It is generally believed, and some evidence does exist, that humic materials consist of a skeleton of alkyl/aromatic units cross-linked mainly by oxygen and nitrogen groups with the major
functional groups being carboxylic acid, phenol and alcohol hydroxyl groups, ketone, and quinone
groups. The structures of fulvic acids are somewhat more aliphatic and less aromatic than humic
acids, and fulvic acids are richer in carboxylic acid, phenolic acid, and ketonic groups. This is
responsible for their higher solubility in water at all pH values, and the role of water in the geochemical cycle for petroleum formation should not be ignored (Siskin and Katritzky, 1991). Humic
acids, being more highly aromatic, become insoluble when the carboxylic groups are protonated at
The Chemistry and Technology of Petroleum
Native protein is rare outside the living cell owing to the ease with which it loses structural
organization (denaturation). In many instances solubility is greatly diminished after denaturation,
and incorporation in the sediment is distinctly favored. Studies on the long-term thermal stability
of amino acids indicate that these substances should survive under the anticipated low-temperature
Earth conditions for long periods of time. Indeed, amino acids in recent sediments, as well as in
shales, are estimated to be some 30 million years old.
Lignin is a mixture of complex, high molecular weight, amorphous substances and forms the cell
wall structure of plants, particularly those of woody type (Sarkanen and Ludwig, 1971). Despite the
extensive work on the lignin of the higher land plants, its structure has not been defined except that
it is probably built up of phenylpropane units containing methoxyl and hydroxyl groups. In sediments it is frequently found as a complex with protein or carbohydrate, but so little is known about
the lignin of marine plants that the ligneous substances in sediments cannot definitely be typed as
to source.
Sterols, which are based on a saturated, condensed ring structure are widely distributed in animals and plants and are also believed to be source material for petroleum although very little is
known of their natural degradation reactions. Additional classes of compounds that also make up
the source material are lipids and terpenoids and, possibly, humic substances, specifically humic
acids and, to a lesser extent, fulvic acids (Gaffney et al., 1996).
Briefly, humic acids are those organic compounds found in the environment that cannot be classified as any other chemical class of compounds (e.g., polysaccharides, proteins, etc.). They are traditionally defined according to solubility. Fulvic acids are those organic materials that are insoluble
in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values
(pH < 2) but are soluble at higher pH values. Humin is the fraction of natural organic materials that
is insoluble in water at all pH values. These definitions reflect the traditional methods for separating
the different fractions from the original mixture.
The humic content of soils varies from 0 to almost 10% w/w. In surface waters, the content
of humic material, expressed as dissolved organic carbon varies from 0.1 to 50 ppm in darkwater swamps. In ocean waters, the dissolved organic carbon varies from 0.5 to 1.2 ppm at the
surface, and the dissolved organic carbon samples from deep groundwater vary from 0.1 to
10 ppm. In addition, about 10% of the dissolved organic carbon in surface waters is found in
suspended matter, either as organic or organically coated inorganic particulate matter (Gaffney
et al., 1996).
Humic materials have a wide range of molecular weights and sizes, ranging from a few hundred
to as much as several hundred thousand atomic mass units. In general, fulvic acids are of lower
molecular weight than humic acids, and soil-derived materials are larger than aquatic materials
(Stevenson, 1989). Humic materials vary in composition depending on their source, location, and
method of extraction; however, their similarities are more pronounced than their differences. The
range of the elemental composition of humic materials is relatively narrow, being approximately
40%–60% carbon, 30%–50% oxygen, 4%–5% hydrogen, 1%–4% nitrogen, 1%–2% sulfur, and
0%–0.3% phosphorus (MacCarthy and Suffet, 1989). Humic acids contain more hydrogen, carbon,
nitrogen, and sulfur and less oxygen than fulvic acids. Studies on humins have shown that they
are similar to humic acids except that they are strongly bound to metals and clays, rendering them
insoluble (Schnitzer and Kahn, 1972).
It is generally believed, and some evidence does exist, that humic materials consist of a skeleton of alkyl/aromatic units cross-linked mainly by oxygen and nitrogen groups with the major
functional groups being carboxylic acid, phenol and alcohol hydroxyl groups, ketone, and quinone
groups. The structures of fulvic acids are somewhat more aliphatic and less aromatic than humic
acids, and fulvic acids are richer in carboxylic acid, phenolic acid, and ketonic groups. This is
responsible for their higher solubility in water at all pH values, and the role of water in the geochemical cycle for petroleum formation should not be ignored (Siskin and Katritzky, 1991). Humic
acids, being more highly aromatic, become insoluble when the carboxylic groups are protonated at