Relationships between coal-quality and organic-geochemical parameters: A case study of the Hafik coal deposits (Sivas Basin, Turkey)

International Journal of Coal Geology 83 (2010) 396–414

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International Journal of Coal Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / i j c o a l g e o

Relationships between coal-quality and organic-geochemical parameters: A case study of the Hafik coal deposits (Sivas Basin, Turkey) N. Yalçın Erik ⁎, S. Sancar Cumhuriyet University, Department of Geological Engineering, Sivas, Turkey

a r t i c l e

i n f o

Article history: Received 3 June 2009 Received in revised form 30 April 2010 Accepted 21 May 2010 Available online 31 May 2010 Keywords: Sivas Basin Turkey Organic geochemistry Organic petrography Biomarker Tertiary coals

a b s t r a c t This study provides coal-quality, organic-petrographic and organic-geochemical data on Tertiary subbituminous coal of the Hafik area, northwestern part of the Sivas Basin, Turkey. Coal-petrological studies along with proximate and ultimate analyses were undertaken to determine the organic-petrographic characteristics of the Hafik coals. Huminite reflectances were found to be between 0.38 and 0.48% (corresponding to an organic-material-rich and coal layers), values characteristic of low maturity. This parameter shows a good correlation with calorific values (average 21,060 kJ/kg) and average Tmax (422 °C) mineral-matter diagenesis, indicating immaturity. The studied coals and organic material underwent only low-grade transformation, a consequence of low lithostatic pressure. Therefore, the Hafik coals are actually subbituminous in rank. Rock-Eval analysis results show types II/III and III kerogens. The organic fraction of the coals is mostly comprised of humic-group macerals (gelinites), with small percentages derived from the inertinite and liptinite groups. In this study, organic-petrographic, organic-geochemical and coal quality data were compared. The Hafik deposit is a high-ash, high-sulfur coal. The mineral matter of the coals is comprised mainly of calcite and clay minerals. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Since the 1950s, many investigations have indicated the possibility that terrestrial sediments (especially coal and bituminous shale) have a potential to generate oil or gas by thermal maturation of organic compounds when buried in a sedimentary basin (Hubbard, 1950). Organic-geochemical studies, such as Rock-Eval pyrolysis, show that humic coals have a chemical potential to generate hydrocarbons (Durand and Paratte, 1983; Espitalié et al., 1985; Mukhopadhyay et al., 1989; Littke et al., 1990; Powell et al., 1991; Boreham and Powell, 1993; Clayton, 1993; Littke and Leythaeuser, 1993; Powell and Boreham, 1994; Clayton, 1998; Kalkreuth et al., 1998). Oil versus source-rock correlations and basin modeling indicate that, in general, Jurassic to Tertiary coals have high oil-generative capacity (Wilkins and George, 2002). Generally speaking, accumulation of oil is not fully associated with coals but oil is associated with coaly shales (Gipssland Basin Upper Cretaceous–Tertiary humic coals (Smith and Cook, 1984), Australia and Indonesia basins (Hunt, 1995); moreover, coals mainly produce gas, such as at Gronnigen, northern Netherlands, in the Australian Cooper Basin, and in the Deep Western Canada Basin (Snowdon, 1980; Thompson et al., 1985; Hunt, 1995; Hu et al., 1997). Also, some studies have indicated the oil-generative potential of different coal macerals (Dai et al., 2000).

⁎ Corresponding author. Tel.: +90 3462191010*1308; fax: +90 3462191171. E-mail addresses: [email protected], [email protected] (N.Y. Erik). 0166-5162/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2010.05.007

Limited, small-scale coal resources in Turkey have been worked by private companies but are insufficient to be exploited in an economic, industrial fashion. Ongoing increases in energy demand and costs requires that local sources be utilized much more efficiently, therefore geochemical studies to determine hydrocarbon production capacities — from especially coals — have acquired great importance in Turkey (Yalçın et al., 2007). Most of the Tertiary coal deposits located in the Sivas Basin are near the towns of Kangal, Divriği, Şarkışla and Gemerek. However, no detailed and systematic organic-petrographic and organic-geochemical investigations have been carried out for these coal areas. The aims of the present study were to acquire detailed organicgeochemical, organic-petrographic and coal quality data from borehole and surface samples, to perform a geochemical characterization of source-rock potential, to determine the relationships between organic-geochemical and coal quality data, and to evaluate the industrial utilization potential of the Hafik coal sequences. 2. Geological setting There have been numerous geological studies in the Sivas Basin (Kurtman, 1973; Poisson et al., 1996 etc.), but most of these studies have focused on basic geological features. Investigations of hydrocarbon potential were carried out by Sungurlu and Soytürk (1970) and Altunsoy and Özçelik (1998). In particular, no detailed, systematic organic-petrographic and organic-geochemical investigations have been carried out on the coal area around Hafik (Sivas).

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The Sivas Tertiary Basin is one of the central Anatolian basins that formed over the collision zone between the Pontides and the Anatolide–Tauride belts (Görür et al., 1998); it developed mainly after the closure of the northern branch of Neotethys in the Early Tertiary. During the Late Eocene–Oligocene, the basin became an uplifted foreland basin (Poisson et al., 1996; Görür et al., 1998). The basin developed during the Late Paleocene–Middle Eocene on southward-obducted ophiolitic mélange sheets. This sedimentation includes fan-delta conglomerates with nummulitic limestones and marls (Kurtman, 1973). The Upper Paleocene–Lower Eocene Bahçecik conglomerate rests unconformably on the Upper Cretaceous Tekelidağı mélange; these units are overlain by the Hafik Formation which contains white to gray, thickly bedded massive gypsum. This formation is overlain by shallow-marine deposits of the Lower Miocene Karacaören Formation, which is in turn unconformably overlain by fluvial deposits of the Early Pliocene İncesu Formation, the Quaternary Karacahisar Formation, and more recent alluvial deposits (Figs. 1 and 2). Following the Alpine–Himalayan orogeny, which was relatively strong and widely influential in what is now Turkey, most of the Paleozoic and Mesozoic lithostratigraphic units were uplifted such that a number of terrestrial areas developed. Among these terrains, previously shallow-marine-lagoonal, later Cenozoic basins, developed in limnic and continental as well as volcanic facies. Some coal, gypsum and halite beds formed in the sediments of this basin. Closure of the northern branch of Neotethys in Turkey, in which the coal formation of the region occurred, also took place at the end of this period (Ketin, 1983). The studied organic-material-rich interbeds and coal seams are located at the base of the Bahçecik Formation and, generally speaking, the upper part of this formation contains more clastic beds such as conglomerate, sandstone, claystone and clayey-carbonatic horizons.

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Organic-material- rich interbeds and coal seams are exposed at Bahçecik village, northeast of the town of Hafik (Fig. 1). The coal resources of this area are exploited by open-cast mining. This coal was deposited in a very stable, peat-favoring depositional environment where the rate of accumulation of plant debris was regulated by slow, uniform subsidence of coal-forming peat, resulting in a moderately thick coal deposit. Locally, coal sub-levels are interbedded with relatively organic-material-rich carbonaceous levels, suggesting erratic subsidence of the basin. The more-clastic beds at the top of this sequence are a consequence of increasing environmental disturbance.

3. Sampling and analytical procedures In the study area, two coal seams (67 and 80 cm in thickness) and one borehole (approximately 45 m long) were sampled at Tozluburun hill near Bahçecik village (Fig. 1). For this study, 40 samples were collected from varying depths (from top to the bottom of the seam) in the borehole and from the seams for organic-petrographic and geochemical analyses (Fig. 2). A simplified stratigraphic section illustrating sedimentary deposition of the Hafik area is given in Fig. 1. Seam sections of 5–10 cm were incorporated into a single sample. Portions of the collected coal and organic-material-rich samples were ground and homogenized prior to further work on them. X-ray diffractometry (XRD) was used to characterize inorganic phases. The mineralogical compositions of 28 samples were determined by X-ray diffractometry using a Rigaku DMAX IIIC apparatus in the Cumhuriyet University MİPJAL Laboratory (Sivas-Turkey), and the clay-fraction mineralogies of 8 samples were also determined in the same facility.

