NATURAL METHANE IN THE KAROO:
ITS OCCURRENCE AND ISOTOPE CLUES TO ITS ORIGIN
A.S. Talma1 and C.Esterhuyse2
1
2
Consultant, Pretoria; email siep.talma@gmail.
SRK, Kimberley; email: [email protected]
Abstract
The possible future exploitation of methane in the Karoo has stimulated work from various disciplines to
examine its occurrence, exploitability and exploitation risks. Groundwater issues are vital in this context
because of its possible use during exploration and exploitation and, more important, to understand the
risks of its pollution during and after, all these activities. This paper presents the experiences of the
authors to document the presence of methane in the Karoo based on data from boreholes, springs,
tunneling and deep drilling. There have been frequent anecdotal reports of explosive gas in boreholes,
both dry and wet in the Karoo. In some cases the gas is identified as methane. Thermal spring waters in
the Karoo invariably contain some amounts of methane. Methane pockets have been found in the Karoo
during tunneling projects and in some deep Soekor boreholes. A groundwater study in the vicinity of the
Gariep Dam indicated substantial quantities of methane in warm groundwater and an association with
helium. The isotope concentrations of carbon and hydrogen in methane characterize the methane forming
processes. Such analyses in samples from the central Karoo basin are consistent with that of thermogenic
gas found elsewhere in the world. Towards the edges of the basin, lower 13C values indicate that methane
there is produced by microbial processes at shallower depths. The presence of thermogenic methane
together with helium on the surface is likely to give clues to pathways from depth.
1. INTRODUCTION
Much interest in the country is now directed towards the Karoo because of its potential to deliver shale
gas as future fuel source (DMR 2012; Steyl and van Tonder 2013). The exploitation of this shale gas has
the potential of polluting shallow groundwater resources (de Wit 2011; Steyl et al 2012) and preliminary
studies and environmental impact assessments will be required, even before exploration boreholes are
drilled.
Methane in local groundwater at levels above 10mg/L is known to occur worldwide in aquifers above
coal and shale gas deposits (Grossman et al 1989; Zhang et al 1998; Thyne 2008; Osborn et al 2011), in
soils (Darling and Gooddy 2006; Levy et al 2012) and in wetlands (Otter and Scholes 2000). The opposite
is also found, since Warner et al (2013) reported the absence of dissolved methane in groundwater above
the Fayetteville shale deposit in Arkansas, USA. It must be noted, however, that the isotope
concentrations of methane dissolved in groundwater above a shale deposit does not necessarily represent
that of the deep shale gas but may be derived from shallower coal deposits (Molofsky et al 2013).
This paper presents information on the occurrence of natural methane in the Karoo environment, its
isotope content and its distribution in the Karoo. The data have to date not been widely known and may
provide guidelines to obtain baseline levels from the Karoo deposits.
2. FORMATION AND ISOTOPIC COMPOSITION OF METHANE
Methane (CH4) is a colourless, odourless gas produced from organic material. It is combustible in air in
concentrations between 5 and 15%. Methane is not poisonous, but can be dangerous when it displaces
oxygen and the victim can become asphyxiated. Methane is an efficient heat absorber and therefore
considered to be a greenhouse gas. The present-day atmospheric CH4 content is in the order of 1.8 ppmv
and rising. Methane is the first in the series of alkanes. The higher compounds in this series are ethane
(C2H6), propane (C3H8), butane (C4H10), etc.
Methane is the main component of natural gas and is produced in landfills, marshes, high-organic soils,
coal, oil and shale deposits. In general the conversion of organic matter is accomplished by bacteria or
archaea in association with bacteria (Schoell 1980, 1984; Whiticar 1996). The most common methane
type is so called ‘marsh gas’ where methane is formed in wet reducing environments. Biogas, methane
produced by fermentation of organic matter as in land-fill sites, is another variation of the marsh gas
process. Ruminants produce large amounts of methane during the process of converting organics in their
rumen (Crutzen et al 1986). Methane is also produced in coal and oil deposits and even there, in some
cases, methanogenesis is accomplished by biological intervention.
