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ISSN 0024-4902, Lithology and Mineral Resources, 2021, Vol. 56, No. 3, pp. 236–247. © Pleiades Publishing, Inc., 2021.
Russian Text © The Author(s), 2021, published in Litologiya i Poleznye Iskopaemye, 2021, No. 3, pp. 243–256.
Polycyclic Aromatic Hydrocarbons in Rocks
and Soils of the Siljan Impact Crater, Sweden
Yu. I. Pikovskya, *, N. I. Khlyninaa, *, and V. G. Kutcherovb, **
a Faculty of Geography, Lomonosov Moscow State University, Moscow, 119899 Russia
b Gubkin University, Moscow, 119991 Russia
*e-mail: lummgu@mail.ru
**e-mail: vladimir.kutcherov@energy.kth.se
Received April 3, 2020; revised August 4, 2020; accepted December 23, 2020
Abstract—New data were used to analyze the distribution of polycyclic aromatic hydrocarbons (PAH) in the
lithological complex and soils of the Siljan impact crater area (Scandinavian Shield, central Sweden). Ten
individual unsubstituted PAHs were identified, including diphenyl, fluorene, phenanthrene, anthracene,
pyrene, chrysene, fluoranthene, benzo[a]anthracene, benzo[a]pyrene, benzo[ghi]perylene, as well as a num-
ber of substituted naphthalene homologs. The PAHs were analyzed using the Shpolsky spectroscopy. The
studies were carried out at the crater edge (an annular morphostructural depression) and in the adjacent
areas. The depression is characterized by traces of hydrothermal activity and modern oil and gas seepages. In
the gas seepage area at depths of 267‒485 m, nine out of eleven studied PAHs were identif ied (concentration
from 20 to 890 μg/kg in total) in the igneous rock complex. Sedimentary rocks at depths from 10 to 250 m
contain only naphthalene homologs, phenanthrene, and pyrene. The PAH concentration in sedimentary
rocks in the oil seepage area is two times higher than in the gas seepage area, and its composition (naphthalene
homologs, phenanthrene, pyrene, diphenyl, chrysene) is close to the PAH composition in oil. In general,
soils of the crater show a hydrocarbon dispersal halo, which is presumably caused by the oil and gas seepages
and traces of hydrothermal activity. Characteristics of this halo are differentiated in space and make it possible
to predict hydrocarbon seepages in unexplored areas.
Keywords: Siljan impact crater, oil and gas seepages, polycyclic aromatic hydrocarbons
DOI: 10.1134/S0024490221030068
INTRODUCTION
Polycyclic aromatic hydrocarbons (PAH) in natu-
ral objects bear important information. These hydro-
carbons have global distribution and different genesis.
They serve as geochemical indicators of lithospheric
and anthropogenic hydrocarbon fluxes, markers of
their migration in the Earth’s crust and environment
(Geokhimiya …, 1996). Associations of individual
PAHs are considered by different authors as evidences
for the presence of petroleum reservoirs (Pikovsky
et al., 1991; Calhoum, 1995), indicators of the inf lu-
ence of thermal flows on the organic matter of sedi-
mentary rocks and bottom sediments (Sorokin et al.,
1986; Alexsander et al., 1986), signs of the hydrother-
mal transformation of dispersed organic matter and
formation of “hydrothermal oil” (Simoneit, 1986,
1990; Kawka and Simoneit, 1990; Garrigues et al.,
1988; Radke, 1988), and proxies of haloes of mineral
formation and hydrothermal alteration of rocks (Flor-
ovskaya et al., 1968; Kaminsky et al., 1985; Geptner
et al., 1999; Chernova et al., 1999, 2001; Fetzer et al.,
1995). Relationships between individual PAHs are
used to characterize the anthropogenic sources of
environmental change (Tsibart and Gennadiev, 2013;
Khaustov and Redina, 2017, Konstantinova et al.,
2018).
The PAHs have been studied in different natural
conditions, but virtually no data are available on their
behavior in such geological objects as impact craters
representing the explosion annular structures from
hundreds of meters to hundreds of kilometers in size,
which were produced in different geological epochs by
impact of large cosmic bodies. Impact-induced explo-
sions formed an additional fault and fracture network
and served as local heat sources, which triggered the
geodynamic and hydrothermal activity during both
impact and subsequent geological time up to the pres-
ent (Donofrio, 1998; Osinski et al., 2013).
Impact craters are characterized by peculiar mor-
phology and signs of high-pressure rock melting. They
are associated with syngenetic and epigenetic ore and
non-ore deposits (Melosh, 1994; Masaaitis, 2008).
Over 10 economic petroleum reservoirs have been dis-
covered in the impact craters and adjacent areas,
including large reservoirs confined to the brecciated
sedimentary rocks and fractured rocks in the sedimen-
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
POLYCYCLIC AROMATIC HYDROCARBONS IN ROCKS AND SOILS 237
tary cover and crystalline basement (Kutcherov and
Krayushkin, 2010). Impact craters are regarded as
promising for the discovery of new hydrocarbon
resources (Donofrio, 1981, 1998; Curtiss and Wavrek,
1998).