Fig. 1. Location map, geological map and generalized stratigraphic succession of investigated area (modified from Temiz, 1994).

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Fig. 2. Coal seams and drill section of investigated coals and sample intervals, sample numbers.

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For proximate and ultimate analyses, selected coal samples were purified according to American Society for Testing and Materials (ASTM) guidelines (D3174; D3175; D3302; 5373, 2004). These analyses were performed in the Turkish General Directorate of Mineral Research and Exploration laboratory (MTA-MAT Laboratory, Ankara, Turkey) using standard analytical procedures. Coal quality data (total moisture, ash, volatile matter, fixed carbon and calorific value) were obtained using an IKA 4000 adiabatic calorimeter; sulfur, carbon, hydrogen and nitrogen contents were determined using a LECO analyzer in the same laboratory. The Hafik coal samples were prepared for organic petrographic analysis using ICCP Standards (1998, 2001). To study the macerals and mineral compositions, the samples were examined via reflected and transmitted white-light and fluorescent microscopy. For organicpetrographic studies of the coals and interbeds, samples were crushed to a maximum size of 1 mm and the crushed material embedded in epoxy; sample preparation and reflectance determinations were performed in the MTA-MAT Laboratory (Ankara, Turkey). Eleven coal samples were prepared for maceral analysis, and reflectances of huminite were measured on standard blocks and analyzed petrographically (as explained in Stach et al., 1982). Huminite reflectance (Ro, Random) measurements were performed using a Leitz MPV-Geor system with reflected white light. Huminite mean and maximum reflectance values (Rmax) were measured at 546 nm using a Leitz MPV-Spectra microscope, a glass standard Leitz Rair 1.24%, and sapphire standard Roil 0.534%, a halogen lamp, and oil-immersion objectives — 32× and 50×. The results were interpreted using the “MPGeor” software system. Reflectance measurements were restricted to the particles that appeared to be well-preserved and wellpolished. The number of reflectance measurements on interbedded sediments depended on huminite abundance. In each sample, at least 500 points were counted under reflected white light in order to determine the maceral as well as the mineral-matter and pyrite contents. Kerogen was isolated from the organic-matter-rich host samples using standard palynological preparation procedures (Durand and Nicaise, 1980; Tissot and Welte, 1984). Organic-matter (kerogen) petrographic analyses were undertaken using a transmitted-light microscope in the laboratories of Cumhuriyet University (SivasTurkey) and of the Research Group of the Turkish Petroleum Corporation (TPAO, Ankara, Turkey). Organic-matter contents, organic-matter types, diagenetic evaluation levels and hydrocarbon source-rock potential characteristics of the Hafik coal samples were determined using Rock-Eval pyrolysis data (Espitalié et al., 1985; Peters, 1986). This technique has been used on our coal samples and has been shown by other workers to be useful in organic-petrographic explanations (Teichmüller and Durand, 1983; Fowler et al., 1991; Altunsoy et al., 2004; Korkmaz and Kara Gülbay, 2007). To define the organic geochemistry of the coals, TOC-Rock-Eval pyrolysis studies were performed on selected coaly and intercalated carbonate and clayey samples. The samples were pulverized and then pyrolyzed using a TOC-Rock-Eval-6 analyzer. Data interpretation followed guidelines outlined by Espitalié et al (1985), Lafarqué et al. (1998) and Peters (1986). S1 (mg HC/g rock), S2 (mg HC/g rock), S3 (mg CO2/g rock), Tmax (°C), hydrogen index (HI; mg HC/g TOC), oxygen index (OI; mg CO2/g TOC), production index (PI) and S2/S3 values were determined. The samples were analyzed in the laboratories of the Turkish Petroleum Corporation (TPAO) Research Group. For biomarker analyses, four representative coal samples from the borehole and coal seams were extracted for approximately 40 h using Dichloromethane (CH2Cl) in an ASE 300 (accelerated solvent extraction). Following extraction, yields were analyzed by Agilent 6850 whole-extract gas chromatography (“GC”) analyses in the Turkish Petroleum Corporation Research Group laboratories in

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Ankara, Turkey, according to ASTM (D 5307-97, 2002). Individual compounds were identified by comparison of mass spectra and retention times in the total-ion-current chromatogram. Relative concentrations of different compound groups in the studied coal samples and the saturated hydrocarbon fractions were calculated using peak highs from gas chromatograms. The aliphatic composition and carbon preference index (CPI) values were calculated from the resulting gas chromatograms. The saturate fractions were also analyzed using an Agilent 7890A/5975C GC–MS spectrometer. Sterane and triterpane distributions of bitumen were determined for selected samples on the basis of peak descriptions on m/z 191 and m/z 217 chromatograms. 4. Results 4.1. Mineral-matter content The coal series comprises coaly, clayey and organic-matter-rich carbonate levels (Fig. 2). The mineral-matter content varies in the 28 coal and interbed samples. Major minerals or mineral groups in evidence are carbonates (calcite 5 to 100 vol.%; average value: 47.25 vol.%; dolomite 8 vol.%), sulfides (framboidal pyrite and pore infillings; 5 to 78 vol.%; average 8.5 vol.%), quartz (2 to 46 vol.%; average 9.13 vol.%), feldspars 8 to 49 vol.% (average 9 vol.%) and clay minerals (13 to 84 vol.%; average 28.6 vol.%), with minor amounts of hornblende (8 vol.%, in a single sample). The mineral matter of the Hafik coal sequences consists mainly of carbonate components and clay minerals (Table 1). These minerals are found in all of the macerals as bands of various types and thicknesses, or as pore- and crackfillings. These mineral-rich zones indicate decreasing levels of organic matter in the peat-forming environment. From XRD clay-fraction diffractograms, the identified clay minerals in 8 samples were illite, smectite and chlorite. Clay-mineral abundances are as follows: illite (15–84 vol.%; average value 36.13 vol.%), smectite (24–67 vol.%; average value 20.4 vol.%). Chlorite (15–72 vol.%; average value 39.1 vol.%), palygorskite (in a single sample, 19 vol.%), serpentine minerals (9–51 vol.%; average value 19.5 vol.%) and sepiolite (in a single sample, 49 vol.%) (Table 1). Ophiolitic components derived from the Tekelidağ complex, mainly concentrated in samples from near the base of the unit, moved vast amounts of serpentine minerals and, therefore, contributed to the development of palygorskite and sepiolite. In coaly and clayey layers, the pyrite content is relatively high, with a mean value of 8.5 vol.%. Pyrite occurs mainly as framboidal pyrite, suggesting enrichment via the activity of sulfur-reducing bacteria, probably related to the presence of carbonate- and sulfaterich waters in the basin during peat formation (Kuder et al., 1998). This pyrite indicates a marine influence and suggests that the Hafik coals were in communication with a marine environment during peat deposition. Sulfur enrichment has been reported by Querol et al. (1996) in the Mesozoic and Tertiary coal deposits of the Mediterranean region where high sulfur contents have been related to alkaline depositional environments, which developed in basins during Alpine cycles in Circum-Mediterranean mobile belts, as in the case of the Çan coals (Çanakkale, Turkey) (Gürdal, 2008). 4.2. Proximate and ultimate analysis Table 2 summarizes the data obtained from proximate and ultimate analyses of air-dried samples. Coal quality (moisture, ash, total sulfur and volatile matter) and organic-petrographic data in this table are given together with the results of calorific and mean random huminite reflectance values. Carbon ranges from 53 up to 75.2 wt.% (dry, ash-free; daf), hydrogen from 3.9 up to 5.5 wt.% (daf), sulfur

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Table 1 Mineralogical components of Hafik coal samples. Sample no

Calcite (vol.%)

Quartz (vol.%)

Feldsp. (vol.%)

Clay min. (vol.%)

Pyrite (vol.%)

Illite (vol.%)

Smectite (vol.%)

Chlorite (vol.%)

Serpantine (vol.%)

K-1 K-2 K-3 K-4 K-5 K-6 K-7 K-8 K-9 K-10 K-11 K-12 H-1 H-2 H-3 H-4 H-5 H-6 H-7 H-9 H-14 H-16 H-24 H-25 H-26 H-27 H-28 H-30

5 21 90 12 100 24 27 36 20 32 63 34 97 96 33 98 65 98 55 n.d. 10 28 52 n.d. 24 6 32 37

11 20 10 43 n.d. 15 29 16 n.d. 2 n.d. 5 3 4 7 2 5 2 4 22 3 46 2 18 12 5 21 13

49 20 n.d. 17 n.d. 13 25 10 12 11 n.d. 19 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 12 n.d. 8 n.d. n.d. 13 27

35 31 n.d. 28 n.d. 30 13 38 56 42 29 34 n.d. n.d. 47 n.d. 30 n.d. 32 n.d. 70 n.d. 31 68 53 84 27 23

n.d. 8 n.d. n.d. n.d. 18 6 n.d. 12 13 8 8 n.d. n.d. 13 n.d. n.d. n.d. 9 78 17 14 15 6 n.d. 5 7 n.d.

