Use of petrological and organic geochemical data in determining hydrocarbon generation potential of coals: miocene coals of Malatya Basin (Eastern Anatolia-Turkey)

With this study, the hydrocarbon generation potential of Miocene aged coals around Arguvan-Parçikan in the northern district of Malatya province was evaluated with the aid of petrological and organic geochemical data. According to organic petrography, coal quality data, and low thermal maturity, the Arguvan-Parçikan coals are of high-ash, high-sulfur subbituminous B/C rank. The organic fraction of the coals is mostly comprised of humic group macerals, with small percentages derived from the inertinite and liptinite groups. The mineral matter of the coals is comprised mainly of calcite and clay minerals. The total organic carbon (TOC, wt%) values of the shale and coal samples are between 2.61 wt% and 43.02 wt%, and the hydrogen index values are between 73 and 229 mg HC/g TOC. Pyrolysis (Tmax, PI), huminite/vitrinite reflectance (Ro, %), and biomarker ratios (CPI, Pr/Ph ratio, Ts/(Ts + Tm) ratio, C32 homohopane ratio (22S/22S + 22R) and C29ββ/(ββ + αα sterane ratio) indicate that the organic matter of the studied coals is thermally immature. When all these data are taken together, Miocene aged coals around Arguvan are suitable for hydrocarbon generation, especially gas, in terms of organic matter type (Type III and Type II/III mixed), organic matter amount (> 10 wt% TOC), however, low liptinitic macerals (< 15%–20%), low hydrogen index (< 200 mg HC/g TOC) and low thermal maturity values inhibit the hyrocarbon generation.


Introduction
Energy is one of the most fundamental and driving needs of the countries' social and economic development, and the concepts of ''energy security'' and ''sustainable energy'' are the main support for the stability of economic life and for ensuring national security in a global or regional significance. Although the energy consumed in the world today is derived from different sources, fossil fuels (oil, coal and natural gas) have the largest share in these resources, accounting for nearly 87% (TP Energy Report 2015) of all the sources. Turkey, though quite poor in terms of oil reserves, has different coal reserves in all its geographical regions, notably in Western Anatolia . However, approximately 68% of the lignite/sub-bituminous coal in Turkey are of low calorific value (nearly \ 1500-2000 kcal/kg) (Palmer et al. 2002;. In Turkey, for which coal is so important in terms of meeting the energy needs, this resource needs to be reevaluated in accordance with the industrial and energy sector development in the world as well as industrial and economic scope. In particular, the suitability of coals for liquefaction and gasification processes or the determination of their coal-derived hydrocarbon potential are important issues. Few geological studies have been carried out on the Tertiary coals of Anatolia (Artova, Zile-Tokat, Hafik-Sivas; Ç ayırlı-Erzincan, Oltu-Erzurum; Soma-Manisa; and Ş arkikaraagaç-Isparta), and thus a limited number of and organic geochemical and petrographic evaluation has been made (Yalçın Erik and Sancar 2010;Hoş Ç ebi and Korkmaz 2013;Yalçın Erik and Ay 2013;Kara Gülbay 2015;Ü nal et al. 2014;Hökerek and Ö zçelik 2015;Bechtel et al. 2016;Ü nal and Ö zçelik 2017).
Although some geological and stratigraphic investigations have been made concerning the coals around Arguvan-Parçikan (Malatya) which are the subject of this study (Yoldaş 1972;Sun 1987;İçel 1988;Türkmen and Aksoy 1998;Erdogan 2004;Türkmen et al. 2004;Koç-Taşgın 2011;Booth et al. 2014), there is not any study on the evaluation of petrology, organic geochemistry and hydrocarbon generation potentials.
This study focuses on the Miocene Arguvan-Parçikan (Malatya) coal bearing units located in the East Anatolian Fault Zone (EAFZ), Eastern Anatolia, Turkey. The principal aim of the study is to determine factors that control coalification environment and hydrocarbon generation potentials by means of bulk coal analyses (proximate-ultimate), mineralogical composition, coal petrography, and organic geochemistry.
2 Geological and stratigraphic properties 2.1 Regional geology The formation of Central Anatolian basins between the two continental units, Sakarya continent and Kırşehir Massif, with the closure of the northern branch of Neo-Tethysin in the Cretaceous-Eocene time period is very important for the regional geological evolution of Turkey, (Ş engör and Yılmaz 1981;Görür et al. 1998) (Fig. 1a, b). These basins, which indicate the paleotectonic period, are divided into two groups as magmatic arc (pre-arc and intra-arc) and collision-related interior basins (Görür et al. 1998). Malatya Basin, included in the study area, is the continuation of Hekimhan Basin and started to form in Late Cretaceous (Okay and Tüysüz 1999). The collision of Anatolian and Arabian plates during the Early Miocene completed its evolution with the closure of the Neo-Tethys. The coexistence of Neogene continental expansion and strike-slip tectonic movement in Eastern Anatolia caused the formation of limited intermountain basins such as Malatya Basin (Görür et al. 1998). Gravity tectonics in the study area are represented by growth faults that began in the early Miocene, and an anticlinal found in Middle Eocene sediments, and the Malatya fault, which cuts the Malatya Basin, with a NE-SW direction, with a left-directional pulse, and the rupture faults that cut the southern growth fault of the basin are structural elements that express neo-tectonic movements. (Okay and Tüysüz 1999;Ö nal 2009;Türkmen et al. 2004). This fault-controlled basin is an area where active tectonic activities still continue nowadays (The latest important earthquake occurred in the magnitude of 5.3 around Malatya-Elazıg on 24 January 2020). As a result of displacements between Eastern Anatolia Fault Zone (EAFZ) and Pliocene and Early Pleistocene, fluvial and lacustrine sediments occurred in these basins. There are also small economic coal deposits and/or coal layers as part of the basin development Türkmen et al. 2004;Ö nal 2009).

