Indications for an active petroleum system in the Laptev Sea, NE Siberia

Abstract

The shallow shelf of the Laptev Sea offshore NE Siberia is characterized by a number of rift basins more than 10 km deep. These basins are filled with sedimentary rocks of predominantly Cenozoic age and are likely sites for petroleum generation and accumulation. One objective of the BGR97 Arctic cruise to the Laptev Sea was to explore for near-surface indications of petroleum, and for this purpose water samples and near-surface sediments were collected for geochemical analysis. Gaseous hydrocarbons adsorbed in near-surface sediments include thermally-generated gas which has probably migrated upwards from deeper sedimentary strata. The hydrocarbons’compositions together with stable carbon isotope ratios indicate an origin from a marine source rock at a maturity of between 0.9 and 1.3% vitrinite reflectance. On reflection seismic profiles, zones of poor reflectivity were observed locally, also suggesting the presence of ascending gas. These geophysical indications for gas occur most frequently in the northern part of the Laptev Sea; here, seepages of thermogenic methane were detected in the sea water at two locations. Refraction seismic and multichannel data indicate the existence of sub-sea permafrost down to a depth of 500m, which probably prevents gas from escaping into the water column in most areas. The greater water depths at the northern edge of the shelf may have prevented the formation of the permafrost layer, allowing the upward migration of hydrocarbons to occur.

Full text

1
Journal of Petroleum Geology, Vol. 28(4), October 2005, pp 1 - 15
INDICATIONS FOR AN ACTIVE PETROLEUM SYSTEM
IN THE LAPTEV SEA, NE SIBERIA
The shallow shelf of the Laptev Sea offshore NE Siberia is characterized by a number of rift
basins more than 10 km deep. These basins are filled with sedimentary rocks of predominantly
Cenozoic age and are likely sites for petroleum generation and accumulation. One objective of the
BGR97 Arctic cruise to the Laptev Sea was to explore for near-surface indications of petroleum,
and for this purpose water samples and near-surface sediments were collected for geochemical
analysis. Gaseous hydrocarbons adsorbed in near-surface sediments include thermally-generated
gas which has probably migrated upwards from deeper sedimentary strata. The hydrocarbons’
compositions together with stable carbon isotope ratios indicate an origin from a marine source
rock at a maturity of between 0.9 and 1.3% vitrinite reflectance. On reflection seismic profiles,
zones of poor reflectivity were observed locally, also suggesting the presence of ascending gas.
These geophysical indications for gas occur most frequently in the northern part of the Laptev Sea;
here, seepages of thermogenic methane were detected in the sea water at two locations. Refraction
seismic and multichannel data indicate the existence of sub-sea permafrost down to a depth of
500m, which probably prevents gas from escaping into the water column in most areas. The
greater water depths at the northern edge of the shelf may have prevented the formation of the
permafrost layer, allowing the upward migration of hydrocarbons to occur.
INTRODUCTION
Circum-Arctic sedimentary basins are thought to have
significant petroleum potential (Haimila et al., 1990).
They include the basins beneath the poorly explored
eastern portion of the Russian Arctic Ocean, including
the shelfal Laptev, East Siberian and Chukchi Seas
(Fig. 1). The petroleum potential of these basins has
been inferred from the results of regional geophysical
studies, and from observations made on nearby islands
(Nikitin et al., 1999). However, no deep wells have
yet been drilled in the Laptev and East Siberian Seas.
The epicontinental Laptev Sea is located between
the Taimyr Peninsula to the west and the New Siberian
Islands to the east (Fig. 1). With an area of 460,000
sq. km (Holmes and Creager, 1974) and an average
water depth of 53m (Timokhov, 1994), the Laptev Sea
B. Cramer*+ and D. Franke*
*Federal Institute for Geosciences and Natural
Resources (BGR), Stilleweg 2, 30655 Hannover,
Germany.
+ author for correspondence, email: b.cramer@bgr.de
is one of world’s largest and shallowest shelf areas.
The tectonic structure of the Laptev Sea shelf is linked
to the opening and development of the oceanic Eurasia
Basin (Fig. 1). Seafloor spreading in the Eurasia Basin
started in the Late Paleocene to Early Eocene (e.g.
Srivastava and Tapscott, 1986; Kristoffersen, 1990;
Jackson and Gunnarsson, 1990). The resulting
extensional regime led to the development of a series
of sedimentary basins on the Laptev Sea shelf
(Drachev et al., 1998, 1999; Sekretov, 2000; Franke
et al., 2001). These basins are filled with
predominantly Cenozoic sedimentary rocks which are
10km or more thick. The structural setting and
geological history of the Laptev shelf was described
by Franke et al. (2001), and the structure of the basins
on the Laptev and East Siberian shelves was discussed
by Franke and Hinz (2005) with reference to recently
acquired geophysical data.
Deeply buried organic matter may have generated
hydrocarbons in these basins. Nikitin et al. (1999)
and Sekretov (2000), for example, reported that the
Laptev Sea has considerable potential for natural gas
originating from clastic sediments of terrestrial origin
containing considerable amounts of coaly organic