36 51 n.d. 27 n.d. 15 n.d. 31 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 28 n.d. n.d. n.d. n.d. 84 17 n.d.

24 n.d. n.d. n.d. n.d. 42 n.d. n.d. 39 56 n.d. 67 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 37 n.d.

28 49 n.d. 46 n.d. 15 n.d. 69 38 20 n.d. 33 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 72 n.d. 59 n.d. n.d. 53 26 n.d.

12 n.d. n.d. 27 n.d. 9 n.d. n.d. 23 24 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 41 n.d. 51 47 20 n.d.

n.d.: not detected.

ultimate analyses and rank determinations classify these coals are subbituminous B/C (ASTM D388, 1992). This type of coal is characterized by low nitrogen but high oxygen and relatively moderate sulfur contents. In particular, with increase in calorific values, fixed carbon, volatile matter and carbon values also increase, but ash values decrease. Fixed-carbon values were calculated on an ash-free basis and in accordance with the total organic-carbon value determined via the Rock-Eval pyrolyser. Carbon contents increase with increasing hydrogen contents, but oxygen contents decrease. In the evaluation of coal quality, the most important relationship is that between volatile matter and ash content, and in the studied samples this relationship was found to be negative (Fig. 3). The sulfur contents of coal are explained by various theories: for example, it has been suggested that high sulfur contents are related to a marine and brackish environments during coal deposition (Sachsenhofer et al., 2003); another theory points to a high water

from 2.3 to 7 wt.%, and nitrogen from 0.7 to 1.1 wt.% (daf); the average values of these components are 70.1, 5.2, 5.3 and 1 wt.%, respectively. In the studied samples, ash contents are high and range from 7.9 to 54.5 wt.% (dry), confirming variability in mineral-matter content; this could be a result of intense organic-matter degradation which would concentrate the inorganic fraction (Teichmüller et al., 1998; Davis et al., 2007). The coals are characterized by moderate sulfur contents (average value 5.3 wt.%) and high ash values, consistent with those of highly peneplaned coastal areas (Stefanova et al., 2002). The volatilematter contents range between 36.7 and 48 wt.% (daf), and ultimate analysis data (Table 2) are in accordance with rank. However, variations in volatile-matter contents and hydrogen ratio could be due to different macerals and maceral groups. Gross calorific values vary between 7729 and 27,658 kJ/kg (average value 21,060 kJ/kg) in the Hafik coal samples. To determine ASTM coal rank, the calorific values were transformed from BTU/lb and calculated on a dry, mineral-matter-free basis. The results of Table 2 Proximate and ultimate analysis result of Hafik coal samples. Sample Total Ash Volatile Fixed Cdaf no moisture contentdaf matterdaf carbondaf contentdaf

H-1 H-3 H-9 H-10 H-19 H-22 H-25 H-29 H-34 H-37 H-39

Hdaf

Ndaf

Odaf

Sdaf

Vitrinite Liptinite Inertinite Mineral Pyrite Huminite matter reflectance (huminite) value

Gross calofiric value db

(wt.%)

(wt.%)

(wt.%)

(wt.%)

(wt.%) (wt.%) (wt.%) (wt.%) (wt.%) (Mean Rmax %)

(vol.%)

(vol.%)

(vol.%)

(vol.%)

(vol.%) kJ/kg

BTU/lb

15.57 9.60 n.d. 18.78 8.91 15.08 16.60 8.22 15.06 13.33 10.01

54.48 26.00 n.d. 17.03 31.78 17.78 16.81 35.12 14.63 24.81 7.87

38.31 47.98 n.d. 44.56 36.73 42.59 43.31 47.29 45.84 38.93 47.98

7.21 26.02 n.d. 38.41 31.49 39.63 39.88 17.59 39.53 36.26 44.15

53.01 67.98 n.d. 72.96 69.73 73.03 74.21 66.65 74.65 73.66 75.21

n.d. n.d. 81 82 85 83 78 n.d. 65 80 81

n.d. n.d. 6 5 4 4 4 n.d. 12 6 5

n.d. n.d. 4 3 3 2 2 n.d. 13 4 7

n.d. n.d. 7 7 8 7 14 n.d. 7 8 6

n.d. n.d. 2 2 1 4 2 n.d. 3 2 2

3322.8 8112.6 n.d. 10575 8373.6 10503 10706.4 6665.4 10924.2 9531 11890.8

db: dry basis, daf: dry, ash-free basis n.d.: not detected.

3.93 4.80 n.d. 5.38 5.09 5.41 5.37 4.91 5.38 5.45 5.37

0.65 0.82 n.d. 0.99 0.99 1.11 1.08 0.95 1.08 0.98 0.93

40.07 21.48 n.d. 14.97 17.21 15.14 13.72 23.93 13.22 13.77 12.04

2.34 4.92 n.d. 5.70 6.98 5.31 5.62 3.56 5.67 6.14 6.45

n.d. n.d. 0.48 0.44 0.44 0.38 0.40 n.d. 0.43 0.44 0.44

7729 18870 n.d. 24598 19477 19213 24903 15504 25410 22169 27658

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Fig. 3. Comparative diagrams of ultimate and proximate analysis results.

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table and high pH and low Eh conditions (Stach et al., 1982), with proximity of the peat to marine waters during deposition, such that sulfate ions in seawater constitute an abundant source of sulfur. High sulfur content may result in bacterial-fixing sulfur in freshwater environments as well (Moore, 1991). Marine (brackish) peats, and fresh-water peats with marine roof rocks are generally sulfurrich. Teichmüller and Teichmüller (1982) emphasized that “freshwater coals deposited in calcium-rich environments have similar composition as brackish-marine coals” (Markic and Sachsenhofer, 1997). On the other hand, sulfur contents may be related to the infiltration of salts from Hafik Formation evaporitic sediments that crop out in the basin. 4.3. Organic-petrographic determinations Macroscopically, organic-petrographic determinations were made according to the nomenclature of Stach et al. (1982). The coal is composed predominantly of dull, banded-dull and banded-coal lithotypes; such bands are related to the high ash contents of this coal. Three maceral groups (vitrinite/huminite, liptinite and inertinite) were determined in these samples and the results have been plotted on ternary diagrams (Fig. 4a). Organic-petrographic determinations show the heterogeneous characteristics of the herbaceousplant conversion process in these samples; the mean values are 80 vol.% huminites, 6 vol.% liptinites and 5 vol.% inertinites (Table 2). The coal is dominated by huminite macerals (65–85 vol.%), mainly gelinites. The gelinites do not exhibit cellular structures but mostly occur as a gelified form of huminite. The liptinite contents range from 4 to 12 vol.% and are dominated by sporinite, resinite and cutinite macerals that are quite resistant to degradation. Inertinite contents range from 2 to 13 vol.%, and this group of macerals mainly consists of micrinite and fusinite; these data indicate that the coal may generate more gas than oil. The characteristic maceral types and various maceral associations within the studied samples are also shown in microphotographs (Fig. 5). Liptinite macerals such as resinite (Fig. 5a), sporinite (Fig. 5b) and cutinite are probably related to accumulation in a reed-type swamp. A high content of gelinite (Fig. 5c) is characteristic of calcium-rich coals, and inertinite macerals such as fusinite and macrinite (Fig. 5a and d) indicate periods of low water level or the occurrence of swamp fires (Flores, 2002; Scott and Glasspool, 2007). Coals rich in sporinite and clay minerals (Fig. 5e) are thought to have accumulated in a reed marsh, under subaquatic conditions and with a high degree of bacterial activity. The presence of cutinite, suberinite and resinite suggests an accumulation in a forest-type swamp, while the presence of liptodetrinite and sporinite indicates reed-marsh vegetation (Flores, 2002). The mineral-matter contents range from 6 to 14 vol.% (average value is 8 vol.%), and the predominant mineral phases are carbonates and clays along with silica, and pyrite (Fig. 5f) resulting from biological activity. The coal is characterized by microlayering of clayey and organic-matter-rich intervals indicating that the water level was such that it periodically brought organic material into the depositional environment. These samples have high calcium contents, and this type of coal indicates an alkaline depositional environment that allowed bacteria to cause severe structural decomposition leading to formation of humic gels and coalification products that are relatively rich in nitrogen and hydrogen, such as this subbituminous coal (Teichmüller et al., 1998). Data for bacterial activity are supported by biomarker results; this characteristic is the same as that of Pliocene coal in the Amynteo Basin of northwestern Greece (Iordanidis and Georgakopoulos, 2003). In facies analysis, the most-used parameters are tissue preservation index (TPI) and gelification index (GI). On this evaluation method, Calder et al. (1991) and Lamberson et al. (1991), in addition, especially, Kalkreuth et al. (1991) with made modifications for lower

