GeolCarp_Vol70_No1_62

GEOLOGICA CARPATHICA

, FEBRUARY 2019, 70, 1, 62–74

doi: 10.2478/geoca-2019-0004

www.geologicacarpathica.com

Organic petrological and geochemical properties of jet

from the middle Triassic Mogila Formation, West Bulgaria

ALEXANDER ZDRAVKOV



, GEORGE AJDANLIJSKY

, DORIS GROSS

and ACHIM BECHTEL

Department of Geology and Exploration of Mineral Resources, University of Mining and Geology “St. Ivan Rilski”, 1700 Sofia, Bulgaria;



alex_zdravkov@mgu.bg

Department of Geology and Geoinformatics, University of Mining and Geology “St. Ivan Rilski”, 1700 Sofia, Bulgaria; g.ajdanlijsky@mgu.bg

Department Angewandte Geowissenschaften und Geophysik, Montanuniversität Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austria;

doris.gross@unileoben.ac.at; achim.bechtel@unileoben.ac.at

(Manuscript received November 27, 2018; accepted in revised form January 23, 2019)

Abstract: The paper presents the results of the petrographic and organic geochemical studies of a jet sample recovered

from a Mid-Triassic carbonate succession from the West Balkan tectonic zone in Bulgaria. Total organic carbon contents

(TOC = 92 % daf) and high vitrinite reflectance (Ro = 1.9 %) indicate semi-anthracite coalification rank. Very high T

max

(577 ºC) and low HI (~10 mg HC/g TOC) further support the overmature organic matter. Extractable organic matter is 

characterized by high portions of NSO compounds and asphaltenes (> 75 %). Hydrocarbons constitute about 20 % and 

are characterized by the predominance of the saturated hydrocarbons over the aromatics. n-Alkanes distribution, dominated

by short-chain compounds (n-C

17–18

),  is  consistent  with  the  woody  origin  of  the  jet  and  the  thermal  maturity  of 

the organic matter. The predominance of PAHs with condensed structure over their alkylated isomers is considered to be

a result of the complex reaction occurring within the organic matrix during the catagenesis, rather than to the presence of

combustion-derived organic matter. Based on the distribution of the diterpenoids, a tentative identification of a possible

Voltziales conifer family source is identified. Low Pr / Ph ratio (0.88) and aryl isoprenoids outline anoxic conditions of jet 

formation, whereas the presence of organic sulfur compounds and tri-MTTchroman suggest marine depositional

environment with normal salinity.

Keywords: Bulgaria, Anisian jet, depositional environment, organic geochemistry, biomarkers.

Introduction

Recently, the occurrence of jet within the early Middle Triassic

(Anisian)  carbonate  succession  of  Mogila  Fm.  from 

the western part of the Balkan tectonic zone in Bulgaria, was

noted (Ajdanlijsky et al. 2018). Jet occurs rarely in the sedi-

mentary record and is entirely constrained to Jurassic and

 Cretaceous rocks (Howarth 1962; Minčev 1978, 1980, 1982; 

Minčev  &  Nikolov  1979;  Minčev  &  Šiškov  1986;  Suárez-

Ruiz et al. 1994 a, b; Bechtel et al. 2001b; Helfik et al. 2001; 

Marynowski et al. 2011b; Markova et al. 2017). Its formation 

proceeds through the deposition of drift wood fragments under

anoxic conditions. Subsequent impregnation with bituminous

substances, generated either within the resin-impregnated

woods during maturation (Suárez-Ruiz et al. 1994b; Bechtel et 

al. 2001b), or derived externally from the surrounding envi-

ronment  (Suárez-Ruiz  et  al.  1994a)  aids  organic  matter 

preservation. The molecular composition of the jet could be

further influenced by bituminous matter, originating from

the bacterial degradation of the organic matter (Bechtel et al.

2001b). 

In this paper, the petrographic, bulk geochemical data and

molecular composition of the non-polar extract fractions of

the jet within the Anisian carbonate succession of Mogila Fm.,

is reported and used in order to infer its origin and depositional

settings.

Geological settings

The Late Alpine Balkan Tectonic zone (Fig. 1a) is characte-

rized by widely distributed Triassic successions, which are

part of the main regional tectonic structures and cover lower

Palaeozoic high-grade metamorphosed and upper Palaeozoic

sedimentary, igneous and volcanic rocks. Stratigraphically,

the Triassic rocks are subdivided into three units: i) the Petrohan 

Terrigenous  Group  (Tronkov  1981)  consisting  mainly  of 

 fluvial  and  rare  alluvial  siliciclastic  deposits;  ii)  the  Iskar 

Carbonate  Group  (Tronkov  1981)  composed  of  shallow- 

marine carbonates and mixed siliciclastic-carbonate rocks; and

iii) the Moesian Group (Chemberski et al. 1974) represented 

by terrigenous-carbonate and carbonate rocks. Tronkov (1983) 

defined four regional stratigraphic levels in the lower part of

the Iskar Carbonate Group, i.e. the Tenuis, the Zhitolub,

the Sfrazhen and the Sedmochislenitzi Beds (Fig. 1b). The log-

ged interval belongs to the Tenuis Bed from the base of

the Iskar Carbonate Group and is situated about 10 m above

the base of the Mogila Formation (Opletnya Member; Assereto

et al. 1983). Recently, the rocks were chronostratigraphically 

constrained to the earliest Middle Triassic (Anisian;

Ajdanlijsky  et  al.  2018).  Cyclic  sedimentary  succession  of 

mainly allochemic and mictritic limestone, alternating with

dolomitized limestone and dolomite, is the most prominent

feature of the Opletnya Member.