2Indication for a petroleum system in the Laptev Sea, NE Siberia
matter. Belonin et al. (1999) estimated that the Laptev
Sea has total in place petroleum reserves of 400
million tons. They inferred that up to 12 separate fields
could be present, each containing more than 30 million
tons of petroleum, with oil mainly occurring in the
western part of the Laptev Sea and gas-condensate in
the Ust’ Lena Rift graben to the east. The basis for
these calculations was however highly speculative.
Kos’ko and Trufanov (2002) reported marine shales
of Eocene to Oligocene age at outcrop on the New
Siberian Islands in the east of the Laptev Sea (Fig. 1).
The occurrence of these shales within adjacent deep
sedimentary basins is likely and may contribute to the
area’s oil potential.
MATERIALS AND METHODS
One aim of the BGR97 cruise was to explore the
petroleum system of the Laptev Sea. Because of the
lack of information from deep wells, near-surface
indications for hydrocarbons were investigated as
follows: (i) reflection seismic data were investigated
for acoustic images of ascending gas; (ii) traces of
gaseous hydrocarbons were extracted from near-
surface sediments; and (iii) sea water was analysed to
identify locations where gas had escaped from the
subsurface. Seismic data were also used to identify
the extent of permafrost in the Laptev Sea. Because
of the low permeability of frozen ground (Olovin,
1988), the distribution of permafrost above a
sedimentary column is a key factor controlling
migration of petroleum into near-surface sediments
(Abrams, 1996; Cramer, 1997).
Detailed information concerning seismic field
parameters and processing of seismic data from BGR
cruises was presented in Franke :et al. (2001).
Geochemical surface methods have been used
widely in petroleum exploration for several decades,
although their applicability is still somewhat
controversial (e.g. Tillman, 1987; Sundberg, 1994).
On the one hand, it is widely accepted that
hydrocarbons migrating upward from depth can be
detected in near-surface sediments. There is often a
good correlation between surface geochemical data
and the geological setting including source rocks and
related maturities (e.g. Faber et al., 1997). On the other
hand, the precise mechanism of hydrocarbon
migration from deeply-buried source rocks into near-
surface sediments is still enigmatic (e.g. Klusman and
Saeed, 1996; Matthews, 1996; Saunders et al., 1999).
Another problem in geochemical exploration is
presented by near-surface microbial processes such
as methane generation or hydrocarbon oxidation
which may mask the primary geochemical signature
of upward migrating gas (e.g. Völz and Schwartz,
1962; Abrams, 1996).
In the Laptev Sea, we used a methodology which
was intended to differentiate in situ generated
microbial gas from thermogenic gas ascending from
depth (Faber et al., 1997). In a first step, free gas in
pore spaces, composed mainly of microbial methane,
was washed out of sediment samples. Then, gas
adsorbed on the solid phase was released from the
sediment by treatment with phosphoric acid and
heating to the temperature of boiling water under a
vacuum (Faber and Stahl, 1983). This gas is referred
to as “adsorbed gas” and in most cases represented
thermally-generated hydrocarbons (Faber et al., 1997).
These hydrocarbons could be related to deeply buried
source rocks by the application of stable carbon
isotope analyses.
For investigating the methane in sea water, 151
water samples were collected at 18 locations in the
Laptev Sea. Water was sampled from various depths
using 5 litre Niskin bottles, and dissolved gas was
extracted from the water by a vacuum-ultrasonic
treatment (Schmitt et al., 1991). Methane
concentrations were determined on site with GC-FID
measurements and the remainder of the gas was stored
in evacuated glass containers for later analyses in the
BGR laboratory.
For the determination of stable carbon isotope
ratios, methane, ethane and propane were separated
on a GC column, oxidized to CO2 and analysed with
an isotope-ratio mass spectrometer (IR-MS) according
to the method described by Dumke et al. (1989).
Isotope ratios are given in standard delta notation
normalized with respect to the PDB standard.
MARINE PERMAFROST
Marine permafrost occurs throughout the shallow
Alaska shelf (e.g. Neave and Sellmann, 1982;
Sellmann and Hopkins, 1984; Lachenbruch et al.,
1988). The permafrost was formed subaerially during
cold periods in the Quaternary when relative sea levels
were low. Sea-level fluctuations for the Laptev Sea
during Quaternary times were determined by Holmes
and Craeger (1975) based on radiocarbon dating of
sediment cores. Their data indicated that the sea level
was 50 to 55 m lower about 15,000 years b.p. than it
is today, and the shoreline of the Laptev Sea was close
to the edge of the shelf. Deep permafrost developed
below the shelf at this time (Delisle, 1998, 2000). A
subsequent marine transgression took place until the
present-day shoreline was reached. The permafrost
was preserved due to the cold bottom waters in the
Laptev Sea and today forms an impermeable barrier
for groundwater and gas ascending from depth.
In seismograms recorded by ocean-floor
instruments in the central Laptev Sea, a high-
amplitude first break with a velocity of about 3.0 to

