View/Open - ARAN - National University of Ireland, Galway

1.1.3 Chemical Composition
Introduction
Crude oils consist of a large mixtures of aliphatic, aromatic, and high molecular
weight organic compounds. Despite this multitude of components, the proportions
of the elements in crude oils vary over fairly narrow limits. The carbon content
normally is in the range 83-87%, and the hydrogen content varies between 10 and
14% (by weight) [14]. Varying small amounts of nitrogen, oxygen, sulfur and metals
(Ni and V) are also found in crude oils [7]. A comparison between kerogen and crude
oil in terms of elemental composition is shown in Table 1.1. During the process of
thermal cracking of kerogen to oil, there is an increase in the % hydrogen and a
corresponding decrease in Sulphur, Nitrogen and Oxygen.
Figure 1.2 shows the main constituents of crude oils in relation to their distillation
fractions. The alkanes dominate the gasoline fraction of crude oils and are most
abundant in highly mature oils. Moving from the gasoline fraction to the heavier
fractions, there is a marked increase in amount of cycloalkanes, after which the
aromatic content slowly rises until reaching the lubricating oil range. Note the
absence of Alkenes and alkynes. These compounds are not found in crude oils as
they are readily reduced to alkanes by H2 or thiols by H2S [7]. The residuum is
the heaviest part of the crude at the bottom of a distillation tower. This residuum
comprises of resins, waxes and asphaltenes and will be described in the next section.
Table 1.1: Typical elemental composition of oil and kerogen (weight %). Reproduced from
[7].
Oil Kerogen
Carbon 84.5 79
Hydrogen 13 6
Sulphur 1.5 5
Nitrogen 0.5 2
Oxygen 0.5 8
Total 100 100
6