Wild fires have been considered for a long time as a
main source of the global PAH distribution over
impact craters (Laf lamme and Hites, 1978; Venkate-
san and Dahl, 1989). The elevated amount of carbon
buried in rocks of impact craters at the Creta-
ceous‒Cenozoic boundary was thought to be related
to the fallout of soot particles during global fires trig-
gered by meteorite falls (Wolbach et al., 1988). How-
ever, the fire hypothesis has been strongly questioned.
First, the studies of conserved soils subjected to fires
showed that the burning of forests and grasses is
accompanied by significant dispersal of the burning
products and erosion. Thus, the significance of long-
term fire-induced PAH accumulation is significantly
overestimated (Tsibart and Gennadiev, 2011). Second,
the study of morphology of carbon particles and PAH
composition in impact craters indicates the combus-
tion of hydrocarbons, not plant biomass (Belcher
et al., 2009). One of the important tasks in decipher-
ing the nature and indicator role of PAH in impact
craters is the study of all geochemical processes that
occurred in these structures during their geological
history.
The aim of this work is to analyze the lithologi-
cal‒geochemical features of PAH distribution and
their possible nature in the Europe’s largest Siljan
impact crater (Central Sweden), which is known for its
modern oil and hydrocarbon gas seepages and traces
of hydrothermal activity. First data were obtained to
identify and quantify the unsubstituted individual
PAHs in the magmatic and sedimentary rocks that
compose the upper 500-m part of the geological
sequence of the impact structure, as well as in soils of
annular depression surrounding the crater.
GEOLOGICAL CONDITIONS
AND PETROLEUM POTENTIAL
OF THE SILJAN IMPACT CRATER
Geological Structure and Morphology of the Crater
The Siljan impact crater is an annular structure in
the central part of the Scandinavian crystalline shield.
Host rocks are Archean and Proterozoic magmatic
and metamorphic rocks. It is suggested that the struc-
ture was produced by the impact of a large bolide (Fig. 1).
Based on isotope data, this event is dated by the termi-
nal Devonian (360 Ma) (Costano, 1993). The center
of the annular structure has coordinates of 61° N and
15° E. The central part of the structure is complicated
by an explosion-caused compensation uplift in the
form of Proterozoic granite massif, which is sur-
rounded by tectonic low—an annular depression
occupied by uneroded remnants of the Lower Paleo-
zoic sedimentary rocks, which in the past covered the
entire crater. The crater, including the surrounding
depression, has a present-day diameter of 52 km,
while the explosion funnel has a diameter of 26–34 km
and a depth of 3–4 km. Sedimentary rocks are made
up of Ordovician limestones, sandstones, siltstones,
and mudstones as well as Silurian limestones and
mudstones. They compose brecciated blocks with lin-
ear size from a meter to kilometer, locally with over-
turned stratigraphic succession and maximum thick-
ness up to 350 m based on seismic data (Collini, 1988).
The width of exposed sedimentary rocks varies from
9–10 km in the west to 5–7 km in the east of th e st r uc-
ture. The sedimentary rocks are covered by a 300-m
Quaternary fluvioglacial sequence. The present-day
structure of the crater is the final product of complex
processes that remain not completely understood yet
(Lund et al., 1988). The annular depression is ascribed
to the system of concentric faults, which provided the
subsequent activation of tectonic movements.
The annular zone of depressions around the
uplifted central block represents a morphostructural
boundary of the Siljan crater. This is the most mobile
and likely most permeable part of the crater. It is trun-
cated by transverse and extensional boundaries of
small blocks. The transverse boundaries of morpho-
structural blocks of different rank come from different
directions to the annular depression. The orientation
of block boundaries mainly coincides with the orien-
tation of faults (Pikovsky et al., 2017).
Deep Gravberg-1 and Stenberg-1 Wells were
drilled, respectively, in the northeastern and central
parts of the granite block in the central uplift of the
crater. The Gravberg-1 Well (6779 m deep) recovered the
following sequence of Precambrian crystalline rocks
(from top downward): granite porphyries (1635 Ma), Sil-
jan and Yarna granites (1760–1670 Ma). These rocks,
in turn, are penetrated by younger quartz monzonites
(1450 Ma) and dolerite sills (970–900 Ma). According
to drilling data, the intensity of rock destruction
strongly varies, but usually decreases with depth (Cos-
tano, 1993).
Hydrothermal Phenomena and Petroleum Potential
Hydrothermal phenomena. Like other impact cra-
ters, the Siljan crater shows numerous evidences for
hydrothermal activity, which proceeded both prior to
and after bolide impact (Komor et al., 1988). These
evidences are expressed in the secondary mineral
assemblages, which were precipitated in rock fractures
and cavities, as well as in the character of fluid inclu-
sions in minerals. Prior to meteorite impact, the
hydrothermal activity was caused by the heat of granite
and dolerite intrusions. Traces of this activity are
recorded in the high-temperature (up to 350–750°C)
inclusions in granite grains. The most intense hydro-
thermal activity was related to the local heating of
rocks and waters, which is supported by the formation
238
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
PIKOVSKY et al.
of quartz veins in the blocks of sedimentary megabrec-
cia. Inclusions of low-temperature hydrothermal liq-
uids in crystals, as well as peculiar deformations of
rocks and minerals with traces of fluid inclusions,
indicate that the hydrothermal system was activated
long after the cosmic impact (Komor et al., 1988). The
hydrothermal activity in the Siljan crater was also
expressed in the formation of lead‒zinc deposits in
the eastern (Boda) and southwestern (Soilerön) parts
of the annular depression filled with Paleozoic sedi-
mentary rocks. Isotope data indicate that the ore mat-
ter was extracted by hydrothermal solutions from host
granites and sedimentary rocks (Johansson, 1984).