3
B. Cramer and D. Franke
Fig. 1. Regional map of the Laptev Sea, NE Siberia, with locations where water and sediment samples were taken and the location of the geophysical and
geochemical profiles presented in Figs 2, 3, 4, and 7.
120°E
130°E
140°E
150°E
160°E
72°N
74°N
74°N
76°N
76°N
-3000 m
-2000m
-1000 m
K2
K6
K5
K7 K9
K1
K8
K3
K4 G6
AB
G11
G17
seismic line
A
A
B
AB
ULR
NSB
Fig.4a Fig.4b
Fig.3
OBH21-2
OBH21-2
OBH1-2
OBH1-2
OBH5-4
OBH5-4
Laptev Sea
Taimyr
New
Siberian
Islands
Lena Delta
Eurasia
Basin
Sediment thickness
> 2.5 s (TWT)
> 5.0 s (TWT)
Contour
G water samples
K/J sediment
samples
gas in water
section
OBH
Fig.9

4Indication for a petroleum system in the Laptev Sea, NE Siberia
3.5 km/s can be identified (Fig. 2). This break shows
rapidly decreasing amplitudes with offset and
disappears at offset distances greater than about 5km.
A second break after a time gap of about 2 seconds
shows a slightly higher velocity than the first one. These
observations indicate a thin, anomalous high velocity
layer that acts as an effective screen, e.g. frozen ground
over soil (Palmer, 1986). The high velocity layer is
thin in relation to the depth of the main refractor, and
is therefore not an adequate refractor itself. As a result,
the layer produces critically refracted arrivals
exhibiting a higher frequency content and rapid
attenuation (Donato, 1965). The amplitudes decrease
in such a way that the arrivals vanish before those from
a deeper layer with a higher seismic velocity can
overtake them, and the seismogram typically exhibits
delays or shifts (Palmer, 1986). We interpret the high
velocity break in the wide angle data as an expression
of a submarine permafrost layer.
The widespread existence of ice-bound permafrost
on the Laptev Sea shelf was first suggested by Solov’ev
(1981) and later confirmed by Hinz et al. (1998) and
Hubberten and Romanovskii (2003). Accordingly, ice-
bearing and partly also ice-bonded permafrost occurs
continuously on the shallow shelf of the Laptev Sea
(down to the 50-60m isobaths), and is replaced by
discontinuous permafrost further into the sea. On our
multichannel seismic data, a horizon can be recognized
which correlates in depth with the thickness of the
assumed permafrost layer estimated from the seismic
wide-angle data (Fig. 3). This possible expression of
the base of the permafrost appears to be widespread in
the southern Laptev Sea and also along structural highs
(Hinz et al., 1998). In seismic data from the northern
shelf, however, these indications for permafrost are
missing. From these observations, we infer that in
southern and central parts of the Laptev Shelf,
permafrost generally prevents the emission of gas.
Beneath the permafrost of the Laptev Shelf, gas
hydrates might be present (Delisle, 2000), also
immobilizing ascending gas. Free migrating
hydrocarbons are able to penetrate the permafrost zone
only locally along active faults or through permafrost
windows (taliks) which may be present on the Laptev
Sea shelf (Delisle, 1998). In permafrost-free areas in
the north, however, hydrocarbons can more readily
migrate from depth to the surface.
REFLECTION SEISMIC INDICATIONS
FOR MIGRATING GAS
Possible evidence for hydrocarbons on reflection
seismic data includes acoustic wipe-out zones and
reflection pull-downs or pull-ups. A gas cloud or
chimney can appear as a low-quality seismic response
(Fig. 3), with vertical bodies having varying
dimensions. Their shape and distribution may vary,
although cigar-shapes and association with fault
zones are common. Internally, they show a chaotic
low energy reflection pattern which is mainly caused
by scattering effects.
On reflection seismic data from the Laptev Sea
shelf, local zones of poor reflectivity are observed
which are interpreted as indications of migrating gas.
In Fig. 3, the approximately 500m wide example at
SP 1920, which is accompanied by a reflection pull-
up, can be correlated with a fault which cuts through
the entire Neogene sedimentary succession to the
basement. A second example (SP 2000-2030) is much
wider and shows no clear correlation with a fault.
Instead, this chimney appears to be correlated with a
bright spot at the depth at which marker horizon LS3
is interpreted i.e. 0.9 s twt (SP 2010).
While there are only a few examples of such
chimneys in the data from the southern and central
Laptev Shelf, zones of poor reflectivity are observed
more frequently in the northern Laptev Sea,
suggesting a greater amount of free gas in the
sediments (Fig. 4). Here, wide chimneys are also
present within rather undisturbed sediments
accompanied by reflection pull-ups. Partially bright
spots may be interpreted (e.g. Fig. 4a: SP 1560, 0.2 s
twt; Fig. 4b: SP 590-600, 0.2 s twt).
GAS IN NEAR-SURFACE SEDIMENTS
During the BGR97 Arctic cruise in the Laptev Sea,
near-surface sediments and water samples were
collected onboard the Russian icebreaker Kapitan
Dranitsyn. The samples were collected during the
short Arctic summer between 21st August and 6th
October 1997. Due to the presence of submarine
permafrost, sediment sampling was restricted to the
unfrozen top of the sedimentary section and only
about one half of our attempts to obtain sediment
cores were successful. Sediment cores were
recovered at nine locations (Fig. 1) using a free-fall
gravity corer. The thickness of the unconsolidated
and unfrozen top of the sediments, approximated by
the length of the sediment core, was usually less than
25cm, but longer cores of 96cm to 126cm length were
sampled at three locations (Fig. 1: K6, K7, K9). Sea
floor sediments in the Laptev Sea consist of dark-
olive coloured silts deposited during the Holocene
(Dehn et al., 1995).
The yield of adsorbed light hydrocarbons was
determined in 18 sediment samples, and stable carbon
isotope ratios of methane, ethane and propane were
measured in 24 samples (Table 1). A clear signal of
C1 to C5 hydrocarbons was detected in all sediments
with no obvious regional variations. The yield of
absorbed hydrocarbons in sediments from depths less