ORGANIC PETROLOGICAL AND GEOCHEMICAL PROPERTIES OF JET FROM THE MOGILA FORMATION

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, 2019, 70, 1, 62–74

High-resolution lithostratigraphic description of the studied

section is presented in Figure 1c. The sequence- and cyclo-

stratigraphic interpretation followed the concepts proposed by

Strasser  et  al.  (1999).  The  sequence  boundaries  (SB)  were 

positioned into dolomitic beds with massive to poorly lamina-

ted structure, which are considered to represent the shallowest

facies deposits. The transgressive surfaces (TS) are erosive, with 

amplitude in the range 8–10 cm, and mark the beginning of

a deepening-up trend. The transgressive deposits (TSd) are com-

posed of massive to trough/planar cross-bedded allochemic to

bioclastic limestones and dolo-limestones. Angular to round

peb bly to cobble dolo- and limestone intraclasts, forming small

lags or occurring along the foresets, are also observed. Well

preserved bivalves, gastro- and cephalopod shales can also be

recognized. Upwards, nodular to laminated and massive wacke-

and mudstones are interbedded by centimeter-thick mainly

fine grained beds of mixed carbonate– terrigenous deposits.

The maximum-flooding surfaces (mfs) represent the deepest 

facies and are manifested by increasing bioturbation.

The  shallowing-up  highstand  deposits  (HSd)  are  repre-

sented by massive and horizontally laminated, rarely nodular,

mud- and wackestones with increasing upwards dolomite con-

tent. Up to 5 centimeter-thick (< 12 cm) beds with sigmoidal 

structure and concave up top surface, having lateral extension

of < 30–40 m, are also common in this part of the elementary 

sequences.

The thickness of the individual sequences from the Opletnya

Member vary from 1.6 to over 4.3 m. Deposition within

an unrimmed carbonate platform under peritidal settings is

considered by Chatalov (2000).

Tectonically, the area belongs to the eastern part of the West

Balkan tectonic zone (Fig. 1a). As part of the Alpine thrust belt 

the latter suffered multiple collisional and compressional

events (in Late Triassic, Mid-Late Jurassic, Mid-Cretaceous,

Late  Cretaceous  and  Mid-Eocene)  due  to  the  accretion  of 

proximal and exotic continental fragments to Eurasia during

the closure of the Tethys ocean (Dabovski et al. 2002). 

Material and methods

The studied sedimentary succession is located about 35 km

north of the capital city of Sofia, in the vicinity of the Lakatnik

Railway Station within the Iskar gorge (43°05’18.2”N,

23°23’01.6”E).  Two  small  jet  fragments  were  noted  within 

a relatively thin (~ 1.6 m) elementary sequence from the base 

of the Tenuis Bed (Fig. 1c). The first one represents ~ 4 cm 

long and ~ 0.7 cm thick extraclast within the lower transgres-

sive part of the cycle (Fig. 1c). Because of the small size, how-

ever, this jet fragment was not sampled as it would not yield

enough material for the analytical procedu res. The second jet

fragment is consistently larger (15×3 cm) and was recovered 

from a massive micritic limestone bed just above the maxi-

mum flooding surface (Fig. 1c). 

For microscopic investigation, three individual fragments

(0.5 – 0.8 mm) of this jet were mounted in epoxy resin, ground 

and polished. Semi-quantitative maceral analysis was per-

formed on Leica DM 2500P microscope using reflected white

and blue excitation light under oil immersion following stan-

dard  procedures  (Taylor  et  al.  1998).  Maceral  identification 

was done after ICCP (1998, 2001). Vitrinite reflectance was 

measured on 60 points using MIDAS MSP 200 spectrometer,

attached to the same microscope. The calibration was done

using  Spinel  (R = 0.421 %),  YAG  (R = 0.902 %)  and  GGG 

(R = 1.716 %) reflectance standards. EDS analyses were perfor-

med on JEOL JSM-6010 PLUS/LA scanning electron micro-

scope in order to study the composition of the established ore

minerals. SEM was operated at reduced vacuum, back-

scattered electron detector and 15 kV accelerating voltage.

The total organic carbon (TOC) and sulphur (S) contents were 

determined with an Eltra Helios C/S analyzer. Moisture and

ash yield were measured following standard procedures (ISO

17246:2010). Rock-Eval pyrolysis was performed using a Rock- 

Eval 6 instrument. The value

of S

 (mg HC/g rock) was used to 

calculate  the  hydrogen  index  (HI = 100 × S

/ TOC  [mg  HC/g 

TOC];  Espitalié  et  al.  1977). The  temperature  of  maximum 

hydrocarbon generation (T

max

)  was  recorded  as  a  maturity 

parameter.

For GC-MS analysis about 5 g of the jet sample were mixed

with inert diatomaceous earth and homogenized. Extraction

was performed by a Dionex ASE 200 equipment using

dichloromethane for 1 hour at 75 ºC and 75 bar. The extract

was concentrated using a Zymark Turbo Vap 500 device.

Extract was dissolved in a solvent mixture of hexane:dichloro-

methane (80:1) and asphaltenes were subsequently separated 

by centrifugation. The hexane-soluble organic compounds

(maltenes) were subdivided into saturated and aromatic hydro-

carbons and NSO components using a Köhnen-Willsch MPLC

(medium pressure liquid chromatography) instrument (Radke 

et al. 1980).

The fractions of saturated and aromatic hydrocarbons were

analyzed by Thermo-Fisher Trace GC Ultra analyzer, equipped

with a 60 m silica capillary column (DB-5MS). Oven tempera-

ture was programmed from 40 –310 ºC with steps of 4 ºC/min,

followed by isothermal period of 30 min. Helium was used as

carrier gas. The device was set in electron impact mode with

a scan rate of 50 – 600 Daltons (0.5 sec/scan). The results were 

processed with the software Thermo-Fisher Xcalibur v.1.4.

Identification of biomarkers is based on retention time and

comparison of mass spectra with published data. The determi-

nation of absolute concentrations of biomarkers was done using

internal standards (deuterated n-tetracosane for the aliphatic

fraction and 1,1’-binaphthyl for the aromatic fraction) and values 

were normalized against the total organic carbon contents.

