Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 13-16 |
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The Central Graben in the North Sea is a mature petroleum
province with Upper Jurassic - lowermost Cretaceous marine
shale of the Kimmeridge Clay Formation and equivalents as
the principal source rock, and Upper Cretaceous chalk as the
main reservoirs. However, increasing oil prices and develop-
ments in drilling technologies have made deeper plays
depending on older source rocks increasingly attractive. In
recent years exploration activities have therefore also been
directed towards deeper clastic plays where Palaeozoic depo -
province with Upper Jurassic - lowermost Cretaceous marine
shale of the Kimmeridge Clay Formation and equivalents as
the principal source rock, and Upper Cretaceous chalk as the
main reservoirs. However, increasing oil prices and develop-
ments in drilling technologies have made deeper plays
depending on older source rocks increasingly attractive. In
recent years exploration activities have therefore also been
directed towards deeper clastic plays where Palaeozoic depo -
sits may act as petroleum source rocks. Carboniferous coaly
sections are the most obvious source rock candidates. The gas -
fields of the major gas province in the southern North Sea
and North-West Europe are sourced from the thick Upper
Carboniferous Coal Measures, which contain hundreds of coal
seams (Drozdzewski 1993; Lokhorst 1998; Gautier 2003).
North of the gas province Upper Carboni-ferous coal-bearing
strata occur onshore in northern England and in Scotland,
but offshore in the North Sea area they have been removed by
sections are the most obvious source rock candidates. The gas -
fields of the major gas province in the southern North Sea
and North-West Europe are sourced from the thick Upper
Carboniferous Coal Measures, which contain hundreds of coal
seams (Drozdzewski 1993; Lokhorst 1998; Gautier 2003).
North of the gas province Upper Carboni-ferous coal-bearing
strata occur onshore in northern England and in Scotland,
but offshore in the North Sea area they have been removed by
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 13-16. Available at:
www.geus.dk/publications/bull
Are Carboniferous coals from the Danish North Sea
oil-prone?
oil-prone?
Henrik I. Petersen and Hans P. Nytoft
13
Fig. 1.
A
: Simplified map showing Carboniferous basins in the North Sea area. The Danish Central Graben is also shown (grey area).
ADB
, Anglo-
Dutch Basin;
FAB
, Forth Approaches Basin;
IMFB
, Inner Moray Firth Basin;
MNH
, Mid Netherlands High;
MNSH
, Mid North Sea High;
MV
, Midland
Vally;
NTB
, Northumberland/Tweed Basin;
NWGB
, North-west German Basin;
OMFB
, Outer Moray Firth Basin;
RB
, Ruhr Basin;
WGG
, Witch Ground
Graben. Based on Ziegler (1990), Besly (1998) and Bruce & Stemmerik (2003).
B
: Present-day distribution of Lower Carboniferous littoral/fluvial/deltaic
and shallow marine/deltaic deposits in the southern North Sea area. The positions of the Gert-2, Gert-3, P-1 and Svane-1 wells are shown. Modified
from Lokhorst (1998).
C
: Sedimentological log of the coal-bearing interval of the Gert-2 well. The coaly interval is underlain by fluvial sediments and
overlain by fossiliferous marine mudstones of the so-called Marine Unit. Slightly modified from Petersen & Nytoft (2007).
erosion. However, Lower Carboniferous strata are present
offshore and have been drilled in the Witch Ground Graben
and in the north-eastern part of the Forth Approaches Basin
(Fig. 1A), where most of the Lower Carbon iferous sediments
are assigned to the sandstone/shale-dominated Tayport For -
mation and to the coal-bearing Firth Coal Formation (Bruce
& Stemmerik 2003). Highly oil-prone Lower Carboniferous
lacustrine oil shales occur onshore in the Midland Valley,
Scotland, but they have only been drilled by a single well off -
shore and seem not to be regionally distributed (Parnell
1988).
offshore and have been drilled in the Witch Ground Graben
and in the north-eastern part of the Forth Approaches Basin
(Fig. 1A), where most of the Lower Carbon iferous sediments
are assigned to the sandstone/shale-dominated Tayport For -
mation and to the coal-bearing Firth Coal Formation (Bruce
& Stemmerik 2003). Highly oil-prone Lower Carboniferous
lacustrine oil shales occur onshore in the Midland Valley,
Scotland, but they have only been drilled by a single well off -
shore and seem not to be regionally distributed (Parnell
1988).