The Martanberg copper deposit and Slattberg nickel
deposit were mined in the vicinity of the Siljan crater,
10 and 15 km southeast of the annular depression,
among the brecciated Proterozoic granite gneisses,
amphibolites, and quartzites. Coarsely crystalline cal-
cite veins, locally developed quartz crystals, and bitu-
men occurrences at these deposits also indicate post-
crater events (Wickman, 1994).
Post-impact hydrothermal phenomena on the Sil-
jan crater systematically occur mainly in the annular
depression, which is filled by brecciated Paleozoic
sedimentary rocks. In these rocks, hydrothermal min-
erals are associated with oil fluid, which migrates
along fractures or is present in fluid inclusions
together with vapor and water bubbles (Hode et al.,
2003). On the outer boundary of the central uplift, the
hydrothermal minerals are mainly represented by
quartz and epidote, which form veins and fill cavities
in breccias related to granitic rocks. The Paleozoic car-
bonate rocks are also intersected by calcite fluorite,
galena, and sphalerite veins.
In the oil seepage zone in the eastern part of the
annular depression, fluid inclusions in rocks and min-
erals contain liquid oil. The fluid inclusions are repre-
sented by at least two generations. One generation is
mainly represented by aqueous inclusions forming iso-
lated groups in the calcite and fluorite crystals, while
other inclusions are dominated by oil inclusions
arranged along healed fractures. Water and oil occur
either in the individual inclusions of one generation or
as separate phases in the same inclusion. In the f luid
inclusions in calcite and fluorite, hydrocarbons repre-
sent a component of two- and three-phase systems.
The two-phase system consists of vapor and liquid
phases (water or oil), while the three-phase system
includes vapor, water, and oil liquid. The homogeni-
zation temperature of liquid inclusions corresponds to
the low-temperature hydrothermal system and varies
from 75 to 137°C (Hode et al., 2003).
Oil seepages within the eastern part of the Siljan
annular structure have long been known. Its artisanal
mining from shallow wells in the Ordovician lime-
stones on Mt. Osmund has been mentioned as early as
the 18th century (Kudryavtsev, 1959). At present, the
petroleum seepage can be observed on the east of the
annular depression zone in the Solberg limestone
quarry. Oil floats above groundwater at a depth of 1–
1.5 m. Its samples can be collected from shallow wells
drilled in the quarry bottom. Oil incrustations are seen
in the vertical fractures traced over the entire height of
exposed high quarry walls. According to the author’s
data, the oil density is 860 kg/m3. According to (Vlier-
boom et al., 1986), the oil is heavier with a density of
16°–18° API (around 950 kg/m3).
Hydrocarbon gas seepages were found during the
drilling of hydrogeological wells in the depression
zone in the western part of the crater (Mt. Mora area).
In many wells, gas oversaturates the ground and frac-
ture waters and is released as free gas. According to
author’s data, the free gas has hydrocarbon (CН4
90‒94%, heavy hydrocarbons 0.0002%, nitrogen 6–
9%, helium 0.01–0.02%) and nitrogen‒hydrocarbon
(CН4 45–65%, heavy hydrocarbons 0.001–0.12%,
nitrogen 34–44%, helium 0.5–0.8%) compositions.
In the stem of the deep Gravberg-1 Well drilled in
the central granite block, free gases were absent. Drill-
ing mud contains two types of absorbed hydrocarbon
gas, which are related to the dolerite and granite intru-
sions. It was assumed that the hydrocarbons are
formed through the Fisher‒Tropsch type reaction
(Laier, 1988) or owing to the activity of microorgan-
isms (Drake et al., 2019). According to (Castaño,
1993), the central part of the crater releases methane
traces with light carbon isotope composition (δ13C
‒60‰), while the oil seepage zone in its eastern part
contain methane with heavy carbon isotope (δ13C
‒45‰).
SAMPLES AND METHODS
The studies in the Siljan crater area were carried
out mainly along a wide annular depression zone,
which is associated with petroleum seepages and
hydrothermal ore formation. Core samples (in total
43 samples) from two hydrogeological wells were ana-
lyzed in this zone: from 485-m Mora Well drilled in
the gas seepage area and 257-m Solberg Well drilled in
the oil seepage area. In addition, we studied samples of
podzolic soils collected from a depth of 1 m in the
depression zone and from areas adjoining its boundar-
ies from the external and internal walls of the crater (in
total, 194 samples). We also analyzed oil from the oil
seepage zone in the Solberg quarry and mud samples
from the deep Stenberg-1 Well (5 samples).
