Geochemical characteristic of the Lutetian black shale (Bayburt, NE-Turkey): amount, nature, origin of organic matter and palaeo-environment conditions

Abstract

To define the amount, thermal maturity and type of organic matter (OM), a comprehensive analysis of major and trace elements, organic carbon isotope and organic geochemistry was conducted on Lutetian black shales found in the Everek (Bayburt) region of northeastern Turkey. Total organic carbon (TOC) levels in the shale samples range from 0.62% to 6.75%, and type II–III to type III kerogen is generated, displaying a combination of high terrigenous and low marine OM. The δ¹³C values (ranging from –28.22‰ to –28.23‰), aromatic hydrocarbon compounds (methyl phenanthrene, dibenzothiophene, tri-aromatic and monoaromatic steroids), saturated hydrocarbon compounds (sterane and terpane), acyclic isoprenoids, n-alkane distribution (n-C₁₃–n-C₃₆) and inorganic geochemical characterization support that the black shales were deposited in a terrestrial-marine transition environment and had a high proportion of terrestrial OM with small amounts of marine OM preserved in relatively arid to hot climate and oxic to suboxic conditions. The analysis of biomarker thermal maturity markers, T_max (ranging from 449–458 ^oC) and estimated vitrinite reflectance (varying from 0.92 to 1.08%) values suggest that the black shales have reached the oil window. As a result, black shales are thought to contain low to high amounts of TOC, have a mixed kerogen type, reach a high thermal maturity level and produce little hydrocarbons.

1. Introduction

Black shales are typically thin laminated dark-coloured sedimentary rocks with fine-grained particles (Wignall, 1994; Eric et al. 2023). The origin, palaeo-climatic conditions, palaeo-depositional environment, palaeo-salinity and palaeo-redox conditions may all be inferred from the geochemical composition of shales (McLennan et al. 1993, Tanaka et al. 2007; Liang et al. 2018; Li et al. 2020; Wu et al. 2021)

Everyone agrees that the world’s petroleum supply will ultimately run out of resources. It makes sense for the government and energy sectors to rekindle their interest in oil shale as a viable alternative supply of oil or a complement to fuel (Cirilli et al. 2018; Spina et al. 2018 Ilevbare & Adeleye, 2023). Because they have the potential to be sources and reservoirs of hydrocarbon accumulation, black shale sedimentary rocks – also known as organic-rich mudstone and shale – are of enormous interest to people all over the world (Jarvie et al. 2007; Spina et al. 2021). Nonetheless, because of rising energy demands, both marine and continental black shales have drawn more interest from conventional and unconventional petroleum resource developers. It is well known that most black shale deposits were primarily deposited in anoxic bottom-water environments in marine and continental sedimentary basins and that they contain significant levels of organic matter (OM) – more than 1 weight per cent (Hakimi et al. 2011; Makeen et al. 2015; Hatem et al. 2016; Ahmed et al. 2022).

However, many variables, including continental weathering, biological productivity, clay mineralogy, sedimentation rates, sea level change, water column oxygen levels and depositional environment, severely restrict the accumulating of OM in shale (e.g. Li et al. 2008; Zonneveld et al. 2010; Bechtel et al. 2012; Shu et al. 2013; He et al. 2017; Zhang et al. 2019). Prior studies have exhibited that the primary causes of OM enrichment are the productivity of organisms as well as the preservation and resolution of OM (Calvert & Pedersen, 1993; Bordenave et al. 1993; Carroll & Bohacs, 1999). OM preservation is significantly influenced by redox conditions. Furthermore, there might be fluctuating restrictions on OM enrichments due to the terrigenous input and sedimentation rate. Some of the sources of Al and Ti are heavy minerals and aluminosilicate minerals. It is proposed that both components serve as reliable stand-ins for detrital inflow (Murphy et al. 2000). In addition, primary productivity is increased in warm, humid climates that support biological growth (Meng et al. 2012; He et al. 2017; Li et al. 2019). This means that there is a definite relationship between climate and productivity.

Sediments provide good information on the palaeo-climate and palaeo-environment after several representatives, including elemental, isotopic and molecular organic geochemical ones, are investigated. These biomarkers are frequently employed in the reconstruction of palaeo-environmental conditions (Meyers & Ishiwatari, 1993; Meyers, 2003; Xie et al. 2004; Zhou et al. 2005; Ortiz et al. 2010 & 2021; Koralay 2021). N-alkanes are widely used for this purpose because the origin of n-alkanes in the lithological record can be inferred from their distribution, e.g. primary sources of short-chain n-alkanes (C₁₇ and C₁₉) are cyanobacteria and algae (Cranwell et al. 1987). Terrestrial plants have n-alkanes in their leaf wax that are richer in C₂₇ to C₃₁, while aquatic macrophytes have homologues of C₂₃ and C₂₅ predominating (Eglinton & Hamilton 1967; Ortiz et al. 2021). The δ¹³C composition of certain chemicals has been utilized for palaeo-environmental interpretations. The δ¹³C composition of individual n-alkanes provides more specific information on past variations in C3 and C4 plant abundance than the δ¹³C of bulk OM. It has been utilized to deduce source inputs and biochemical pathways (Street-Perrot et al. 1997; Ficken et al. 1998; Huang et al. 2000; Sinninghe Damste et al. 2011; Sun et al. 2013).

The Lutetian sedimentary sequence outcropping in the Everek area (Bayburt) contains black shale layers in places. Since it is thought that these black shale levels may be rich in OM, it was found worth investigating. Eocene aged marl/shale outcropping around Çalıdere, approximately 20 km away from the Everek region, was previously examined to feature the OM and sedimentation environment and to evaluate its hydrocarbon production potential (Saydam Eker, 2022). With the previous study, approximate information was obtained about the type, amount and degree of maturation of OM in marl/shale. However, questions about productivity, the relationship between productivity and OM, the relationship between the palaeo-environment and its conditions and OM, the factors causing high T_max and the basin model have not been answered due to the lack of aromatic biomarkers and major and trace element analyses.

The main purpose of this study is to define the amount of OM contained in black shales, kerogen type, maturation level of OM, source of OM and affecting factors based on organic geochemical analysis (Rock-Eval/TOC, stable carbon isotope, gas chromatography (GC), GC-mass spectrometry (GC-MS)). In addition, palaeo-climate, palaeo-productivity, palaeo-redox and palaeo-salinity conditions and therefore palaeo-depositional environment and conditions were interpreted based on inorganic (whole-rock trace and major elements analysis) and organic geochemical data of the black shales.