In the southern part of the Norwegian and UK Central
Graben and in the Danish Central Graben a total of only
nine wells have encountered Lower Carboniferous strata, and
while they may have a widespread occurrence (Fig. 1B; Bruce
& Stemmerik 2003) their distribution is poorly constrained in
this area. The nearly 6000 m deep Svane-1/1A well (Fig. 1B)
in the Tail End Graben encountered gas and condensate at
depths of 5400-5900 m, which based on carbon isotope val-
ues may have a Carboniferous source (Ohm et al . 2006). In
the light of this the source rock potential of the Lower
Carboniferous coals in the Gert-2 well (Fig. 1C) has recently
been assessed (Petersen & Nytoft 2007).
nine wells have encountered Lower Carboniferous strata, and
while they may have a widespread occurrence (Fig. 1B; Bruce
& Stemmerik 2003) their distribution is poorly constrained in
this area. The nearly 6000 m deep Svane-1/1A well (Fig. 1B)
in the Tail End Graben encountered gas and condensate at
depths of 5400-5900 m, which based on carbon isotope val-
ues may have a Carboniferous source (Ohm et al . 2006). In
the light of this the source rock potential of the Lower
Carboniferous coals in the Gert-2 well (Fig. 1C) has recently
been assessed (Petersen & Nytoft 2007).
Lower Carboniferous strata in the Danish
Central Graben
Central Graben
In the Danish Central Graben, Lower Carboniferous strata
were drilled by the Gert-2, Gert-3 and P-1 wells (Fig. 1B). The
depth to the Lower Carboniferous ranges from 3289 m in the
P-1 well to 4840 m in the Gert-2 well. Whereas the P-1 well
reached Caledonian basement after penetrating about 67 m
of Carboniferous sediments, the Gert-2 well drilled 192 m of
Carboniferous strata before drilling terminated at about 5000
m depth within the Carboniferous. The drilled Carbon iferous
section in the Gert-2 well is principally non-marine (Fluvial
Unit) and contains a coaly interval at about 4890 m (Fig. 1C)
were drilled by the Gert-2, Gert-3 and P-1 wells (Fig. 1B). The
depth to the Lower Carboniferous ranges from 3289 m in the
P-1 well to 4840 m in the Gert-2 well. Whereas the P-1 well
reached Caledonian basement after penetrating about 67 m
of Carboniferous sediments, the Gert-2 well drilled 192 m of
Carboniferous strata before drilling terminated at about 5000
m depth within the Carboniferous. The drilled Carbon iferous
section in the Gert-2 well is principally non-marine (Fluvial
Unit) and contains a coaly interval at about 4890 m (Fig. 1C)
that constitutes a transition to marine shales and shoreface
and tidally influenced sandstones of the Marine Unit
(Petersen & Nytoft 2007). The coals overlie a fluvial fining-
upward succession and are overlain by fossiliferous marine
shales (Fig. 1C). The coals formed in peat-forming coastal
plain mires as shown by high sulphur contents (average 5.3
wt%) and the presence of framboidal pyrite (Petersen &
Nytoft 2007). High contents of vitrinite (65-82 vol.%),
derived from degradation of higher land plant woody material,
indicate waterlogged, oxygen-deficient conditions in the pre-
cursor mires. Although the proportion of more oil-prone lip-
tinite constituents is generally small (4-8 vol.%), the paralic
peat-forming conditions may be favourable for the oil gene -
ration potential of the resulting coals (Petersen 2006). This
raises the question: are the coals encountered in the Gert-2
well oil- or gas-prone?