Stratigraphy
Stratigraphic sequence of the study area consists mainly of Paleozoic and Cenozoic aged sedimentary rocks and volcanics (Figs. 2a,3). Paleozoic aged limestones are dark blue gray colored, medium-thick bedded limestones and calcschists. The Cenozoic sequence starts with Eocene series and is considered to be two lithostratigraphic units, Eocene (Lutetian) conglomerate and Limestone-Shale  (İçel 1988). The polygenic conglomerates forming the base of the Eocene are approximately 100 m thick and consist of thick-bedded, brown colored, tightly cemented conglomerates (Fig. 2a). In the limestone-shale alternation, the limestones are yellow-light brown colored and highly fossiliferous. Oligocene aged unit, which is red, gray-green colored, generally consists of conglomerate-sandstone-shale intercalation, and the thickness is around 50-100 m (Figs. 2a, 3) (İçel 1988). The Burdigalian aged unit consists of red claystone, mudstone, siltstone-mudstone intercalation with coal levels, limestone and marls (Türkmen et al. 2004). This unit, which shows lateral and vertical transitions with Malatya volcanic, gives a sample in the Karaca Anticline (KA) core in the south (Fig. 3). It was described as the Alibonca Formation by Ö nal (2009). Volcanic rocks in the study area consist mainly of trachyandesite, andesite and basalts. The Late Miocene series with a lacustrine character consists of three levels: Mü1 (Conglomerate-Sandstone-Claystone), Mü2 (Limestone-Tuffite-Basalt Currents, coal layers), Mü3 (Conglomerate-Limestone-Claystone-coal layer) (Figs. 2a, 3) (İçel 1988).

Analysis methods
The properties of the coals in the study area (thickness, distribution area, lithotype changes and tectonic effects) were evaluated by field surveys and compared with other coal occurences in this basin. Thin coal veins are seen around Arguvan, mainly around Parçikan, Kuyudere, Göçeruşagı and Akören villages (Fig. 3). However, the only economically operating site is the Parçikan coal field. As the coal vein in this area is relatively thick (2 m) and the relationship between the floor and overlying rocks is more clearly seen, detailed investigations and sampling were made in the vicinity of Parçikan village (in the Arguvan-Parçikan road cut- Fig. 4a, d and Parçikan coal mining gallery- Fig. 4b).
Six channel samples of coal and other organic rich mudstones, coaly shales from all investigated areas were collected according to ASTM D-2234 and used for this study. A systematic sampling based on variation in lateral and vertical succession of the sedimentary facies was carried out. The samples were taken at about 1 m interval vertically after removing weathered surfaces. Lithological features of each of the studied coal sections  were macroscopically described and the lignite lithotype was determined according to guidelines established by the International Committee for Coal and Organic Petrology (ICCP 1994), as well as by Taylor et al. 1998.
Leitz MPV-SP reflective light microscope and 50X objective were used for petrographic evaluations. Leica DM2500 P and MSP200 windows-based program were used for random huminite/vitrinite reflection measurements and they were evaluated according to ICCP (1998), ISO 11760 (2005. Random reflectance measurements were performed using the Zeiss RS-III microscope with 40 9 oil immersion objective according to ISO 7404-5 (2009) standards, and results have been presented as mean random reflectance values (R o , %). The GI, TPI, WI, and GWI parameters were determined according to the formulas prepared for Tertiary aged lignites, by Kalaitzidis et al. (2004). The maceral nomenclature was based on the ICCP System1994 (ICCP 2001;Sýkorová et al. 2005;Pickel et al. 2017).
The standart proximate and ultimate analyses of the coal samples were conducted according to ASTM standards (ASTM D3174, 2004;D3302, 2004;D5373, 2004) in M.T.A. The analyses were carried out in the laboratories of the General Directorate of Mineral Research and Exploration MAT Department (MTA, Ankara). The gross and net calorific values of coal samples were determined using an IKA 4000 adiabatic calorimeter (ASTM D5865, 2004); also, sulfur, carbon, hydrogen and nitrogen contents were determined with LECO TGA 701 analyzer in the same laboratory.
Mineralogical composition was determined in the 28 powder coal samples and their clay fractions (Ø \ 2 lm) using X-ray diffraction spectrometers with CuKa radiation (4-70 2H range in powder-samples and 0-30 2H range in clay-fractions) in the laboratories at Cumhuriyet University (Sivas) and the General Directorate of Mineral Research and Exploration (MTA, Ankara).
Rock-Eval/TOC analysis was made for ten coal and coaly shale samples that were collected systematically from measured sections in the Arguvan-Parçikan coal areas (Figs. 2b,3). These analyses of all the samples were made using a Rock-Eval 6 instrument equipped with a TOC module. The samples were heated from 300°C (hold time 3 min) to 650°C at a rate of 25°C/min. Using the TOC and Rock-Eval pyrolysis results as a basis for selection, ten samples were extracted in ASE 300 with Dichloromethane (CH 2 Cl 2 ) for approximately 40 h in the TP AR-GE laboratory (Ankara). The whole extract was analyzed using an Agilent 6850 gas chromatograph equipped with a flame photometric detector and flame ionization detector. In addition, saturated portions were analyzed by Agilent 7890A/5975°C GC-MS, terpane and sterane molecules were studied in SIM Scan mode using 191 and 217 m/z ratios, respectively (TP AR-GE Lab., Ankara).  (Fig. 4c, e). Macroscopically, the coal section displaying blackish colour is dull and brittle, and contains gastropod shells at certain places (Fig. 4f). Clayey coal levels showing soft sediment deformation under the influence of active tectonic movements are present below this level (Fig. 4e). There are abundant fossiliferous, clayey limestones particularly on the floor and ceiling of the thick coal vein. Black colored very fossiliferous claystones with 8 cm thickness and gray-black colored fossiliferous coal levels are observed on matt, graydark brown colored claystones. Approximately 50 cm of cream colored, occasionally dark brown, claystones with abundant-cracks are followed by the main coal vein. At the base of this vein is a 20 cm brown, calcite filled thin coal layer followed by a bright black colored 30 cm coal vein and 15 cm brown colored, mostly clayey coals. The side rocks of the coal vein are composed of gray-dark yellow, brownish-green colored clayey and carbonated parts and alteration structures are intensely observed in places (Fig. 4a, d).
Coals around Akören village are seen as alternation of black, dark brown color, mostly calcite and claystone. The lower parts of the coal vein are matt black, with abundant cracks; medium levels are creamy-yellowish black, hard and layered, the top layer of coal is cracked and matt black and 145 cm thick (Fig. 4a). The coal bearing section around Kuyudere village is approximately 200 cm thick and has a matt black and brown color, mostly carbonate host rock, clayey, and it has abundant cracks. Above the coal layer, there are abundant fractured and brittle claystones, and at the top are layered limestones (Fig. 4b). It is observed that coal layer thicknesses are around 30-40 cm and some carbonated and clayey levels are frequently intercalations. In the coal sample observed around the Göcerusagı, generally there are ''mixed'' levels with soil cover, irregular ''thickness and diffuse'', and dense clayey layers. There are coal levels with an approximate thickness of 195 cm. It was found that the matte black and brown coals, although the layer is difficult to follow, 0.3-0.85 cm thick coal layer, are mostly carbonate host rock and claystone intercalation due to the presence of a cover layer and a thin soil layer.