Methane can also be produced by thermal decomposition of organic matter. This thermogenesis is
essentially a breakdown of complex organic molecules into the simplest possible hydrocarbon. This is the
source of methane emission in oil fields and also produces coal bed methane (Schoell 1984; Coleman et
al 1995). It is also the process responsible for production of methane in shale deposits. The temperatures
required are in excess of 70 oC (Kaplan et al 1997).
The different methane source processes produce differing patterns in the chemical composition and the
isotope ratios of the product gases. This approach has been very successful in the identification of sources
of natural gas and understanding some of the processes taking place underground (Schoell 1980, 1984;
Whiticar 1996; Kaplan et al 1997). Each of the natural gas production methods produce a specific
chemistry and isotopic pattern that can be interpreted profitably. The main chemical parameter considered
is the ratio of higher alkanes to methane. This is expressed as C2+, defined as (C2H6+C3H8)/CH4 (Schoell
1980, 1984). Its inverse is sometimes also being used (Bernard et al 1976; Whiticar 1996). The useful
isotope parameters are the 13C/12C and the D/H (=2H/1H) isotope ratios of the methane part of the gas.
More complex analyses include the isotope ratios of the higher alkanes as well (Whiticar 1996).
The most common biogas formation process is the fermentation of organic material in anaerobic
conditions. Acetic acid is formed and converted by methanogenic bacteria into methane (Kaplan et al
1997). The gas produced in this way contains mainly methane with some H2S, CO2 and higher alkanes.
δ13C(CH4) ranges from -60 to -35‰PDB, δD(CH4) between -300 and -250‰SMOW (Figure 1).
Fermentation methane is found in landfill sites, fresh water marshes, waterlogged soils and similar
environments (Hackley et al 1999). A different biological production process of methane is the reduction
of carbon dioxide by hydrogen gas by specific bacteria or archaea. This mechanism is probably most
important in marine and estuarine environments (Schoell 1984, Coleman et al 1995), but is also found in
exhalation from ruminant animals (Kaplan et al 1997). This methane has lower δ13C (-80 to -45‰) but
higher δD (-250 to -150‰). In contrast to bacterial processes, thermogenesis produces methane with
isotope ratios much closer to that of the source material: its δ13C ranges from -55 to -25‰PDB, δD
between -250 and -140‰SMOW (Schoell 1980, 1984, Whiticar 1996, Kaplan et al 1997).
These isotope patterns can be demonstrated on a 13C-D plot and specific fields assigned to the different
sources (Figure 1). Since these are generalizations from sources all over the world there are uncertainties
in the interpretation of sources and, inevitably some overlaps. There are possibilities of mixing of
different sources (e.g. Whiticar 1996; Ward et al 2004) and local secondary effects. Additional clues to
methane provenance are provided by the higher alkanes (Schoell 1984, Whiticar 1996). One example is
the fact that thermogenesis at lower temperatures will produce ‘wetter’ (higher C2+) gas than that from
higher temperatures. The practical problem is that transport of gas may cause sorption of the higher
hydrocarbons along the way, thereby rendering a surface gas sample unrepresentative. There is, however,
no evidence that the isotopic composition of methane will change during transport (Schoell 1984).
Figure 1. Distributions of deuterium and carbon isotope values in methane for the three processes of
origin: Near-surface microbial (fermentation), Sub-surface microbial (carbon reduction) and
Thermogenic. (Schoell 1980; Coleman et al 1995)
3. DATA SOURCES
The authors have attempted to consolidate all information of local methane occurrences. The data
presented in this paper have been derived from different sources. The selection comprises methane in
boreholes, springs and mines but is limited to the Karoo in South Africa.
