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Organic geochemical characteristics and depositional environment of the Soma-Eynez (Manisa) coals, Western Anatolia, Turkey Selin Karadirek, and Orhan Ozcelik Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03137 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on February 7, 2019
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Energy & Fuels
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Organic geochemical characteristics and
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depositional environment of the Soma-Eynez
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(Manisa) coals, Western Anatolia, Turkey
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Selin Karadirek*, Orhan Ozcelik
5
Akdeniz University, Department of Geological Engineering, Antalya, Turkey
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ABSTRACT: The organic geochemical characteristics of Soma-Eynez coals located in the
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southwest section of the Soma Neogene Basin were examined and the depositional environment
8
and maturity levels were evaluated. The analyzed samples were drilling samples and they were
9
taken from the Lower Coal Seam (KM2) to represent the basin. The coal's total organic carbon
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(TOC) content is 28.45-73.38%, Potential yield (PY) value is 22.43-168.29 and extractable
11
organic matter (EOM) yield is 1204-29774 ppm; which are all indicators that the source rock has
12
excellent hydrocarbon-production potential. Tmax values are between 389-430 0C indicating
13
immature stage for hydrocarbon production. It is further supported by biomarker maturity
14
parameters (a high odd to even predominance index (OEP), low 20S/(20S+20R) sterane ratio,
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low ββ/(ββ+αα) sterane ratio, low 22S/(22S+22R)(C32) homohopane ratio, low C29Ts/C29
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Ts+C29H ratio). Unresolved compex mixture (UCM) observed in gas chromatograms point to
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immature-low mature non-biodegraded organic matter. The saturated biomarker fractions of
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KM2 coals are characterized by high Tm/Ts ratio, high Pr/Ph ratio, dominant C29 regular sterane 1 ACS Paragon Plus Environment
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presence, and high C30*/C29 Ts ratio. All biomarker parameters showed that terrestrial higher
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plants' materials and algal/bacterial source organism collectively deposited in the paleo-swamp
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and under suboxic-oxic conditions.
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KEYWORDS: Coal, Saturated Biomarker, Soma Formation, Organic Geochemistry, Turkey
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1. INTRODUCTION
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While Turkey's natural gas and oil reserves are rather limited, there are 1.3 billion tons hard
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coal and 15.6 billion tons lignite reserves. This amount constitutes 1.8% of coal reserves in the
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world. Moreover, the size of the country's lignite reserves are 7.1% of the world's reserve.1 Coal
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exploration works by public institutions and private enterprises gained momentum after 2005 and
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significant increase was seen in reserves. The calorific value of Turkish lignites are very low and
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90% of all lignite resources have calorific value below 3000 kcal/kg, and more than half of lignite
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resources contain moisture above 20%. Therefore, lignites are mostly used as fuel for thermal
14
power plants. On the other hand, lignites with high calorific value are used for heating housing
15
and for industrial purposes.2
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A great part of the coal basins in our country are located in the Western Anatolia, Central
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Anatolia and Thrace regions. With a total reserve of 720 million tons, the Neogene Soma
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(Manisa) coal basin located in the Western Anatolia region has the 5th largest lignite reserve in
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the country. Production is carried out using underground coal mine and open-pit techniques and
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new coal seams continue to be surveyed. The study area and its surroundings have been discussed
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in many different studies to date as it contains economically ranked coal deposits.
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organic geochemical studies on the Soma-Eynez coals have remained very limited. This study
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attempts to define the n-alkane, isoprenoid and saturated biomarker distribution of coals and to
3-29
However,
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shed light on the organic geochemical characteristics, depositional environment conditions and
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the maturity level of organic matter by drawing from other data. Furthermore, it investigates the
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hydrocarbon-production potential of coal/coaly units and organic matter type.
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2. GEOLOGICAL SETTING
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The study area was affected by the NE-SW, N-S directed extensional regime in Western
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Anatolia in the Neo-tectonic period, is located between the İzmir-Ankara suture which borders
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the Tauride-Anatolide block in the north and the Bornova Flysch Zone, and is within the borders
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of Yaylaköy, Kalemköy, Eynez situated SW of Soma (Manisa) (Figure 1).