5
B. Cramer and D. Franke
Fig. 2. Data from wide-angle reflection/refraction measurements in the Laptev Sea showing a high-velocity
(3.0-3.5km/s) near-offset first break. The high velocity break was interpreted to result from a sub-sea
permafrost layer. For location, see Fig. 1.
than about 80 cm was quite low, between 32 and 91
ppbw (parts per billion by weight) for methane (Table
1). The yield of light hydrocarbons increased strongly
with sample depth to a maximum of CH4 = 356 ppbw.
The δ13C values of methane, ethane and propane
showed no systematic variation with depth (Table 1).
δ13C ranged between -37 and 43‰ for methane,
between -32 and -37‰ for ethane, and between 27
and 33‰ for propane.
Hydrocarbon gas concentrations combined with
stable isotope composition can be used to describe
the origin and level of thermal maturity of natural
gases (e.g. Rice and Claypool, 1981; James, 1983;
Whiticar, 1994). A “Bernard diagram” is widely used
to classify light hydrocarbon gases and is a plot of C1/
(C2+C3) versus methane δ13C (Bernard, 1978). Fig. 5
is a Bernard plot for our data from the Laptev Sea,
and shows that all of the adsorbed gases display a
similar ratio of methane to higher hydrocarbons
(average C1/(C2+C3) = 10.8 ± 1.2). This observation
points to a single source for the gas which, according
to the classification scheme in Fig. 5, is probably a
thermal gas generated from marine sedimentary
organic matter.
Thermogenic gas can be related to source rock
type and maturity by comparing the stable carbon
isotope signatures of methane versus ethane and of
ethane versus propane (e.g. Stahl and Carey, 1975;
James, 1983; Faber, 1987; Shen et al., 1988; Whiticar,
1994). In Fig. 6, the δ13C values of adsorbed
hydrocarbons from the Laptev Sea are plotted together
with the semi-empirical maturity relationships of
010
20
-20 30-30 -10 Distance [km]
0
2
4
Time - x/6 [s]
OBH 21-2
High velocity break
010 20
-20 30-30 -10 Distance [km]
0
2
4
Time - x/6 [s]
OBH 1-2
High velocity break
010
20
-20 30
-30 -10 Distance [km]
0
2
4
Time - x/6 [s]
High velocity break
OBH 5-4