Results and discussion

Petrography and ash yield

The studied jet represent single highly compressed fossil

wood fragment, about 15 cm long and about 3 cm thick

ORGANIC PETROLOGICAL AND GEOCHEMICAL PROPERTIES OF JET FROM THE MOGILA FORMATION

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, 2019, 70, 1, 62–74

(Fig. 2a). It lies parallel to the bedding, within a thin (20 – 25 cm) 

laminated micritic limestone bed, constrained between allo-

chemic limestone at the base and allochemic dolomitic lime-

stone at the top (Fig. 1). Multiple millimeter sized (< 3 mm) 

carbonate-filled fractures indicate postdepositional deforma-

tions of the wood. Micropetrographic investigations revealed

strongly gelified organic tissues (V; Fig. 2 b–e) significantly 

fragmented by at least two fracture systems. Neither liptinite

nor inertinite macerals were detected. The fractures are mostly

filled by carbonate minerals, but some host also pyrite (± pyr-

rhotite) – sphalerite  hydrothermal  mineralization  (Fig.  2 b, c). 

The latter is typically constrained to the fractures and only

rarely  is  found  within  the  organic  matter  (Fig.  2e).  Hydro-

thermal pyrite always contain certain amount of zinc (pro-

bably in the form of micrometer-sized sphalerite inclusions;

Fig. 2h), whereas sedimentary pyrite was found to be free of 

admixtures (Fig. 2i). Based on this observation sedimentary 

pyrite was tentatively differentiated from the hydrothermal

one. Sedimentary pyrite is mostly represented by scarce

micrometer-sized euhedral crystals with octahedral and penta-

gondodecahedral habit (Fig. 2f, i), scattered within the organic 

matter  (Fig.  2d).  Only  in  one  of  the  studied  jet  fragments, 

the presence of several clusters of framboidal and euhedral

pyrite was established close to one of the sides of the fragment

(Fig. 2f, g), which is assumed to be the outer side of the jet. 

Following the criteria of Wiese & Fyfe (1986) and Kortenski 

& Kostova (1996), and considering the co-occurrence of both 

framboidal and euhedral aggregates, mostly inorganic forma-

tion of pyrite can be assumed, although some of the framboids

can be also considered of bacterial origin (e.g., Fig. 2g). Fram-

boids are typically poorly mineralized, which together with

the very low amount of sedimentary pyrite (~ 0.1 %; Table 1) 

argue for a limited pyrite formation. Considering the abun-

dance of sulfur-containing organic compounds, the negligible

pyrite formation indicates iron-deficient marine sedimentary

environment.

Vitrinite reflectance measurements (avg. Ro = 1.9 %; Table 1) 

indicate low volatile bituminous to semi-anthracite coalifi-

cation stage. However, because of the bituminization, jet is

typically characterized by reduced vitrinite reflectance in

com parison to coals of same maturity degree (Minčev 1978, 

1980; Minčev & Nikolov 1979; Petrova et al. 1985; Minčev & 

Šiškov 1986; Suárez-Ruiz et al. 1994a, b; Bechtel et al. 2001b). 

Therefore, in the present case vitrinite reflectance is presumed

to be suppressed by the bituminization of the drift wood,

and therefore, the actual coalification degree is expected to

be higher.

Because of the enhanced postdepositional mineralization,

the ash yield of the studied jet is quite high (19 wt. %; Table 1). 

However, only a small fraction of it is expected to be contri-

buted from the living wood. Major elements, like Ca, Mg, Si,

Al etc., which play a crucial role in growth or form structural

supporting elements in the living tissues, are typically estab-

lished in living woods, but generally the total amount of ash do

not exceed 0.5 wt. % (Rowell et al. 2005). Although the amount 

of inorganic matter in the woods vary due to taxonomy and

environmental conditions of growth, it is highly unlike that

the amount of wood ash in the studied jet sample will be higher

than about 2 wt. %. Therefore, it is here assumed that the post-

depositional contribution to the jet’s ash yield is about 17 %, 

and all the geochemical parameters were recalculated in order

to exclude this contribution.

Bulk geochemical parameters

Bulk organic geochemical parameters, together with the nor-

malized yield of extractable organic matter (EOM) and pro-

portions of saturated, aromatic and polar compounds and

asphaltenes, are summarized in Table 1.

The  amount  of  total  organic  carbon  (TOC)  is  quite  high 

(92.4  wt. %; Table  1),  arguing  for  a  bit  higher  coalification 

rank (anthracite) than evidenced from the vitrinite reflectance. 

Very high T

max

 value of 577 °C (Table 1) suggests overmature 

organic matter. However, since the amount of hydrocarbons

generated during the pyrolysis is quite low (S

= 9.6 mg HC/g 

rock; Table 1) the high T

max

might be erroneous. Nevertheless,

the  very  low  hydrogen  index  (HI = 10.4  mg  HC/g  TOC)  is 

consistent with the presence of inert organic matter. Since

no inertinite was detected, the low HI unequivocally points

towards overmature organic matter outside of the main

hydrocarbon generation window. It is therefore possible to

assume that the jet was subjected to temperatures over 150 °C.

The pre sence of hydrothermal ore mineralization within

the fractured jet suggest that organic matter might have been

thermally influenced by the hydrothermal activity. Over 40

small-scale copper and lead-zinc deposits (mostly strata-

bound)  of  presumably  Upper  Cretaceous  age  (Campanian–

Maastrichtian),  exerting  strong  tectonic  control,  have  been 

established within the western Balkan orogen (Mincheva-

Stefanova  1988).  How ever,  the  temperatures  of  main  ore 

formation in one of the big

gest Pb–Zn deposits, i.e.