10 11
Figure 1. Geological setting of Western Anatolia and the major sutures, continental blocks of
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Turkey. BFZ: Bornova Flysch Zone (modified after Yağmurlu et al.,30 Okay and Tüysüz31)
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Researchers suggested four different models for the origin of and reason behind the extensional
2
regime in Western Anatolia. These are Back-arc spreading model,32-37 Tectonic escape model,38-
3
42
Orogenic collapse model,43-47 and Episodic (two-stage graben model).48-58
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Lignite-bearing basins are generally bordered by growth faults and consist of volcanic and
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sedimentary rock assemblages.30 The basement rock of the study area comprised of Mesozoic
6
clayey, schist-greywacke; unconformably overlain by recrystalline limestone (Figure 2).
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Figure 2. Geological map of the southwest of Soma (Manisa) basin and generalized stratigraphic
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column of study area (modified after Tan et al.,24 Hokerek and Ozcelik59).
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The basement rocks were consecutively overlain by the Early to Middle Miocene Soma
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Formation with unconformity and by the Late Miocene Deniş Formation with unconformity.
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Characterized by non-marine, lateral spread lacustrine units with dominant limestone, marl and
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claystone; the Soma and Deniş Formations shape the sedimentary sequence of the basin.15, 17, 22
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The basin has 3 coal seams: Upper coal seam-KP1, Middle coal seam-KM1 and Lower coal
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seam-KM2. The Soma and Deniş Formations are cut by Upper Miocene andesitic-basaltic dykes,
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and are discordantly covered by Upper Miocene agglomerate-tuff and andesite-basalt units, as
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well as by Quaternary slope debris and alluvium (Figure 2). In this study, evaluations were
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carried out with regard to the lower coal seam (KM2). Tuff and tuffites observed within KM2 are
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thought to be a product of short-lived volcanic activity taking place at long distances.24 Slopes at
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KM2 vary between 7o-18o. In places close to the tectonic line, slopes of 21o-28o can be observed.
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Towards the S/SW of the basin, units' slopes increase. Layer slopes generally decrease from top
13
to bottom and are dipped 2° to 30° towards S-SW.24 According to data from the drilling on the
14
study area, the unit's thickness increases towards S-SW of the area.
15 16
3. MATERIAL AND METHODS
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Drilling samples were obtained from KM2 seam of Lower Miocene Soma Formation of ES332,
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ES334, ES338, ES331A, ES342, ES340, ES344A, ES349B, ES352 and ES359 wells located in
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the study area (Figure 2). 30 samples collected from 10 drillings were analyzed for Rock Eval
20
pyrolysis. Each sample (100 mg) was analyzed using a Rock–Eval/TOC 6 version. The samples
21
were dried, pulverized and heated in stages. Guidelines outlined by Espitalie et al.,60 Peters,61
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Lafargue et al.,62 Peters and Cassa,63 Behar et al.64 were followed for data interpretation.
23
Following pyrolysis analysis, 10 samples were chosen for further organic geochemistry (gas
24
chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analyses that are 5 ACS Paragon Plus Environment
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used to evaluate paleoenvironment, lithology, geologic age, maturity and biodegradability of
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source rock. Saturated hydrocarbon was extracted from powdered rocks by bitumen extraction
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that was performed using ASE 300 (accelerated solvent extraction) for 40 h with
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dichloromethane (CH2Cl2). Extracts of coal and coaly samples were analyzed using Agilent 6850
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gas chromatograph (GC) instrument equipped with flame ionization detectors (FID), according to
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ASTM D5307-97(2007).65 HP-1 capillary column was utilized and helium was the carrier gas.
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The GC–MS analysis was carried out on saturated fractions using Agilent 5975C quadruple
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mass spectrometer coupled to a 7890A gas chromatograph and a 7683B automatic liquid sampler.
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The gas chromatograph was equipped with an HP-1MS capillary column and helium was the
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carrier gas. Fragmentograms for steranes (m/z 217) and triterpanes (m/z 191) were recorded for
11
analysis of biomarkers. Individual components were identified by comparison of mass spectra
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and retention times in the single ion- current chromatogram. Relative abundances of triterpanes
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and steranes were calculated using peak heights from GC-MS. Whole analysis were carried out in
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the Laboratories of Turkish Petroleum (TP, Ankara).
