187
8
Chemical Composition
8.1 INTRODUCTION
Petroleum is not usually found where the precursors were laid down, but in reservoirs where accumulation occurs after it has migrated from the source rocks through geologic strata (Chapter 3)
(Tissot and Welte, 1978 and references cited therein).
Knowledge of the composition of petroleum allows the geologist to answer questions of precursor–product relationships and conversion mechanisms. Biomarkers, molecules that retain the
basic carbon skeletons of biological compounds from living organisms after losing functional
groups through the maturation process, play an important role in such studies. The distribution
of biomarker isomers can not only serve as fingerprints for oil/oil and oil/source correlation
(to relate the source and reservoir) but also give geochemical information on organic source
input (marine, lacustrine, or terrestrial source), age, maturity, depositional environment (clay or
carbonate, oxygen levels, salinity, etc.), and alteration (water washing, biodegradation, or other
processes) (Tissot and Welte, 1978 and references cited therein).
It is, therefore, not surprising that petroleum is not a uniform material. In fact, its chemical and
physical (fractional) composition can vary not only with the location and age of the oil field but also
with the depth of the individual well. Indeed, two adjacent wells may produce petroleum with markedly different characteristics. On a molecular basis, petroleum is a complex mixture of hydrocarbons with small amounts of organic compounds containing sulfur, oxygen, and nitrogen, as well as
compounds containing metallic constituents, particularly vanadium, nickel, iron, and copper. The
hydrocarbon content may be as high as 97% w/w, for example, in the lighter paraffinic crude oil or
as low as 50% w/w in heavy crude oil and bitumen.
Nevertheless, crude oil with as little as 50% hydrocarbon components is still classified as a mixture of naturally occurring hydrocarbons. It will retain most of the essential characteristics of the
hydrocarbons even though the nonhydrocarbon portion of the crude may actually consist of molecules containing one or perhaps two atoms of elements other than carbon and hydrogen.
With the necessity of processing heavy oil, bitumen, and residua to obtain more gasoline and
other liquid fuels, there has been the recognition that knowledge of the constituents of these higher
boiling feedstocks is also of some importance. Indeed, the problems encountered in processing the
heavier feedstocks can be equated to the chemical character and the amount of complex, higher
boiling constituents in the feedstock. Refining these materials is not just a matter of applying knowhow derived from refining conventional crude oils but requires knowledge of the chemical structure
and chemical behavior of these more complex constituents.
However, heavy crude oil and bitumen are extremely complex and very little direct information
can be obtained by distillation. It is not possible to isolate and identify the constituents of the heavier
feedstocks (using analytical techniques that rely upon volatility). Other methods of identifying the
chemical constituents must be employed. Such techniques include a myriad of fractionation procedures (Chapter 9) as well as methods designed to draw inferences about the hydrocarbon skeletal
structures and the nature of the heteroatomic functions (Chapter 11).
Nevertheless, the theory that the petroleum precursors form a mix that is often referred to as
protopetroleum (also referred to as primordial precursor soup or petroleum porridge) is an acceptable generalization (Chapter 3). And, the molecular types in any specified fraction are limited by
8
Chemical Composition
8.1 INTRODUCTION
Petroleum is not usually found where the precursors were laid down, but in reservoirs where accumulation occurs after it has migrated from the source rocks through geologic strata (Chapter 3)
(Tissot and Welte, 1978 and references cited therein).
Knowledge of the composition of petroleum allows the geologist to answer questions of precursor–product relationships and conversion mechanisms. Biomarkers, molecules that retain the
basic carbon skeletons of biological compounds from living organisms after losing functional
groups through the maturation process, play an important role in such studies. The distribution
of biomarker isomers can not only serve as fingerprints for oil/oil and oil/source correlation
(to relate the source and reservoir) but also give geochemical information on organic source
input (marine, lacustrine, or terrestrial source), age, maturity, depositional environment (clay or
carbonate, oxygen levels, salinity, etc.), and alteration (water washing, biodegradation, or other
processes) (Tissot and Welte, 1978 and references cited therein).
It is, therefore, not surprising that petroleum is not a uniform material. In fact, its chemical and
physical (fractional) composition can vary not only with the location and age of the oil field but also
with the depth of the individual well. Indeed, two adjacent wells may produce petroleum with markedly different characteristics. On a molecular basis, petroleum is a complex mixture of hydrocarbons with small amounts of organic compounds containing sulfur, oxygen, and nitrogen, as well as
compounds containing metallic constituents, particularly vanadium, nickel, iron, and copper. The
hydrocarbon content may be as high as 97% w/w, for example, in the lighter paraffinic crude oil or
as low as 50% w/w in heavy crude oil and bitumen.
Nevertheless, crude oil with as little as 50% hydrocarbon components is still classified as a mixture of naturally occurring hydrocarbons. It will retain most of the essential characteristics of the
hydrocarbons even though the nonhydrocarbon portion of the crude may actually consist of molecules containing one or perhaps two atoms of elements other than carbon and hydrogen.
With the necessity of processing heavy oil, bitumen, and residua to obtain more gasoline and
other liquid fuels, there has been the recognition that knowledge of the constituents of these higher
boiling feedstocks is also of some importance. Indeed, the problems encountered in processing the
heavier feedstocks can be equated to the chemical character and the amount of complex, higher
boiling constituents in the feedstock. Refining these materials is not just a matter of applying knowhow derived from refining conventional crude oils but requires knowledge of the chemical structure
and chemical behavior of these more complex constituents.
However, heavy crude oil and bitumen are extremely complex and very little direct information
can be obtained by distillation. It is not possible to isolate and identify the constituents of the heavier
feedstocks (using analytical techniques that rely upon volatility). Other methods of identifying the
chemical constituents must be employed. Such techniques include a myriad of fractionation procedures (Chapter 9) as well as methods designed to draw inferences about the hydrocarbon skeletal
structures and the nature of the heteroatomic functions (Chapter 11).
Nevertheless, the theory that the petroleum precursors form a mix that is often referred to as
protopetroleum (also referred to as primordial precursor soup or petroleum porridge) is an acceptable generalization (Chapter 3). And, the molecular types in any specified fraction are limited by