and tidally influenced sandstones of the Marine Unit
(Petersen & Nytoft 2007). The coals overlie a fluvial fining-
upward succession and are overlain by fossiliferous marine
shales (Fig. 1C). The coals formed in peat-forming coastal
plain mires as shown by high sulphur contents (average 5.3
wt%) and the presence of framboidal pyrite (Petersen &
Nytoft 2007). High contents of vitrinite (65-82 vol.%),
derived from degradation of higher land plant woody material,
indicate waterlogged, oxygen-deficient conditions in the pre-
cursor mires. Although the proportion of more oil-prone lip-
tinite constituents is generally small (4-8 vol.%), the paralic
peat-forming conditions may be favourable for the oil gene -
ration potential of the resulting coals (Petersen 2006). This
raises the question: are the coals encountered in the Gert-2
well oil- or gas-prone?
Source rock quality and hydrocarbon
generation capacity
generation capacity
The average Tmax of the coals is 448°C, which corresponds
to a vitrinite reflectance of ~0.95%R
to a vitrinite reflectance of ~0.95%R
o
indicating that the
coals are at the threshold of, or slightly within, the so-called
`effective oil window' (in which efficient oil expulsion occurs;
Sykes 2001; Petersen 2006). In addition the Hydrogen Index
(HI) values of the coals are very close to their HI
`effective oil window' (in which efficient oil expulsion occurs;
Sykes 2001; Petersen 2006). In addition the Hydrogen Index
(HI) values of the coals are very close to their HI
max
values.
During initial maturation the HI of coals increases to a max-
imum value, which is considered to be a better estimate of the
generation potential of coal (Sykes & Snowdon 2002;
Petersen 2006). Thus, at first glance HI values from 171-219
mg HC/g TOC may suggest some potential for liquid petro-
leum formation (Fig. 2). The type of generated petroleum is,
however, determined by the paraffinicity of the organic mat-
ter, i.e. the proportion and length of hydrogen-bearing car-
bon chains (aliphatic chains) in the kerogen structure. The
ability to generate and expel typical waxy terrestrial crude oil
requires the presence of long-chain aliphatics with more than
~20-25 carbon atoms (Isaksen et al . 1998; Killops et al .
1998). Fourier transform infrared spectroscopy (FTIR) of the
Gert-2 coals clearly reveals a response in the aliphatic stretch-
ing region, but the response can mainly be assigned to iso-
lated CH
imum value, which is considered to be a better estimate of the
generation potential of coal (Sykes & Snowdon 2002;
Petersen 2006). Thus, at first glance HI values from 171-219
mg HC/g TOC may suggest some potential for liquid petro-
leum formation (Fig. 2). The type of generated petroleum is,
however, determined by the paraffinicity of the organic mat-
ter, i.e. the proportion and length of hydrogen-bearing car-
bon chains (aliphatic chains) in the kerogen structure. The
ability to generate and expel typical waxy terrestrial crude oil
requires the presence of long-chain aliphatics with more than
~20-25 carbon atoms (Isaksen et al . 1998; Killops et al .
1998). Fourier transform infrared spectroscopy (FTIR) of the
Gert-2 coals clearly reveals a response in the aliphatic stretch-
ing region, but the response can mainly be assigned to iso-
lated CH
2
compounds, which are of no importance to the
liquid petroleum generation potential (Petersen & Nytoft
2006). Quantification of the proportion of long-chain ali -
phatics in the kerogen structure of the Gert-2 coals by com-
prehensive chemical treatment (so-called ruthenium tetroxide
catalysed oxidation; see Petersen & Nytoft 2006, 2007)
demonstrates a negligible or extremely low amount of ali -
phatic chains with more than 18 carbon atoms. The domi-
nance of shorter aliphatic chains strongly indicates that the
coals are gas- and condensate-prone.
2006). Quantification of the proportion of long-chain ali -
phatics in the kerogen structure of the Gert-2 coals by com-
prehensive chemical treatment (so-called ruthenium tetroxide
catalysed oxidation; see Petersen & Nytoft 2006, 2007)
demonstrates a negligible or extremely low amount of ali -
phatic chains with more than 18 carbon atoms. The domi-
nance of shorter aliphatic chains strongly indicates that the
coals are gas- and condensate-prone.