Analytical data
Dr Leslie Kent, former director of the (then) Geological Survey, conducted research into thermal springs
all over the country over many years (summaries in Kent 1949, 1969). He assembled data from earlier
workers and arranged sampling and analyses of many additional springs and warm boreholes. Methane
was analysed as part of the dissolved gas fraction of the water. Kent only found methane in water sources
emanating from Karoo formations (Table 1). Springs from other formations contained nitrogen, carbon
dioxide and occasionally helium, but no methane. Methane production can be significant: Kent (1969),
for example, reported that at one time methane from the Aliwal North hot spring was actually collected
and used in the town for heating.
Table 1. Methane content of spring water associated with Karoo formations (Kent 1949)
Source
Formation Methane concentration
(% of dissolved gas)
VKI, OFS (borehole()
Ecca
80.8
Florisbad, OFS
Ecca
71.5
Fort Beaufort
Beaufort
100
Cradock
Beaufort
83.4
Tarka Bridge, CP, (borehole)
Beaufort
94.0
Aliwal North, CP
Beaufort
55.2
Lake Mentz, CP
Dwyka
39
Grasrand, Graaff Reinet, CP
Beaufort
98.5
Vredenburg, Marais siding, CP
Beaufort
33.3
Kruitfontein, Edenburg, OFS
Beaufort
79
Gruisfontein, Devon, Tvl
Ecca
82.1
During the drilling of Soekor boreholes, samples were also taken of the gas released in the boreholes. As
reference data, gas samples were also collected from boreholes in the Karoo formations all over the
country. The gas samples were collected in 300 ml glass double stopcock flasks. Sample details are not
known any more, but based on general practice at that time, they probably involved arrangements with
funnels and rubber tubes and displacement of the gas with water or mercury (!). The samples were sent to
the Physics Department of the University of Groningen in the Netherlands. The head of the isotope
laboratory at that time was Dr John C Vogel, a South African, who had just established a stable isotope
facility at that university. He had good contacts with Soekor staff and he was keen to apply environmental
isotope techniques in different disciplines.
A student, DJW Nijborg, developed a method of gas analysis on an isotope mass spectrometer (Atlas
M86) by peak height evaluation. He also developed a method to remove CO2 from the gas by NaOH and
subsequently oxidized the methane to obtain CO2 for analysis of 13C in the methane. He analysed a few
samples and produced a ‘scriptie’ (MSc dissertation) (Nijborg 1965) which unfortunately does not exist
anymore. Nijborg’s laboratory note book is, however, still available. In 1968, the first author of this paper
improved Nijborg’s method to also separate the higher hydrocarbons, ethane and propane, from the gas
mixture. He then analysed the remaining samples (Talma 1969). Talma also collected water from the
methane combustion which enabled deuterium analysis in the Council for Scientific and Industrial
Research (CSIR) Laboratory in Pretoria (Schiegl 1970). The analytical results of all of these analyses are
presented in Table 2. Analytical precisions of the isotope analyses are probably better than 1‰ for 13C
and 10‰ for D.
In the mid 1970s the isotope laboratory at the CSIR in Pretoria (then known as the Natural Isotopes
Division of NPRL) undertook a groundwater investigation around Venterstad in the Northern Cape. The
project was triggered by the earlier flooding of the Orange Fish tunnel when anomalous water
occurrences were found in fractures through which the tunnel was excavated. Parts of the tunnel were
flooded and required extensive grouting and pumping for construction to continue. During the CSIR
project, groundwater was sampled from some of the many boreholes that had been drilled around the
inflow location, and from there along a line of 130 km northeast towards Reddersburg. 26 groundwater
samples were collected during this project (Vogel et al 1980). Apart from hydrochemistry and the usual
isotopes, dissolved gases were also analyzed by extraction of gas from a vacuum flask of water and mass
spectrometer analysis (Heaton and Vogel 1979). The results indicated that half the samples from this area
contained significant quantities (>1 mg/l) of methane dissolved in the water, that methane is correlated
with elevated helium content in the water (Figure 2) and, in many cases, with lower 14C content in the
associated groundwater (Figure 3). Isotope analysis of methane from two of these samples yielded δ13C =
-35.7 (sample 3b) and –32.2‰PDB (sample 11b). This places the samples within the thermogenic origin
range (Figure 1).