6Indication for a petroleum system in the Laptev Sea, NE Siberia
NW SE
2.0
0
4.0
1.0
3.0
TWO-WAY TIME [s]
SP
1800 1900 2000 2100
GAS?
LS1
LS1
LS2
LS2
LS3
LS3
LS3
LS3
LS1
LS1
LS2
LS2
base of the
permafrost?
05
Km
BGR97-20a
time
migrated
base of the
permafrost?
Fig. 3. Interpreted seismic section from the central Laptev Sea shelf (line BGR97-20A), showing the
permafrost break and a small zone of poor reflectivity (probably associated with a fault), suggesting the
presence of gas within the Neogene basin fill. LS2 and LS3 represent marker horizons in the Cenozoic
stratigraphy (see Franke and Hinz, 2005); LS1 is the top-basement reflector. The data are shown without gain.
For location, see Fig. 1.
Berner and Faber (1996). The figure shows that, in
accordance with the Bernard classification (Fig.5), δ13C
values of ethane and propane display a trend similar
to that of thermogenic gas derived from marine source
rocks whose maturity ranges between 0.9 and 1.3%
vitrinite reflectance (Fig. 6A). By comparison, some
of the methane δ13C values (Fig. 6B) are shifted
towards isotopically heavier (less negative) values than
would be expected for this type of gas (up to +4‰
difference). This heavier isotope signature for methane
can be interpreted to be the result of mixing with a
thermogenic gas from a more mature terrestrial source
rock, indicated by the mixing corridor on Fig. 6b.
Thus in summary, hydrocarbon compositions and
stable carbon isotope signatures of light hydrocarbons
adsorbed in near-surface sediments in the Laptev Sea
indicate a signal of thermogenic gas generated from a
mature, deeply buried, marine source rock.
METHANE IN SEA WATER
The distribution of methane
in the central Laptev Sea
In order to differentiate the gas seepages from
background, the background distribution and isotope
characteristics of methane in water from the Laptev
Sea was assessed. Fig. 7 shows the typical distribution
of methane in sea water along a profile in the central
Laptev Sea without gas seeping from the underlying
sedimentary column. The profile traverses three
submarine valleys which represent the palaeo
channels of the Rivers Olenek, Western Lena and
Eastern Lena (Fig. 7).
Water temperatures along section A-B (Fig. 7a)
indicates stratification of the water body which is
typical for the summer period in the central Laptev
Sea, cold bottom waters being overlain by warmer
surface waters. The 0°C isotherm was measured at
water depths between 15 and 25m. Regional
variations in temperature were observed in the near-
surface water layers, and the temperatures were higher
by more than +2°C in regions above the submarine
valleys. This warmer water was also distinguished
by a lower salt content, and represents the outflow of
the Rivers Lena, Olenek and Yana (Dmitrenko et al.,
1995).
The methane content along profile A-B (Fig. 7b)
varies widely between about 20 nl/l and 500 nl/l.
Interestingly, the thermal stratification of the water
body is not directly mirrored by variations in the

7
B. Cramer and D. Franke
Fig. 4. Interpreted seismic sections from the northern Laptev Shelf (line BGR97-06), showing wide zones of
poor reflectivity, suggesting the presence of gas within the Neogene basin fill. LS1, LS2 and LS3 represent
marker horizons (see Franke and Hinz, 2005). The data are shown without gain. For location, see Fig. 1.
500 600 700 800
SW NE
2.0
0
4.0
1.0
3.0
TWO-WAY TIME [s]
SP
1500 1600 1700 1800
SW NE
2.0
0
4.0
1.0
3.0
TWO-WAY TIME [s]
SP
BGR97-06
time
migrated
BGR97-06
time
migrated
LS1
LS1
LS2
LS2
LS3
LS3
GAS?
LS1
LS1
LS2
LS2
LS3
LS3
LS3
LS3
LS1
LS1
GAS?
05
Km
05
Km
(a)
(b)