The PAH samples for studies were extracted by
normal hexane from the air-dry aliquots of rocks and
soils mechanically crushed to 0.25 mm at room tem-
perature. The hexane extract represents bitumen, into
which hydrocarbons, including PAHs, resins, and
other light heteroatomic compounds are transformed.
The application of this solvent allows analyzing PAHs
without additional procedures of solvent purification,
because hexane simultaneously serves as a matrix for
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
POLYCYCLIC AROMATIC HYDROCARBONS IN ROCKS AND SOILS 239
the frozen PAH molecules during analysis by the
Shpolsky spectroscopy.
The content of hexane bitumens was determined
using a modified V.N. Florovskaya technique of lumi-
nescence‒bituminological analysis on a Flyuorat-02-
2M liquid analyzer (Lyumeks, St. Petersburg) with
exchangeable color filter.
The identification and quantitative determination
of individual PAHs were carried out in hexane bitu-
mens by the Shpolsky spectroscopy using spectrof luo-
rimetric analysis in a frozen n-hexane matrix at
‒196°C (Alekseeva and Teplitskaya, 1981; Rovinsky
et al., 1988; Geokhimiya …, 1996; Nurmukhametov
et al., 2015). The analysis was carried out on a Fly-
uorat-Panorama device with an additional LM-3
monochromator. The PAH concentrations were cal-
culated from the height of peaks of characteristic flu-
orescence wavelength by comparison with a certified
SRM 2260a standard (USA) containing PAH mixture
solution in toluene.
The luminescence output in the UV range
(300‒340 nm) was measured at excitation within
240‒280 nm. The total luminescence output in the
visible region was measured within wavelength of
400‒580 nm at excitation with 360 nm maximum.
Concentrations of matter in the solution were calcu-
lated using references with close luminescence char-
acteristics.
Identification was mainly performed for the unsub-
stituted PAHs that are characteristics of objects sub-
jected to thermal influence typical for impact craters
and, in particular, hydrothermal phenomena.
In all samples, we identified and determined quan-
titatively 11 unsubstituted individual PAHs, including
the sum of alkyl-substituted homologs of two-nuclear
naphthalene. The PAHs in crude oil were identified in
the diluted solutions in n-hexane (up to 10‒6 g/mL),
similarly to the analysis of hexane bitumens from rocks
and soils. The quantitative analysis of PAHs in oil was
not carried out.
The studied complex of individual PAHs and some
luminescence‒spectral characteristics are given in
Table 1.
All PAHs analyses were carried out in the Labora-
tory of Carbonaceous Matter of Biosphere, Faculty of
Geography, Lomonosov Moscow State University.
The Shpolsky spectroscopy was chosen as the most
optimal method, which provides signal from intact
individual molecules, as well as large-scale studies of
PAHs in geochemical samples. The high sensitivity
and selectivity of the method make it possible to use
small aliquots (up to 1‒2 g). This method reliably
identifies the unsubstituted and substituted individual
PAHs in minerals, rocks, and soil in contents more
than 0.5 ng/g. A method of “spectral fractionation”
involving the choice of selective excitation and charac-
teristic lines of fluorescence for each individual com-
pounds was applied for the diluted PAH solutions to
avoid losses during the preliminary chromatography
(Alekseeva and Teplitskaya, 1981).
RESULTS
PAH Distribution in the Upper Part of Geological Section
of the Siljan Impact Crater
Table 2 presents data on the distribution of bitu-
mens and PAHs in the core of the Mora (gas seepage
area) and Solberg (oil seepage area) hydrogeological
wells.
The 500-m Mora Well was drilled in the western
part of the crater in the gas seepage area in the annular
depression zone. In this and neighboring wells, the
ground and fracture waters are saturated with the dis-
solved hydrocarbon gas freely released on the surface.
Table 1. Characteristic luminescence lines (nm) for identification of the studied PAHs (Alekseeva and Teplitskaya, 1988)
PAH Molecular
formula
Number of rings
in molecule Excitation lines, (λe) nm Fluorescence line, (λfl) nm
Naphthalene homologs C10Н82 290 320‒328
Diphenyl C12Н10 2 278 315
Fluorene C13Н10 3288 301.6
Phenanthrene C14Н10 3 255, 293 346
Anthracene C14Н10 3 253, 357 377.4
Pyrene C16Н10 4337 372
Chrysene C18Н12 4 269 360.0/360.4
Benz[a]anthracene C18Н12 4290 383.9
Fluoranthene C16Н10 4362 437
Benz [a]pyrene C20Н12 5367 403
Benz[ghi]perylene C22Н12 6367 419
240
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
PIKOVSKY et al.
The well recovered sedimentary rocks represented by
Ordovician and Silurian limestones and mudstones
within 10‒250 m and Precambrian magmatic rocks
(porphyrites and gabbronorites) within 267‒485 m.
The total concentration of bitumens and PAHs in sed-
imentary rocks is much higher than those of magmatic
rocks. At the same time, the composition of individual
PAHs in the magmatic rocks is more diverse than in
sedimentary rocks.