2. Geological setting

The Bayburt area is in the Eastern Pontides orogenic band of the Alpine–Himalaya fold system in northeastern Anatolia. The Carboniferous aged Pulur Metamorphic Massif, which comprises migmatite, amphibolite, gneiss, phyllite, marble, meta-chert and metavolcanic, is the oldest geological block in the Bayburt region. Upper Carboniferous–Permian rocks consist of the Carboniferous Cebre Rhyolite including high K calc-alkaline rhyolites and detrital rocks containing conglomerate, mature sandstone, limestone and shale (Topuz et al. 2004; Eyüboğlu et al. 2017; Dokuz et al. 2017). The Early Jurassic unit thickness varies between 1500 and 1800 m and consists of an alternation of conglomerate, sandstone, siltstone, marl, shale and tuffite. This alternation is accompanied by bands and lenses of red-coloured fossiliferous limestone, coal and acidic-basic rocks, dykes and sills. This sequence is unconformably layered above these basement rocks (Saydam Eker et al. 2015; Saydam Eker et al. 2016a). The Late Jurassic to Early Cretaceous aged limestone, which is separated into three units in the research region, is unconformably layered above these strata. Unit I is primarily made up of thick to medium-bedded grey and beige-coloured sandstone, gravel stone and sandy limestone; Unit II is made up of thick-bedded grey and beige-coloured oolitic, wackestone, packstone and grainstone and Unit III is mostly made up of thin-bedded yellow-coloured dolomite. Reefal limestones conformably overlie earlier limestone units, whereas the Upper Cretaceous series conformably overlies the carbonate rocks. Late Cretaceous rocks show great diversity lithological and have been examined under four different units. Unit I consists of an alternation of brown, yellowish sandstone, red pelagic limestone, mudstone and claystone. Unit II consists of andesite, basalt, andesitic-basaltic, tuff and tuffite. Unit III is a mélange and contains sandstone, marl, radiolarite, chert, serpentinite, gabbro, basalt and limestone blocks. Unit IV is a white, grey-coloured reef limestone and contains abundant coral (Keskin et al. 1990). Unit IV is overlain unconformably by the Pleistocene sedimentary rock, which is made up of marl, sandstone, sandy limestone and grey-coloured, thin, medium-layer pelagic limestone. Keskin et al. (1990) divided Eocene rocks into four formations named Sığırcı Formation (thickness varies between 275 and 470), Sırataşlar Formation (thickness about 750 m), Yazyurdu Formation (thickness varies between 250 and 300) and Rize Pluton (thickness varies between 800 and 1000). Sığırcı Formasyonu (the subject of this study) begins with basement conglomerate. Lutetian sedimentary rocks are classified as turbidite units, consisting of grey, brown and greyish-green thin- to thick-layered, fine- to medium-grained sandstones intercalated with marls and shales that are grey and dark grey in hue (Saydam Eker, 2012). There is also an upper interval of coal-containing claystone in some places (Saydam Eker et al. 2016b). Sırataşlar Formasyonu consists of grey, beige, yellow, dirty yellow, light green, coloured, medium-thick layered, limestone, sandstone, occasionally marl and sandy limestone levels. Yazyurdu Formasyonu is characteristic of volcano-clastic rocks composed of andesitic-basaltic rock, marl, sandstone and tuff. Rize Pluton generally consists of volcanic and plutonic rocks. The basaltic to andesitic rocks have an affinity for calc-alkaline to tholeiitic rocks, whilst the intrusive rocks are composed of granodiorite, tonalite and diorite. (Kaygusuz & Öztürk, 2015; Eyuboğlu et al. 2017; Gücer, 2021; Kaygusuz et al. 2022). Eocene rocks unconformably overlie Miocene rocks, which include limestone, marl, conglomerate, siltstone, sandstone and gypsum. Travertine is formed when carbonic acid-rich water percolates from rocks in limestone. The alluvium is from rounded and subrounded gravel, clay, silt and sand, mainly derived from non-altered sources and not chemically mature (Saydam Eker & Demirkol Kiliç, 2018; 2024), the older rocks are covered unconformably by Quaternary units (Figs. 1a, b, 2a).

Figure 1. (a) Simplified geological map of the eastern Pontides (after Güven, 1993) and location map of the study area. (b) Geological map of the study area (after Musaoğlu, 1987).

Figure 2. (a) Stratigraphic section of Eocene rocks, (b) measured stratigraphic sections of Lütetian detrital rocks, (c) and (d) photographs of black shale and (e) photograph of plant fragments.

3. Samples and methods

During the field study, a stratigraphic section was measured (Measured stratigraphic section= MSS) from Everek (about 40^o7′59″ north and 40^o22′59″east), where the Lutetian black shales are best exposed. The Everek measured stratigraphic section starts with sandstones at the base and contains alternations of sandstone, marl and shale upwards (Fig. 2a). For geochemical analysis, 10 samples were collected from black shales (Fig. 2b). Whole-rock trace, major elements and Rock-Eval/TOC analyses were applied to 10 shale samples. Based on the results of the Rock-Eval/TOC analyses, stable carbon isotope (δ¹³C), GC–MS of aromatic and saturated biomarkers and GC analyses were also applied to two shale samples (E-15 and E-16, Fig. 2c, b).

3.a. Inorganic geochemistry analysis

At Vancouver, Canada’s commercial Acme Analytical Laboratories Ltd. (now Bureau Veritas Minerals), major and trace elements were identified on ten shale samples. 0.2 g of sediment fused with 1.5 g of LiBO2 and dissolved in 100 ml of 5% HNO3 were used to evaluate the major and trace element compositions using inductively coupled plasma mass spectrometry (ICP-MS) and ICP optical emission spectrometry. Dried samples were heated to 1,000 °C for 15 minutes to calculate loss on ignition. For trace elements, the detection limits ranged from 0.1 ppm to 10 ppm. Calibration and verification standards together with reagent blanks was added to the sample sequences. In-house certification of STD SO-18 was performed against 38 certified reference materials, which included CANMET SY-4 as well as established external standards USGS AGV-1, G-2, GSP-2 and W-2. The analytical precision exceeds 4%.

Since chemical weathering is closely related to palaeo-climate, chemical index alteration (CIA) was used. Nesbitt & Young (1982) first proposed CIA, which is widely used to investigate the palaeo-climate (Feng et al. 2003; Yadav & Rajamani, 2004; Goldberg & Humayun, 2010; Wang et al. 2017; Xu & Shao, 2018; Liang et al. 2020; Saydam Eker & Arı, 2020). The CIA is calculated using the following formula:

(1) $${\rm{CIA}} = {\rm{mole }}\left[ {{\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}}{\rm{/}}\left( {{\rm{A}}{{\rm{l}}_{\rm{2}}}{{\rm{O}}_{\rm{3}}} + {\rm{CaO}} + {\rm{N}}{{\rm{a}}_{\rm{2}}}{\rm{O}} + {{\rm{K}}_{\rm{2}}}{\rm{O}}} \right)} \right] \times 100$$

(2) $${\rm{CaO*}} = {\rm{mole}}\ {\rm{CaO\ mole\ }}{{\rm{P}}_{\rm{2}}}{{\rm{O}}_{\rm{5}}} \times {\rm{10/5}}$$

Where CaO* indicates the sum of CaO in silicates.

Furthermore, the distribution and content of a few main and trace elements in sedimentary strata may reveal palaeo-climatic changes (Hu et al. 2017). Some research effectively used the C-value as a proxy for climate change (e.g. Zhao et al. 2007; Fu et al. 2016; Wang et al. 2017)

The C-value is calculated by the following formula:

(3) $$\small{\eqalign{& {\rm{C - value}} =\cr & {\Sigma ( {{\rm{Fe}} + {\rm{Mn}} + {\rm{Cr}} + {\rm{Ni}} + {\rm{V}} + {\rm{Co}}} )}{/} \Sigma ( {{\rm{Ca}} + {\rm{Mg}} + {\rm{Na}} + {\rm{K}} + {\rm{Sr}} + {\rm{Ba}}})}}$$

3.b. Stable carbon isotope analysis

Two samples of black shale (E-15 and E-16 samples containing the highest TOC) underwent stable carbon isotope analysis (δ¹³C). A system called Elementary Vario Pyro Cube Isoprime Vision Elemental Analyzer– Isotope Ratio Mass Spectrometry (EA-IRMS) was used to perform this analytical technique. Calibration was performed using Sorghum Flour, EMA-PI and NB-22-approved reference materials. The isotope data for nitrogen, carbon and sulphur are provided by Vienna Canyon Diablo Troilite VCDT, Vienna Pee Dee Belemnite (VPDB) and AIR. Every sample undergoes at least two analyses. The Turkish Petroleum Corporation’s Oil and Organic Geochemistry Laboratory (TPAO, Ankara) performed the analyses.