“Sedmochislenitsi” deposit (located ~10 km NE of the studied 

section), most probably did not exceeded 80 –100 °C. In addi-

tion, no thermal changes can be established within the studied

sedimentary sequence, thus suggesting that the temperature

of the hydrothermal fluid might have been even lower.

On the other hand, Botoucharov (2014) provide data for over-

mature  (Ro > 2 %)  organic  matter  within  the  Late  Triassic 

rocks at the front of the Balkan thrust zone (southern Moessian

Platform  margin)  in  central  north  Bulgaria.  Furthermore, 

based on subsidence and thermal modelling, the same author

indicates that the maximum degree of maturity of the organic

matter was achieved by the end of late Aptian, i.e. significantly

earlier than the presumed hydrothermal activity. Therefore,

considering the complex geodynamic evolution of the Balkan

orogen  (Dabovski  et  al.  2002),  burial  of  the  sedimentary 

sequence is here presumed to have exerted the main control on

the organic matter maturity.

Total  sulfur  content  was  recorded  (Table  1),  but  due  to 

the epi

genetic sulfide hydrothermal mineralization and

the impos sibility to assess its share, the data cannot be inter-

preted in terms of environmental settings.

ORGANIC PETROLOGICAL AND GEOCHEMICAL PROPERTIES OF JET FROM THE MOGILA FORMATION

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, 2019, 70, 1, 62–74

Molecular composition of organic matter

The contents of extractable organic matter (EOM) in the stu-

died sample is low (0.4 mg/g TOC; Table 1) and is consistent 

with  the  (semi-)anthracite  rank  of  the  jet.  The  EOM  is 

 dominated  by  polar  compounds  and  asphaltenes  (Table  1). 

Predominance  of  the  saturated  (15.1 %)  over  the  aromatic 

hydrocarbons (7.3 %; Table 1) is established. 

n-Alkanes and isoprenoids

The total ion current (TIC) chromatogram of the saturated 

hydrocarbon fraction is shown in Figure 3. The concentrations

and proportions of short-, mid- and long-chain n-alkanes are

presented in Table 2.

The sample is characterized by the presence of n-alkanes in

the range n-C

to n-C

 (Figs. 3, 4). The distribution is uni-

modal and is characterized by the predominance of short-chain

hydrocarbons with maximum concentrations at n-C

17-18

(Table 2, Fig. 4a). Prominent peaks at n-C

and n-C

 (Fig. 4a) 

point toward the terrestrial origin of the organic matter.

Long-chain n-alkanes (> n-C

) are common components of 

the  epicuticular  waxes  of  the  vascular  plants  (Eglinton  & 

Hamilton  1967)  and  are  therefore  widely  used  to  infer 

ter restrial origin of organic matter. On contrary, short-chain

homologs  (< n-C

)  mostly  originate  from  algae  and  micro-

organisms (Blumer et al. 1971; Cranwell 1977), whereas mid-

chain n-alkanes (n-C

21-25

)  are  related  to  submerged  aquatic 

plants  and  mosses  (Cranwell  1977;  Ficken  et  al.  2000), 

although bacterial lipids from sulfate-reducing bacteria also

contain mid-chain homologs (Han & Calvin 1970). However, 

n-alkane distributions are known to be influenced by bacterial

degradation (Allen et al. 1971; Johnson & Calder 1973) and 

thermal maturation (Tissot & Welte 1984; Peters et al. 2005) 

of the organic matter. The former results in the progressive

removal of the n-alkanes, starting with the lighter ones,

whereas the latter results in progressive thermal cracking of

the long-chain homologs and the formation of shorter-chain

n-alkanes during maturation. Considering the abundance of

short-chain homologs in the studied sample, bacterial degra-

dation of the organic matter is unlike. Furthermore, the trace

amounts of hopanes in the studied jet sample do not support

the hypothesis of extensive biodegradation. Considering

the over mature organic matter, however, thermal cracking of

the long-chain hydrocarbons is expected. On the other hand, it

has been shown that jet, similarly to other fossil palaeowoods

(Fabiańska  &  Kurkiewicz  2013),  is  in  most  cases  characte-

rized by short-chain n-alkane distribution patterns (Bechtel et

Parameter

Value

Units

Ash

19.0

wt. %

Total organic carbon (TOC)

92.4

wt. %

Sulfur

2.7

wt. %

9.6

mg HC/g rock

Hydrogen Index (HI)

10.4

mg HC/g TOC

max

577.0

°C

Vitrinite

100.0

vol. %, mmf

Pyrite (sedimentary)

< 0.1

vol. %

1.9

Extractable organic matter

0.4

mg/ g TOC

Ʃ Saturated HC

15.1

Ʃ Aromatic HC

7.3

Ʃ Polar

40.6

Ʃ Asph.

37.0

Table 1: Petrographic and organic geochemical characteristics of

the studied jet.

Saturated hydr

ocarbons

n-Alkanes

Sum

2.21

μg/g TOC

n-C

15-19

63.72

n-C

21-25

15.55

n-C

27-31

8.85

Isoprenoids

Sum

0.42

μg/g TOC

Pr/Ph

0.88

Pr/n-C

0.41

Ph/n-C

0.38

Diterpenoids

Sum

0.50

μg/g TOC

Abietane-type

9.54

Pimarane-type

36.85

Phyllocladane-type

53.61

Regular Steranes

Sum

0.02

μg/g TOC

Hopanes

Sum

0.02

μg/g TOC

Table 2: Molecular composition of the aliphatic fraction of the jet

extract.

Fig. 3. Gas chromatogram of the saturated hydrocarbon fraction of the jet sample with partial chromatogram of diterpenoids distribution (a). 

Std. = standard.