15 16
4. RESULTS AND DISCUSSION
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4.1. Source rock properties
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In this study, Lower Miocene KM2 coals' organic matter richness, organic matter type and the
19
maturity level of the organic matter were determined, and the source rock characteristics were
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evaluated. Data concerning hydrocarbon production potential were evaluated using Rock-Eval
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pyrolysis data and this data was shown in Table 1. TOC values varied between 28.45 and 73.38
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wt.%. S1, S2, S3 and Tmax were measured in the pyrolysis analysis and these values were used to
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measure Hydrogen index (HI), Oxygen Index (OI), Production Index (PI), S2/S3 and Potential
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Yield (PY) values (Table 1). Kerogen type, an important parameter for hydrocarbon generation, 6 ACS Paragon Plus Environment
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is determined using the HI and OI values. For the examined samples, HI values were between 67-
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246 mg HC/g TOC and OI values were between 11-28 mg CO2/g TOC.
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Table 1. Results of Rock Eval/TOC analysis Pyrolysis data Well No
ES332
Depth
TOC
(m)
(wt.%)
S1
S2
(mg HC/g Rock)
(mg HC/g Rock)
S3
(mg CO2/g rock)
Tmax (0C)
HI
OI
PI
mg HC/g TOC
mg CO2/g TOC
S1/(S1 +S2)
S2/S3
PY (S1+S2)
624.80
32.51
0.64
21.79
9.26
421
67
28
0.03
2
22.43
626.40
63.16
1.20
83.29
16.81
427
132
27
0.01
4
84.49
641.90
38.87
0.98
47.49
9.78
416
122
25
0.02
4
48.47
ES334
584.90
32.97
1.08
50.30
8.71
422
153
26
0.02
5
51.38
ES338
769.00
50.05
1.60
75.07
10.25
425
150
20
0.02
7
76.67
771.10
65.21
1.18
108.20
10.48
416
166
16
0.01
10
109.35
776.20
39.36
1.00
56.64
8.03
429
144
20
0.02
7
57.64
842.20
66.98
0.24
44.88
15.18
429
67
23
0.01
2
45.12
852.00
66.62
0.89
57.35
12.28
416
84
18
0.02
4
58.24
846.90
67.42
1.19
109.50
10.79
412
162
16
0.01
10
110.72
857.00
64.23
2.50
86.66
10.36
403
135
16
0.03
8
89.16
666.50
65.69
0.81
77.85
15.88
415
119
24
0.01
4
78.66
671.10
70.23
7.78
160.5
7.670
389
229
11
0.05
20
168.29
673.80
67.16
1.04
63.85
17.70
422
95
26
0.02
3
64.89
835.90
69.69
0.92
48.18
19.61
425
69
28
0.02
2
49.10
843.70
52.62
1.31
45.48
14.59
430
86
28
0.03
3
46.79
850.00
58.52
0.98
51.81
12.17
417
89
21
0.02
4
52.79
770.40
28.45
1.68
33.35
6.49
419
117
23
0.05
5
35.03
774.65
71.02
0.99
69.54
13.79
422
98
19
0.01
5
70.53
781.70
64.71
1.76
108.90
9.50
414
168
15
0.02
11
110.70
790.40
44.36
1.04
109.10
6.43
413
246
14
0.01
16
110.16
764.00
58.61
1.20
90.42
6.56
419
154
11
0.01
13
91.62
774.65
62.92
0.69
60.46
11.14
421
96
18
0.01
5
61.15
779.75
43.61
0.73
43.05
9.70
429
99
22
0.02
4
43.78
788.80
29.74
0.93
54.60
6.36
428
184
21
0.02
8
55.53
1059.40
64.12
1.02
77.86
10.04
423
121
16
0.01
7
78.88
1064.10
73.38
1.12
87.20
11.45
417
119
16
0.01
7
88.32
1069.50
72.66
2.52
113.50
8.14
396
156
11
0.02
13
116.05
1074.20
66.67
0.99
67.85
8.77
415
102
13
0.01
7
68.84
541.70
46.92
0.90
67.17
12.06
405
143
26
0.01
5
68.07
ES331A
ES342
ES340
ES344A
ES349B
ES352
ES359
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The S2 value was between 21.79 and 160.51 mgHC/g rock. According to the S2 value, it shows
2
very good-excellent hydrocarbon generation potential.63 In the S2-TOC diagram, all but 2
3
samples were situated in the Type III kerogen area (Figure 3). The two samples (ES342-671.10
4
and ES344A-790.40) were located on the Type II area, as their HI values were higher than 200
5
mgHC/g rock. The slope of the regression line in this diagram was identified as 1.12 and it was
6
determined that the coals had an 11.2% pyrolysable hydrocarbon ratio.