3.c. Organic geochemistry analysis

Rock-Eval 6 equipment fitted with a TOC module has been used for Rock-Eval pyrolysis/TOC analysis of shale materials. The samples were heated at a rate of 25 °C per minute from 300 °C (hold time: 3 min) to 650 °C. For oxidation, the crushed sediment was heated at a rate of 25 °C per minute from 400 °C (hold time 3 min) to 850 °C (hold time 5 min). Two samples (E-15 and E-16) were subjected to a 40-hour Soxhlet extraction process using dichloromethane (CH₂C_l2) to yield extracts. An Agilent 6850 gas chromatograph fitted with a flame photometric detector and flame ionization detector was used to examine the entire extract. Helium was used as the carrier gas for a fused capillary column (100m, 0.25 mm i.d.) coated with cross-linked dimethylpolysiloxane (JandW, 0.50 ⁰m film thickness) used for separation. The oven’s temperature was set to rise at a rate of 4 ^oC per minute from 40 ^oC (hold time: 8 min) to 270 ^oC (hold time: 60 min). Using liquid chromatography, the extract samples were divided into fractions containing Nitrogen, Sulfur, Oxygen (NSO) compounds, aromatic hydrocarbons and saturated hydrocarbons. To elute the fractions, N-hexane, toluene and methanol were utilized, in that order. The two samples (E-15 and E-16) with the highest extract were subjected to GC-MS studies. Shale extracts’ saturated fractions were used for the GC-MS analyses. An Agilent 5975C quadrupole mass spectrometer was connected to an automated liquid sampler (7683 B) and a gas chromatograph (7890 A). An HP-1MS fused silica capillary column measuring 60 m in length, 0.25 mm in diameter and 0.25 µm in film thickness was fitted with the gas chromatograph. The carrier gas in this case was helium. Programming for the oven temperature included a range of 50 ^oC (hold time 10 min), 200 ^oC (hold time 15 min), 250 ^oC (hold time 24 min) and 280 ^oC (hold time 24 min) at 2 ^oC/min. At last, the oven was raised to 290 ^oC (hold time: 40 min) at a rate of 1 ^oC per minute. At a source temperature of 300 ^oC and an ionization energy of 70 eV, the mass spectrometer was run in the EI mode. Biomarker contents are at m/z 191 for tri-, tetra- and pentacyclic triterpenes, at m/z 217 for steranes and rearranged steranes, at m/z 253 for monoaromatic steranes, m/z 231 for tri-aromatic steranes, phenanthrene, and methyl-phenanthrene at 178 and 192, dibenzothiophene (DBT) and methyl- DBT at m/z 184 and 198 determined using the record and ion fragmentogram definitions are given in SI table. Elution order matching and retention time were used to identify compounds. The Turkish Petroleum Corporation’s Oil and Organic Geochemistry Laboratory (TPAO, Ankara) performed the analyses.

Additionally, vitrinite reflectance (Ro %) values were calculated based on T_max (Jarvie et al. 2001). The Ro % is calculated using the following formula:

(4) $${\rm{Ro}}\% = \left( {0.018\times{{\rm{T}}_{{\rm{max}}}}} \right)-7.16$$

4. Results

4.a. Lithology

In this investigation, the shale samples were collected from the Lutetian age sequence from the Everek area. Here the sequence starts with sandstones at the base and keeps going with sandstone, marl and shales. The layer thickness of sandstones generally varies between 10 and 100 cm. The layer thicknesses of marl and shales are quite variable and marl layers vary between 4 cm and 50 cm, and shale layers vary between 2 cm and 30 cm. The total thickness of the sequence is 380 m. It starts with brown-grey, thick-layered sandstone (about 1 m thick) and continues for 20 metres (Fig. 2a, b). Throughout to the top, the section is formed of thin-thick-bedded brown-grey sandstone, thick-medium-bedded green-grey marl and medium, thin-bedded grey-dark grey-black shale (Fig. 2c, d). Therefore, while sandstone dominates in the lower parts of the stratigraphic section, shale and marl become dominant towards the top. The shales show a dark grey to black colour, especially in the central parts of the sequence, and are usually laminated. Intra-layer sedimentary structures (lamination and gradation), and sub-layer sedimentary structures (flute, load and rill marks) have been detected in the section and many remarkably well-preserved plant fragments (Fig. 2e). These sedimentary structures are the primary indicators of sequences transported and deposited by turbidity currents. It is possible to classify these turbidity currents into two groups: a) hyperpycnal turbidites and low-density turbidites (Gani, 2004; Mutti et al. 2009; Yang et al. 2015; Zhang et al. 2021). Hyperpycnal turbidity currents are high-density flows that carry high concentrations of sediment particles; they are observed at the bottom of the section. Because coarse-grained and thick-layered sandstones are more dominant in the lower part of the section than in the upper part, an upward-fining grain sequence is observed above the higher energy deposits; this corresponds to the energy attenuation zone with limited erosion capacity. Moderate, low-density flows have a low erosional capacity and mostly form fining-upward sequences, as observed at the top and medium of the section, respectively. These flows can erode, and ripples are produced by bedload reworking and low-amplitude bed waves. In this study, low-density turbidites contain higher amounts of TOC than hyperpycnal turbidites.

4.b. Major and trace elements

Trace and major element concentrations of the studied black shale are listed in Table 1. Trace and major elements protect a wealth of palaeo-redox and palaeo-climate knowledge beneficial to the reconstruction of the early diagenetic history and depositional environment of fine-grained detrital rocks (Wu et al. 2021; Li et al. 2020; Lin et al. 2019; Zhang et al. 2018).

Table 1. Concentrations of elements in the analyzed groups of black shale

SiO₂ and Al₂O₃ contents of the black shale range from 48.83 to 53.30 wt% (mean 51.36 wt%) and 14.73 to 17.52 wt% (mean 15.90 wt%), respectively. Fe₂O₃ content of the samples is extremely similar to one another and varies between 8.09 and 8.51 wt% (mean 8.29 wt%). MgO, CaO and K₂O contents of the samples range from 4.36 to 5.45 wt% (mean 4.90 wt%), 2.09 to 5.26 wt% (mean 4.01 wt%) and 1.87 to 2.87 wt% (mean 2.48 wt%), respectively. Other major element oxides such as TiO₂, Na₂O, MnO, P₂O₅ and Cr₂O₃ have low contents, those of Na₂O and TiO₂ are higher than 1 wt%, and others are less than 1 wt% (Table 1). The mean concentrations of the chosen trace element in the samples indicate the following trends Ba (306ppm) > V (191ppm) > Ni (175ppm) > Sr(151ppm) > Cu (62.8ppm) > Co (36.5 ppm) > Th (28.0 ppm) > Zn (22.2 ppm) > U (1.93 ppm) > Mo (0.33 ppm) > Cd(0.20 ppm) (Table 1).

