Environmental Engineering Reference
In-Depth Information
Since 1945 its only major users are South African SASOL
plants.
missed by all but a few Western geologists. The isotopic
composition of hydrocarbons—their content of
13
C
matches that of terrestrial and marine plants and is lower
than the isotope's presence in abiotically generated CH
4
(Hunt 1996)—is the best proof of their biogenic origin,
and crude oil production from weathered or fractured
crystalline rocks can be explained by migration from a
flanking or overlying source rock.
Crude oils range from light mobile liquids of reddish-
brown color to highly viscous black materials. Their
composition is dominated by three homologous series of
hydrocarbons: cycloalkanes (naphtenes or cycloparaffins
in the oil industry, C
n
H
2n
), alkanes (commonly called
paraffins, C
n
H
2n
þ
2
), and arenes (aromatics, C
n
H
2n
6
,
starting with benzene). Cycloalkanes are the most abun-
dant compounds in crude oil (typically half of the
weight), and methylcyclopentane (C
5
H
11
) and methylcy-
clohexane (C
6
H
13
) are the two most common cycloal-
kanes. The lightest alkanes, methane (CH
4
) and ethane
(C
2
H
6
), are gases at atmospheric pressure; propane
(C
3
H
8
) and butane (C
4
H
10
) are also gases but are easily
compressible to liquids (hence, known as liquid petro-
leum gases, LPG). Chains with five (pentane) to sixteen
carbons are liquids, and the remainder are solids. Aro-
matics are unsaturated, highly reactive liquids named
after the members with pleasant odors that share at least
one benzene ring (C
6
H
5
) to which are attached long,
straight side chains. Carbon accounts for 83%-87% of
crude oil and hydrogen for 11%-15%, and the H/C ratio
is 1.4-1.8, compared to about 0.8 for bituminous coals.
Sulfur is the most common contaminant; sweet crude oils
contain less than 0.5%, very sour crudes more than 4%.
Other elements present are N and O (both
<
0.5%), and
there may be traces of metals (Al, Cu, Cr, Pb). Oil's en-
ergy content is fairly uniform at 42-44 MJ/kg.
8.2 Hydrocarbons: Crude Oils and Natural Gases
Chemically, hydrocarbons are the simplest organic com-
pounds composed of straight-chained, branched, or cy-
clic molecules that contain only carbon and hydrogen.
In energetics, hydrocarbons are liquid and gaseous fuels
that contain mixtures of these compounds. Crude oils
exist as liquids both in underground reservoirs and after
they have been brought to the surface. They were
formed by heat and pressure transformation of marine
biomass in sedimentary (mostly marine and also lacus-
trine) basins and are found either in continuous basin-
centered accumulations or in structural or stratigraphic
traps. This generally accepted genesis of crude oils has
been challenged by an abiogenic theory advanced by
some Russian and Ukrainian geologists (Kudryavtsev
1959; Simakov 1986) and promoted by Thomas Gold
(1999). Instead of considering highly oxidized low-
energy organic molecules as the precursors of all highly
reduced high-energy hydrocarbons, as does the standard
account of the origins of oil and gas, this theory attrib-
utes the formation of such highly reduced molecules to
high pressures encountered in the Earth's mantle.
Accordingly, hydrocarbons should be found, and some
were, in such nonsedimentary regions as crystalline base-
ments, volcanic and impact structures, and rifts as well as
in strata deeper below the already producing reservoirs.
Moreover, if the origin of hydrocarbons is abyssal and
abiotic, then there is a possibility that existing producing
reservoirs can be gradually replenished, a phenomenon
that would remove many oil and gas deposits from the
category of nonrenewable energy resources (Mahfoud
and Beck 1995; Gurney 1997). The theory has been dis-