7 8
Figure 3. Plot of S2-TOC for Eynez-KM2 samples (modified after Peters and Cassa63)
9 10
The HI-Tmax diagram is another evaluation to determine the type and maturity level of organic
11
matter. The Tmax value differs depending both on the maturity level and type of the kerogen.66
12
The Tmax values for the analyzed samples varied between 389-430 0C and the average was 420 0C
13
(Table 1). Organic matter has not yet reached maturity. It is seen that many of the samples are
14
located in the Type III kerogen area in the HI-Tmax diagram, while a few are found in the Type II
15
kerogen area and the diagram shows the immature kerogen area (Figure 4). The coal petrography
16
studies carried out in the Soma basin15, 29, 67-69 are in accordance with pyrolysis data. According to
17
the maceral analysis, huminite (type III kerogen) has been identified as the dominant maceral
18
group whereas liptinite is very common and inertinite is rare in the Eynez-KM2 seam.29 8 ACS Paragon Plus Environment
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Figure 4. Plot of HI-Tmax for Eynez-KM2 samples, showing kerogen types and maturity70
3 4
PI values were between 0.01-0.05 and the average was 0.02 (Table 1). According to the PI
5
classification, these values indicate immature organic matter. The PY value obtained from Rock-
6
Eval data was between 22.43-168.29 mgHC/g rock and the average was determined as 73.76 mg
7
HC/g rock. According to these values, it is possible to say the samples show good condition
8
source rock characteristics.
9 10
4.2. Bitumen analysis
11
The amount of extractable organic matter (EOM) obtained by the bitumen analysis is listed in
12
Table 2. The EOM yield in KM2 coals is between the range of 1204-29774 ppm and indicates
13
hydrocarbon-generating source rock.63 This value is also supported by S2-TOC (Figure 3). The
14
bitumen/TOC ratio is generally between 0.002-0.051.
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Parameters
ES340
ES352
ES331A
ES344A
ES342
ES338
ES359
Sample ID
ES349B
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
Table 2. Bitumen extraction and n-alkane parameters for Eynez-KM2 samples.
ES334
1
ES332
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 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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TOC (wt.%)
63.16
32.97
58.61
58.52
72.66
67.42
71.02
70.23
65.21
46.92
Σ Extract (ppm)
1204
2165
5064
29774
3914
3873
5147
28739
7446
3079
Bitumen/TOC
0.002
0.007
0.009
0.051
0.005
0.006
0.007
0.041
0.011
0.007
Pr/Ph
3.70
1.00
4.40
2.33
6.29
3.21
6.15
1.00
3.00
0.82
Pr/n-C17
2.47
0.95
2.10
3.18
3.45
4.39
4.04
3.00
1.20
0.53
Ph/n-C18
0.77
1.00
0.51
0.86
0.40
0.36
0.62
1.50
0.67
0.42
CPI
3.44
2.94
2.78
3.25
2.34
2.76
2.67
1.83
2.75
3.53
OEP
1.75
2.04
1.67
2.51
1.56
2.19
1.92
1.65
1.29
2.63
Short chain n-alkane (C27>C28. The C29>C28>C27 ranking can be seen in the sample
12
numbered ES349B and ES352, but abundance of the C28 and C27 steranes have close values.
13
Dominance of the C29 sterane shows terrestrial organic matter input.86-89 Values for the C29/C27
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sterane ratio are between 2.54 and 8. The C27, C28, C29 triangle diagram illustrates the distribution
2
of samples belonging to the KM2 seam (Figure 9).
3
4 5
Figure 9. Ternary diagram of regular C27-C28-C29 steranes showing depositional environment and
6
source input (modified after Huang and Meinschein86)
7 8
Sterane/hopane ratio presents with high (>1) values in the samples numbered ES332, ES334,
9
ES344A and ES342; but is low in the other samples, and this signifies that terrestrial organic
10
matter is predominant and also that algae is present in the depositional environment.75, 90 While
11
the diasterane/sterane ratios provide high values, low values are observed in two different
12
maturity parameters, the C29 20S/(20S+20R) sterane ratio and the C29 ββ/(ββ+αα) sterane ratio
13
(Table 3).