Mineralogical compositions
The coal bearing units in the studied areas are mainly composed of coal, organic-rich carbonated rock, coaly claystones and clayey levels (Figs. 2b,4a,b,d). The investigated samples contain a variety of minerals at variable proportions. Identifiable mineral constituents include pyrite, quartz, clay minerals, calcite, and jarosite (Fig. 4d). The dominant component is carbonates (calcite; 4 wt%-96 wt%; dolomite; 7 wt%) and clay (11 wt%-82 wt%) in the mineralogical composition of the studied samples. In addition, sulphides (framboidal pyrite and cavity fillings; 4 wt%-7 wt%), quartz 3 wt%-48 wt%, feldspar 5 wt%-46 wt% are other minerals in the composition. The predominance of silicate minerals along with the high ash yields implies that the clastic input was high during peat depositions. The clay minerals are mostly aggregated with other clastic minerals (e.g., quartz, feldspars) and attrinite as well. Clay minerals determined by XRD-clay fraction diffractograms are illite (12 wt%-76 wt%), smectite (21 wt%-63 wt%) and chlorite (11 wt%-75 wt%). Framboidal pyrite crystals are common in the studied samples, whereas massive pyrite crystals are barely identified.  (Table 1). The ultimate analysis shows that the coal samples are characterized by moderate hydrogen (up to 1.67%, on an air dry basis) contents (Tables 1 and 2). Carbon values on dry basis were between 12.63% and 65.53% (av; 43.41% on an air-dry basis), hydrogen 4.18%-7.06% (av. 5.53% on an air-dry basis), nitrogen 0.13%-2.07% av. 1.23% (on an air-dry basis), H/C ratios vary between 0.91 and 6.71 and O/C ratios range between 0.06 and 0.90 ( Table 2). The gross calorific value varies between 318 and 2027 kcal/kg (av. 1001 kcal/kg), and the volatile matter yield is between 5.56% and 27.89% (av. 15.45%). 1282-2027 kcal/kg values were determined only from the hand piking samples taken from the coal bands operated in the Parçikan coal field. The calorie, total sulfur and fixed carbon values of the samples in the other sites were lower and the ash yields were higher in comparison with the Parçikan coal area samples (Table 1).

Lithotype characteristics
When broken, the sorfaces of the Parçikan coals appear blackish-brown, while the layer is seen to be darker black at some levels. After surface moisture is gone, prismatic cracks and concoidal fractures develop in the coal structure ( Fig. 4c, e). The coals in the investigated locations are macroscopically different from each other due to their mainly seam thickness, and physico-chemical degradation processes. Coals in Akören, Göçeruşagı and Kuyudere sections are generally much more clayey; they are seen in the form of very thin bands or lenses and differ from Parçikan coals in macroscopic view. In the field observations of the coals and in the detailed macro examination of the samples in the laboratory, the durain level is clearly monitored with a few mm clarain band with vitrain bands of approximately 5-6 cm thickness (Fig. 4c, e). Plant tissues, fossil shells (gastrapods) and jarocyte minerals, which have remained intact in fine coal seam, have also been found (Fig. 4d, f).

Microlithotype and maceral composition
The quantitative distribution of different macerals (on mineral-free basis) and associated mineral matter from the coal samples are shown in Table 3. Representative microphotographs of various macerals are illustrated in Fig. 5. Petrographic observations reveal that the Arguvan coals are rich in huminite (49 vol%-67 vol%, mean; 60.29 vol%), in which textinite and ulminite (telohuminite subgroup), attrinite, densinite (detrohuminite subgroup), and corpohuminite (gelohuminite subgroup) are more abundant than other macerals. (Table 3 and Fig. 5). The amount of the macerals of the liptinite group is very low (3 vol%-9 vol% (av. 5.86 vol%), and they are represented by sporinite, resinite suberinite and dominantly liptodetrinite. The color of sporinite in fluorescent light is yellow. Inertinite macerals are rare 1 vol%-4 vol% (av. 2.29 vol%) and represented by fusinite, funginite and inertodetrinite. The inertinite group macerals occur in low concentrations (av. 6.64 vol%), and inertodetrinite is the most common maceral of this group (Fig. 5). The distributions of the maseral groups of the studied coal samples in the triangular diagram are given in Fig. 6a, b.
The mineral matter content, as determined microscopically, ranges from 11 vol% to 42 vol%, and consists mainly of clay minerals, followed by carbonate (calcite), sulphate (gypsum) and iron (pyrite) minerals. The most common occurrence of pyrite in the form of crack and void filling or granular formations observed in the coals is framboidal pyrites and they have an average composition of 3.71 vol% (Table 3) (Fig. 5). Framboidal pyrite crystals are generally syngenetic and related to reducing conditions within the palaeomire (Kalaitzidis et al. 2004;Siavalas et al. 2009). Although the coal formations around Arguvan appear physically different from each other, there is no an important difference in petrographic composition. Only Akören coals can be said to have more inertodetrinites, except that the maceral composition properties are similar (Table 3). Use of petrological and organic geochemical data in determining hydrocarbon generation… 517 Table 3 Maceral composition (vol%), mineral matter content (vol%), and petrological parameters (GI, TPI, GWI, VI) of Arguvan coals Humınıte group macerals (vol% on mineral matter free basis) Liptinite group macerals (vol% on mineral matter free basis) Coal section