14
Fig. 2. Hydrogen Index versus T
max
plot of non-extracted and solvent
extracted coal samples from the Gert-2 well. The average Hydrogen
Index of the non-extracted coals is also shown.
Carboniferous coals are
inherently gas-prone
inherently gas-prone
The above results are in line with the
findings of Petersen & Nytoft (2006),
who showed that Carboniferous coals in
general contain very minor proportions
of long-chain aliphatics in the range
C
findings of Petersen & Nytoft (2006),
who showed that Carboniferous coals in
general contain very minor proportions
of long-chain aliphatics in the range
C
19-35
and are therefore inherently poorly
suited to generate oil. Thus, for Carbon -
iferous coals only an effective gas/con -
den sate window exists.
iferous coals only an effective gas/con -
den sate window exists.
Of the total amount of aliphatic
chains in the range C
12-35
, Carbon
-
iferous coals contain on average about
20% in the C
20% in the C
19-35
range (Fig. 3A). In
contrast, Jurassic coals from the Søgne
Basin in the North Sea contain about
26%, while Cenozoic coals contain on
average as much as 55% (Fig. 3A). The
significantly higher proportion of long-
chain ali-phatics in the youngest coals
seems to be related to the high amount
of organic de-trital groundmass (Fig.
3B). The groundmass consists of detrital
vitrinitic and liptinitic organic matter
that can be positively correlated to the
long-chain ali-phatics in the kerogen
structure (Petersen & Nytoft 2006). The
oil-proneness of the Cenozoic coals thus
seems to be related to the evolution of
more diversified plant communities, in -
cluding the appearance of angio sperms in
the Late Cretaceous.
Basin in the North Sea contain about
26%, while Cenozoic coals contain on
average as much as 55% (Fig. 3A). The
significantly higher proportion of long-
chain ali-phatics in the youngest coals
seems to be related to the high amount
of organic de-trital groundmass (Fig.
3B). The groundmass consists of detrital
vitrinitic and liptinitic organic matter
that can be positively correlated to the
long-chain ali-phatics in the kerogen
structure (Petersen & Nytoft 2006). The
oil-proneness of the Cenozoic coals thus
seems to be related to the evolution of
more diversified plant communities, in -
cluding the appearance of angio sperms in
the Late Cretaceous.
Limited expulsion efficiency
and implications for en -
hanced gas-proneness
and implications for en -
hanced gas-proneness
In agreement with the kerogen structure
the generated hydrocarbons from the
Lower Carboniferous Gert-2 coals are
dominated by shorter-chain aliphatics.
These do not facilitate expulsion (Isak -
sen et al . 1998), and the generated hydro -
carbons remain trapped in the coals.
This is sustained by a pronounced diffe -
rence in the HI of the non-extracted
coals and the HI of the extracted coals:
upon extraction the HI is on average
reduced by 30% (Fig. 2; Petersen &
the generated hydrocarbons from the
Lower Carboniferous Gert-2 coals are
dominated by shorter-chain aliphatics.
These do not facilitate expulsion (Isak -
sen et al . 1998), and the generated hydro -
carbons remain trapped in the coals.
This is sustained by a pronounced diffe -
rence in the HI of the non-extracted
coals and the HI of the extracted coals:
upon extraction the HI is on average
reduced by 30% (Fig. 2; Petersen &
15
Fig. 3.
A
: The proportion (%) of C
19-35
long-chain aliphatics of the total amount of C
12-35
aliphatics
in a number of Carboniferous, Jurassic and Cenozoic coals.
B
: The proportion (vol.%) of ground-
mass composed of detrital vitrinite and liptinite in a number of Carboniferous, Jurassic and
Cenozoic coals.