Magmatic rocks with a frequency of 80% and more
contain naphthalene homologs, phenanthrene,
pyrene, and chrysene. Fluorene, diphenyl, fluoran-
thene, and benz[ghi]perylene were identified in the
studied samples with a frequency of 21‒64%. One
sample contains benz[a]anthracene. Fluoranthene in
gabbros and porphyrites accounts for from 19 to 38%
of the total PAHs, while diphenyl in some samples
accounts for 20‒72%. Only naphthalene homologs
and phenanthrene occur in sedimentary rocks with a
frequency over 80%. Pyrene and chrysene were identi-
fied in 50‒67% samples. Only 2 of 12 samples contain
fluorene, benz[a]anthracene, benz[a]pyrene, and
benz[ghi]perylene in amounts of 1–2 μg/kg rock.
Diphenyl and fluoranthene were not found in sedi-
mentary rocks.
The Solberg Well was drilled in the oil seepage area
in the eastern part of the annular depression. The well
passed through Ordovician limestones, mudstones,
and breccias and recovered granites at a depth of 298.5 m.
Sedimentary rocks from the Solberg Well have an
order of magnitude higher bitumen content and two
times higher PAH contents compared to the sedimen-
tary rocks from the Mora Well. Most frequently (in
71–100% cases), sedimentary rocks from the Solberg
Well contain naphthalene, phenanthrene, and pyrene
homologs. Diphenyl was found in 50% of samples,
while chrysene, in 29% samples. Fluoranthene,
benz[a]anthracene, benz[ghi]perylene, and anthra-
cene were identified in three or four samples out of
seventeen studied. Oils from this area contain naphtha-
lene homologs, phenanthrene, anthracene, pyrene,
chrysene, benz[a]anthracene, and benz[ghi]perylene.
In general, the PAH composition in the Solberg Well
is similar to that of oil from this area. In particular,
anthracene present in oil was identified only in indi-
vidual rock samples from the “oil” Solberg Well, but
was not found in rocks from the “gas” Mora Well.
Table 2. PAH distribution in the upper part of the geological section of the Siljan impact crater (μg/kg)
(min) minimum value; (max) maximum value; (me) median; (N) occurrence frequency of this compound (%) in the total number
of samples; (0) compound is either unidentif ied or its concentration is below 1 μg/kg.
Parameters
Hexane
bitumen, mg/kg
Total PAH
Naphthalene
homologs
Diphenyl
Fluorene
Phenanthrene
Anthracene
Pyrene
Chrysene
Fluoranthene
Benz[a]anthracene
Benz[a]pyrene
Benz[ghi]perylene
Mora Well (gas field)
Depths 10‒246 m, sedimentary rocks, 12 samples
min 9 1200 0.02 0 0 0 0 0 0 0 0 0 0
max 2010 16600 8100 0 2 16300 000 0 30 140 0 2 1 1
me 400 5800 400 0 0 2400 0 2 4 0 0 0 0
N, % 100 100 100 0 8 83 0 67 50 0 17 8 17
Depths 267‒485 m, magmatic rocks, 14 samples
min 720 3 00 00100000
max 315 890 680 50 4 820 0 10 8 200 2 0 2
me 7523020 13 1700220001
N, % 100 100 93 50 21 79 0 100 86 43 7 0 64
Solber Well (oil field)
Depth 10‒257 m, sedimentary rocks, 17 samples
min 700 1600 300 0 0 0 0 0 0 0 0 0 0
max 36 000 76 600 324000 25900 0 17500 –300 2500 70 1 0 940
me 5400 7000 2300 0 0 1600 0 3 0 0 0 0 0
N, % 100 100 100 47 0 82 18 71 29 18 24 0 18
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
POLYCYCLIC AROMATIC HYDROCARBONS IN ROCKS AND SOILS 241
PAH Distribution in Soils
of the Siljan Impact Crater Area
Previous (prior to deep drilling) analysis of soil
samples from the Siljan Crater revealed the presence
of geochemical halo with clear traces of methane and
heavy hydrocarbons in soils. The gas halo coincides
with the distribution of vanadium and nickel in soils
and only partially with sedimentary rocks from the
annular depression zone. The halo character indicates
its relation with migration processes in deep parts of
the impact crater (Karlsson, 1988).
To study the possible PAH migration from litho-
logical complex of the crater to the surface and their
retention in soils as on sorption barrier, we studied the
areal PAH distribution in soils of the annular depres-
sion zone and in the adjacent areas. Soil samples were
collected from a depth of 1 m with a hand drill.
The mineral component of podzolic soils and soil-
forming debris is mainly represented by fluvioglacial
sands and light loam, which cover the underlying orig-
inal magmatic and sedimentary rocks. The content of
organic carbon in 70% of studied samples was less
than 1%. The syngenetic formation of PAHs in such
objects is hardly probable. Sources of anthropogenic
PAH influx are absent in soils of this area.
In bitumens extracted by hexane from soils, all
individual PAHs were identified and then analyzed.