Table 1. Chemical and isotopic analyses of gases from the Karoo (from Nijborg 1965(1); Talma 1969(2),
1991(3); Ward et al 2004(4) and unpublished results)
depth
(ft)
%CH4
%C2+
δ13C
CH4
δD
CH4
Molteno
69
0
-41.3
-183
2
Beaufort
98
0
-32.4
-212
2
Beaufort
75
0
-34.2
-236
2
-57.8
-210
1
-155
2
Surface
geology
Location
Source
Aliwal North
Thermal spring
Fort Beaufort
Hayfields
Thermal spring
Spring near
dolerite
Borehole near
dolerite
Borehole near
dolerite
Jamestown
Dry borehole
Loeriesfontein
Borehole
Mentz meer
Middeldam, Kimberley
Spring
Dry borehole
near dolerite
Soutpansfontein
Borehole
Hlotse BTR107
Lesotho
CR 1/68
Soekor
Glen
Soekor
KL 1/65
Soekor
6
SA 1/66
Soekor
20
Sutherland 2 (KL1/65?)
Soekor
Sutherland 3 (KL1/65?)
Soekor
VR 1/66
Soekor
8278
Kalabasvlakte, Dannhauser
Artesian in coal
1200
Ecca
Brakfontein, Kestell
Roodewal, Camden BH
C25/59
In coal
Artesian in coal;
dolerite
5000
246
Blinkpan seam 4
Coal mine
Northfield lower seam
Coal mine
Dannnhauser 10
Grasrand
Gruisfontein, Devon
880
Deep geology
144
Beaufort
Middle Ecca
Beaufort/
dolerite
40
0
-28.3
200
Molteno
Molteno
81
0
-31.7
Ecca
Witteberg
98
-30.6
15
Witteberg
34
-58.6
204
Dwyka shale
63
0
-57.1
-182
Ecca
Dolomite
56
0.0
-57.9
-184
Basalt
Ecca
18
0.9
-29.1
-141
-22.0
-238
280
2886
78
55
14
C
CH4
Ref.
1
-205
1
1
2
2
2.4
3
0.0
Beaufort
83
4.9
-26.5
Beaufort
93
2.9
-26.5
-224
81
0.0
Dwyka shale
78
3.7
-32.1
-203
1
Beaufort
Shist
93
0.5
-40.9
-243
2
Dolerite
Ecca
51
0.0
-55.6
-171
1
Ecca
74
0.0
-71.1
-181
1
Ecca
99
0.0
-40.3
-218
1
Coal mine
Ecca
72
Middelbult Colliery
MB Sec49
Karoo
93
0.0
-45.5
-213
4
Middelbult Colliery
MB 531005
Karoo
95
0.0
-53.2
-206
4
Middelbult Colliery
MB W135704-1
Karoo
94
0.0
-53.7
-197
4
Middelbult Colliery
MB W135704-2
Karoo
73
0.0
-53.3
-206
4
MB W135035
Karoo
83
0.0
-55.3
-199
4
Middelbult Colliery
1 foot = 0.304 metre
-33.6
1
Figure 2. Correlation between dissolved methane and helium content in groundwater collected during the
CSIR Venterstad project (Heaton & Vogel 1979). The sulphuretted water, having high methane and
helium contents, are indicated with triangles. (1 ml STP/kg = 0.71 mg CH4/L)
Figure 3. Helium concentrations in groundwater of the Venterstad project versus 14C content (Heaton &
Vogel 1979). Models of mixing (solid lines) and ageing (broken lines) are indicated.
During the drilling of the transfer tunnel linking the Khatse Dam and the Ash River (part of the Lesotho
Highlands Project), methane was found in the Ecca rocks along the tunnel traverse. The first author was
asked to sample gas from a nearby borehole and analyse it for its isotope content (Talma 1991). The
stable isotope results indicated that the methane was of thermogenic origin (Table 2). The low 14C content
(2 pmc) precluded methane development from recent material.