Bulk geochemical characteristics
Rock-Eval pyrolsis data were interpreted according to Peters (1986) and Lafarqué et al. (1998), on the amount of organic matter, organic matter type (kerogen type), diagenetic process (thermal maturity) and hydrocarbon generation potential of coal and coaly samples taken from the vicinity of Arguvan. Total organic carbon (TOC) values range from 2.61 wt% to 43.02 wt%. The total organic carbon and hydrogen index values are seen to be higher especially in the Parçikan and Akören area samples (Table 4). Coal samples are characterized by a low hydrogen index (HI = 73-229 mg HC/g TOC). T max values ranging from 409 to 436°C agree with the low rank of the coal.
4.6 Molecular geochemistry reviews 4.6.1 n-alkane and isoprenoid compound properties The extract values of the investigated samples are generally low (Table 5) In the studied samples, n-alkanes are the dominant peaks of gas chromatograms, and unimodal distribution is observed (Fig. 7a). The relative abundance of the oddnumbered ones in the n-alkanes in the C 11 -C 35 range and the n-C 29 and n-C 31 components is dominant (Table 5 and Fig. 7a). In particular, long chain n-alkanes (n-C 27 to n-C 32 ) are typically found in the highest relative abundances in low-rank coals in Turkey (Yalçın Ay 2010, 2013;Ü nal et al. 2014;Kara-Gülbay 2015). High molecular weight ([ n-C 27 ) straight chain lipids, which are  (Eglinton and Hamilton 1967). These data bring about a controversial issue in terms of the formation of long-chain n-alkanes from vascular plants, but they are suitable for the immature character of organic matter (Peters and Moldowan 1993). Low molecular weight n-alkanes (\ C 20 ) consisting of major algae and microorganisms are found in very low relative ratios, such as 1% of total n-alkanes (Peters et al. 2005) (Fig. 7). The intermediate molecular weight n-alkanes (n-C 21-25 ), which are reported to be originated from aquatic macrophytes (Ficken et al. 2000), are found in studied samples in proportions between 19% and 29% relative to total n-alkane concentrations (Table 5 and Fig. 7a). The acyclic isoprenoids pristane (Pr) and phytane (Ph) are present in the saturated hydrocarbon fractions of all samples in low concentrations (Pristane; 0.09%-0.97%, and phytane; 0.12%-0.54%) ( Table 5, Fig. 7a).

Coal type and rank
Physical and chemical properties of coal are crucial for the classification of coal, quality determination and definition of the potential economic value. Rank (maturity) of the Arguvan coals are specified by reflectance measurement on ulminite (huminite) maceral. The mean random reflectance (Ro) value ranges between 0.38% and 0.51% (Table 4) across the seams, suggesting that the studied coals have attained 'Brown coal' (German Standard) or 'Sub-bituminous' stage/rank (ASTM) and are of low rank B/C ( ISO: 11760, 2005), and fall in the early diagenetic zone of methane generation (Taylor et al. 1998). It is consistent with other maturation parameters and indicates that the units studied have not matured yet.

Evaluation of hydrocarbon generative potential
of coals with petrological and biomarker data