Soils of the studied area of the Siljan impact crater
are characterized by the uneven PAH distribution. The
total content of identified PAHs in soil samples varies
within four orders of magnitude from 8 to 5800 μg/kg,
with their average content over an area from 157 to more
than 1800 μg/kg, while the content of individual PAHs in
soils varies from a unity and lower to 5700 μg/kg.
The widest spread PAHs in soils are naphthalene
homologs (occur in 100% samples) and phenanthrene
(88‒100% samples).
The next in the detection frequency in soils are
diphenyl and fluorene (43‒90% samples), pyrene
(30‒82% samples), chrysene (27‒63% samples), and
benz[ghi]perylene (10‒64% samples).
The PAH distribution in soils is controlled by the
position of the area in the crater structure, the compo-
sition of underlying bedrocks, as well as epigenetic
geochemical processes that have occurred in the crater
throughout its geological history. For better under-
standing obtained data, the sampling area was subdi-
vided into nine sites, which differ in the position in the
block structure of the crater, type of underlying bed-
rocks, and peculiarities of oil and hydrocarbon seep-
ages and hydrothermal ore occurrences (Table 3).
Data presented in Table 4 show that the detection fre-
quency of definite individual PAH and its concentra-
tion in soils is correlated neither with position of sam-
pling locality in the crater structure nor with the type
of underlying bedrocks.
Site 1 with gas seepages and adjacent external site 6
are almost identical to soils in the distribution of indi-
vidual PAH. Fluorene and chrysene are most devel-
oped in soils (63‒68% samples) after phenanthrene
and naphthalene homologs.
Sites 2, 5, and 7 united into “oil field” are charac-
terized by six times higher average contents of phenan-
threne, naphthalene homologs, and total PAHs and
3‒4 times higher average concentrations of diphenyl,
fluorene, pyrene, and chrysene compared to the cor-
responding hydrocarbons of “gas field”. In soils of the
“oil field”, pyrene is characterized by the highest
occurrence frequency (71–79%) after phenanthrene
and naphthalene homologs.
Site 4 located at the boundary of central granite
block and annular depression shows traces of intense
hydrothermal activity. Soils of this site, in addition to
phenanthrene and naphthalene homologs, usually
contain fluorene (90% samples) and diphenyl (80%
samples). The distribution of each of other PAHs is
not more than 30% of studied samples.
The northern blocks of the annular depression (site
3) and external site 8 adjoining from the northeast, in
addition to phenanthrene and naphthalene homologs,
shows the wide distribution of pyrene (74‒82%) and
fluorene (61‒72%). Soils from the northwestern site
9 have the highest concentrations of phenanthrene
(average 1804, maximum up to 5702 μg/kg), which are
several orders of magnitude higher than concentra-
tions of other PAHs. The other PAHs (naphthalene
homologs, fluorene, diphenyl, fluoranthene, pyrene,
and chrysene) occur with similar frequency (43%
each), averaging from 1‒3 to 14‒28 μg/kg.
DISCUSSION
The uneven distribution of PAHs in the upper part
of the geological section of the Siljan crater and the
wider diversity of individual molecules of these hydro-
carbons in the magmatic rocks compared to sedimen-
tary rocks testify to the migration nature of PAHs in
the impact crater. This conclusion is supported by the
PAH discovery in the lower deep layers of the crater,
which were recovered by the deep Stenberg-1 well.
Due to higher porosity and sorption capacity, sedi-
mentary rocks have much higher content of bitumi-
nous matters and PAHs than magmatic rocks.
Thus, we may suggest that the PAH carriers are
mainly gas in the gas field and oil components, which
become heavier near the surface, are floated, and
accumulated above the ground-fractured waters, in
the oil field.
The studied soil samples from the Siljan crater have
both common and different features in the distribution
of individual PAHs in the hydrocarbon gas and oil
seepage areas and traces of hydrothermal activity. The
common features are the ubiquitous presence of
phenanthrene and naphthalene homologs in soils,
242
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
PIKOVSKY et al.
Table 3. Characteristics of soil sampling sites
Site nos. Site location Underlying bedrocks Geochemical features of site
1 Depression zone, western block
(Mt. Mora area)
Ordovician and Silurian
limestones and shales
Free and dissolved
hydrocarbon gas seepages.
Hydrothermal ore occurrences
2 Depression zone, southeastern block
(from Ratvik to Solberg)
Ordovician limestones Oil seepage at the groundwater level
and in fractures along quarry walls
(“oil site”)
3 Depression zone, northern blocks
(from Ore to Orsa)
Ordovician and Silurian
sandstones, shales, and limestones
Unexplored
4 Internal eastern boundary
of the central block
and depression zone (Mt. Boda)
Ordovician and Silurian
sedimentary rocks,
granites of the central block
Hydrothermal ore occurrence,
old indications of oil seepage
on Mt. Osmundberg
5Eastern part
of the depression zone
Ordovician and Silurian
sedimentary rocks
Adjoining the “oil site”
from the southwest ”
6 External blocks adjoining
the depression zone
from the southwest and south
Proterozoic granites Adjoining Lake Siljan with gas
seepage from the south
7 External blocks adjoining
the depression zone
from the east and southeast
Proterozoic granites Copper and nickel deposits
in the vicinity of the site
8External block northeast
of the depression zone
Migmatites Unexplored
9 External block northwest
of the depression zone
Ordovician and Silurian
porphyrites and sedimentary rocks
Unexplored
with the overwhelming predominance of phenan-
threne among other PAHs. Differences consist in the
distribution of other PAHs in areas with different
peculiarities of geochemical processes. Thus, we may
conclude that PAHs in soils of the crater confirm the
presence of large dispersal hydrocarbon halo caused
by the presence of oil and gas in the Earth’s interior
and present-day hydrocarbon migration. The main
pathway for fluid migration likely is the annular
depression zone expressed by the wide small-block
morphostructural boundary of the crater.