8Indication for a petroleum system in the Laptev Sea, NE Siberia
sample latitude N longitude E depth CH4C2H6C3H8C1/
(
C2+C3
)
δ13CH4δ13C2H6δ13C3H8
location cm b.s.f. ppbw ppbw ppbw ‰ ‰ ‰
K2 76° 23.90' 152° 47.47' 3 -37.6 -34 -27.2
12 -37.4 -32.2
K3 77° 04.12' 126° 11.62' 3 -39.7 -31.4 -29.5
K4 77° 38.14' 130° 08.62' 7.5 58.0 7.7 4.2 9.1 -39.7 -32.1 -28.3
K5 78° 06.22' 137° 07.42' 8.5 -39.9 -32.6 -31
K6 76° 39.90' 132° 50.51' 1 62.2 7.7 4.4 10.9 -38.5 -32 -30
10 -41.9 -36.5 -30
20.5 35.0 3.9 1.9 12.6 -42.8 -34.5 -31
30 55.5 7.0 3.1 11.5 -38.7 -33.4 -31
40 32.1 3.9 1.9 11.6 -42.1 -34.1 -32.6
50 52.5 5.9 2.6 12.8 -40 -34.1 -30.7
60 -39.6 -32.3 -29
80 134.7 18.1 9.9 10.2 -40.6 -32.8 -30.5
91 220.0 26.3 14.0 11.5 -39.8 -33.2 -30
K7 76° 41.78' 132° 26.31' 1 69.3 8.4 5.6 10.7 -39.7 -32.4 -31
30 82.8 10.7 7.1 10.0 -40.6 -30.9 -29
60 51.3 7.1 3.8 10.0 -40.9 -33.6 -30.3
106 249.0 26.9 16.0 12.4 -38.8 -30.1 -28.2
K8 75° 05.19' 128° 31.60' 2.5 50.3 8.0 4.6 8.5 -40.1 -32.2 -29.7
17.5 46.3 7.4 4.4 8.3 -41 -33.8 -33
K9 76° 40.62' 132° 49.39' 7.5 53.3 7.2 3.6 10.3 -40 -32.3 -28.9
17.5 91.4 11.6 6.5 10.7 -39.5 -31.4 -30.9
77.5 64.1 8.3 4.4 10.6 -39.6 -32 -28.4
101.5 355.8 44.4 23.5 11.0 -39.3 -31.5 -28.5
average 10.8 -39.9 -32.7 -29.9
std. deviation 1.2 1.3 1.4 1.4
1
10
100
1000
10000
-80 -60 -40 -20
δ
13
C - methane (‰)
thermal
microbial
marine
terrestrial
adsorbed gas
C /(C +C )
123
Fig. 5. “Bernard diagram” (i. e. plot of d13C methane versus C1/ (C2+C3)) for adsorbed gas from near-surface
sediments of the Laptev Sea.
Table 1. Geochemical characteristics of gas desorbed from near-surface sediments in the Laptev Sea. For
sample locations, sea Fig. 1. The yield of gas components is given in parts per million by weight (ppbw).
Sample depth is given in centimetres below sea floor (cm b.s.f.).

9
B. Cramer and D. Franke
Fig. 6. Plot of carbon isotope ratios of ethane versus propane (A) and methane versus ethane (B) for
adsorbed gas from sediment samples from the Laptev Sea. Maturity relationships for gas from marine and
terrestrial source rocks are taken from Berner and Faber (1996) with δ13C values of organic matter identified
by these authors. The mixing trend in the methane - ethane plot (B) indicates an admixture of thermally-
generated natural gas from highly mature terrestrial source rocks.
methane content, and the highest methane
concentrations were observed above the submarine
valley systems. This methane was characterized by
light stable carbon isotope ratios, with δ13C1 values
below -50‰ In general, the methane content
decreased and the δ13C1 values increased both above
and below these cells with high methane
concentrations, although the methane content again
increased within the deepest parts of the submarine
valleys. Here, methane becomes isotopically lighter
(Fig. 7c). The isotopically lightest methane with a δ13C1
of -72‰ was detected in the deepest sample from the
Western Lena submarine valley at a water depth of
50m.
The spatial distributions of methane concentrations
and methane δ13C ratios in the central Laptev Sea as
represented in Fig. 7 are not influenced by methane
seeping from the sedimentary column into the water.
Rather, the distribution can be interpreted in terms of
balanced microbial methane generation and microbial
methane oxidation within the water column. The
dominance of one or other of these processes
0.6
1.0
1.5
2.0
VR (%)
marine
source rock
VR (%)
marine
source rock
0.5 1.0
1.5
2.0 2.5
VR (%)
terrestrial
source rock
VR (%)
terrestrial
source rock
-45
-35
-25
-15
13
C - ethane (‰)
-40 -30 -20 -10
13C - propane (‰)
adsorbed gas
maturity lines for
natural gas (Berner
& Faber 1996)
-50
-40
-30
-20
-45 -35 -25 -15
0.6
1.0
1.5
2.0
0.5 1.0 1.5 2.0 2.5
13
C - ethane (‰)
13C - methane (‰)
methane
in water
at G6/G11
mixing
A
B
δ
δ
δ
δ