Type of organic material
In the Van Krevelen (HI-OI) and HI-T max diagrams prepared according to the Rock-Eval pyrolysis data of the studied samples, most of the samples were scattered in Type II-III (terrestrial and marine) and Type III kerogen (terrestrial, residual organic matter) in the area (Fig. 8a, b). This data is consistent with the results obtained in the petrographical evaluation of coal samples, and shows the dominance of terrestrial organic matter (Table 3, Fig. 6a). N-alkane and isoprenoid ratios are also used so as to determine the type of organic matter (Peters and Moldowan 1993;Peters et al. 2005). In the studied samples, the nalkane distributions are in the range of n-C 11 and n-C 35 and the long chain hydrocarbons in the range of nC 25 -nC 31 are the dominant peaks (72.75%-89.06%). Medium chain hydrocarbons in the range nC 21 -nC 25 in the total n-alkane composition are 7.62%-17.82% and short-chain n-alkanes are found in the ratio of 2.26%-9.43% (Table 5). Shortchain n-alkanes are formed from algae and microorganisms and as shown in Table 3; the low rate of alginite in the composition (1%-3%) is also supported by low hydrogen index and high oxygen index values (Ficken et al. 2000) ( Table 4).
The abundance of long-chain n-alkanes in immature coals indicates that the primary organic material is terrestrial plant waxes (leaves) rich in cuticular and waxy constituents of long-chain fatty fatty acids, alkanoids and their esters (Eglinton and Hamilton 1967;Tissot and Welte 1984;Zdravkov et al. 2011;Yalçın Erik 2011;Fabianska et al. 2012). According to Schwark et al. (2002), the increase in nC 27 and nC 29 components in long chain nalkanes indicates high terrestrial plants in forests, and nC 31 indicates herbaceous components.
In varying proportions of the composition, medium-chain n-alkanes are composed of vascular plants, microalgae, cyanobacteria, Sphagnum spp (algae) and aquatic plants (Ficken et al. 2000;Nott et al. 2000). The biomarker parameters calculated for the Arguvan samples are listed in Table 5. The carbon preference index (CPI) and Odd-to-even predominance (OEP) may be used as an indicator of the source of the OM (Misra et al. 2020). The carbon preference index (CPI 24-34 ; Bray and Evans 1961) values range between 2.51 and 5.06, and indicate higher plant source for the organic matter. In studied samples, even though the values are in accordance with the low rank, the slightly low values could be caused by high However, it is stated in recent studies that CPI 24-34 value also affects petrographic composition. For example, in Poland (Konin and Turolszow coals), in the Czech Republic and some coals in Greece, it was observed to be different in organic geochemical evaluations, and CPI 24-34 values were also different because of lithotype differences (Siavalas et al. 2009;Zdravkov et al. 2011;Fabianska et al. 2012;Havelcova et al. 2012). In general, the average CPI value of xylitic coals 2.8, in detritic and 4.2 in detoxylitic coals and fusinitic lignites were found to be 1.88-1.78 on average (Fabianska et al. 2012). Possible reasons for this can be the changes in the amount of low fatty acid containing mainly n-alkanes with a single carbon number in cuticular waxy constituents rather than changes in environmental conditions or maturity (Fabianska et al. 2012).
The Paq determines the input of submerged/floating aquatic macrophytes relative to emergent and terrestrial organic matter. The proxy aqueous (Paq; Ficken et al. 2000) index of the samples varies from 0.24 to 0.35. In modern plants, these values correspond to the emergent macrophytes. However, in sediment extracts these values can indicate a mixed organic matter source.
In the studied samples, the nC 17 /nC 31 ratio (0.02-0.11) (Bray and Evans 1961;Hunt 1995) and degree of wax (Connan and Cassou 1980), P wax [ P aq and nC 23 /(nC 27-? nC 31 ) values also support the result of domination of terrestrial organic matter. However, although the terrestrial plants are dominant, wood/plant ratio (Q w /p) and weed/plant ratio (Q g /p) of grassy and grass type short, low cellulose ratio of plants living around the lake show the enrichment (Peters et al. 2005) (Table 5).
The acyclic isoprenoids pristane (P r ) and phytane (P h ) are observed in relatively low concentration. The pristane/phytane ratio ranges between 0.28 and 1.43.
In addition, sterane and terpan distributions reflect the types of organic matter (Tissot and Welte 1984;Peters et al. 2005). The presence of C 29 [ C 28 [ C 27 aaa (20R) sterane in the Arguvan coal samples indicates that mainly high terrestrial plants and later herbaceous plants are dominant in peat formation and small amounts of lacustrine algae are added to the composition (Peters et al. 2005) (Fig. 6b).
However, terpane compositions and paleoenvironment indexes may also indicate the type of organic matter as well as the maturation value (Peters and Moldowan 1993;Peters et al. 2005). The Arguvan coal samples are characterized by the high relative abundance of pentacyclic triterpenes with a maximum abundance of hopanoids. Very low concentrations of C 21 , C 23 and C 28 tricyclic terpane in coal samples from Arguvan, and the high amount of C 19 -C 21 tricyclic relative to C 23 (C 19 ? C 20 /C 23 tricyclic terpan ratio) refer to the input of terrestrial organic matter (Peters et al. 2005) (Table 6). Furthermore, the presence of the C 29 Ts18a (H)-Norhopane (C 29 Ts) component (Fig. 7b, c), and C 29 Ts 18 a (H)-Norhopane/(C 29 Ts18 a (H)-Norhopane ? C 29 17 a (H), 21 b (H)-Norhopane (C 29 Ts/C 29 Ts ? Norhopane) with ratios, and the presence of C 29 17 b (H),21 a (H)-30Normoretane and C 30 17 b (H), 21 a (H)-Moretane show that organic matter is of terrestrial origin (Philip and Gilbert 1986). Abundance of hopanoids in the studied samples indicates the considerable contribution from bacterial source to these sediments. Abundance of 17 a (H), 21 b (H)-homohopane (R) has been reported previously from low rank coals (Singh et al. 2017a).
This also suggests that these sequences might have been deposited in a more oxic and acidic environment (Ź ivotić et al. 2014). Extended hopanes are represented by 17 a (H), 21 b (H)-homohopanes (both 22R and 22S epimers), 17 b (H), Bacteriopolyhopanol, a compound present in the prokaryotic cell membrane is found to be the precursor of homohopanes (Waples and Machihara 1991;Peters et al. 2005). The identified triterpenoids other than hopanoids include tetra-and pentacyclic compounds with oleanane, and lupane skeletons. Also, they are found to constitute very low amount in the composition of oleanane (Fig. 7c), and this indicates that plant material is denser by gymnospermderived plants rather than angiosperms (Peters et al. 2005). According to all these data, it can be said that terrestrial plants are dominant in the paleo-swamp environment. However, beginning from the reed plants, grasses near the water, herbaceous components near the marsh near the drier forest area, a developing vegetative accumulation, and to a lesser extent, woody components with high cellulose content, and high terrestrial plants and algae were also included in the composition (Peters and Moldowan 1993;Peters et al. 2005).