The ratios of Th/U, C-values and CIA values used as palaeo-climate indicators vary from 3.63 to 4.76, 0.64 to 1.12 and 42 to 62 (Table 2), respectively. The ratios of Cu/Zn, Ni/Co and V/(V+Ni) used as palaeo-redox indicators range from 2.10 to 2.48, 4.14 to 5.42 and 0.51 to 0.56, (Table 2), respectively. The ratios of Sr/Ba and TS/TOC used as palaeo-salinity indicators vary from 0.35–0.65 to 0.07–0.49 (Table 2), respectively. The ratios of Ni/Al, Cu/Al and Zn/Al utilized as palaeo-productivity indicators are between 0.019 and 0.022, 0.006 and 0.007 and 0.002–0.003, respectively. The ratio Ti/Al is used as a detrital input indicator and ranges from 0.07 to 0.13 (Table 2). However, it is known that the trace and major elements in the sediments originate from hydrothermal, authigenic, clastic, hydrogenous and biogenic, origins in mixed proportions (Dymond, 1981; Brumsack, 2006; Xu et al. 2012; Tripathy et al. 2014; Liang et al. 2020). When the element is authigenic, it can be used to investigate palaeo-productivity, palaeo-redox and palaeo-salinity conditions (Tribovillard et al. 2006).

Table 2. Concentrations of elemental ratios in the analyzed groups of black shale

4.c. δ ¹³ Corg isotopes

Because this parameter is used by plant communities for photosynthesis, stable carbon isotopic parameters (δ¹³C) are used to identify the source of the OM (aquatic vegetation, dissolved OM, particulate OM, algae, microbes and terrestrial vascular plants). (Whiticar, 1996; Lamb et al. 2006; Samanta et al. 2013; Kumar et al. 2021).

The bulk fraction of δ¹³Corg isotope content of the samples is close to each other, measured as −28.23 for E-15 and −28.22 for E-16 (Table 4).

Table 3. Rock-Eval and carbon isotope analysis results and calculated parameters

Table 4. The parameters were calculated from gas chromatograms (GC) for selected shale samples

CPI = 2 × Σ tek n-C_23–29/(Σ çift n-C_22–28 + Σ çift n-C_24–30);

OEP = (n-C₂₁ + 6 × n-C₂₃ + n-C₂₅)/(4 × nC₂₂ + 4 × n-C₂₄); WI =Σ n-C_21-31/Σ n-C_15-20;

TAR = (n-C₂₇ + n-C₂₉ + n-C₃₁)/(n-C₁₅ + n-C₁₇ + n-C₁₉);

Pr = Pristane; Ph = Phytane.

4.d. Bulk organic geochemical characteristics

4.d.1. TS, TOC and Rock-Eval pyrolysis

The consequences of the Rock-Eval pyrolysis, TOC analysis and TS values are given in Table 3. TS values of the studied shale samples vary between 0.05 and 0.83% (mean of 0.30%). The TOC abundance of shale samples from Everek varies considerably, ranging from 0.62 to 6.54 % (mean 1.57 %), and the S₁+S₂ values range from 0.44 to 13.12 mg/g (mean 2.21 mg/g). The HI indicates a narrow variation in the shale samples, varying from 73 to 192 mgHC/g TOC (mean 93 mgHC/g TOC). The highest TOC, HI and S₁+S₂ values are observed in the middle part of the section (E-15 sample). In addition, HI, S₁+S₂ values show a linear relationship with TOC (Fig. 3). T_max and Ro values of the samples are high, ranging from 449^oC to 458^oC (mean 455^oC), and 0.91 and 1.08, respectively. T_max is similar throughout the section, and unlike TOC, S₁+S₂ and HI, the lowest value was measured in the E-15 sample (Fig. 3). The production index (PI) and bitumen index of the samples range from,0.03 to 0.09 (mean = 0.07) and 0.03 – 0.13 (mean = 0.08), respectively.

Figure 3. Organic geochemical proxy profiles of the black shale samples.

4.d.2. n-alkanes and isoprenoids

For sample E-15, n-alkanes in the range n-C₁₃–n-C₃₆ are characterized by monomodal distributions, with the maximum peak appearing at n-C₂₅ (Fig. 4a), while for E-16, n-alkanes are characterized by monomodal distributions in the range n-C₁₅–n-C₃₄ and the maximum peak also appears at n-C₂₅ (Fig. 4b).

Figure 4. Gas chromatograms of saturated hydrocarbons of two representative black shale samples.

Because they capture the redox conditions during sedimentation and diagenesis as well as the palaeo-environmental conditions of the source rocks, phytane (Ph) and pristane (Pr) are often regarded as the most significant acyclic isoprenoid hydrocarbons (Didyk et al. 1978; Chandra et al. 1994). In both samples, Pr and Ph are the acyclic isoprenoids record, with Pr being very dominant over Ph (Fig. 4a, b). Therefore, Pr/Ph ratios are calculated as 7.48 for E-15 and 5.86 for E-16 (Table 4). Pr/n-C17 ratios are calculated as 2.35 and 4.03 for E-15 and E-16, respectively, and Ph/n-C₁₈ ratios are calculated as 0.32 and 0.41 for E-15 and E-16, respectively. The carbon preference index (CPI, Peters & Moldowan, 1993) and odd-to-even predominance (OEP, Scalan & Smith, 1970) values are calculated as 1.11 and 1.03 for E-15, respectively, and 1.07 and 1.02 for E-16, respectively (Table 4). Terrigenous/aquatic ratio (TAR; Bourbonniere & Meyers, 1996) values are higher than 1 and are calculated as 1.42 for E-15 and 1.74 for E-16. The n-C₁₇/n-C₂₇ ratio represents the marine/aquatic input and is calculated as 0.62 and 0.49 for E-15 and E-16, respectively. The WI values, indicating the contribution of terrestrial plants to the OM (Waxiness index, Peters et al. 2005), are calculated as 2.61 and 3.07 for E-15 and E-16, respectively (Table 4).

4.d.3. Terpanes and steranes

Hopanes (pentacyclic terpane), C₂₄ Tet (tetracyclic terpane) and C₁₉-C₂₆ Tri (Tricyclic Terpane) are observed in E-15 and E-16 samples (Fig. 5a, b). The ratios of C₁₉/C₂₃ Tri, C₂₄/C₂₃ Tri, C₂₆/C₂₅ Tri of the E-15 sample are higher than the E-16 sample. The ratios of C₁₉/C₂₃ Tri, C₂₄/C₂₃ Tri and C₂₆/C₂₅ Tri are calculated as 1.02, 0.87 and 1.10 and 0.37, 0.55 and 0.87 for E-15 and E-16 samples, respectively (Table 4). The C₂₄Tet/(C₂₄Tet+ C₂₃Tri) ratios of the analyzed samples were close to each other and were calculated as 0.41 and 0.34 for E-15 and E-16, respectively. C₂₉H/(C₂₉H+C₂₉M), C₃₀H/(C₃₀H+C₃₀M) and Ts/(Ts+Tm) ratios are widely used as thermal maturity parameters (Seifert & Moldowan, 1978; George et al. 2001). The ratios C₂₉H/(C₂₉H+C₂₉M), C₃₀H/(C₃₀H+C₃₀M)and Ts/(Ts+Tm) are calculated as 0.87, 0.87 and 0.23 for E-15, respectively, and 0.91, 0.88 and 0.30 for E-16, respectively. The Gammacerane/H₃₀ ratio is utilized to appraise the salinity of the sedimentation environment and the ratios of the Gammacerane/H₃₀ of the analyzed samples are highly low and varying from 0.02 to 0.03 for E-15 and E-16, respectively. C₃₁22S/22(S+R) ratios are calculated as 0.59 for E-15 and E-16 samples. C₃₂22S/22(S+R) ratios for E-15 and E-16 are calculated as 0.58 and 0.60, respectively. While the homohopane index (HHI) was calculated as 0.03 for both samples, the C₃₅/C₃₄ ratio was calculated as 0.62 for E-15 and 0.47 for E-16 (Table 4).

