Page 94 - Petroleum Geology

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molecular weight on distillation, and gas condensate, which leaves no residue
on distillation. Petroleums are mixtures of hydrocarbons (compounds of
hydrogen and carbon only) usually with some contaminant non-hydrocarbon
compounds of nitrogen, sulphur and oxygen in small amounts; also of vana-
dium and nickel. The hydrocarbons form a great range of compounds, and
each accumulation of oil and most accumulations of gas are unique in that
they differ in the proportions of the constituent hydrocarbons and the con-
taminants. They differ also in that each hydrocarbon compound may have
the atoms of its molecule arranged in two or more different spatial arrange-
ments, called isomers. The isomers of a compound differ slightly in physical
and chemical properties while sharing the general properties of the Series.

The Alkane (Paraffin) Series

A single carbon atom can attach itself to other atoms through four valency
bonds, and the simplest hydrocarbon molecule is therefore one carbon atom
to which are attached four hydrogen atoms. This is methane (CH,), the com-
monest petroleum gas. A molecule with two carbon atoms may have six
hydrogen atoms attached.

H
I
H-C-H H-C-C-H
I I I
H H H
Methane (CH,) Ethane (CzHg)

The attachment of a third carbon atom leads to yet another compound with
eight hydrogen atoms (propane, C3H,), and continuing along these lines, we
have a hydrocarbon series with the general formula CnH2n+2 - the Alkane
Series. The next member of the series, butane (C4H,0), is also a gas; but those
with carbon numbers from 5 to 16-20 are liquids, after which they are
waxy solids. Members of a series are said to be homologous.
When each carbon atom is linked to the next by a single bond and the re-
maining bonds are to hydrogen atoms, the hydrocarbon is said to be saturated.
Most hydrocarbons in nature are saturated.
The Alkane Series is divided into two: the straight-chain normal-alkanes
(n-alkane) and the isomeric branch-chain iso-alkanes (i-alkanes), for example
(leaving out the hydrogen atoms).