Organic maturity
As a result of the evaluation of Hydrogen Index (HI) and T max (Mukhopadhyay et al. 1995), most of the samples are suitable for vitrinite/huminite reflection values (R max values 0.38%-0.51%) and indicate the immature stage (Fig. 8b). Production index (PI) which is another maturity parameter obtained by pyrolysis analysis is in the range of 0.02-0.07 and together with the T max data, and it indicates the diagenesis stage for immature sediments and hydrocarbon generation (Peters 1986;Hunt 1995;Wilkins and George 2002) (Table 4). Besides these data, low extracts of samples, bb hopans, 17 b (H) trisnorhopane, presence of olefinic components and triterpanes, CPI 24-34 values, C 32 22S/22S ? 22R homohopane ratios, C 23 tricyclic terpane/ (C 23 Tricyclic terpane ? C 30 hopane) Ts/Ts ? Tm ratio, moretane/hopane ratio, 20S/20S ? 20R and bb/bb ? aa, diasterane index values also indicate immature phase especially for hydrocarbon formation (Seifert and Moldowan 1986) (Tables 5 and 6).

Paleo-mire and coal facies characteristics
Facies analysis, palynology and sedimentological data are widely used in the determination of the coalification process and the characteristics of the depositional environment. The most common used petrological parameters in the paleo-environment and facies assessment of coals are tissue preservation index (TPI) and gelification index (GI), vegetation index (VI) and groundwater effect index (GWI). Changes have been made in this evaluation method by Calder et al. (1991) and Lamberson et al. (1991), in addition, some changes were made for coal with low carbonization degree by Kalkreuth et al. (1991).
Although there is a disagreement among the researchers regarding the interpretation of these data (Moore and Shearer 2003;Sen et al. 2016), TPI/GI and GWI/VI data continue to be used for paleo-depositional interpretation Calder et al. 1991;Kalkreuth et al. 2000;Flores 2002;Siavalas et al. 2009;Singh and Singh 2000;Yalçın Erik 2011;Sia and Wan Hasiah 2012;Singh et al. 2013;Bechtel et al. 2014;Sen et al. 2016;Hoş Ç ebi and Korkmaz 2013). However, a more accurate interpretation can be obtained by multidisciplinary approaches (Scott 2002;Moore and Shearer 2003).
In Arguvan coal samples, TPI, GI, GWI and VI indices were calculated according to modifications introduced by Kalaitzidis et al. (2004) and Scott (2002) for Miocene lowrank coals. The TPI measures the degree of humification of the peatforming organic matter. A high TPI value ([ 1) reflects equilibrium between the growth and accumulation of plant materials, rise of the water table, and domination of tree vegetation. A low TPI suggests either predominance of herbaceous plants in the mire or large-scale degradation of plant tissues as a result of advanced humification (Diessel 1992). The TPI also indicates the pH conditions of palaeomires because in a low pH conditions, microbial activity is weak and plants can be well preserved, and vice versa. Tissue Protection Index (TPI) value of Arguvan field samples ranged from 0.14% to 0.33% (Table 3).
The GI is the ratio of gelified versus non-gelified macerals and indicates the wetness in peat-forming environment. High GI value is indicative of a high moisture/water level and higher subsidence rate and vice versa. The gelification index (GI) value in the investigated samples ranges between 0.38% and 2.86%.
The VI is related to the type of vegetation that dominated the mire. It is dependent on the type of peat-forming plant communities (e.g. trees and bushes). VI generally indicates an aqueous environment in which herbaceous plants develop, as well as an area with woody plants.
TPI values are generally low; GWI is greater than 1 and VI is generally less than 1, also gastropod shells with high pyrite content indicate limnotelmatic environment (Fig. 9b). Low TPI and GI values were either dependent on the type of plant material (high angiosperm/gymnosperm ratio) or they developed according to low tissue protection conditions. As determined in the Arguvan coals, medium-high GI-GWI values indicate variable water level, and relatively medium-high sulfur content refers to alkaline, calcium-rich waters and changing P h values (Casagrande 1987;Bechtel et al. 2003). In addition, according to Flores (2002) organic facies characteristics, the coals around Arguvan are rich in herbaceous plants and deposited in a wet forest and dry-wet mixed swamp environment.
In this environment, the water level usually covers the paleotopography and generally suboxic-anoxic conditions prevail. In wet forest facies, humotelinite ratio (ulminite) is abundant, and in mixed marshes, humodetrinites are dominant, and to a lesser extent, resinites and subberinites are accompanied. Abundance of inertinite macerals such as fusinite after humunite maceral group shows fires and decreasing water level in the marshes, thus surface oxidation (Flores 2002). The richness of the studied coals in terms of sporinite and clay minerals also indicates degradation due to reeds and underwater conditions as well as bacterial activity.
The medium-high sulfur content in the coals studied can be explained by the composition of the primary organic material or by marine or brackish water conditions affecting the depositional environment. Framboidal pyrites are related to the presence of sulfate-rich waters, and anaerobic bacterial activity during the formation of peat showed that the paleo temperature value could be between 100 and 125°C (Rort, 0.38%-0.45%) according to the reflection value of Arguvan coals (Boggs 1987).
Isoprenoid and n-alkane data of the studied samples are also used in the interpretation of the precipitation medium. For example, the Pr/Ph ratio is directly related to the redox conditions of the precipitation medium (Didyk et al. 1978;ten Haven et al. 1987;Tissot and Welte 1984;Peters et al. 2005;Fabianska et al. 2012;Powell et al. 1991). High P r / P h ratio ([ 3.0), oxidized, low rates (\ 0.6) anoxic, values between 1.0 and 3.0 indicate suboxic conditions (Peters and Moldowan 1993). For example, in coal in SE Asia, it was reported that the P r /P h ratio was [ 4 and it showed peat marsh in oxic conditions (Zulkifli et al. 2008). The same situation is laso valid for Pinangah coals (Malaysia) (Alias et al. 2012;Hakimi et al. 2013) and many Tertiaryaged Turkish coal gave similar results (Yalcin Erik and Sancar 2010;Yalcin Erik 2011;Yalcin Erik and Ay 2013;Bechtel et al. 2014;Hoş Ç ebi and Korkmaz 2013). P r /nC 17 (0.26-2.4) and P h nC 18 (0.28-1.43) values were found to be similar to Tertiary coal in Bulgaria and Poland (Zdravkov et al. 2011;Havelcova et al. 2012). The P r /nC 17 and P h /nC 18 diagrams indicate oxidation-reduction conditions in the precipitation medium (Fig. 10). Fabianska et al. (2012) indicated the water washing effect of coals and stated that these results in Tertiary aged coal in Poland reflect the alteration effect and do not fully show the environmental characteristics. According to Fabianska et al. (2012), the water washing effect during or after carbonization is characterized by features such as low extract value in organic matter, destruction of methylnaphthalenes and sometimes removal of low carbon number n-alkanes. The low levels of nC 17 in the organic composition are supported by the low amount of alginite in the petrographic composition, for this type of paleo bog wet, forest-reed bog sedimentation medium (in the lower delta plain) was determined. However, as in the Parcikan coal samples, P r /nC 17 ratios varying generally from moderate to high (greater than 1 or nearly 1) are related to coal formation in the continental and limnotelmatic environments. Similar results were obtained in Tertiary aged coal in Sumatra basin (Indonesia) by Amijaya and Littke (2005). Sterane and terpane data of coals are important to explain the paleoenvironment characteristics (Peters 1986;Peters et al. 2005;Alias et al. 2012). Homohopans are present in all samples of the Arguvan coals, and the decrease in the 17 a (H), 21 b (H)-homohopane ratios in the C 31 -C 35 range is typically observed for clastic facies (Waples and Machihara 1991;Peters and Moldowan 1993) (Fig. 7b, c). The presence of hopanoids indicates the presence of bacterial organisms (Simoneith 1986), which is especially typical of coals in Indonesia (Amijaya et al. 2006).
Tm/Ts ratios may also reflect paleoenvironment properties; high values indicate oxic conditions during sedimentation (Peters et al. 2005). The high Tm/Ts values determined in the studied samples are consistent with the P r /P h ratios and indicate that the swamp formed by atmospheric precipitation (ombrogenous paleomire) is exposed to oxic conditions showing periodic change in the upper part (Ten Haven et al. 1987;Peters et al. 2005). A similar situation was also determined by Amijaya et al. (2006) in coal in Malaysia. However, it is seen that low proportional changes in Ts and Tm values may be related to maseral composition. The highest rates are in the coal rich in humocollinite (Amijaya and Littke 2005;Stojanović et al. 2012;Misra et al. 2020). In addition, high C 29 17a (H), 21b (H)-30-Norhopane values accompanying, low ''17a (H) -28,30-Bisnorhopane'' deltaic deposition conditions and low rate of marine organic matter doped, refer to sovereign terrestrial organic matter accumulation, and low diasterane/ sterane, low C 23 tricyclic terpane/C 23 tricyclic terpane ? C 30 hopane rates also support this data (Peters et al. 2005).
It can be said that the suboxic-anoxic conditions of Arguvan coals change periodically with high ash and sulfur content, topogenic, eutrophic swamp (in an elevated terrestrial area) and high mineral matter rate. It can also be said that there is an autochthonous-hypotoctonous coal formation that develops in limnotelmatic environment where there is not much intake of epigenetic clastic material. In addition, petrographic data of these coals are drawn in ternary diagrams. In these diagrams, the presence of mixed vegetation in the paleomire environment is indicated (Fig. 6c, d). Besides, the abundance of detrohuminite mixed up with liptodetrinite in some samples indicates significant contribution of reed/sedge vegetation. Appropriate pH and water-level conditions can be another Almost all samples are projected on the lowest part of the Mukhopadhyay's diagram indicating relatively strong anoxic conditions in the northern part of the deposit in comparison to the eastern one; this suggests rather high and relatively stable water table in the palaeomire during peat accumulation (Fig. 6c) (Diessel et al. 2010;Silva et al. 2008).