Figure 5. The m/z 191 mass fragmentograms (on top) and m/z 217 mas fragmentograms (below) saturated hydrocarbon fractions of two representative black shale samples.

The sterane distributions are indicated in Fig. 5 c, d; their concentrations are lower than hopanes with the sterane/hopane ratio calculated as 0.32 for E-15 and 0.20 for E-16 (Table 4). C₂₇Dia(Dia+Regular) sterane ratios are calculated as 0.40 for E-15 and 0.70 for E-16. The ααα20Rsteranes are characterized by lower proportions of C₂₇ (20 % for E-15 and 20 % for E-16) compared to C₂₈ (20 % for E-15 and 24 % for E-16), and C₂₉ (60 % for E-15 and 56 % for E-16). The equilibrium point of the C₂₉ ββ/(ββ + αα) ratio varies between 0.67 and 0.71 (Seifert & Moldowan 1986). The calculated C₂₉ ββ/(ββ + αα) ratios of E-15 and E-16 samples are 0.61 and 0.55, respectively.

4.d.4. Aromatic hydrocarbons

In the m/z 192 and 178 mass chromatograms of the E-15 and E-16 samples, phenanthrene (P) constitutes the dominant peak, and methyl phenanthrene abundances are approximately similar (Fig. 6a, b). While DBT has the highest abundance in the m/z 184-198 mass chromatograms of both samples, methyl dibenzothiophenes (MDBTs) show approximately similar distribution (Fig. 6c, d). In the m/z 231 mass chromatogram of both samples, the highest value belonged to the C₂₈ tri-aromatic steroid, while the lowest value was recorded in the C₂₇ tri-aromatic steroid (Fig. 6 e, f). The TAI/(TAI+TAII) ratio was calculated as 0.78 for the E-15 sample and 0.80 for the E-16 sample (Table 5). In the m/z 253 mass chromatogram of both samples, while C₂₉ monoaromatic steroid has the highest value, the lowest value belongs to C₂₇ monoaromatic steroid (Fig. 6g, h). The order is monoaromatic steroid C₂₉ > C₂₈ > C₂₇ (57 %, 27 %, 16 % in the E-15 and 48 %, 26 %, 26 % in the E-16) and the C₂₉/C₂₇ ratio is calculated as 3.59 for E-15 and 1.89 for E-16 (Table 4). The MAI/(MAI+MAII) ratio is not very high and is calculated as 0.25 for E-15 and 0.21 for E-16 (Table 4).

Figure 6. The m/z 178-192, m/z 184-198, m/z 231 and m/z 253 ion fragmentograms aromatic hydrocarbon fractions of two representative black shale samples.

Table 5. Selected saturated and aromatic biomarker parameters for the selected black shale samples

Tri = tricyclic terpanes. Tet = tetracyclic terpanes. H = hopane. M= Moretan. HHI = homohopane index ((HHI. C₃₅/ΣC₃₁-₃₅ 22S and 22R homohopanes; Peters and Cassa, 1994)). Steranes/hopanes = C₂₇-C₂₉ regular steranes/C₂₉-C₃₅ 17α-hopanes. Dia: diasteranes; Reg: regular steranes; Ts: 17α- 22.29.30-trsinorhopane; Tm: 18α-22.29.30-trisnorhopane, MPI= (2MP+3MP)/(1MP+9MP), TAI/(TAI+TAII)= (C₂₀+C₂₁)/(C₂₀+C₂₁+C₂₆+C₂₇+C₂₈), MA(I)/(MAI+MAII)= (C₂₁+C₂₂)/(C₂₁+C₂₂+C₂₇+C₂₈+C₂₉).

5. Discussion

5.a. Nature of organic matter and deposition environment

To define the OM type of black shale samples, the S₂/S₃ ratios (hydrocarbon type index) and hydrogen index (HI) values obtained from TOC and pyrolysis analysis results were used. The HI vs T_max diagram developed by Lafargue et al. (1998) is widely used to determine the kerogen type. Kerogen type, which is commonly classified under three groups (Type I, II and III), provides very important information about the source from which OM is derived. Type III kerogen is primarily composed of terrestrial plants, Type I kerogen is derived dominantly from alginite and Type II kerogen has a mixed source of marine organisms and higher terrestrial plant debris (Zhou et al. 2023).

In the HI vs T_max diagram, 7 samples of black shale were in the Type II–III kerogen and oil-gas-prone region, while three samples of shale were in the Type II kerogen and oil-prone region (Fig 7a). S₂/S₃ ratios of the analyzed samples are widely distributed and vary between 1.28 and 62.45 (Table 3). According to the classifications of Clementz et al. (1979) and Peters & Cassa (1994), the kerogen content of these samples changes between Type III and Type I/Type II, Type III and Type I, respectively. Based on this, we can talk about mixed OM, with OM predominantly coming from terrestrial plants. Additionally, the high value of the correlation coefficient and the linear distribution of the samples in the S₁ vs S₂ graph (r = 0.96, Fig 7b) indicate that a single source is dominant for OM. However, only terrestrial OM is characterized by relatively higher S₃ values and lower S₂/S₃. The samples in the TOC-S₂/S₃ diagram show linear spread in a narrow range (ratios (r = 0.78, Fig 7c) (Mallick et al. 2022), confirming that a single source is dominant. The fact that the S₂/S₃ ratios of all the samples are >1 (1.28 – 62.45) suggests mixed type OM. This shows the limited contribution of marine OM, as well as the intense OM input coming from land during the deposition of shales, which is supported by C isotope values as explained below.

Figure 7 (a) The plot of T_max vs. HI indicates kerogen types for the black shale samples (Lafargue et al. 1998), (b) the plot of S₁ vs. S₂ and (c) TOC vs. S₂/S₃ for the black shale samples.

OM obtained from terrestrial vascular C3 plants has δ¹³C isotope values ranging from −22 to −33, whereas the δ¹³C isotope values of C3 aquatic plants vary between −13 and −27‰ (Whiticar, 1996). δ¹³C isotope values of bacteria in coastal and open sea environments vary between −12 and −27‰ depending on their source (Coffin et al. 1989). The analyzed samples’ δ¹³C isotope levels are in the range of values seen in OM obtained from terrestrial vascular C3 plants. In this context, the OM contained in the studied shales is thought to be predominantly of terrestrial origin.

Gas chromatograms of selected samples have low to high-weight n-alkanes and the n-alkanes distribution varies between n-C₁₃-n-C₃₆ for E-15 and n-C₁₅-n-C₃₄ for E-16. This distribution reflects a mixture of algae/bacteria (Hunt, 1996) and terrestrial plants (Eglinton & Hamilton, 1967; Killops & Killops, 2005; Saydam Eker, 2013; Qiao et al. 2021). The high n-C₁₇/n-C₂₇ ratio reflects the dominance of marine OM over terrestrial OM. In this study, the n-C₁₇/n-C₂₇ ratios are < 1 (vary between 0.62 and 0.49) indicating OM of terrestrial is more predominant. TAR values of the samples are >1, supporting that terrestrial OM is more dominant than marine OM. These values mentioned above reflect the existence of both marine and terrestrial OM in the depositional environment. However, in the Pr/n-C₁₇ vs. Ph/n-C₁₈ graph, which is widely used to interpret the origin of OM and the depositional environment, the samples fell into the terrestrial OM area. This can be explained by the high amount of pristane due to the oxic-suboxic environment (Fig. 8).

Figure 8. The plot of Pr/n-C17 vs. Ph/n-C18 of the black shale samples (Shanmugam, 1985).

