Geology Reference
In-Depth Information
Carbon
two hydrogen atoms - these are called
normal alkanes
(
n-alkanes
for short) - while other alkanes have
branched or ring structures. Alkanes have the general
formula C
n
H
2n+2
(C
n
H
2n
for ring-shaped
cycloalkanes
).
Natural gas consists mainly of methane. Most of the
world's natural gas supply comes from conventional
hydrocarbon reservoirs, but recently
shale gas
reserves
tapped by hydraulic fracturing ('fracking') have trans-
formed the US gas market. Potential reserves of meth-
ane also exist in the form of
methane hydrate
(methane
trapped in ice) in marine sediments on continental
margins and in onshore permafrost regions.
Methane is a greenhouse gas with a higher global
warming potential than carbon dioxide (Table 9.7.1):
each molecule is over 20 times more powerful as
a greenhouse gas than a molecule of CO
2
. Major
anthropogenic sources of atmospheric methane
include the oil and gas industry, farming (ruminant
livestock, rice paddies), biomass burning and land-
fills. The main process removing methane from
the atmosphere is reaction with hydroxyl ( ∞OH )
free radical
s in the troposphere as summarized in
Equation 3.18. This reaction also occurs in the upper
atmosphere where it is the chief source of strato-
spheric water.
Alkanes are examples of
saturated
organic com-
pounds, consisting of molecules constructed entirely
with single bonds. Although not shown in the simpli-
fied diagrams above, such molecules have a zigzag
shape in three dimensions, owing to the tetrahedral
(sp
3
-based) disposition of bonds around each carbon
atom (Box 9.4). An example is hexane:
Carbon is unique among the chemical elements for the
huge variety of compounds it can form: the number of
known carbon compounds exceeds the number of
compounds associated with all of the other elements
considered together. So extensive is the chemistry of
carbon and so vital to life that - on its own - it const-
itutes a separate branch of chemical science called
organic chemistry
. Sedimentary rocks commonly con-
tain 0.2 to 2% of organic matter, the highest concentra-
tions occurring in shales (mudrocks).
From the point of view of its occurrence in rocks, it is
helpful to consider the chemistry of carbon under two
headings: organic carbon and inorganic carbon.
Organic carbon
In a topic of this size, one can only scratch the surface
of a subject as vast as organic geochemistry. Good
summaries written from a geoscientist's viewpoint are
given by Krauskopf and Bird (1995) and White (2013);
a fuller introduction can be found in the opening
chapters of the topic by Killops and Killops (2005).
The names of organic compounds used here con-
form to modern systematic nomenclature (recognized
by the
International Union of Pure and Applied Chemistry
).
Where names are given in brackets, they are the corr-
esponding traditional or 'trivial' names, which may be
more familiar.
Hydrocarbons
Hydrocarbons are compounds of carbon and hydro-
gen alone. The simplest are the
alkanes
(paraffins), a
family of
polymers
related to methane:
H
HHHHH
Hexane
C
C
H
C
C
C
C
H
H
Tetrahedral grouping
of atoms around carbon
HH
HH
HH
HC
H
H
HC
H
HCH
H
Unsaturated
compounds are those containing one
or more double C = C bonds or triple C≡ C bonds.
Examples are ethene (ethylene), C
2
H
4
, belonging to a
family called
alkenes
:
H
C
H HCH
HCH
H
H
Propane
H
Methane
Ethane
H
H
Many alkanes are chain-like polymers in which each
carbon atom (except those at the ends of the chain)
is attached to two neighbouring carbon atoms and
C
C
H
H
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