A more recent study of the isotope composition of methane by Ward et al (2004) from the Witwatersrand
Basin indicated a microbiological origin for most of the methane found in five mines. There appears to be
a second component consisting of high-13C methane probably produced by thermogenic methanogenesis.
This study also included five samples from Middelbult Mine, near Kinross, Mpumalanga. The isotope
data from this mine exhibit clear coal-mine isotopic signatures (Table 2).
Anecdotal information
The authors questioned a number of individuals knowledgeable of Karoo groundwater about the existence
of methane in boreholes. All of them confirmed that the occurrence of ‘gas’ in boreholes is quite
common. Since methane is not detectable by humans, the observation that gas was detected does not
necessarily imply that the gas is methane. In some cases, however, the gas was found to be explosive or
combustible. There are, in fact, quite a few anecdotal reports of farmers and drillers igniting the gas from
boreholes.
One such example is on the farm Orange Puts approximately 25 km north-east of Williston (Northern
Cape Province). A water supply borehole, drilled by the farmer during the early 1970s, intersected a
dolerite sill from the surface to a depth of 70 m. Groundwater and gas were struck directly below the sill
and the borehole would burn, if set alight. This phenomenon continued until about ten years ago
whereafter it no longer produced combustible gas. The borehole is equipped with a windpump without an
air-tight seal at the top which means that it was producing free flowing gas for thirty years.
Another incident occurred on the evening of 16 October 2009 when a tremor registering 3.5 on the
Richter scale occurred approximately 24 km west of Williston. At the town is a spring which originates
on the upper contact of a saucer-shaped dolerite sill. During the tremor event, a fire started at the spring
although it was flowing at the time. It is likely that the earth tremor released a natural gas pocket.
Many of the gas occurrences in the central and western Karoo (and likely elsewhere as well) are
associated with the lower contacts of dolerite sills. These sills trap the up-moving gasses and form gas
pockets underneath it.
Seen together, these data and observations indicate that methane in the Karoo is quite likely a very
common feature and has been so for quite some time. The distribution of confirmed methane occurrences
liable at this stage (Figure 4) does not conform to any surface geology feature. The cluster of methane
localities in the Venterstad/Reddersburg area is the product of intensive sampling for dissolved gas in that
area (Heaton and Vogel 1979; Vogel et al 1980). The same may, or may not be, the case in any other part
of the Karoo. Some methane occurrences are definitely associated with coal (Table 2) and coal may also
be present in those cases where the sampler did not record its presence. Natural methane emissions are
found elsewhere: in Pennsylvania US, above the well-known Marcellus shale source, methane emissions
on the surface have been known for 200 years, long before any drilling (Molofsky et al 2013).
Figure 4. Karoo map with confirmed occurrences of methane indicated. Open circles indicate boreholes
where combustible gas was found. Closed circles represent groundwater where methane was actually
identified by analysis.
4. ISOTOPIC INDICATORS TOWARDS THE ORIGIN OF KAROO METHANE
The isotopic contents of the samples that have become available from the Karoo (in its geological sense)
indicate a wide range of values contained in both the fields of thermogenic and biogenic reduction (subsurface sources) (Figure 5). Based on the source classification of Kaplan et al (1997), some of the data
plot within the thermogenic field. Other samples are positioned within the bio-reduction field or in
between. The samples that were analysed did not show any bio-fermentation process at work.
A well-known type example of thermogenic methane is from the Marcellus shale in the US (Figure 5)
where much work has been done (Molofsky et al 2013).Thermogenic gas is known to be formed at
temperatures in excess of 70oC (Kaplan et al 1997). This would imply that the minimum formation depth
of half the samples in Figure 5 is located deeper than 2½ km (using a thermal gradient of 20oC/km). The
samples from the Soekor boreholes, with higher δ13C of methane (Table 2), would, in this model be
derived from even greater depths.