Fig. 4. Organic matter type ternary diagrams of Hafik coal samples.

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Fig. 5. a. Organic petrographic features of Hafik coal samples (the most common gelinite maceral (gray groundmass), a resinite, sporinite and macrinites). b. The most common macerals and mineral matter in Hafik coals. c. Gelinite, sporinite, macrinite and pyrites. d. Gelinite and Fusinite. Fusinite exhibit pecular structures. e. Micro laminations of mineral matter (mostly clays) with coals are common in the coals. f. Tiny framboidal pyrites are abundant are disseminated in Hafik coals.

rank coals, and many investigators have used TPI/GI data in order to develop ideas about the depositional environments of coals (Paul et al., 1989; Calder et al., 1991; Kalkreuth et al., 1991; Amijaya and Littke, 2005; Siavalas et al., 2009). Recently, there have been serious doubts regarding the data that are appropriate for elucidating depositional environments (Wüst et al., 2001; Moore and Shearer, 2003). As with all analytical data, TPI/GI parameters alone may not exhibit absolute or definite values, but they may provide additional data when other values are also taken into consideration.

In this study, the TPI (tissue preservation index), VI (vegetation index) (Georgakopoulos and Valceva, 2000) and GWI (groundwater index) formulae for calculating the GI (gelification index) of Diessel (1986) were used. Low TPI values can be a result of vegetation type (high angiosperm/gymnosperm ratio), or due to less favorable conditions for tissue preservation (Kolcon and Sachsenhofer, 1999). In these samples, the TPI value is 0.02 to 0.09 (mean value 0.07). The GI values for the Hafik samples (varying between 5 and 41.5, with a mean value of 24) indicate high water table and/or weakly acidic pH

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Table 3 Total organic carbon (TOC) and Rock-Eval pyrolysis results of the Hafik coal area. Sample no

TOC

S1

S2

S3

S2/S3

Tmax

HI

Ol

Pl

PY

RC

PC

MinC

H-1 H-3 H-5 H-9 H-10 H-16 H-19 H-22 H-25 H-29 H-31 H-34 H-37 H-39 K-6 K-8 K-10 K-12

16.11 26.31 8.03 10.26 43.59 0.75 40.58 70.49 60.57 23.75 14.18 63.76 57.25 72.45 0.32 0.51 0.48 0.41

0.32 0.56 0.20 0.77 1.13 0.03 0.93 1.07 1.45 1.18 0.54 0.81 1.20 1.15 0.01 0.02 0.03 0.02

46.97 68.20 30.98 10.67 50.54 0.75 54.68 108.54 78.30 95.49 62.73 69.25 97.66 125.85 0.07 0.21 0.08 0.09

10 14.68 3.96 2.7 8.91 0.65 8.02 12.26 14.29 4.88 3.44 14.75 11.67 11.9 0.55 0.44 0.73 0.86

4.7 4.65 7.83 3.95 5.68 1.15 6.82 8.85 5.48 19.5 18.24 4.7 8.3 10.5 0.13 0.48 0.11 0.1

430 430 431 423 417 426 418 412 418 426 429 424 419 417 418 426 416 418

292 259 386 104 116 100 135 154 129 402 442 109 171 174 22 41 17 22

62 56 49 26 20 87 20 17 24 21 24 23 20 16 172 86 152 210

0.01 0.01 0.01 0.07 0.02 0.04 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.15 0.08 0.31 0.02

47.29 68.76 31.18 11.44 51.67 0.78 55.61 109.61 79.75 96.67 63.27 70.06 98.86 127.0 0.08 0.23 0.11 0.36

11.64 19.76 5.20 9.08 38.42 0.65 35.2 60.26 52.64 15.34 8.67 56.72 48.03 60.7 0.29 0.47 0.44 0.05

4.47 6.55 2.83 1.18 5.17 0.10 5.38 10.23 7.93 8.41 5.51 7.04 9.22 11.75 0.03 0.04 0.04 0.03

9.62 6.62 12.04 0.59 1.63 6.22 1.72 1.77 1.72 6.99 10.48 1.80 4.54 1.92 5.96 4.56 6.38 3.96

TOC; Total Organic Carbon (wt.%), S1; Free Hydrocarbons in rock (mg HC/g rock), S2; Hydrocarbon generated from the thermal breakdown of kerogen (mg HC/g rock), S3; CO2 value (mg CO2/g rock), Tmax; Maximum temperature of pyrolysis (°C), HI; Hydrogen Index (mg HC/g TOC), OI; Oxygen Index (mg CO2/g TOC), PI; Production Index (S1/S1 + S2), S2/S3; Hydrocarbon Type Index, PY; Potential Yield (S1 + S2; mg HC/g rock), RC; Rezidual Organic Carbon, PC; Pyrolysable Organic Carbon, MinC; Mineral Carbon.

level insofar as gelification requires continuous water flow and microbial activity requires low acidity (Kolcon and Sachsenhofer, 1999), but these data should be corroborated by paleobotanical and/ or palynological data. GWI values vary between 1.07 and 1.22 (mean value 1.12), and VI values vary between 0.07 and 0.32 (mean value 0.13). The TPI values are b0.5 and the GI values are N5. Values of GWI N 1 and VI b 1 indicate a limnic stage; thus, the Hafik coal formed in a limnic swamp. Besides the TPI and GI values indicating the environment of the coals as limnic, sedimentological evidence also supports this designation. The major Turkish coals and their environmental conditions were described by Toprak (2009), and the Hafik coals have exactly the same environmental and depositional conditions as those coals. From organic-petrographic observations, it can be stated that the Hafik coal developed in an autochthonous to hypoautochonous paleoenvironment. Coalification occurred during high groundwater levels and with a moderate subsidence rate; there were highly alkaline, reducing conditions and a marine influence. Low TPI values indicate high bacterial activity and high pH conditions, and the presence of gastropod shells indicates an alkaline condition, similar to that reported for the Amyneto Basin, Greece (Iordanidis and Georgakopoulos, 2003). High pyrite contents probably resulted from both high water level and abundant gastropod shells. The dominance

of n-C31 indicates the presence of either grasses or warm-climate plant waxes, as in the Bourgas coal area (Bechtel et al., 2005). 4.4. Organic-geochemical determinations 4.4.1. Total organic carbon Total organic carbon (TOC wt.%) was determined for 18 selected samples. TOC contents vary from 0.32 to 72.45 wt.%, with an average TOC content of 28.32 wt.% (Table 3). The total organic-carbon contents of the shale and carbonate intervals are between 0.32 and 16.11 wt.%, while those of the coaly levels are between 38.42 and 60.70 wt.% (Table 3). High SOM (concentration of free lipids) yields (100–200 mg/g Corg) were measured for two samples (77–5412 ppm). The SOM mainly comprises resins and asphaltenes. 4.4.2. Type of organic matter Rock-Eval analyses were performed on 18 selected samples (Table 3). The hydrogen index values for the Hafik coals vary between 17 and 442 mg HC/g TOC (average 187.5 mg HC/g TOC). Oxygen index values range from 16 to 210 mg CO2/g TOC (average 60.28 mg CO2/g TOC) (Table 3). Some higher oxygen indices (oxygen index value N150 mg CO2/g TOC) are possibly related to mineral matrix effects or mineral decomposition during the Rock-Eval pyrolysis procedure. It is well

Fig. 6. Comparative diagrams of organic petrographic data vs. Rock-Eval pyrolysis results.