14 15
4.4. Thermal maturity
16
The maturity level of the Soma-Eynez KM2 units were evaluated using pyrolysis, vitrinite
17
reflectance (Ro%) and biomarker distributions. The fundamental maturity parameters obtained
18
from pyrolysis are Tmax, Production Index (PI). It shows Tmax (389-430 0C) and PI (0.01-0.05)
19
values, and indicates an immature level. The values of this study are within the immature kerogen 18 ACS Paragon Plus Environment
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area in the HI-Tmax diagram as well. Average vitrinite reflectance (Ro%) values for all coals are
2
relatively low and point to the immature-early mature (0.21-0.65%) phase.
3
The bitumen/TOC ratio of 0-0.05 indicates the immature-early mature range. The ratio of odd-
4
carbon-numbered alkanes to even-carbon-numbered alkanes obtained from n-alkane distributions
5
can be used to determine thermal maturity. CPI and OEP were used to achieve this.71, 72, 79 Both
6
ratios are characteristic of maturity, but are affected by source input and biodegradation. If these
7
ratios are bigger than 1, they indicate low thermal maturity. Samples belonging to the
8
investigation area had CPI between 1.83-3.53 and OEP between 1.56-2.63, and these were placed
9
in the immature-early mature level (Figure 10a and Table 2). The Pr/nC17-Ph/nC18 diagram
10
contains similar interpretations (Figure 6). High hump formations are also observed in resulting
11
chromatograms due to low maturity of the samples. This is a typical characteristic in coal
12
samples, especially those that have low maturity.80
13 14
Figure 10. a) Cross plot of CPI versus OEP showing thermal maturity of the analyzed samples.
15
b) Cross plot of 20S/(20S+20R) C29 sterane versus ββ/(ββ+αα) C29 sterane showing thermal
16
maturity of analyzed samples.
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The 20S/(20S+20R) and ββ/(ββ+αα) sterane ratio increases along with maturity and reaches
2
equilibrium at 0.55 and 0.7 respectively.66,
76, 91-94
3
20S/(20S+20R) sterane ratio was 0.04-0.12, while ββ/(ββ+αα) sterane ratio was 0.27-0.40 (Table
4
3 and Figure 10b). These values have not reached equilibrium and are at immature-early mature
5
level. The diagram in Figure 10b shows the early mature section. The 22S/(22S+22R) (C32)
6
homohopane ratio also increases depending on increasing maturity and reaches equilibrium at
7
0.6, which points us towards the early maturity level.92,
8
homohopane isomerization ratio is between 0.1-0.3 and has not yet found a balance, meaning it
9
has not matured. While the Ts/(Ts+Tm) ratio increases with maturity,97-99 the moretane/hopane
10
ratio decreases as maturity advances.100-102 A high moretane/hopane ratio (0.31-0.63) and a low
11
Ts/(Ts+Tm) ratio (0.02-0.24) characterize that the coals have not matured. The C29Ts/C29
12
Ts+C29H ratio shows tendency to increase along with maturity. The C29Ts/C29 Ts+C29H ratio for
13
KM2 coals is 0-0.16 and thus not matured. Considering all these evaluations, it can be said that in
14
general the examined samples exhibit immature-early mature characteristics. Differences
15
between maturities may be due to tectonism-related different depths of burial, the effect of
16
volcanism and facies changes.
For KM2 units in the Soma-Eynez site,
95, 96
The 22S/(22S+22R) (C32)
17 18
4.5. Depositional environments of coal
19
In this study, the source input and depositional environment conditions of KM2 coals were
20
defined using biomarker distributions (n-alkane, isoprenoids, triterpanes and sterane
21
distributions).
22
CPI values (1.83-3.53) obtained from gas chromatogram for coal samples show terrestrial
23
higher plants' input.76-78, 80 The Pr/Ph (>3) ratio shows that terrestrial plants are predominately the
24
source of organic matter, while at the same time reflecting oxic conditions. While a low Pr/Ph 20 ACS Paragon Plus Environment
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Energy & Fuels
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