The primary sources of the biomarker compounds sterane and hopane, respectively, are bacteria and plants/algae (Rohmer & Outrissan 1976). While high amounts of C₁₉ and C₂₀ TT indicate terrestrial OM (Peters et al. 2005), the dominance of C₂₃ TT exhibits a reducing marine carbonate environment and marine OM input (Waples & Machihara, 1991; Tao et al. 2015; Qiao et al. 2021). While the E-15 sample’s C₁₉/C₂₃ TT ratio is calculated as >1, indicating the dominance of terrestrial OM, the E-16 sample’s C₁₉/C₂₃ TT ratio is calculated as <1, indicating the dominance of marine OM (Table 4). For this reason, it can be said that there is both marine and terrestrial OM in the depositional environment. The high C₂₄Tet/(C₂₄Tet+C₂₃TT) ratio indicates that OM includes a mixture of aquatic algal-bacterial and terrestrial OM (Alexander et al. 1983; Connan et al. 1986; Peters et al. 2005). The C₂₄Tet/(C₂₄Tet+C₂₃TT) ratio of the analyzed black shale samples is not very low and varies between 0.34 and 0.41 (Table 4), confirming the dominance of terrestrial OM as well as the presence of aquatic algae and bacteria. While C₂₉ steranes derive from continental higher plants, C₂₇ steranes are generally of algal origin (Huang & Meinschein, 1979; Moldowan et al. 1986; Volkman, 2003;). In the studied samples, in order C₂₉ααα20R>C₂₈ααα20R>C₂₇ααα20R is observed (Table 4). In this context, the dominance of C₂₉ααα20R steranes over C₂₈ααα20R and C₂₇ααα20R steranes in the black shale samples indicates the dominance of terrestrial OM (Fig. 9).

Figure 9. The ternary diagram indicating the distribution of C₂₇, C₂₈, C₂₉ ααα 20R steranes (modified from Huang and Meinschein, 1979, after Qiao et al. 2021).

Although some phytoplankton and microalgae produce the aromatic component C₂₉ sterol, the major origin of this sterol is terrestrial plants (Volkman et al. 1999; Volkman, 2003; Kostova et al. 2022;). C₂₈ sterols are highly high in microalgae of marine (Barrett et al. 1995; Volkman et al. 1998; Volkman, 2003), while C₂₇ sterols are typically derived from marine phytoplankton (Brassell & Eglinton, 1981; Volkman, 1986). Therefore, the C₂₉/C₂₇ sterol ratio is accustomed to determining the proportion of terrestrial/phytoplankton OM in the environment. C₂₉/C₂₇ sterol < 1 indicates that phytoplankton/algal input is more dominant in the sedimentation environment. The C₂₉/C₂₇ ratio of the studied black shale samples is > 1, indicating the dominance of terrestrial OM in the depositional environment. However, C₂₉ sterols are represented by stigmastanol (a chemical compound found in many plants) rather than sitosterol (one of the phytosterols whose chemical structure is like cholesterol), which means that plenty of even-numbered n-alkanes are observed over low-weight n-alkanes in anaerobic environments (Welte & Ebhardt, 1968; Welte &Waples, 1973). In this context, it should be kept in mind that a high C₂₉/C₂₇ ratio indicates not only the dominance of terrestrial plants but also the presence of aquatic plants, especially in anoxic environments. In the study, it is thought that the depositional environment was oxic-suboxic and the high C₂₉/C₂₇ ratio represents terrestrial OM, and the order C₂₉MA>C₂₈MA>C₂₇MA also supports this.

5.b. Palaeo-climate

Th/U ratios and C-values are widely used to interpret palaeo-climatic conditions. Cao et al. (2012) suggested that Mn, Fe, Cr, Co, Ni and V are relatively enriched under humid conditions. Contrarily, Ca, Mg, K, Na, Sr and Ba are relatively enriched under arid conditions. Warm, humid climates are indicated by Th/U ratios less than 4, whereas hot, dry climates are indicated by higher values (Xu et al. 2015; Zhang et al. 2021). Low CIA values (70–50) show weak chemical weathering under arid climates, medium CIA values (80–70) reflect moderate chemical weathering under humid and warm climates and high CIA values (100–80) indicate hard chemical weathering under humid and hot climates (Yan et al. 2010).

In this study, while the C-values are low (mean of 0.88), the Th/U ratios are generally higher than 4 (mean of 4.14), indicating that the palaeo-climate was arid and hot. The low CIA values of the studied samples indicate that chemical weathering is weak, and the region has an arid and hot climate. The highest CIA value was calculated in sample E-26 (at the top of the MSS), and the lowest CIA value was calculated in E-16 (in the middle of the MSS) (Table 2, Fig. 10).

Figure 10 (a), (b). Inorganic geochemical and proxy profiles of the black shale samples.

5.c. Palaeo-productivity and detrital influx

Although some researchers (Goldberg & Arrhenius, 1958; Dehairs et al. 1980; Dymond et al. 1992; McManus et al. 1999; Cardinal et al. 2005) have noted a clear relationship between Ba and OM abundance, the biogenic contraption of Ba is thought to be some contentious (Liang et al. 2020). In this study, the negative relationship between Ba and TOC and the lack of any correlation with Al made the source of the Ba uncertain, therefore, the Ba was not used as a productivity proxy. The elements Cu, Ni and Zn in sediments react with OM (which is usually present in living things) to form complexes that eventually lead them to precipitate at the bottom of the water (Martin and Knauer, 1973; Piper and Perkins, 2004). Therefore, Zn, Cu and Ni were considered proxy productivity indicators due to their micronutrient behaviour (Brumsack, 2006). During deposition, element ratios like Zn/Al, Ni/Al and Cu/Al are commonly employed to assess the palaeo-productivity of source rocks. Increased Ni/Al, Zn/Al and Cu/Al ratios signify higher palaeo-productivity (Algeo and Maynard, 2004; Schoepfer et al. 2015; Shen et al. 2015)

In this study, Cu/Al, Ni/Al and Zn/Al are quite low, indicating that the productivity is not high. It is known that an arid climate decreases primary productivity, whereas a humid and warm climate increases primary productivity (Meng et al. 2012). This means that there is a definite relationship between climate and productivity. During the deposition of the shale, arid and hot climatic conditions existed in the study area, resulting in low productivity.

It is thought that Ti and Al serve as proxies for the input of detrital material (Hatch & Leventhal, 1992; Canfield, 1994; Algeo & Maynard, 2004; Wu et al. 2021). Al is mainly found in aluminosilicate minerals such as feldspar and clay (Rimmer, 2004), while titanium often exists in heavy minerals and clays (Kidder et al. 2001). As a result, the Ti/Al ratio indicates the energy of sediment deposition, and Ti and Al abundances aid in identifying the provenance of sediment (Murphy et al. 2000; Liang et al. 2018; Wu et al. 2021).

The Ti/Al ratio of the samples varies between 0.07 and 0.13, and the mean is calculated as 0.08 (Table 2). This value is considered moderate and indicates a medium-energy flow of clastic sediment, representing moderate terrestrial detrital input during this period. Consequently, the OM enrichment was not significantly affected by the dilution of OM caused by detritus sediment influx, as shown by the low correlation relationship between TOC and Ti/Al (r = 0.227).