10 Indication for a petroleum system in the Laptev Sea, NE Siberia
40
40
40
20
20
20
depth (m)depth (m) depth (m)
AB
Olenek
valley Western Lena
valley Eastern Lena
valley
(c) C (‰)δ
13
(a) T (°C)
T (°C)
δ13CH (‰)
4
CH (nl/l )
4H2O
> 4
< -60
> 400
3 .. 4
-60 .. -52.5
2 .. 3
200 .. 300
300 .. 400
1 .. 2
-52.5 .. -45
0 .. 1
-45 .. -37.5
100 .. 200
-30 .. -37.5
50 .. 100
< 0
> -30
0 .. 50
(b) CH (nl/l)
4
Fig. 7. Measured water temperatures (a), methane concentrations (b) and stable carbon isotope ratios (c) of methane dissolved in water along a
transect through the central Laptev Sea. For location, see Fig.1.

11
B. Cramer and D. Franke
0
50
100
150
200
250
-70-80 -60-60 -50-40 -40-20 -70 -60 -50 -40 -30 -20
00
5
10
10
15
20
20
25
30
30
35
40
00 0200100 200400200 400600300 600 800 1000
(b) C (‰)δ
13
(a) C (‰)δ
13
(c) C (‰)δ
13
concentration (nl/l) concentration (nl/l)concentration (nl/l)
water depth (m)
water depth (m)
water depth (m)
thermal gas
-36.9 ‰
thermal gas
-37.6 ‰
microbial gas
-68.6 ‰
G6G17 G11
conc.
conc.
conc.
δ
13
C
δ
13
C
δ
13
C
Fig. 8. Methane concentration and δ13C values of methane in sea water at three locations (for locations see
Fig. 1). Values at location G17 are typical for the Laptev Sea with a predominance of microbial methane. At
locations G6 and G11, the signature of deep, thermally-generated gas emanating into the water column was
detected.
determines whether methane concentrations and stable
carbon isotope ratios are high or low. Clearly, the best
conditions for microbial methane generation occurred
at the interface between the upper and lower water
layers and in the deepest parts of the submarine
valleys.
In addition, it is very likely that some microbial
methane originating from swamps and tundra
environments on the Siberian continent was washed
into the Laptev Sea with river run-off streams.
Gas seeps in the northern Laptev Sea
In contrast to the microbial methane signature (i.e.
light stable carbon isotope ratios) at locations of high
methane concentration (Fig. 8a: G17), seeps of
thermogenic gas from the subsurface can be
distinguished by comparatively heavy isotope
signatures, as shown for the adsorbed gases in Figs 5
and 6. Gas samples from two locations in the Laptev
Sea with high methane contents showed such
signatures (Fig. 8b, c; G6 and G11).
At location G6 (Fig. 8b), the highest methane
concentration (510 nl/l) was detected directly above
the sediment surface, indicating gas sourced from
below. Here, the δ13C value of methane (δ13C1 = -37‰)
is almost the same as that of thermal methane adsorbed
in near-surface sediments (Fig. 6). Within the BGR97-
08 profile, the G6 location is covered by reflection
seismic data (Fig. 9). From this, it is obvious, that G6
is underlain by the Kotel’nyi Horst, which is separated
from the narrow Neben Basin and the New Siberian
Basin (see Franke and Hinz, 2005) by deep-reaching
faults. These faults probably provide preferential
pathways for upwards migrating gas. In fact, zones
of disturbed seismic reflectivity in the thin sediment
layer above the horst structure close to location G6
(Fig. 9, SP 880) indicate upward migrating gas near
the sediment surface. We propose that this gas reached
the water column and was detected by our
geochemical analyses at G6.
At the other location with an isotope signal of deep
thermogenic gas (G11), the highest methane
concentration (840 nl/l) was observed at a water depth
of 100m (Fig. 8). The location of G11 coincides with
a major SW-NE trending transfer or strike-slip fault,
namely the Sever’nyi Transfer or Northern Fracture
(e.g. Drachev et al., 1998, 1999; Franke et al., 2001).
This fault may also provide a pathway for upward
migrating gas.
The fact that the highest methane concentration at
G11 was not measured at the sea floor, as would be
expected for seeping gas, could be explained in terms
of a laterally asymmetrical plume of methane seeping
from the steep continental slope.