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known that pyrolysis data is affected by mineral-matter content (Peters, 1986). For this purpose, the graphs were compared with the other data as well. For example, while there is a negative relationship between hydrogen index and liptinite composition, when huminites are added to the liptinites (liptinite + huminite), there is a positive relationship between these parameters (Fig. 6). Mineral-matter contents, along with hydrogen index, TOC, pyrolysed carbon, residue carbon values, seem to have negative relations, but their correlation-coefficient values are fairly low; consequently, these data are not presented graphically. The best parameters for comparison are total organic carbon and carbon (wt.%) values (Fig. 6). An S2/S3 ratio (hydrocarbon type index) b5 may indicate type III kerogen, suitable for gas and condensate. Some S2/S3 ratios N5 indicate type II kerogens, suitable for oil production. The production index (PI) (Hunt, 1995), S1/S1 + S2, is significant when it is higher than 0.05–0.1, but the analyzed samples from the Hafik area have an average PI of 0.04, with a maximum value of 0.31 (Table 3). Cross plots of hydrogen index (HI) and oxygen index (OI) on van Krevelen and HI vs Tmax diagrams (Figs. 7 and 8) demonstrate that most of the samples are scattered in the types II–III mixed and type III areas (some marine/algal input); this recognition is supported by organic-matter determinations on palynological preparations, wherein woody and coaly materials are dominant. The types of organic matter in the studied samples are 5–10% algal, 5% herbaceous, 5–15% woody, 75–95% coaly (Fig. 4b). This evaluation is essential to show that the organic-petrographic and microscopic analyses support one another. In particular, the coaly samples in Figs. 7 and 8 tend to be different from the samples rich in organic materials, and are thus distributed especially in the type III kerogen and immature fields. Mineral-matter effects may significantly influence the quantity and the composition of pyrolysates, especially when samples have high clay and carbonate contents, such as in the case of the Hafik coal samples (Peters, 1986; Langford and Blanc-Valleron, 1990). In this study, organic-petrographic, organic-geochemical and coal quality analyses were performed, and an attempt was made to

Fig. 7. Van Krevelen (Hydrogen Index vs. Oxygen Index) diagram of Hafik coal samples.

405

determine the interrelationships of some important parameters for coal-derived hydrocarbon development. There is a strong positive relationship between total organic carbon (important with respect to hydrocarbon potential) and gross calorific value (important with respect to coal quality). Of the Rock-Eval parameters, with increase in residual carbon there is increase in fixed carbon and carbon, but decrease in ash contents. There is a positive relationship between pyrolysed carbon values and fixed carbon and oxygen index-oxygen, and a negative relationship between S3-ash and oxygen index-gross calorific value (Fig. 9a). In addition, with respect to hydrocarbongeneration potential, hydrogen index is important, and in our samples there is a negative relationship between hydrogen and gross calorific value; similarly, Tmax values are negatively related to gross calorific value, carbon and fixed carbon (Fig. 9b). In gas chromatograms of the Hafik coal samples, there is no evidence of lower carbon n-alkane distribution and there are no nalkanes of carbon number higher than C32 that would be indicative of both marine and terrestrial organic-matter input. From chromatographic analysis of three samples, it was found that carbon values are low and that n-C6 as well as n-C17 contents are quite low; these results indicate that the organic material developed in high-level terrestrial plants (Fig. 10). Biomarker analysis of selected Hafik coal and carbonaceous-shale samples indicate that n-alkane distributions are dominated by the occurrence of high molecular-weight (C20+) n-alkanes and a distinct odd–even carbon number preference over the C25–C31 range typical of

Fig. 8. Classification of the kerogen types by using Hydrogen Index vs. Tmax (°C) diagram (Mukhopadhyay et al., 1995).

Fig. 9. a. Comparative diagrams of Rock-Eval pyrolysis and ultimate, proximate analysis results. b. Comparative diagrams of Rock-Eval pyrolysis and ultimate, proximate analysis results.

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Fig. 9 (continued).

Fig. 10. Typical GC traces for Hafik coals.

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organic matter derived from higher terrestrial plants. Also, predominance of C29 sterane over C27 and C28 indicates a terrestrial source. The sterane/hopane ratios of the two studied samples are b1, thus reflecting a marine influence; in one sample, the sterane ratio is higher than that of hopanes, indicating a higher input of terrestrial material. The high abundance of C29 αααR isomers of steranes indicates immature organic matter and an abundance of terrestrial organic matter. 4.4.3. Organic maturity The thermal maturities of the coals and interbedded layers were evaluated in terms of Tmax, huminite reflectance (Rmax), volatilematter content and carbon content. The Tmax (°C) values range between 412 and 431 °C, with an average value of 422 °C, indicating that the coaly levels and organic-matter-rich interbeds are immature to early mature; these maturity levels are typical of a diagenetic zone and are insufficient for oil and gas generation. On the HI vs Tmax diagram (Fig. 8), most of the samples plot in the early mature and immature zones. Moreover, the same samples have PI values b 0.10 and most of samples suggest an immature-zone assignment. Huminite reflectance values are given in Table 2. Rmax values range from 0.35 to 0.49% (Table 2), indicating that the coals should be classified as subbituminous. Correlations among average huminite reflectance values and calorific values indicate that the range fits for the immaturediagenetic stage to bordering on maturity (with sample ash values below 15 wt.%); that is, high ash content affects these values. There is no good linear relationship between the huminite reflectance (Rmax) and Tmax values of the samples; this relation was not clearly seen due to organic-petrographic compositional differences. Conversely, both parameters suggest either early stages of maturation or immature organic material. 20(S)/(20S + 20R) and ββ/(ββ + αα) sterane ratios increase with increasing maturity (Waples and Machihara, 1991; Hunt, 1995), and that observation is in accordance with our Tmax and Rmax values. These values are b1 (Table 4), indicating the presence of immature organic matter in the studied samples. Ts/Tm ratios are between 0.52 and 1.89. Ts/Tm = 1 has been considered a threshold between immature (Ts/Tm b 1) and mature (Ts/Tm N 1) organic matter (Seifert and Moldowan, 1978). In these samples, 18α (H)-22, 29, 30-trisnorneohopane (Ts)/(Ts + Tm) varies between 0.34 and 0.65 (Table 4). Typically C31 or C32 homohopanes Table 4 Biomarker parameters calculated from m/z 217 and m/z 191 mass chromatograms. Biomarker parameters

Sterane/hopane ratio C32 22S/(22S + 22R) ratio Moretane/hopane ratio C29/C30 hopane ratio Ts/(Ts + Tm) ratio C23/C24 ratio Gammacerane ındex Diasterane/sterane ındex Ββ/(ββ + αα) sterane ratio C29 20S/(20S + 20R) ratio % C27 % C28 % C29 C27/C29 C28/C29 sterane ratio C25/C26 tricyclic terpane αββ/(αββ + ααα) sterane ratio Ts/Tm Pr/nC17 Ph/nC18 Pr/Ph

Sample no H-22

H-32

H-39

0.14 0.36 0.37 0.58 0.65 0.94 1.04 4.20 n.d. 0.29 43 14 44 0.98 2.41 1.11 0.11 1.89 n.d. n.d. n.d.

1.72 0.21 0.47 0.67 0.34 1.68 1.64 2.89 n.d. 0.44 40 14 46 0.86 0.30 1.07 0.13 0.52 n.d. n.d. n.d.