16
Nytoft 2007). Hence, the measured HI values of the Gert-2
coals are strongly influenced by the trapped petroleum in the
coals. The limited (or lack of ) expulsion maintains a low sat-
urate/aromatic hydrocarbon ratio of the trapped petroleum,
which according to Pepper & Dodd (1995) is less thermally
stable than expelled oil that is dominated by saturated
(aliphatic) hydrocarbons. For source rocks with HI values
below 300 mg HC/g TOC, intra-source rock cracking of
hydrocarbons commences from 115-145°C (Pepper &
Dodd 1995). The average HI of the Gert-2 coals is 193 mg
HC/g TOC, and the vitrinite reflectance values suggest bur-
ial temperatures of 124-132°C, implying that intra-source
rock oil-to-gas cracking of the trapped hydrocarbons may
enhance the gas-proneness of the coals. Observation of
pyrolytic carbon in the coals may provide direct evidence for
gas generation (Petersen & Nytoft 2007).
coals are strongly influenced by the trapped petroleum in the
coals. The limited (or lack of ) expulsion maintains a low sat-
urate/aromatic hydrocarbon ratio of the trapped petroleum,
which according to Pepper & Dodd (1995) is less thermally
stable than expelled oil that is dominated by saturated
(aliphatic) hydrocarbons. For source rocks with HI values
below 300 mg HC/g TOC, intra-source rock cracking of
hydrocarbons commences from 115-145°C (Pepper &
Dodd 1995). The average HI of the Gert-2 coals is 193 mg
HC/g TOC, and the vitrinite reflectance values suggest bur-
ial temperatures of 124-132°C, implying that intra-source
rock oil-to-gas cracking of the trapped hydrocarbons may
enhance the gas-proneness of the coals. Observation of
pyrolytic carbon in the coals may provide direct evidence for
gas generation (Petersen & Nytoft 2007).
Concluding remarks
As is the case with other Carboniferous coals, the drilled
Lower Carboniferous coals encountered in the Gert-2 well,
located at the northern margin of the Danish Central
Graben, are gas-prone. The gas-proneness is inherited from
the coaly organic matter, which generally contains a small
amount of oil-prone kerogen due to the lack of long-chain
aliphatics. Limited expulsion efficiency maintains a low satu-
rate/aromatic ratio of the generated and trapped petroleum.
The thermally less stable petroleum mixture promotes intra-
source rock oil-to-gas cracking of the trapped hydrocarbons
in the coals, which enhances their gas-proneness. The thin
coaly section, present in the Gert-2 well, has no economic
significance. However, provided that the Lower Carbon -
i ferous coaly section is regionally distributed and the section
elsewhere is thicker with a larger number of coal seams
and/or thick sections of coaly shale, it can potentially be a gas
source for deep plays in the Danish Central Graben and adja-
cent areas. This is supported by the encountered gas in the
Svane-1/1A well (Ohm et al . 2006).
Lower Carboniferous coals encountered in the Gert-2 well,
located at the northern margin of the Danish Central
Graben, are gas-prone. The gas-proneness is inherited from
the coaly organic matter, which generally contains a small
amount of oil-prone kerogen due to the lack of long-chain
aliphatics. Limited expulsion efficiency maintains a low satu-
rate/aromatic ratio of the generated and trapped petroleum.
The thermally less stable petroleum mixture promotes intra-
source rock oil-to-gas cracking of the trapped hydrocarbons
in the coals, which enhances their gas-proneness. The thin
coaly section, present in the Gert-2 well, has no economic
significance. However, provided that the Lower Carbon -
i ferous coaly section is regionally distributed and the section
elsewhere is thicker with a larger number of coal seams
and/or thick sections of coaly shale, it can potentially be a gas
source for deep plays in the Danish Central Graben and adja-
cent areas. This is supported by the encountered gas in the
Svane-1/1A well (Ohm et al . 2006).
Acknowledgements
The study was part of a larger project financially supported by the
Danish Natural Science Research Council (grant 21-04-0605).
Danish Natural Science Research Council (grant 21-04-0605).
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Authors' address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
hip@geus.dk