Source rock potential
As in the examples in the study area, humic coals contain richer organic matter content (generally \ 10% TOC) than conventional oil/gas source rocks, predominantly composed of type III and Type II/III kerogen, and have gas generation potential (Hunt 1995;Wilkins and George 2002;Petersen 2002). However, for Hedberg (1968), Sykes (2001), Petersen (2002) the hydrogen index (HI), which indicates the abundance of hydrogen in coals is more important than the total organic carbon (TOC) value. In general, they stated that values higher than [ 200 mgHC/g (Hunt 1995) are necessary for generation. The TOC values of Arguvan samples evaluated within this scope vary between 2.61% and 43.02%, hydrogen index (HI) values between 73 and 229 mgHC/g and H/C values between 0.91 and 6.71. In addition, S2/S3 values are between 0.59 and 3.34, and production index (PY) is 5. 57-87.22. S1 values are between 0.32 and 2.31, and S2 values are between 5.25 and 84.91 mgHC/gTOC. According to these values, it can be said that only P-08 and A-04 samples have gas generative potential (Tissot and Welte 1984;Peters 1986;Hunt 1995;Sykes and Snowdon 2002) (Fig. 11). However, as stated by Killops et al. (1998), the bitumen index of these samples is quite low for species (BI [ 10) and removal (BI [ 10) (0.02-0.12) ( Table 4). Considering the rank related increase in HI of low-rank coals (Sykes and Snowdon 2002), none of the samples exceed the minimum HI of 300 mg HC/g TOC required for oil generation (Pepper and Corvi 1995) when their thermal maturity reaches the onset of oil expulsion (''effective HI'' of Sykes and Snowdon 2002) (Fig. 11).
The hydrocarbon potential in the samples was evaluated mainly by pyrolysis analysis data, and petrological data for coals are also important (Petersen 2002;Fabianska et al. 2012). Generally, the ratio of liptinite macerals [ 15%-20% has been found to be very important for hydrocarbon generation from coals (Stach et al. 1982;Petersen 2002;Sykes and Snowdon 2002;Wilkins and George 2002;Alias et al. 2012). For the humic coals of the Dong Ho Basin (Vietnam), which offer petrographic and chemical properties similar to Arguvan coals, and the oil window onset for Konin and Turszow coals in Poland are equal approximately to 1.03%-1.15% R o . These coals have been mainly derived from oil and gas (Zdravkov et al. 2011;Fabianska et al. 2012). In Arguvan coals, maturation level is not sufficient for hydrocarbon generation according to biomarker maturity parameters and T max and R o values, and thus it was concluded that gas type generation could be possible at higher maturity values.