0.71 0.26 0.30 0.59 n.d. 1.26 2.21 3.62 0.22 0.27 48 20 32 1.5 2.78 0.9 0.11 n.d. 0.04 0.23 0.25

are used for calculations of 22S/(22S + 22R) ratio, and this ratio increases with increasing maturity — from 0 to about 0.6 — during maturation. The values for these samples are 0.21, 0.26 to 0.36. Diasterane/sterane ratios are generally low in immature sediment extracts (Arfaouvi et al., 2007); in the present study, these ratios are 2.89, 3.62 and 4.20. Moretane/hopane ratios are 0.37, 0.47 and 0.30. The moretane/hopane ratio generally decreases with increasing maturity (Kvenvolden and Simoneit, 1990). The low bitumen/TOC ratios of these samples also indicate immaturity. The low concentration of n-alkanes in some of the Hafik coal samples may have influenced the CPI values. Nevertheless, the CPI values indicate that these coals are immature (Table 5). In the studied samples, chromatograms were obtained with intense peak scattering in the sterane and triterpane area, indicating low maturity. A maturity parameter derived from C29 regular steranes is the proportion of 5α (H), 14β (H), 17β (H) C29 sterane and 5α (H), 14α (H), 17α (H) C29 sterane (αββ/αββ + ααα). These ratios are all N1 in the studied coal samples. A single coal sample from the Hafik coal seams has a significant n-C17 peak. Results of GC and GC–MS analyses indicate that the Hafik coals are immature or early mature, and might only generate gas. 4.4.4. Hydrocarbon generative potential In our samples, S1 values are extremely low, ranging from 0.01 to 1.45 mg HC/g rock, with an average value of 0.62 mg HC/g rock. S2 varies between 0.07 and 125.85 mg HC/g rock, with an average value of 50.05 mg HC/g rock (Table 3). While S2 values less than 4.0 mg HC/g rock are generally considered to be source rocks with poor generative potential, yields greater than 4.0 are common in known hydrocarbon source rocks. Thus, these S2 yields indicate that most of our samples have good to very good source-rock potential. Generally, the samples are characterized by lower S1 and S2 values, on the order of 50 mg HC/ g rock of sample. According to this data, the coals have hydrocarbon source-rock potential, but the coaly shales and other organic-matterrich levels have no hydrocarbon source-rock potential. The factor that most affects the potential of coal for generating liquid hydrocarbons is the presence or absence of hydrogen-rich material. According to Hunt (1995), coals and terrestrial organic matter dispersed in shales capable of generating and expelling oil have HI values greater than about 200 mg HC/g TOC. In the present study, the hydrogen contents of the samples were measured by both direct (ultimate analyses; H-wt.%) and indirect methods (Rock-Eval pyrolysis; HI-mg HC/gTOC). Some marine influences were recognized in the studied samples, as characterized by high hydrogen-index values. The organic compounds known as type II kerogen were derived by deterioration of terrestrial or humic organic material (Peters, 1986; Hunt, 1995).

Table 5 Gas chromatography results of Hafik coal samples. Components (%)

H-22

H-32

H-39

K-12

CS2 nC3 nC6 Benzene Ph Pr nC17 nC18 nC27 nC30 nC31 CPI (Bray and Evans, 1961) Total extract (ppm) Bitumen/TOC TOC (wt.%)

99.47 n.d. n.d. 0.52 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 5412 0.008 70.49

92.37 1.55 1.85 0.52 n.d. n.d. 0.47 n.d. 0.66 2.63 1.55 0.94 3999 0.006 63.76

86.47 n.d. n.d. n.d. 0.04 0.16 0.96 0.7 n.d. n.d. n.d. 2.74 4214 0.017 72.45

99.43 n.d. n.d. 0.57 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. 77 0.0008 8.76

n.d.: not detected.

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The humic material in type II formed by deterioration of wax, spore, pollen, cuticle and resin particulates during peat formation (Tissot and Welte, 1984). As in samples from the study area, humic coals containing type III kerogen can generate gas. Thus, these coals might generate gas but their maturity levels are low. Hydrocarbon-generating potential is estimated by measurement of total pyrolitic hydrocarbon yield (PY; S1 + S2). S1 + S2 values are generally similar to TOC values. Potential yield values range between 0.1 and 127 mg HC/g rock, with an average of 50.71 mg HC/g rock. S2/S3 values for five of the studied samples have values lower than 2, implying that the samples may have extremely limited gas-generation potentials. The S2/S3 values of the other samples are higher than 2 and their PI values b 0.1, indicating that the samples could produce liquid hydrocarbons, but that the Tmax values indicate immaturity (Table 3). Some samples are scattered in the poor oil zone in the HI vs TOC diagram (Fig. 11), but some samples indicate gas and some oil potential. The organic-maturation data of the studied coaly and organic-matter-rich samples indicate the presence of sufficient organic matter and that these are of the appropriate type, but that the rocks are characteristic of the early-mature and diagenetic stages. Burial of the sediments at increasing depth results in temperature and pressure increases that, in turn, induce progressive transformation of the organic matter. 4.5. Molecular composition of coals To determine the biomarker characteristics of the aliphatic fractions, GC and GC–MS analyses were performed on the studied samples. Table 5 shows extract yields and their compositions obtained after solvent extraction. The relative proportions of the hydrocarbons in the SOM from the samples are low (77–5412 ppm), consistent with the low maturity of the organic matter. The SOM is mainly composed of resins and asphaltenes. When the bitumen contents of the coal and organic-matter-containing levels are compared with TOC, they show very low values. Sterane and triterpane distributions of bitumen were

Fig. 11. Hydrogen Index vs. Total Organic Carbon diagram of Hafik coal samples (modified from Jackson et al., 1985).

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determined for selected samples; peak descriptions are given in Tables 6 and 7. 4.5.1. N-alkanes, isoprenoids The n-alkanes for the studied samples range from C20 to C32 (Table 5). Typical GC traces for the aliphatic fractions are given in Fig. 10. The n-alkanes are the dominant components of the saturated fractions of whole hydrocarbons. In the GC analysis, low carbon value n-alkanes were determined in two samples; the other organic components are CS2 and benzene (Table 5). The GC–MS data for the studied samples are characteristic of typical saturate fractions (Figs. 12 and 13). The major biomarkers are C25 (22S + 22R) tricyclicterpane, C24 Tetracyclicterpane (seco), C26 22R tricyclicterpane and C28 tricyclicterpane. The presence of these triterpanoids in these samples supports their higher plant origins, and the presence of gammaccerane also indicates a hypersaline depositional environment. In this study, the high proportions of long chain C27–C31 n-alkanes relative to the SOM of the n-alkanes are typical of higher terrestrial plants while short chain n-alkanes (bC20), detected in low amounts, are predominantly found in algae and microorganisms. The studied samples are dominated by intermediate- and high-molecular weight n-alkanes (C21–25). These data indicate derivation from terrestrial and lagoonal organic matter. The low Pr/Ph ratio of a single sample (0.25), as compared to the values generally obtained from terrestrial source rocks, may be explained by low rank and maturity of the Hafik coals. 4.5.2. Steroids, hopanoids From the m/z 217 mass chromatograms of our samples, the relative abundances of the C27, C28, and C29 steranes and their 20S and

Table 6 Peak definitions of triterpanes in the m/z 191 mass chromatograms. Peak

Compound name

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

C19 Tricyclicterpane C20 Tricyclicterpane C21 Tricyclicterpane C22 Tricyclicterpane C23 Tricyclicterpane C24 Tricyclicterpane C25 Tricyclicterpane (22S+22R) C24 Tetracyclichopane (seco) C26 Tricyclicterpane 22 (S) C26 Tricyclicterpane 22 (R) C28 Tricyclicterpane C29 Tricyclicterpane C27 18α (H)-22,29,30-TRISNORHOPANE (Ts) C27 17α (H)-22,29,30-TRISNORHOPANE (Tm) 17α (H)-29,30-BISNORHOPAN C30 Tricyclicterpane 17α (H)-28,30-Bisnorhopane C29 17α (H),21β (H)-30-Norhopane C29 Ts (18α (H)-30-Norhopane C30 17α (H) Dıahopane C29 17β (H),21α (H)-30 Normoratene Oleanane C30 17α (H),21β (H)-Hopane C30 17β (H),21α (H)-Moretane C31 17α (H),21β (H)-30-Homohopane (22S) C31 17α (H),21β (H)-30-Homohopane (22R) Gammacerane Homomoretane Homohopane C32 17α (H),21β (H)-30,31-Bishomohopane (22R) C33 17α (H),21β (H)-30,31,32-Trishomohopane (22S) C33 17α (H),21β (H)-30,31,32-Trishomohopane (22R) C34 17α (H),21β (H)-30,31,32,33-Tetrakishomohopane (22S) C34 17α (H),21β (H)-30,31,32,33-Tetrakishomohopane (22R) C35 17α (H),21β (H)-30,31,32,33,34-Pentakishomohopane (22S) C35 17α (H),21β (H)-30,31,32,33,34-Pentakishomohopane (22R)

n.d.: not detected.