5.d. Palaeo-redox condition

Redox conditions play a significant role in the preservation of OM in the sedimentation environment. The accumulations of some trace elements (e.g. V, U, Cr, Cu, Ni, Zn and Co) express the palaeo-redox conditions (McKay et al. 2007). Cu/Zn, Ni/Co and V/V+Ni ratios are commonly used to interpret palaeo-redox conditions. In a sedimentary environment, the Ni/Co ratios above 5 show suboxic and anoxic conditions, while the Cu/Zn values below 5 show oxic conditions (Jones and Manning, 1994; Liang et al. 2020). In this study, the Ni/Co ratios of six samples are above 5, the others are below 5, while the Cu/Zn ratios of all samples are below 5 with an average of 2.29, indicating oxic to dysoxic and oxic conditions (Table 2, Fig. 10). Redox conditions classed as euxinic, suboxic to anoxic, dysoxic and oxic are shown by V/(V + Ni) ratios of >0.82, >0.60 to ≤0.82, >0.46 to ≤0.60 and ≤0.46, respectively (Hatch & Leventhal, 1992; Jones & Manning, 1994). V/(V+Ni) ratios of black shale samples are above 0.46 and below 0.60 (mean of 0.52), indicating dysoxic conditions. This determination is also supported by the TS vs V/(V+Ni) diagram. In the diagram, all the samples are in suboxic conditions and a mixture of marine-terrestrial areas (Fig.11).

Figure 11. Cross-plot of sulphur (TS) and V/(V+Ni) ratio, indicating highly marine reducing environmental conditions black shale samples (Bechtel et al. 2001).

In addition, Pr/Ph ratios were used to interpret redox conditions in this study. Phytane (Ph) and pristane (Pr) are considered the foremost significant isoprenoid compounds of hydrocarbons in that they indicate the palaeo-environmental conditions of the source rocks and reflect the redox conditions during diagenesis and sedimentation. (Powell & McKirdy, 1973; Didyk et al. 1978; Chandra et al. 1994; Hakimi and Abdullah, 2014). A high Pr/Ph ratio indicates that the deposition environment is oxic throughout the sedimentation of OM (Mello & Maxwell, 1990; Philp, 1994; Huang et al. 2003; Saydam Eker et al, 2016b). Peters & Moldowan (1993) stated that Pr/Ph ratio <0.5 reflects a strongly anoxic environment, 0.5<Pr/Ph<1 exhibits an anoxic environment (aquatic), 1<Pr/Ph<2 reflects a weakly oxic-weakly anoxic aquatic environment and Pr/Ph>2 shows an oxic environment. The Pr/Ph ratios of the analyzed black shales are much higher than 2 (Table 4), indicating an oxic condition. A high C₃₅ homohopane value is one of the most important indicators of evaporitic and anoxic environments (Boon et al. 1983; Connan et al. 1986), so the C₃₅/C₃₄ homohopane ratio is accustomed to interpreting the redox conditions of depositional environments, and low ratios (< 1) show oxic-dysoxic conditions for the source rock (Curiale et al. 1985). The C₃₅/C₃₄ HH ratios were calculated as < 1 for the analyzed samples, indicating that the depositional environment was oxic-dysoxic. In addition, low HHI values indicate oxic and/or sulphate-poor, as well as clastic depositional environments (Köster et al. 1997; Peters et al. 2005). The HHI value of samples E-15 and E-16 was calculated as 0.03, which is low and supports that the deposition environment was oxic and/or sulphate-poor.

5.e. Palaeo-salinity

Various parameters (Sr/Ba and TS/TOC ratios) were used to interpret the palaeo-salinity conditions of the depositional environment (e.g. Deng & Qian, 1993; Wei & Algeo, 2020; Zhang et al. 2021). S is preserved in the sediment through evaporation, microbial sulphate and thermochemical sulphate reduction (Trundinger et al. 1985; Sim et al. 2011), thus providing information about the salinity of the environment. TS/TOC is less than 0.1 in freshwater and greater than 0.1 in marine and brackish facies. Whereas Sr/Ba is >0.5 in marine facies, it is <0.2 in freshwater and 0.2–0.5 in brackish (Wei & Algeo, 2020). In analyzed shale, the Sr/Ba ratios vary from 0.35 to 0.65 (Table 2, Fig. 10), indicating brackish and marine facies and this finding is also supported by the TS/TOC ratio. TS/TOC ratios vary from 0.07 to 0.49, indicating freshwater and brackish and marine facies and deposition environments.

Gammacerane obtained from biomarker analysis also provides useful information about the palaeo-deposition environment. Gammacerane is generally abundant in high-salt marine environments. In addition, the high gammacerane/C₃₀hopane ratio indicates marine depositional environments with high salinity associated with evaporite and carbonate deposition (Peters & Moldowan; 1991). In this study, the gammacerane/C₃₀hopane ratios are low, varying between 0.02 and 0.03, indicating a moderate salinity environment.

5.f. Thermal maturity of organic matter

The characteristics that are most frequently utilized in thermal maturity studies include the colour change of the palynomorph, vitrinite reflectance and T_max, which is obtained by programmed temperature pyrolysis (e.g. Hartkopf-Fröder et al. 2015; Sorci et al. 2020; Spina et al. 2021; Buratti et al. 2024). In the present study, to determine the OM’s thermal maturity, T_max and vitrinite reflectance (Ro%) values calculated based on T_max (Jarvie et al. 2001), aliphatic and aromatic components are used. Aliphatic maturity parameters contain the ββ/(ββ + αα) sterane (reaching an equilibrium value is 0.70), 20S/(20R + 20S) homohopane (value of achieving equilibrium for C₂₉ is 0.55) (Seifert & Moldowan, 1986), and 22S/(22R + 22S) homohopane (C₃₂), and their ratios increase as maturity increases. Likewise, the 22S/(22R + 22S) homohopane ratios are quite reliable in indicating the thermally early mature to immature range (value ranges from 0.55 to 0.60, Waples & Machihara, 1991; Peters et al. 2005). In aromatic components, methyl-DBT ratio and methyl-phenanthrene index (MPI) (MDR = 4MDBT/1MDBT) (Radke & Willsch, 1994) are used as maturity parameters and these rates increase as maturity increases. Additionally, Radke (1987) divided source rock maturity into three groups according to MPI-3 value; MPI-3 > 1 mature, MPI-3 = 0.80–1 medium mature and MPI-3< 0.80 immature.

The T_max value of the studied shale samples was measured as > 435 ^oC and the Ro values were calculated as > 0.70, therefore, the samples of black shale are thermally mature according to the classifications of Mukhopadhyay et al. (1995), Espitalié et al. (1977) and Peters and Cassa (1994) (Schito et al. 2017; Riboulleau et al. 2018). The T_max–PI diagram (Lafargue et al. (1998) also supports this; in this graph, all the black shale samples are clustered in the mature area (Fig 12). In the ββ/(ββ + αα) C₂₉ sterane and ααα20S/(20S+20R) C₂₉ sterane and C₃₂ homohopane 22S/(22S + 22R) and C₃₁ homohopane 22S/(22S+22R) diagrams, samples E-15 and E-16 have reached the equilibrium value and are therefore thermally mature (Fig. 13a, b). In addition, in the diagram of 20S/(20S+20R) C₂₉ sterane and C₃₂ 22S/(22S + 22R) homohopane, both samples were collected with peak oil window maturity area (Fig. 13c). However, MPI-3 values of E-15 and E-16 samples were calculated as 4.85 and 4.18, respectively, and MDR ratios were calculated as 0.96 and 0.92, indicating medium thermal maturity. In short, various aromatic indicators exhibit slight differences, but all values (T_max, Ro %, aliphatic indicators) indicate oil window maturity.

Figure 12. The plot of T_max vs. PI shows hydrocarbon potential for the black shale samples.