ZDRAVKOV, AJDANLIJSKY, GROΒ and BECHTEL

GEOLOGICA CARPATHICA

, 2019, 70, 1, 62–74

al.  2001b;  Marynowski  et  al.  2014;  Markova  et  al.  2017), 

because of the absence (or the minor amounts) of long-chain 

fatty acids within the woods, as these are present mainly in

the leaf waxes. Considering these facts, the observed unusual

n-alkane distribution pattern can be attributed mostly to

the woody origin of the jet, overprinted by the effects of

the thermal maturation of the organic matter.

The studied jet sample is characterized by the presence of

low amounts of isoprenoids in the range i-C

16–20

, among which

pristane (Pr) and phytane (Ph) are the most abundant (Fig. 3; 

Table 2). Both commonly originate from the phytyl side chain 

of chlorophyll-α in photosynthesizing organisms (Peters et al. 

2005), whose degradation is strongly controlled by the Eh set-

tings of the depositional environment (Didyk et al. 1978;

Volkman  &  Maxwell  1986).  Oxygenated  environments  are 

known to favor the formation of pristane, whereas anoxic set-

tings lead to preferential formation of phytane. Following this

interpretation and considering the predominance of phytane

over pristane and the low Pr/Ph ratio (< 1; Table 2) of the stu-

died jet sample, deposition of the drift wood fragments under

reducing environment can be suggested. This hypothesis is

also supported by the low Pr/n-C

and Ph/n-C

ratios (Fig. 4b;

Table  2).  However,  the  amount  of  isoprenoids  might  be 

influen ced by the thermal maturity (Goossens et al. 1984;

Volkman & Maxwell 1986; Koopmans et al. 1999; Peters et al. 

2005), as well as by the isoprenoid precursors (e.g., pristane 

formation from tocopherols (Goossens et al. 1984; ten Haven

et  al.  1987),  and  phytane  formation  from  bacterial  lipids 

(Volkman & Maxwell 1986; Peters et al. 2005). Considering 

the high maturity degree of the organic matter, significant ren-

dering of the pristane concentrations due to the thermal evolu-

tion of the jet can be expected. In addition, isoprenoid

contribution from additional sources cannot be excluded.

Nevertheless, the presumed organic matter deposition under

anoxic conditions is in agreement with the generally accepted

mode of jet formation (e.g., Suárez-Ruiz et al. 1994a; Bechtel 

et al. 2001b). 

Diterpenoids

The studied jet sample is characterized by the presence of

low amounts of tri- and tetracyclic saturated diterpenoids

(0.5 μg/g TOC; Table 2) with abietane (fichtelite, abietane), 

pimarane (norisopimarane, norpimarane, iso-pimarane),  and 

phyllocladane  (α-  and  β-phyllocladane)  skeleton,  among 

which α-phyllocladane and pimarane are dominant (Fig. 3a). 

Their  aromatic  counterparts  are  more  abundant  (1.20  μg/g 

TOC;  Table 3)  and  include  abietane  type  hydrocarbons 

(nor(19)abieta-3,8,11,13-tetraene, nor(19)abieta-8,11,13-triene, 

dehydroabietane, simonellite, tetrahydro(1,2,3,4)retene, diaro-

matic totarane, retene), among which simonellite and retene 

are predominant. Trace amounts of diaromatic totarane-type

diterpenoid is also established (Fig. 5a). 

Diterpenoid biomarkers are widely distributed in various

sedimentary rocks, coal, fossil resins and amber (Simoneit

1977, 1986; Noble et al. 1985, 1986; Sukh Dev 1989; Otto & 

Fig. 4. Histogram, showing the distribution of the n-alkanes (a) 

and cross-plot of Phytane/n-C

versus Pristane/n-C

(b)  marking 

the disoxic/anoxic depositional environment.

omatic hydr

ocarbons

Unsubstituted

PAHs

Sum

17.57

μg/g TOC

Fl/(Fl+Py)

0.44

BaA/(BaA+Tri+Ch)

0.15

IP/(IP+BgP)

0.41

Alkyl-

Naphthalenes

0.00

μg/g TOC

DMN

0.01

μg/g TOC

TMN

0.09

μg/g TOC

TeMN

0.44

μg/g TOC

Alkyl-

Phenanthrenes

1.41

μg/g TOC

DMP

0.15

μg/g TOC

MPI-1

0.19

2.19

Organic S

compounds

DBT

0.85

μg/g TOC

MDBT

0.66

μg/g TOC

DMDBT

0.21

μg/g TOC

BNT

2.28

μg/g TOC

MDR

12.47

Furans

0.05

μg/g TOC

Diterpenoids

1.20

μg/g TOC

Aryl isoprenoids

0.46

μg/g TOC

Chromans

0.01

μg/g TOC

Table 3: Molecular composition of the aromatic fraction of the jet

extract.

ORGANIC PETROLOGICAL AND GEOCHEMICAL PROPERTIES OF JET FROM THE MOGILA FORMATION

GEOLOGICA CARPATHICA

, 2019, 70, 1, 62–74

Wilde 2001), and are common constituents of jets (Bechtel et 

al.  2001b;  Marynowski  et  al.  2014;  Markova  et  al.  2017). 

Diterpenoids are considered indicators of gymnosperm plants,

because of their presence in essential oils and resins. Based on

the distribution of the diterpenoid compounds, a general taxo-

nomic differentiation of the different conifer species may be

possible (e.g., Simoneit 1977; Wakeham et al. 1980; Noble et

al.  1985;  Otto  &  Simoneit  2001;  Otto  &  Wilde  2001). 

