Environmental Engineering Reference
In-Depth Information
4.4 Shale Gas
Shale gas is predominantly, but not exclusively, methane. The source of this
gas is the organic material contained in the sediment at the time of de-
position. Two processes for the formation of methane are commonly recog-
nised: microbial production (biogenic) and non-biological, chemical
production (thermocatalytic or thermogenic).
23
Biogenicgasisformedat
relatively shallow depths and low temperatures by the anaerobic microbial
decomposition of the organic material. Thermogenic gas is formed at much
greater depths and over longer geological timescales through the thermal
cracking of the organic material at high temperatures and pressures. Ther-
mogenic gas is usually associated with the major hydrocarbon reservoirs as oil
is formed by the same process. The biogenic process does not produce oil.
In general, thermogenic gas has a high methane content with low but
significant concentrations of other hydrocarbons such as ethane (C
2
) and
propane (C
3
), with C
1
/(C
2
+C
3
)
o
100. In contrast, biogenic gas contains an
even higher proportion of methane, with a C
1
/(C
2
+C
3
) ratio between 1000
and 10 000. Where a gas contains such a high proportion of methane it is
often described as a 'dry' gas.
Biogenic and thermogenic gases can be readily differentiated and char-
acterised through their geochemistry if the gas has remained close to its
source. One method that is often used is the analysis of the stable carbon
isotope ratios of the methane (
13
C/
12
C).
12
C is the most common isotope but
around 1% is
13
C. Due to differences in the biological, chemical and/or
physical conditions under which the gas forms, differences in the isotopic
ratios occur. The stable isotope ratio (d
13
C ) values are expressed with ref-
erence to an international standard in parts per thousand (permil or %).
Biogenic methane, on average, contains a greater proportion of isotopically
lighter carbon (i.e. is more depleted in
13
C) than thermogenic methane.
Further differentiation is possible, but one potential problem with relying on
stable carbon isotopes is that if the gas migrates from its source it may
undergo other changes, such as oxidation, that can result in changes in the
ratios and introduce the risk of misinterpretation.
24
The d
13
C of thermo-
genic methane lies in the range
110 to
55% and biogenic gas in the range
55 to
20%.
25
In addition to methane and other light hydrocarbons, shale gas may also
contain small amounts of carbon dioxide, oxygen, nitrogen, hydrogen sul-
fide, rare or noble gases (argon, helium, neon and xenon) and radon. The
concentrations of these gases are generally low and do not present a major
hazard, except for radon which is radioactive. This will require appropriate
risk assessment and management.
5 Contaminant Pathways and Receptors
As an activity that takes place on and below the land surface and involves
the use of potential pollutants, there is the potential to pollute groundwater.