There are different points of view concerning the
oil origin in the Siljan crater. F. Vlierboom et al. (1986)
comprehensively studied oil and host rocks. Based on
stable carbon isotopes and terpenoid biomarkers in
“immature” Ordovician—Silurian shales as well as the
occurrence of “more mature” oil in the same area, the
authors suggest that the local thermal inf luence of
cosmic impact led to the geologically instant “matura-
tion” of organic matter in a source rock and rapid gen-
eration and migration of oil. This hypothesis is not
convincing, since the authors analyzed sedimentary
rocks and oils in the hydrocarbon dispersal halo,
where the similarity of organic matter components of
rocks with oil components is predetermined.
Other point of view on the mantle origin of the
modern hydrocarbon seepage in the crater also did not
gain convincing evidence. In particular, this hypothe-
sis is inconsistent with crustal isotope signatures of
helium obtained from the deep well. Some authors
believe that the oil is formed in the crust and has no
mantle contribution (Komor et al., 1988). In any case,
the present-day influx of free weakly degraded oil and
hydrocarbon gas, which formed 300‒400 Ma ago in
the completely eroded rocks, can be explained neither
from geological nor from geochemical point of view.
The most probable hydrocarbon source in the Sil-
jan crater is geologically recent hydrothermal activity
in the Earth’s interior, the traces of which are noted as
oil inclusions in mineral crystals. At present hydrocar-
bons are uninterruptedly supplied from deep zones of
the crater with the formation of a wide geochemical
halo. Polycyclic aromatic hydrocarbons mark this halo
in both rocks and soils. Contrast heterogeneity in the
distribution of the same PAHs in the well core and in
soils shows their extraneous origin.
The widest southwestern part of the depression
zone is characterized by a spontaneous upward migra-
tion of mainly gas components of the fluid, which
bring light bituminous matter and PAHs. The PAH
composition in the water-saturated cap of sedimentary
rocks becomes poorer due to their partial dispersal in
the fractured and ground waters. At the same time,
soils retain specific features, which are typical of only
gas geochemical fields. The widest spread PAHs in the
gas seepage area are fluorene and chrysene. In the
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
POLYCYCLIC AROMATIC HYDROCARBONS IN ROCKS AND SOILS 243
Table 4. PAH distribution in soils of the Siljan crater, μg/kg
PAH,
μg/kg
Location in the crater Annular depression Internal blocks External blocks
site (see Fig. 1)123456789
underlying rocks
sedimentary
rocks
sedimentary
rocks
sedimentary
rocks
sedimentary
rocks,
granites
sedimentary
rocks,
granites
granites
granites
migmatites
porphyrites,
sedimentary
rocks
number of samples 60 23 23 10 7 19 33 11 8
Naphthalene
homologs
Detection frequency, % 100 100 100 100 100 100 100 100 100
Min‒max 2–176 10–314 5–640 4–110 10–168 3–154 7–1884 13–53 7–71
Mean 16 99 67 33 47 33 106 31 28
Standard deviation 24 94 151 36 52 36 320 12 20
Diphenyl Detection frequency, %605243804358615543
Min–max 0–86 0–173 0–122 0–53 0–42 0–75 0–129 0–67 0–82
Mean 13 43 14 27 12 17 27 20 24
Standard deviation 205627161722382530
Fluorene Detection frequency, % 63 61 61 90 43 68 73 73 43
Min‒max 0–9 0–21 0–8 0–26 0–3 0–10 0–6 0–9 0–8
Mean 1.3 3.9 1.8 8 1 3 2 3 3
Standard deviation 1.7 2.8 2 8.5 1 3 2 3 3
Phenanthrene Detection frequency, % 88 100 100 100 100 89 94 100 100
Min‒max 0–1178 15–4278 0–1925 65–2327 19–2999 0–1003 0–847 19–1314 279–5702
Mean 120 722 266 726 485 188 138 309 1804
Standard deviation 230 1085 473 776 1027 247 227 342 1807
Anthracene Detection frequency, % 2 13 0 0 0 0 18 18 0
Min‒max 0–6 0–666 0 0 0 0 0–16 0–2 0
Mean 0.1300000100
Standard deviation0.81360000310
Pyrene Detection frequency, %507474307147798243
Min‒max 0–4 0–20 0–53 0–1 0–3 0–4 0–16 0–10 0–3
Mean 0.73 30.42 1 2 2 1
Standard deviation 0.8 4.5 11 0.2 1 1 3 3 1
244
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
PIKOVSKY et al.