The presence of aromatic abietane type diterpenoids (i.e. simo -

nelite and retene) in recent sediment, however, indicate that aro-

matization proceeds early in the diagenetic process (Simoneit

1998). Cyclization and aromatization of diterpenoids is known 

to be mediated by clay mineral catalysts or microbial activity

(Wakeham  et  al.  1980).  Furthermore,  diterpenoids  are  ther-

mally unstable and commonly aromatize to retene and even-

tually to alkyl-phenanthrenes and phenanthrene with increasing

rank  (Hayatsu  et  al.  1978).  Therefore,  considering  the  high 

level of thermal maturity of the studied jet, the presence of

diterpenoids is surprising. Although the reasons for the occur-

rence of diterpenoids in this study are currently not fully

understood, a combination of a very limited bacterial activity,

which correspond to the negligible hopane concentrations,

as well as a limited availability of clay mineral catalysts in

the carbonate-dominated host rocks, can be considered.

Based on the available geochemical data for the studied jet

sample, hardly any specific chemotaxonomic assignment

could be done, because of the presence of diterpenoid com-

pounds, characteristic for the whole gymnosperm group.

However, it should be noted that the radiation of the modern

gymnosperm families (except Podocarpaceae; Cleal  1993) 

from their common ancestors (e.g., Voltziales conifers) did not 

started earlier than the Late Triassic (Cleal 1993; Taylor et al.

2009). Therefore, considering the early Anisian age of the sedi-

mentary succession, it can be assumed that the drift wood

most likely originated from the Voltziales conifer family.

Indirectly, this hypotheses is supported by the establishment

of dominant Voltziales (e.g., Voltzia, Albertia, Yuccites,

Aethophyllum) species in the early Anisian red beds of the Grès

à Voltzia Formation from the western margin of the German

Basin. The latter is believed to represent one of the first loca-

lities, where the resurgence of the gymnosperms (and particu-

larly the conifers), which survived the harsh arid- to semi-arid 

conditions following the end-Permian life crisis, have occurred

(Grauvogel-Stamm & Ash 2005). On the other hand, Taylor et 

al. (2009) noted that many of the early conifers share common 

botanical features with other families, and it can therefore be

suggested that this poor differentiation is also expressed in

their chemical characteristics.

Steranes and hopanes

Regular steranes and hopanes were detected in equal trace

amounts (0.02 μg/g TOC; Table 2), but because of the difficult 

identification of the individual compounds due to the poor

signal-to-noise ratio, their distribution is not discussed here.

The very low concentrations of hopanes might indicate a very

limited bacterial activity. This is consistent with the presumed

coniferous origin of the drift wood, in which resins typically

act as anti-bacterial agent. However, both hopanes and stera-

nes are thermally unstable at very high maturity degrees (e.g.,

Stout  &  Emsbo-Mattingly  2008)  and  considering  the  semi- 

anthracite rank of the studied jet this might be the main reason

for their general absence.

No rearranged steranes were detected, which is consistent

with the dominant carbonate sedimentation during the drift-

wood deposition. Surprisingly, however, the aromatic fraction

does not provide evidence for the presence of neither aromatic

steranes, nor hopanes, although their aromatization is known

to occur quite early during the diagenesis (Hussler et al. 1984;

Peters et al. 2005). However, since the cyclization and aroma-

tization reactions are favored by the catalytic activity of clay

minerals, the carbonate-dominated depositional environment

might have predetermined their absence.

Polyaromatic hydrocarbons hydrocarbons (PAHs)

The analysis of the aromatics revealed complex composition,

characterized by the occurrence and dominance of the unsub-

stituted three to six rings PAHs, over their alkyl- and

 phenyl- substituted  derivatives  (Fig.  5;  Table  3). Apart  from 

the abundant phenanthrene, the C

PAHs are characterized by

predominance of three and four-ring compounds and notable

bell-shaped distribution, maximizing at chrysene (Fig. 6).

Predominance of condensed over alkyl-substituted PAHs is

often considered to represent contribution from combustion-

derived organic matter (e.g., Laflamme & Hites 1978; Wakeham 

et al. 1980; Yunker et al. 2002). However, such pyrogenic pat-

terns are not uncommon for high-rank coals (semi-anthracite

and  anthracite)  for  which  rearrangement,  fragmentation  and 

condensation reactions during coalification result in progres-

sive loss of alkyl side chains and thus produce significant pre-

dominance of the unsubstituted PAHs (e.g., Radke et al. 1982;

Wang  et  al.  2017).  The  latter  is  especially  pronounced  for 

the two- and three-ring compounds (Chen et al. 2004; Stout & 

Emsbo-Mattingly 2008). In addition, phenanthrene concentra-

tions in high rank coal can also be increased by demethylation

reactions  of    aromatic  diterpenoids  (Hayatsu  et  al.  1987). 

On  the  other  hand,  Bechtel  et  al.  (2001a)  report  significant 

increase of phenanthrene concentrations relative to the sum of

the methylphenanthrenes, together with abundant sulfur and

oxygen heterocyclic hydrocarbons, as a result of demethyla-

tion processes related to oxidative hydrothermal alteration of

Kupferschiefer Formation near the Rote Fäule zone. Although

such explanation for the significant predominance of phenan-

threne in the present study seems unlikely given the detected

very low concentration of furans (Table 3), the possibility of 

triphenylene,  benz[b]fluoranthene,  benz[e]pyrene formation

from dehydrocyclization of alkyl- or phenyl-substituted PAHs

(Grafka et al. 2015) cannot be completely excluded as a reason 

for the decreased methylphenanthrene concentrations. Never-

theless, considering the above discussion, as well as the (semi-) 

anthracite rank of the studied jet, the established PAHs

ZDRAVKOV, AJDANLIJSKY, GROΒ and BECHTEL

GEOLOGICA CARPATHICA

, 2019, 70, 1, 62–74

distribution can mostly be attributed to the thermal transfor-

mation reactions within the organic matrix. However, because

of the presence of unsubstituted PAHs, which have no appa-

rent biological precursors and are mostly considered to form

during burning of organic matter (e.g., pyrene, chrysene,

benz[a]pyrene, etc.; Fig. 5; Chen et al. 2004; Keiluweit et al.