12 Indication for a petroleum system in the Laptev Sea, NE Siberia
DISCUSSION
The permafrost layer beneath the Laptev Sea is up to
several hundred metres thick and was formed
subaerially during a cold period in the Pleistocene.
The presence of submarine permafrost may influence
the migration of hydrocarbons to the surface as
permafrost is believed to act as an effective seal.
However, gas hydrates could be stable within and
beneath the permafrost of the Laptev Sea and could
contain considerable volumes of gas (Delisle, 2000).
Remotely sensed atmospheric plumes above the New
Siberian Islands, with temperatures as low as –45oC,
were believed to be effected by methane degassing
from permafrost and the accompanying hydrates at
the sea floor (Clarke et al., 1986). By contrast,
however, Kerr (1992) reported air-borne
measurements in this area showing no elevated
methane contents and no plumes rising from the sea
surface.
The measurements presented here point to the
influence of permafrost on gas migration from the
sedimentary column beneath the Laptev Sea. The two
gas seepages detected by geochemical methods are
located in the northern part of the Laptev shelf (Fig.
1) where the permafrost is known to be absent. Ilyin
et al. (1998) also detected the highest concentrations
of liquid hydrocarbons in surface sediments in the
north of the Laptev Sea. In contrast to these authors
who only discussed anthropogenic sources for the
hydrocarbons, we interpret them to be traces of natural
seepages from depth. At the transition from the
shallow shelf to the slope, water depths in Pleistocene
times were sufficient to prevent the development of
permafrost. Thus, upwardly migrating hydrocarbons
in this area are not prevented from seeping up into
the water column.
The presence of thermogenic hydrocarbons in
near-surface sediments in the Laptev Sea may indicate
the presence of a source rock containing marine
organic matter (Fig. 6), indicating some potential for
oil exploration in this area. However, it is not
consistent with the presence of terrestrial organic
matter within the Cenozoic succession, as was
proposed for example by Nikitin et al. (1999), which
would probably be more gas prone. In fact, ´13C values
of methane and ethane from surface sediments provide
some evidence for an admixture of hydrocarbons from
a mature terrestrial source rock (Fig. 6). Its
significance in comparison to the marine source rock,
however, appears to be low. Direct confirmation of
our surface geochemical observations is not possible
at present because no deep wells have yet been drilled.
However, reports of marine claystones of Cenozoic
age from the New Siberian Islands (Kos’ko and
Trufanov, 2002) and the Lomonosov Ridge
(Backmann et al., 2004) tend to support our
geochemical data, possibly indicating the oil-prone
nature of the Laptev Sea basins.
CONCLUSIONS
During the BGR97 cruise to the Laptev Sea, near-
surface indications of petroleum migrating from the
underlying sedimentary column were investigated
using geochemical and geophysical methods. The
main results can be summarized as follows:
•On reflection seismic profiles, zones of poor
reflectivity and chimneys indicate the existence of an
active petroleum system within the sedimentary basins
of the Laptev Sea. These features suggest there is
ongoing hydrocarbon generation and migration in
depths down to at least 2-3 s twt.
•The composition and stable carbon isotope data
of adsorbed hydrocarbons from near-surface
sediments from the Laptev Sea display a signature of
thermally-generated gas from a marine source rock
within the maturity range of peak petroleum
generation. Due to the widespread distribution of this
signature within near-surface sediments, these rocks
can be assumed to occur over large areas.
•Seepages of thermal gas from the sedimentary
column into the sea water were detected at two
locations at the northern margin of the shelf, indicating
the rather uneven distribution of petroleum seeps in
the Laptev Sea.
•This uneven distribution probably reflects the
occurrence of sub-sea permafrost. The south and the
centre of the Laptev Sea is underlain by a permafrost
layer several hundreds of metres thick, generally
preventing petroleum from seeping into the water. In
the north, the permafrost is absent, allowing the
migration of hydrocarbons into the water column.
ACKNOWLEDGEMENTS
We are grateful to Captain Agafonov and the crew of
the icebreaker IB Kapitan Dranitsyn for their excellent
cooperation and for the completion of a great deal of
hard work under Arctic conditions. Funding for this
study was provided by the Federal Institute for
Geosciences and Natural Resources (BGR). The
comments of an anonymous reviewer on a previous
version are acknowledged with thanks.
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13
B. Cramer and D. Franke
2.0
0
4.0
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TWO-WAY TIME [s]
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LS1
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16 Indication for a petroleum system in the Laptev Sea, NE Siberia