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Table 7 Peak definitions of steranes in the m/z 217 mass chromatograms. Peak

Compound name

1 2 3 4 5 6 7 8

C27 13β (H),17α (H)-Diasterane (20S) C27 13β (H),17α (H)-Diasterane (20R) C27 13α (H),17β (H)-Diasterane (20S) C27 13α (H),17β (H)-Diasterane (20R) C28 13β (H),17α (H)-Diasterane (20S) C28 13β (H),17α (H)-Diasterane (20R) C28 13β (H),17β (H)-Diasterane (20S) C27 5α (H),14α (H),17α (H)-Sterane (20S) + C28 13α (H),17β (H)-Diasterane (20S) C27 5α (H),14β (H),17β (H)-Sterane (20R) + C29 13β (H),17α (H)-Diasterane (20S) C27 5α (H),14β (H),17β (H)-Sterane (20S) + C28 13α (H),17β (H)-Diasterane (20R) C27 5α (H),14α (H),17α(H)-Sterane (20R) C29 13β (H),17α (H)-Diasterane (20R) C29 13α (H),17β (H)-Diasterane (20S) C28 5α (H),14α (H)-17α (H)-Sterane (20S) C28 5α (H),14β (H)-17β (H)-Sterane (20R) + C29,13α (H),17β (H)-Diasterane (20R) C28 5α (H),14β (H)-17β (H)-Sterane (20S) C28 5α (H),14α (H),17α(H)-Sterane (20R) C29 5α (H),14β (H),17α(H)-Sterane (20R) C29 5α (H),14β (H),17β (H)-Sterane (20R) C29 5α (H),14β (H),17β (H)-Sterane (20S) C29 5α (H),14α (H),17α(H)-Sterane (20R) C29 5α (H),14α (H),17α(H)-Sterane (20S) C30 5α (H),14β (H)-17β (H)-Sterane (20R) C30 5α (H),14β (H)-17β (H)-Sterane (20S) C30 5α (H),14α (H),17α(H)-Sterane (20R)

9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

20R epimers have been determined (Table 4) (Fig. 12). The studied samples are characterized by the occurrence of C27 and C29 steranes. C28 steranes and C28 diasteranes occur in very low quantities C27 N C29 N C28 (Fig. 4c). Algae have been proposed as the predominant primary producers of the C27 sterols, while the C29 sterols are more typically associated with land plants (Huang and Meinschein, 1979; Volkman et al., 1986). C30 sterane was recorded only in minor amounts. Also, richness in C27 indicates a lagoonal environment (McKirdy et al., 2010) and algal organic material in that environment; the C28 levels are quite low and typical of limnic environments (Volkman et al., 1986). Diasterane/sterane ratios are quite low. C20, C21, C23, C24, C26, C28 and C29 tricyclic terpanes are also present in the coal samples. Relatively high concentrations of C24 tetracyclic terpane in these extracts indicates terrigenous input (Peters and Moldowan, 1993). In the coal samples, the C23/C24 ratios rise from 0.94 to 1.68. The C28/C29 sterane ratio varies between 0.30 and 2.78 in the m/z 217 mass chromatograms; these chromatograms show that the hydrocarbons in the Hafik coal-related sediments are in the range 27–32. The asphaltene-free extracts of the coals are dominated by heteroatomic compounds (NSOs). The C27/C29 sterane ratios vary from 0.86 to 1.5; the C27 and C29 steranes are related to marine and terrestrial sources, respectively. The greater marine influence on the coastal reaches of the peat mire is determined in these coal seams via increased ratio of C27 regular steranes relative to C29 and C28 steranes. In the GC–MS analyses, gammacerane was determined, indicating salinity. To Bray and Evans (1961), CPI value is equal to 1 (C24–C34). Our CPI values (C16–C26) range from 0.94 to 2.74. In our m/z 191 mass fragmentograms, very low tricyclicterpanes were recorded in two samples. In the Hafik coal samples, C30 hopane is more abundant than C29 norhopane. Oleanane was not detected, indicating that there was not a significant angiosperm contribution to the organic matter. There were no carbon component values higher than that of the C32 homohopanes in these samples. The C29/C30 hopane ratio is used to distinguish between carbonate and clastic lithologies (Waples and Machihara, 1991); in our study, this ratio was measured as 0.58, 0.59 and 0.67 (Table 4).

Fig. 12. m/z 217 mass chromatograms of investigated coal samples.

4.6. Depositional environment aspects Following the guidelines of ASTM standards (D 3174; D3175; 3302; 5373, 2004), results of proximate, ultimate, calorific and reflectance analyses indicate that the rank of these coals is subbituminous B/C (ASTM D388, 1992). The coals of the study area,

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Fig. 13. m/z 191 mass chromatograms of investigated coal samples.

as mentioned before, are situated at the base of the Upper Paleocene– Lower Eocene Bahçecik Conglomerate as thin and lateral intercalations with claystone, sandstone and carbonatic levels. Lithological and paleontological evidence from the Bahçecik Conglomerate, including coal units and sandstones with plant fragments, indicate lagoonal environmental conditions; conglomerates with coarse blocks, in the

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upper levels of the unit, reveal alluvial-fan properties (Kurtman, 1973). Furthermore, in these levels, the finest-grained clastics contain Nummulites sp. and tend to be laterally intercalated, indicating shallow-marine conditions and transition to a deltaic-fan conditions. These environmental conditions also affected the chemical and organic-petrographic compositions of the studied coal samples. The Hafik samples have an average ash value of 25 wt.%, and an average sulfur value of 5 wt.%. The reason the coals contain high amounts of sulfate ions is that these were probably derived via occasional marine influence and abundant bacterial activity. The presence of gastropod fragments in the coal layer provides further support that these coals were deposited in a limnic environment. High ash yields determined in all samples indicate that periodic flooding affected the depositional environment (Davis et al., 2007); the presence of interlayered clay minerals and quartz are evidence of these flooding events. In the studied samples, low TOC values suggest that a relatively oxidizing depositional environment influenced the amount and composition of organic matter (Hunt, 1995; Sachsenhofer et al., 2006); however, some TOC values are high, reflecting an anoxic environment of deposition (Petersen et al., 1996). The nitrogen contents of coal deposited under marine influence are relatively high as shown in this study (average value 0.96 wt.%). High-Ca levels promoted decay of herbaceous materials and sulfates were reduced by bacteria; collinite and pyrite develop at the end of such a process. This situation is typical of the studied coals. As indicated above, there is abundant pyrite in the studied coals. The pyrite is framboidal, and was probably derived from waters rich either in sulfates or carbonates. At this point, it can be said that sulfatereducing bacterial activity was considerably high (Teichmüller et al., 1998). The abundance of hopanoids is probably related to bacterial contribution to the paleomire. Humic materials were probably broken down by high pH values and abundant bacterial activity. The decomposition of plants occurred under anoxic conditions with a generally high but variable water table; this is confirmed by the predominance of huminite-group macerals (Flores, 2002). The fusinite contents indicate old swamp fires or other oxidizing events. When reflectance values (Rmax %) and burial-temperature values of the coals were determined (Boggs, 1987), it seems clear that the burial temperature was b100 °C. Biomarker analysis of soluble organic matter in the coals has contributed to our understanding of the paleoenvironment of the mires (Waples and Machihara, 1991; Silva et al., 2008). In gas chromatograms of the studied samples, n-alkanes are found in the range of C22–C32, indicating a lagoonal and/or terrestrial environment (Filley et al., 2001; Riboulleau et al., 2007; Silva et al., 2008). 17α (H) — homohopane ratios are essential as a paleoclimate indicator (Waples and Machihara, 1991). Decrease in the C31–C35 extended hopanes reflects a clastic facies, and the C31 hopane rate can be correlated with peat and coal (Villar et al., 1988). Clastic materials and peat-forming environments both imply materials derived from lacustrine terrains. When compared on this point, this ratio is much higher in sample H-22 (Table 4), although there are homohopanes in only two samples. Harmonic decrease in the peak values of homohopane from C31 to C35 is quite typical of clastic facies (Waples and Machihara, 1991). Moreover, in these samples which predominantly comprise lower numbers, it is appropriate to mention suboxic depositional conditions (Hunt, 1995); indeed, the Pr/Ph ratio reflects anoxic environmental conditions. The C29/C30 hopane ratio indicates clastic deposition, and the presence of C29 norhopane indicates the presence of a carbonate/ evaporite lithology (Connan et al., 1986). Gammaceranes are indicators of high salinities (Waples and Machihara, 1991; Peters and Moldowan, 1993; Hunt, 1995). Furthermore, the presence of gammaceranes suggests derivation from a protozoan source. The C25/C26 tricyclicterpane ratios are N1 for the coals and indicate a terrestrial environment; lower values reportedly