-100
Non-coal gas
Coal gas
Soekor gas
-150
MARCELLUS
D in CH4
THERMO
-200
BIO-RED'N
-250
BIO-FERM'N
-300
-90
-80
-70
-60
13C in
-50
CH4
-40
-30
-20
Figure 5. Plot of δD versus δ13C of all samples used in this data compilation. ‘Coal’ samples are identified
as such; ‘non-coal’ carries no identification. The boundaries of sources obtained elsewhere are from
Kaplan et al (1997) and are also shown in figure 1. The displayed Marcellus shale gas data are from
Molofsky et al (2013).
The samples located within the biogenic-reduction field (Figure 5) represent methane produced in what is
described as ‘sub-surface’ environments which includes marine and estuarine deposits and coals. It is
known that individual coalfields have isotope values fairly close to each other; to such an extent that the
isotope data can be used to differentiate between methane produced at different levels (Molofsky et al
2013). Within a coalfield, isotope contents can vary to some extent based on the maturity of the coal and
other factors (Whiticar 1996). This can be of use in coal-bed methane projects.
The distribution map of 13C in methane (Figure 6) shows some patterning. The lowest δ13C values are
found in the coalfields of Mpumalanga and along the southern and western edges of the Karoo. The
thermogenic field (13C > -42‰) includes most of the central Karoo area in the Eastern Cape and towards
the south-west. The likely (Ecca group) shale sources are at their deepest (3-5 km) in the eastern Cape
(Water Expert Group 2012). Coupled with the expected methane formation depth derived from 13C, it
suggests that these gases are derived from the deep shales. The association of methane with helium found
in the Venterstad area (Figure 2) supports the concept of a deep origin for this gas, if it should be present
in other parts of the Karoo.
Despite a very low sampling density, the isotope data therefore show that the Karoo produces methane
from both thermogenic (probably mature shales) and bio-reduction (probably coalfields). The sampling
conditions during this reconnaissance sampling were not such that samples from distinct levels could be
obtained. This is especially regrettable in the case of the Soekor boreholes. In principle the classification
should be confirmed, or expanded, by using the chemical composition of the gas (Schoell 1980, 1984;
Whiticar 1996; Kaplan et al 1997). The chemical analyses of the samples discussed here were, however,
not of good enough quality to make the distinction between ‘dry’ and ‘slightly wet’ gas. Further work in
this area should fill this gap.
Figure 6. Karoo map with 13C contents of borehole methanes (from Table 2) indicated.
5. CONCLUSIONS
The main conclusions drawn from the information presented in this paper are
• Methane in borehole and spring water is fairly common throughout areas with Karoo geology and
has been detected in shallow and deep, wet and dry boreholes and in thermal spring water.
• The correlation between dissolved methane, helium and distinct water quality types, found in the
Venterstad area should be searched for in other parts of the Karoo.
• There are not enough data to indicate to what extent methane production remains constant. The
Soekor data suggest that there are pockets of methane underground that may well decrease their
flow with time.
• The stable isotope patterns of hydrogen and carbon in methane indicates that the gas in the
central Karoo Basin was thermogenically produced at significant depth and very likely originated
from the deep shales.
• On the edges of the Karoo, some methane emissions carry the isotope signal of microbiological
reduction of carbon. This is a feature of a lower temperature environment and may swamp the
isotope signal from lower, if present, depth.
• Depth specific sampling and correlations with water hydrochemistry would be required to
indicate the levels where the gas is actually produced
• Further characterisation of the sources of methane formation can be accomplished through
detailed chemical analyses of the total gas.
• The association of helium and methane in part of the Karoo Basin suggests that these gases could
be indicators of preferential pathways from depth.
6. ACKNOWLEDGEMENTS
This work is primarily based on sampling done in the 1960s by unknown samplers. The late Dr John
Vogel arranged the sampling and the analyses of natural gas out of pure interest. We thank the members
of the Karoo Groundwater Expert Group for discussions on this subject. This interaction and the interest
shown by Shell, provided the motivation to retrieve the old data and re-evaluate them. Peter Rosewarne
reviewed the manuscript.
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