(0) Concentration below 1 μg/kg.
Fluoranthene Detection frequency, % 20 9 17 0 0 21 15 18 43
Min‒max 0–24 0–11 0–70 0 0 0–840 0–53 0–64 0–53
Mean 20.5400483814
Standard deviation 5.6 2 14 0 0 187 10 19 21
Chrysene Detection frequency, %633557305763552743
Min‒max 0–13 0–38 0–58 0–4 0–5 0–5 0–8 0–4 0–4
Mean 1.754122211
Standard deviation 2.6 11 12 1.3 2 2 2 1 1
Benz[a]anthracene Detection frequency, % 8 22 17 0 7 5 67 0 0
Min‒max 0–6 0–4 0–15 0 0–1 0–1 0–7 0 0
Mean 0.10.61000000
Standard deviation0.81.23000100
Benz[a]pyreneDetection frequency, %290075000
Min‒max 0–1.7 0–0.7 0 0 0–1 0–1 0 0 0
Mean 0.070.110000000
Standard deviation0.20.20000000
Benz[ghi]perylene Detection frequency, % 37 30 43 10 43 53 27 64 25
Min‒max 0–2.6 0–3 0–2 0–2 0–2 0–2 0–2 0–4 0–2
Mean 0.50.51011010
Standard deviation0.60.81011111
Sum of 11 PAHs Min‒max 8–1283 38–4670 16–1960 108–2500 31–3172 24–1141 21–1931 37–1412 287–5772
Mean 157 906 362 798 549 291 281 376 1874
Standard deviation 240 1240 487 820 1072 307 404 362 1826
Hexane bitumen,
mg/kg
Min‒max 6–400 15–416 12–370 39–270 15–120 10–390 9–300 15–108 50–200
Mean 56 92 91 120 52 102 87 59 139
Standard deviation 688481893590762448
PAH,
μg/kg
Location in the crater Annular depression Internal blocks External blocks
site (see Fig. 1)123456789
underlying rocks
sedimentary
rocks
sedimentary
rocks
sedimentary
rocks
sedimentary
rocks,
granites
sedimentary
rocks,
granites
granites
granites
migmatites
porphyrites,
sedimentary
rocks
number of samples 60 23 23 10 7 19 33 11 8
Table 4. (Contd.)
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
POLYCYCLIC AROMATIC HYDROCARBONS IN ROCKS AND SOILS 245
eastern, narrowest part of the depression, oil is
squeezed with gas to the surface. Gas is likely released
in atmosphere without traces, while oil is retained by
sedimentary rocks. From the bottom and almost to the
surface, the bitumen and PAHs are well correlated,
which suggests that PAHs migrate with oil in a dis-
solved state. The PAHs in soils at the oil seepages areas
are peculiar in the widest distribution of pyrene and
fluorene, as well as in the anomalous concentrations
of phenanthrene. Soils from this geodynamic zone
show well expressed relation of PAH distribution with
traces of hydrothermal activity. Thus, the PAH geo-
Fig. 1. Geological position and sampling localities of the Siljan crater area, m odified after (Durelius, 1988). (1‒15) Ancient com-
plexes of the Scandinavian Shield, after (Durelius, 1988): (1‒3, 10, 11) Proterozoic granites of different types; (4) Ordovician and
Silurian sedimentary rocks (sandstones, shales, limestones), (5) diabase dikes, (6) migmatites, (7) sandstones and conglomerates;
(8) porphyries, (9) porphyrites, (12) leptites, (13) quartzites, (14) sandstones, (15) gabbro and diorites; (16) internal and external
boundaries of the annular depression in the framing of the impact crater; (17) deep and hydrogeological wells: (1) Gravberg-1,
(2) Stenberg-1, (3) Mora, (4) Solberg; (18) soil sampling localities; (19) conditional boundaries of soil sampling sites; (20) soil
sampling locality numbers.
0510
1500
6750
6800
km
N
W
S
E
15°30′
15°00′
14°30′
14°00′
61°10′
61°00′
60°50′
1450
1400
123456
789
1
1
2
3
4
5
6
7
89
1
2
3
4
10 11 12
13 14 15 16 17 18
19 20
ENVIKEN
Lake Siljan
Lake
Orsa-
Sjön
LEKSAND
D
D
DDD
D
D
D
D
D
DD
D
D
D
BINSSJÖ
MORA
ALVDALEN
RSA RE
BOD
246
LITHOLOGY AND MINERAL RESOURCES Vol. 56 No. 3 2021
PIKOVSKY et al.
chemistry in the impact craters is an excellent tool for
the indication of the hydrocarbon flows in the Earth’s
crust.
CONCLUSIONS
The presented data show that impact craters could
be considered as pathways for hydrocarbon degassing
of the Earth and as promising objects for searching
petroleum reservoirs. Poor results of deep drilling in
the Siljan crater in this respect could be explained by
ill-chosen position of wells. They were drilled not in
the permeable zones favorable for hydrocarbon migra-
tion, but in a dense granite massif devoid of fluid-con-
ductive channels.
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Translated by M. Bogina