Conclusions
Concerning the significance of limited fossil fuel resources of Turkey, lignites/brown coals are of great economic importance as they represent the main source for energy production. A significant number of coal bearing basins with significant coal reserves formed during the Miocene especially in Western Anatolia of Turkey, as a result of different peat-forming conditions. Apart from the economical importance, the hydrocarbon generation potential and geochemical properties of coal-bearing strata have been intensely discussed all around the world. As a result of the floral evolution, especially Cenozoic coals and coaly sediments have higher potential to generate oil compared to older age counterparts.
During this research, the hydrocarbon potential and the paleo-depositional environment characteristics of the lignites from the places around Arguvan in Malatya Tertiary Basin were evaluated based on petrological, bulk organic geochemical parameters and biomarker analysis results. Samples were collected from Parçikan, Akören, Göçeruşagı and Kuyudere coal areas of Arguvan district. Investigated coaly units are composed of rich carbonated and coaly clay levels in coal, clay, organic matter.
Due to the regional intense tectonics, the coaly sequence is folded, broken, and easily fragmented. This lignites are typical humic coals. Calcite bands, pyrite, jarosite, and clay minerals are widely observed in the intermediate levels in the coal units or in veins, lenses, or scattered in the coal vein. The mineralogical composition of the studied samples is dominanted by mineral carbonate and clay (calcite, 4%-96%, dolomite, 7%). The clay minerals in the composition are illite (12%-76%), smectite (21%-63%) and chloride (11%-75%). The ash yield of the samples is quite high (avr. 80.81 wt%). Total sulfur content is 1.46%, total moisture content 28.69%, volatile matter content 15.45%, and the gross heating value is between 318 and 2027 kcal/kg (avr, 1001 kcal/kg). High values such as 1282-2027 kcal/kg have been determined only for the Parçikan coal mining area samples. The carbon content in the dry basis determined by elemental analysis is 43.41% on average, 5.53% on hydrogen average, 1.23% on average nitrogen. According to ASTM (1983) standards, Arguvan coals are of low quality and low maturity (R max %), low calorific, high ash, mineral matter and moisture values and potential for industrial use only as domestic fuel, of the type ''lower bituminous B/C type coal.
The Arguvan coal is dominated by mixed xylitic/attrital and detritic lithotypes and by huminite macerals (gelinite) (avr. 60.29%) with secondary inertinite macerals (avr. 5.57%) and minor liptinite macerals (avr. 5.29%). Although the coals examined seem physically different from each other, they do not differ greatly as petrographic composition. Local and vertical variations in proportions of huminites and inertinites reflect frequent fluctuations in water levels, periodic flooding, dehydration and burning periods of the paleomire. According to the petrological and organic geochemical characteristics, the paleodepositional environment which formed the investigated coals around Arguvan developed more mixed forest marsh where the herbaceous and reed-crop plants cover wide areas, the water level decreases and the trees are concentrated in more distant areas. Peatification proceeded in a freshwater environment under variable redox conditions, from anoxic to slightly oxic.
Recently, detailed organic geochemical analyses in coal extracts have been used as an effecting tool for assessment of the reconstruction of paleobotanical characteristics and palaeoenvironmental conditions in peatlands during the formation of coal-bearing strata. In the examined samples, the n-alkane distributions are in the range of nC 15 and nC 35 , with long and intermediate chain hydrocarbons being dominant. In addition, TAR, ETR, CPI, P wax [ P aq and n-C 23 /(nC 27 ? nC 31 ) values support the terrestrial organic matter dominance in paleomire. It can be said that suboxic-anoxic conditions are effective during the formation of peat according to n-alkane, isoprenoid and biomarker data. The major biomarkers are C 30 tricyclic terpane, hopane, homohopane, moretane. In the studied samples, C 29 [ C 28 [ C 27 steranes indicate the dominance of terrestrial and herbaceous plants, as well as the incorporation of lesser amounts of lacustrine algae. The environment with a high rate of sterane/hopane and a lacustrine or special bacterial activity and a low concentration of pentacyclicterpenes (C 32 -C 35 ) indicates that biodegradation, and autochthonous/aquatic and allochthonous terrestrial organic materials have accumulated in paleomire during the formation of coals. In XRD and organic petrographical studies, it can be said that the tectonic movements in the swamp are influenced by the appearance of clay minerals and detrohuminites in high order and formation of coal. This situation is also clearly observed in units showing coarse and soft sedimentary deformation in the field.
The total organic carbon (TOC, wt%) values of the samples are between 2.61 and 43.02% and the hydrogen index values are between 73 and 29 mgHC/g TOC. The high TOC is corroborated by the hydrocarbon yield (S2) indicating excellent source rock generative potential for the samples. Based on HI, the Arguvan-Parçikan coal can be classified as being capable of generating gas/oil ([ 200 mg HC/g TOC) at higher maturity. Vitrinite reflectance (R o , %), T max and biomarker ratios show that the organic matter of the studied samples are thermally immature. When all these data are considered together, Tertiary aged coals around Arguvan are have good hydrocarbon generation potential, especially gas, in terms of organic matter type (Type III and Type II/III mixed), organic matter amount ([ 10 wt% TOC), however, low liptinitic macerals (\ 15%-20%), low hydrogen index (\ 200 mg HC/gTOC) and low thermal maturity values inhibit the hyrocarbon generation.