2012), the possibility of contribution from combustion- derived 

hydrocarbons should not be completely excluded. In order to

evaluate  such  contribution  the  ratios  Fl/(Fl+Py),  BaA/

(BaA+Tri+Ch) and In/(In+BgP) (Yunker et al. 2002) were cal-

culated (Table 3). Following the criteria of Yunker et al. (2002) 

for separation of diagenetically-derived from combustion-

deriver PAHs, however, somewhat contradictory conclusions

can be drawn. While the ratios Fl/(Fl+Py) and In/(In+BgP) are 

above the boundaries for diagenetically-derived PAHs (0.4 and

0.2  respectively; Yunker  et  al.  2002)  and  thus  indicate  that 

minor contribution from pyrolytic organic matter is possible,

the  low  value  of  the  ratio  of  BaA / (BaA + Tri + Ch)  (< 0.2; 

Table 3) is consistent with the absence of such contribution. 

The results suggest there might be separate sources of

the individual PAHs. As discussed above, the established

PAHs distribution is probably mostly a result of the diagenetic

and catagenetic transformation of the organic matter.

Nevertheless, minor anthropogenic contribution, especially to

the five- and six-ring PAHs, should also be taken into account,

considering the fact that the studied outcrop is adjacent to

a major road and a roadside restaurant.

The alkyl substituted naphthalenes and phenanthrenes are

widely used to reflect the thermal maturity of the organic

matter. Numerous alkyl-naphthalene and alkyl-phenanthrene

ratios have been developed for that purpose (Radke & Welte 

1981; Radke et al. 1982, 1986; van Aarssen 1999; Stojanović 

et al. 2007). Because the observed distribution of the alkyl-

naphthalenes is clearly modified by secondary processes

(i.e. water washing, oxidation or biodegradation; Volkman et

al. 1984; Marynowski et al. 2011a), the methylphenanthrene 

index (MPI-1; Radke & Welte 1981), which is calibrated and 

widely used as maturity indicator of Type-III kerogen, was

used to further support the maturity assessment of the jet.

Following the established empirical relation between MPI-1

and the vitrinite reflectance (Rc = −0.6*MPI-1 + 2.3; Radke et 

al.  1984),  the  calculated  methylphenanthrene  index  (0.19; 

Table 3) can be correlated to equivalent vitrinite reflectance 

(Rc) of 2.19 % (Table 3). The value is close to the measured 

vitrinite  reflectance  (Ro = 1.9;  Table  1)  and  suggest  that 

the sup pressing effect that the bituminization play on Ro

(e.g.,  Suárez-Ruiz  et  al.  1994a, b)  might  be  significantly 

reduced during maturation, possibly as a result of the transfor-

mation of the impregnating resins.

The aromatic fraction is further characterized by the occur-

rence  of  sulfur  heterocyclic  compounds  (Fig.  5;  Table  3). 

These are represented by dibenzothiophene and its alkyl-sub-

stituted  isomers,  as  well  as  by  benzo[b]naphthothiophenes

(Fig. 5; Table 3). These compounds have no obvious biolo-

gical precursors and are therefore considered to originate from

Fig. 5. Gas chromatograms of the aromatic hydrocarbon fraction of the jet sample with partial chromatograms of aromatic diterpenoids (a), 

aryl isoprenoids (b)  and  methyl-  and  dimethyl-dibenzothiophenes  (c).  TMN = trimethylnaphthalenes;  TeMN = tetramethylnaphthalenes; 

P = phenanthrene; MP = methylphenanthrenes; DMP = dimethylphenanthrenes; DBT = dibenzothiophenes; MDBT = methyldibenzothiophenes; 

DMDBT = dimethyldibenzothiophenes;  DBF = dibenzofurans;  BNT = benzonaphthothiophenes;  MFl = methylfluoranthene;  PhN = phenyl-

naphthalene; PhP = phenylphenanthrenes; PhA = phenylanthracenes; Std. = standard.

Fig. 6. Distribution of the unsubstituted PAHs.

ORGANIC PETROLOGICAL AND GEOCHEMICAL PROPERTIES OF JET FROM THE MOGILA FORMATION

GEOLOGICA CARPATHICA

, 2019, 70, 1, 62–74

the reaction of the organic matrix with sulfur during early

diagenesis (White et al. 1988). Although the mechanisms of 

sulfur  incorporation  are  still  debatable  (Wang  et  al.  2017), 

it is generally accepted that the origin and distribution of

the organic sulfur compounds are controlled by the deposi-

tional  conditions  (Hughes  1984;  Hughes  et  al.  1995)  and 

maturity (Radke et al. 1986). Sediments, deposited in marine 

environments typically contain increased concentrations of

sulfur aromatics, although hypersaline settings in lacustrine

basins could also produce such compounds (Hughes et al.

1995;  Radke  et  al.  2000).  Owning  to  the  carbonate  deposi-

tional environment, the presence of sulfur aromatics in

the studied jet is not surprising. On the other hand, Radke et al.

(1986) pointed out that methyl shifts in the alkyl-dibenzothio-

phenes follow the same pathway as in the alkyl substituted

naphthalenes and phenanthrenes, and based on this proposed

the methyldibenzothiophene ratio (MDR) as additional matu-

rity  parameter  (MDR = 4-MDBT/1-MDBT). The  distribution 

of the methyldibenzothiophenes in the examined aromatic

extract is clearly dominated by the thermodynamically more

stable  4-MDBT  (MDR = 12.47),  which  is  another  proof  of 

the enhanced maturity of the jet’s organic matter.