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Fig. 14. Plot of the ratio C28/C29 regular steranes as a function of geological time (Grantham and Wakefield, 1988) of coal samples in this diagram.

indicate marine environments (Hanson et al., 2000). In addition, high sterane and sterane/hopane ratios indicate paralic and marine environments (Peters et al., 2004). Based on the C28/C29 sterane ratios, these coals are Tertiary in age, and this conclusion is in full agreement with the geological age (Fig. 14). Differences in diasterane/ sterane ratios exist within the coal samples (Table 4), and these high ratios show both acidic and oxic environments (Peters and Moldowan, 1993). All samples give hopanoid patterns characterized by the occurrence of 17α, 21β (H)- and 17β, 21β (H)-type hopanes, from C27 to C32, with C28 hopanes absent, as in the similar Maritza-East lignites, Bulgaria (Stefanova et al., 2002). In all of the studied coal samples, no considerable quantities of sesquiterpenoids, diterpenoids, non-hopanoid triterpenoids were observed (Bechtel et al., 2003). Tricyclic terpanes were found in all extracts. Generally, the coal samples have relatively high C25 tricyclicterpanes and low C23 tricyclicterpanes. Relatively high concentrations of C24 tetracyclic terpane in extracts indicates terrigenous input (Peters and Moldowan, 1993). βαMoretane/αβ-hopane (moretane/hopane) ratios indicate the early mature stage (equivalent to Rmax = 0.5%, Peters and Moldowan, 1993) and are related to organic matter deposited in a high-salinity depositional environment; this ratio decreases with increasing thermal maturity. Framboidal pyrite is abundant in all of the studied coal seams, suggesting the activity of sulfur-reducing bacteria and anaerobic conditions. Pr/Ph and diasterane/sterane ratios record variations in redox conditions and the depositional environment (Peters and Moldowan, 1993; Bechtel et al., 2005). In our study, Pr/Ph was only determined in one sample (0.25). Low Pr/Ph (b0.5) and ≤2 with Pr/n-C17 ratios b 0.5 ratio indicate an anoxic, hypersaline environment and marine influence. Low or very low C30 sterane values indicate limnic environments (Peters and Moldowan, 1993). On the basis of these data, the studied coals are believed to have been deposited in a limnic environment which was periodically influenced by marine and fresh-water conditions. 5. Conclusions Geological, organic-geochemical and organic-petrographic investigations of the Tertiary Hafik coals (Sivas Basin, Turkey) led to the conclusions that follow. The coal series comprises coaly, clayey,

organic-matter-rich carbonate levels with clay minerals, namely illite, smectite and chlorite. The Hafik coals are composed predominantly of dull, banded-dull and banded-coal lithotypes; such bands are related to high ash content. The ash and the sulfur contents of the Hafik coals are high and gross calorific values vary between 7729 and 27,658 kJ/ kg (average value 21,060 kJ/kg). The volatile-matter contents and ultimate-analysis data are in accordance with the rank of the coals. These coals are characterized by high huminite (dominated by gelinite) and low liptinite (dominated by sporinite, resinite and cutinite macerals) and inertinite contents (mainly macrinite and fusinite). The total organic carbon contents of the coaly levels are between 38.42 and 60.70 wt.%. A crossplot of HI and Tmax values and organic-matter determinations on palynological preparations show that kerogen is type III (terrestrial) with minor amounts of type II/III, in which woody and coaly materials are dominant. High SOM contents (concentrations of free lipids) were obtained from two samples (77–5412 ppm); the SOMs comprise mainly resins and asphaltenes. The n-alkanes are the dominant components of the saturated hydrocarbon fractions and range from C20 to C32. The presence of C25, C24, C26 and C28 triterpanoids and the predominance of C29 sterane over C27 and C28 indicate a terrestrial source. The high proportions of long chain C27–C31 n-alkanes relative to the SOM contents of the n-alkanes are typical of higher terrestrial plants, while short chain n-alkanes (bC20), detected in minor amounts, occur predominantly in algae and microorganisms. The studied samples are dominated by intermediate- and high-molecular weight n-alkanes (C21–25), and these data indicate derivation from terrestrial and lagoonal organic matter. Tricyclic terpanes were found in all extracts. In general, the coal samples contain relatively high C25 tricyclicterpanes and low C23 tricyclicterpanes. The relatively high concentrations of C24 tetracyclic terpane in extracts indicate terrigenous input. Huminite reflectance values ranging from 0.35 to 0.45%, Tmax values ranging from 412 to 431 °C, Ts/(Ts + Tm) ratios and 22S/(22S+ 22R), 20 (S)/(20S + 20R) and ββ/(ββ+ αα) sterane ratios and high resin and asphalthane contents indicate that the coals are in an early stage of maturation. An attempt to establish a significant correlation among huminite reflectance/vitrinite reflectance parameters, calorific values and Rock-Eval results leads us to classify the coals as subbituminous B/C, corresponding to a low rank of maturity. This situation suggests a low degree of organic-matter transformation and evolution.

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These coals contain abundant framboidal pyrite, indicating the existence of sulfur-reducing bacteria and anaerobic conditions in the depositional environment. The presence of gastropod fragments in the coal layer further supports the idea that these coals were deposited in a limnic environment. The high ash yields obtained from all samples indicate that periodic flooding affected the depositional environment; the presence of interlayered clay minerals and quartz are also evidence of these flooding events. The nitrogen contents of the coals suggest material that was deposited under marine influence. The decomposition of plants occurred under anoxic conditions with a generally high but variable water table; this is confirmed by the predominance of huminite-group macerals. High Ca-levels promoted decay of herbaceous materials and sulfates were reduced by bacteria. The high gelinite contents are characteristic of calcium-rich coals. Coals rich in sporinite and clay minerals are thought to have accumulated in a reed marsh, under subaquatic conditions and with a high degree of bacterial activity; these contents suggest high water levels, and the presence of gastropod shells and sedimentological evidence also indicate that the environment was of limnic nature. Moreover, low or very low C30 sterane values also indicate limnic environments. On the basis of these data, the studied coals are believed to have been deposited in a limnic environment which was periodically influenced by marine and fresh-water sources. From organicpetrographic observations, it can be concluded that the Hafik coals were deposited in an autochthonous to hypautochonous paleoenvironment. Coalification occurred during high groundwater levels and moderate subsidence rate. In general, highly alkaline, reducing conditions and a marine influence prevailed. Low TPI values indicate high bacterial activity and high pH conditions, and the preservation of gastropod shells indicates alkaline conditions.

Acknowledgments This work was financially supported by the Scientific Research Project Fund of Cumhuriyet University, Project Number M-319. The authors are grateful to Prof. Dr. M. Namık YALÇIN, Dr. Dursun Erik, Kayhan Pamuk, H. İsmail İlleez, Dr. Steven Mittwede, Prof. Dr. R. Littke, Dr. T.A. Moore and an anonymous reviewer for their valuable comments, which improved an earlier version of the manuscript.

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