Figure 13. The plots of biomarker parameters sensitive to the thermal maturity of the two black shale samples a) (a) C₂₉ ααα 20S/(20S + 20R) vs. C₂₉ ββ/(ββ + αα), (b) C₃₁ homohopan S/(S+R) vs. C₃₂ homohopan S/(S+R), (George et al. 2001), c)20S(20S+20R) C₂₉ sterane vs. C₃₂ 22S(22S+22R) homohopane (modified from Peters and Cassa, 1994, from Hakimi & Abdullah, 2014).

The studied black shales are Lutetian in age and no younger formations were deposited on them in the study area. Therefore, such a high level of thermal maturation cannot be explained by the burial history. It is known that there were volcanic activities near the study area, contemporary with sedimentation during the Eocene (Keskin et al. 1990). These volcanic (andesitic, basaltic and pyroclastic rocks) and intrusive rocks (granodiorite, diorite and tonalite) (Kaygusuz & Öztürk, 2015; Eyuboğlu et al. 2017; Gücer, 2021) are thought to help increase the temperature of the basin where the black shales were deposited and cause the thermal maturation of the OM.

5.g. Source rocks characteristics and hydrocarbon production potential

Regarding low to moderate HI values and TOC contents, the samples can be featured as fair oil source rocks (Fig. 14a). Potential production (PP = S₁ + S₂) < 2 mg/HC rock value indicates that the rocks do not have source rock potential, 2-6 mg/HC rock value indicates that the rocks have moderate source rock and hydrocarbon production potential and > 6 mg/HC rock value indicates that the rocks have good source rock and hydrocarbon production potential (Tissot & Welte, 1984). The PP value of one (E-15) of the shale samples is >6 mg/HC rock, the PP value of two (E-16 and E-20) of them varies between 2 and 6 mg/HC rock and the PP value of the others is <2 mg/HC rock. Therefore, the studied shale samples vary from poor to very good source rock, and Figure 14b supports this.

Figure 14. The plots of (a) TOC vs. HI (Zhang et al., 2021) and (b) TOC vs S₁+S₂ (Kostova et al. 2022) show source rock characteristics for the black shale samples.

All the studied samples in the T_max and PI diagram (Lafargue et al. 1998) are in the “well-drained source rock” area (Fig. 12). Hunt (1995) stated that 1% TOC is required for oil production and 0.5% TOC for gas production. TOC values of the samples examined show a very strong positive correlation with RC values (r = 0.99, Fig. 15a), and RC values of 3 samples (E-15, 16 and 20) are > 1%, indicating a few generative potentials left. Furthermore, given the positive relationship between TOC and PC values (r = 0.98, Fig. 15b), thermal maturity and mixed kerogen type (mostly terrestrial and marine), some samples with high TOC content may release little hydrocarbons (English et al. 2004; Mallick et al. 2022).

Figure 15. The plots of (a) TOC vs. RC and (b) TOC vs. PC show hydrocarbon potential for the black shale samples.

In summary, it is observed that primary productivity is generally low in the basin (except in some places) and, accordingly, the amount of OM is not very high. It appears that palaeo-conditions were not suitable for OM accumulation at the top and bottom of the sequence. Conversely, OM-rich shales were deposited in the middle part of the sequence, particularly between 150th and 280th metres. This can be explained by the variability of sea water level, OM productivity, palaeo-salinity, palaeo-redox, palaeo-climatic conditions and terrigenous material input in the basin. However, the palaeo-productivity, palaeo-climate conditions and terrigenous material input amounts of the samples taken from the bottom, middle and top parts of the section are generally similar (Table 2, Fig. 10a). The palaeo-redox and palaeo-salinity conditions of the middle part of the section differ slightly from the bottom and top parts and are relatively higher (Fig. 10b). Therefore, it may be said that the OM enrichments in the Lutetian shales were controlled by palaeo-redox and palaeo-salinity conditions. In other words, the fact that the samples in the middle part of the section are rich in OM can depend on the rise in sea level, low-density flows and the provision of a stratified water column. Due to the stratified water column, anoxic conditions may have been temporarily provided, albeit slightly, and OM may have been preserved. Therefore, it is thought that the sea level can be divided into three stages during the deposition of Lutetian clastic rocks. In the I. stage, the sea level is low, there is terrigenous debris matter and low productivity (Fig. 16a). In stage II, the sea level is relatively high, the terrestrial debris material is lower than in stage I and the productivity is higher than in stage I (Fig 16b). Stage III is like stage I, with low sea level, high terrestrial debris material input and low productivity (Fig 16c). However, during the Lutetian, climatic conditions were hot and dry, and chemical weathering was low.

Figure 16. Schematic figure showing the organic matter accumulation and sedimentation model of Lutetian clastic rocks, (a) stage I, (b) stage II, (c), stage III.

6. Conclusion

The following results were reached after a thorough analysis of the geochemical properties of Lutetian shale samples from the Everek area, including elemental composition, C isotopic signature, Rock-Eval parameters, GC and GC-MS saturated and aromatic hydrocarbon parameters.

It was determined that in the HI -T_max covariation, 7 shale samples included Type II-III kerogen, while three shale samples included Type II kerogen. S₂/S₃ ratios of the analyzed samples are widely distributed and vary between 1.28 and 62.45 and according to various classifications, the kerogen content of these samples varies between Type I and Type III. δ₁₃C_org isotope values ranging between −28.23 and −28.22 support this, which illustrates the low contribution of marine OM and the strong OM intake from land during the deposition of shales. GC parameters such as n-C₁₇/n - C₂₇, Pr/n-C₁₇ - /Ph/n-C₁₈ and TAR show that the OM contained in the samples is both terrestrial and marine, but terrestrial OM is more dominant. To clarify the sedimentation environment conditions of the black shales, a variety of biomarker analyses have been conducted. The dominance of C₂₉ααα20R steranes over C₂₈ααα20R and C₂₇ααα20R steranes, low C₃₁22R/C₃₀H ratios, high C₂₉/C₂₇ sterol ratios and the order C₂₉MA>C₂₈MA>C₂₇MA indicate that terrestrial OM is more dominant than marine OM.

Low CIA, C-values, high Th/U ratios (inorganic proxies for palaeo-climate), low Cu/Al, Ni/Al and Zn/Al ratios (inorganic proxies for palaeo-productivity) of the samples indicate hot-arid palaeo-climate conditions and low productivity. The moderate Ti/Al ratios represent moderate terrestrial detrital input during the depositional period. High Ni/Co, Pr/Ph, moderate V/(V+Ni), low Cu/Zn, C₃₅/C₃₄ ratios and low HHI values of the shale samples indicate oxic-dyoxic conditions during the Lutetian period. Sr/Ba and TS/TOC ratios showing a wide range indicate that the Lutetian basin changed between freshwater and marine facies and was a terrestrial-marine transition environment. The low gammacerane/C30hopane ratios of the samples also support this.

According to maturity estimations derived from T_max, calculated vitrinite reflectance values and biomarker maturity criteria, the studied shales have achieved the mature level of the oil window.

Overall, the results obtained, the Lutetian sequence was transported and deposited by low-density turbidite and hyperpycnal turbidite currents. During the deposition of the studied rocks, turbidite flows carried more terrestrial OM to the basin due to hot-dry climate conditions and this caused the amount of terrestrial OM to be more dominant than the marine one. Low-density turbidites contained higher amounts of TOC than hyperpycnal turbidites, suggesting that the turbidite current energy also affects the OM content. In addition, OM enrichment is thought to be controlled by palaeo-redox and palaeo-salinity as well as seawater level.