Aryl isoprenoids and chromans

Small  amounts  (0.46  μg/g TOC; Table  3)  of  C

– C

aryl

isoprenoids with 2,3,6-trimethyl substitution pattern for the aro-

matic ring and a tail-to-tail isoprenoid chain, were tentatively

identified in the aromatic extract fraction (Fig. 5b). The aryl 

isoprenoids are mostly considered to derive from isorenie-

ratene, which is known to be synthesized by photosynthetic

green sulfur bacteria. Since these organisms are phototrophic

anaerobes and require both light and H

S for growth (Pfennig

1977;  Summons  1993),  the  presence  of  aryl  isoprenoids  is 

often interpreted as indication for photic zone anoxia (e.g.,

Summons  &  Powell  1987;  Sinninghe  Damsté  et  al.  2001). 

However, Koopmans et al. (1996) report aryl isoprenoids with 

a 2,3,6-trimethyl substitution as diagenetic transformation

products of β-carotene, thus indicating that isorenieratene is 

not their sole precursor. Considering the very advanced

organic matter maturity of the studied jet, the failure to detect

isorenieratene, which is typically reported from immature

sedi ments (e.g., Grice et al. 1996; Sinninghe Damsté et al.

2001), is not surprising. Because of this and the absence of 

carbon isotopic data, however, the detected aryl isoprenoids

cannot be equivocally assigned to green sulfur bacteria.

Nevertheless, considering the abundance of organic sulfur

compounds, arguing for H

S-rich environment, as well as

the anoxic settings, that are prerequisite for jet formation and

further evidenced from the low Pr/Ph ratio, at least partial

origin of aryl isoprenoids from green sulfur bacteria can tenta-

tively be suggested.

In a series of mono-, di- and trimethylated 2-methyl- 2-

trimethyltridecylchromans (MTTC; Sinninghe Damsté et al.

1987)  only  trace  amount  of  the  tri-MTTC  was  tentatively 

identified  in  the  present  study  (0.01  μg/gTOC;  Table  3). 

Despite of the fact that the origin of the methyl substituted

MTTCs is still controversial (Sinninghe Damsté et al. 1987,

1993) they are widely used as a palaeosalinity indicator (e.g., 

Schwark & Püttmann 1990; Grice et al. 1998; Schwark 1998; 

Bechtel  et  al.  2013).  Empirical  evidences  (e.g.,  Sinninghe 

Damsté et al. 1987, 1993) suggest that formation and preser-

vation of mono-, di- and trimethyl substituted chromans is

strongly controlled by the salinity of the depositional environ-

ment. Thus non-hypersaline environment favor the synthesis

of tri-MTTC, whereas mono-MTTCs are preferentially formed

under hypersaline settings. Considering the presence of only

tri-MTTC in the studied jet, hypersaline settings seem rather

unlike. It is therefore possible to suggest that the deposition

and further transformation of the drift wood occurred under

normal salinity marine environment. The latter is also sup-

ported by the complete absence of gammacerane, which is

also widely used to infer hypersalinity (e.g., Jiamo et al. 1986;

Sinninghe Damsté et al. 1995; Peters et al. 2005), and is in 

agreement  with  the  conclusions  of  Chatalov  &  Stanimirova 

(2001) based on the conditions of early diagenetic dolomiti-

zation of carbonate mud from Mogila Fm.

Conclusions

Early Mid-Triassic jet, occurring within the cyclic carbonate

succession from the lower part of Mogila Formation from

the Western Balkan tectonic zone in Bulgaria, was characte-

rized by petrographic analysis, Rock Eval pyrolysis, and

organic geochemistry proxies. The results indicate that the jet

was formed from wood of vascular plant. Тhe Voltziales coni-

fer family seem the most probable source based on the estab-

lished poor differentiation of the diterpenoid hydrocarbons.

High  total  organic  carbon  content  (TOC = 92 %),  vitrinite 

reflectance  value  (Ro = 1.9 %)  and  T

max

  (577  °C)  argue  for 

overmature  organic  matter  at  (semi-)anthracite  coalification 

rank. The latter is also well expressed in the molecular compo-

sition of the jet. The dominance of short-chain n-alkane homo-

logs (n-C

17–18

) and the low amounts of the long-chain n-alkanes

is considered to reflect the woody origin of the jet, overprinted

by the enhanced thermal maturity. Furthermore, the predomi-

nance of PAHs with condensed structure over their alkylated

isomers most probably reflect the complex fragmentation and

condensation reactions within the organic matrix during

the coalification, rather than the input of combustion-

derived organic matter. The calculated equivalent reflectance

(Rc = 2.14 %), based on the distribution of phenanthrene and 

methylphenanthrenes, is close to the measured one and further

proofs the (semi-)anthracite rank. Because of the timing and 

the presumed low temperature hydrothermal fluids, the estab-

lished epigenetic hydrothermal activity is assumed as of negli-

gible influence on the thermal maturity of the organic matter.

Therefore, the enhanced maturity of the jet is considered to be

a result of the complex geodynamic evolution of the West

Balkan tectonic zone and the deep burial of the sediments.

The  low  Pr/Ph  ratio  (< 1),  as  well  as  the  presence  of  aryl 

ZDRAVKOV, AJDANLIJSKY, GROΒ and BECHTEL

GEOLOGICA CARPATHICA

, 2019, 70, 1, 62–74

isoprenoids, are consistent with drift wood deposition under

anoxic environmental settings. Furthermore, the presence of

tri-MTTC and the absence of gammacerane argue for normal

salinity marine environment of deposition.

Aknowledgments: We would like to express our sincere

grati tude to Prof. Dr. I. Kostova and D. Apostolova (Sofia

 University “St. Kl. Ohridski”) for their support with vitrinite 

reflectance measurements. Additional gratitude are expressed

to Assist. Prof. S. Dobrev and Assist. Prof. G. Lyutov (Univer-

sity of Mining and Geology “St. Ivan Rilski”, Sofia) for their 

helpful comments regarding the hydrothermal mineralization,

and to Prof. Dr. J. Kortenski for the valuable comments on

the origin of sedimentary pyrite. The critical reviews from

three anonymous reviewers are also greatly acknowledged.

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