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Petrography of the coal from the Oranovo–Simitli basin, Bulgaria and indices of the coal facies
International Journal of Coal Geology 57 (2004) 71 – 76 www.elsevier.com/locate/ijcoalgeo
Petrography of the coal from the Oranovo–Simitli basin, Bulgaria and indices of the coal facies Anton Sotirov a,*, Jordan Kortenski b b
a The Carel Corporation, Keller, TX 76248, USA Department of Economic Geology, University of Mining and Geology ‘‘St. Ivan Rilski’’, Sofia 1700, Bulgaria
Accepted 8 August 2003
Abstract Petrographic analyses of 137 Bulgarian coal samples collected from an underground mine in the Oranovo – Simitli basin show vitrinite (huminite) reflectance values near 0.34%. This middle Miocene age coal is mostly made of vitrinite (huminite), with about 10% liptinite and lesser amounts of inertinite. The inertinite group macerals are entirely funginite (sclerotinite) and inertodetrinite. The liptinite group macerals include fluorinite, which is identified for the first time in a Bulgarian coal. The abundant, degraded vitrinite group macerals result in a high gelification index and a high tissue preservation index, which are consistent with a forested, continuously wet raised-bog depositional environment. D 2003 Published by Elsevier B.V. Keywords: Petrography; Paleo-environment; Subbituminous coal; Bulgaria
1. Introduction The Oranovo– Simitli coal basin is in southwest Bulgaria, about 100 km south of the capital city of Sofia, and belongs to the Strouma –Mesta Province (Siskov, 1997). Archaic amphibolites and gneisses occur in the coastal zone of the basin (Marinova and Zagorchev, 1993). Coal occurs in the 150 –200-mthick, middle Miocene age Oranovo Formation. Up to 16 coal beds, ranging from 1 to 53 m thick, are
* Corresponding author. 253 Tzar Ozvoboditel Str., P.O. Box 7, Kyustendil 2500, Bulgaria. E-mail address: [email protected] (A. Sotirov). 0166-5162/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/j.coal.2003.08.004
present in the Oranovo Formation. Other sediments in the Oranovo Formation include alternating layers of sand, sandstone and aleurolite (silt) with sandy clay enriched in bitumen and plant detritus (Vatsev, 1991). Kortenski and Sotirov (2000) examined the minerals in coal from the Oranovo –Simitli basin and observe pyrite, marcasite, chalcopyrite, galena, albite, anorthite, analcime, kaolinite, sericite, illite, kammererite (chlorite), montmorillonite, epidote, titanite, calcite, siderite, aragonite, witherite, dolomite, hematite, braunite, rutile, gypsum and apatite. Other than a few limited studies (Kamenov et al., 1965; Siskov, 1988), the petrographic composition of coal in the Oranovo– Simitli basin is not well known. The purpose of the present investigation is to more completely
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Fig. 1. Texto-ulminite (TU), Eu-ulminite (EU), phlobaphinite (Ph), reflected light, oil immersion, magnification 1500.
Fig. 3. Gelinite (G), phlobaphinite (Ph), suberinite (SB), reflected light, oil immersion, magnification 1500.
examine the maceral composition of coal in the Oranovo –Simitli basin and to use the resulting data to infer the depositional environment of the ancient peat bog.
Polished coal blocks were examined using a Leitz – Wetzlar microscope at 384 white-light magnification, and 300 fluorescence-mode magnification. Vitrinite reflectance (random) was measured on huminite using 546 nm monochromatic light, an oil immersion objective and a sapphire calibration standard (Ro = 0.58%). A minimum of 500 macerals were counted for each specimen. The vitrinite group macerals are described using Stopes – Heerlen brown coal nomenclature (International Committee for Coal and Organic Petrology) described by Taylor et al. (1998). Although the coal is of subbituminous rank, brown
2. Materials and methods The 137 coal specimens examined in this study were collected as channel samples from an underground coal mine within the Oranovo– Simitli basin.
Fig. 2. Attrinite (AT), densinite (D), sclerotinite (SC, now called funginite). Note: the term sclerotinite is obsolete; these macerals are now called either funginite or sclerotinite (Lyons, 2000). Reflected light, oil immersion, magnification 1500.
Fig. 4. Sporinite (S), liptodetrinite (Ld), fluorescent light, oil immersion, magnification 1200.
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Fig. 5. Cutinite (C), fluorescent light, oil immersion, magnification 2500.
Fig. 7. Alginite (A), fluorescent light, oil immersion, magnification 2500.
coal maceral nomenclature is used because it allows easy comparison with results from other Bulgarian coal.
Vitrinite (huminite) reflectance ranged from 0.33% to 0.34%, which is consistent with the subbituminous C rank of these coals determined by Sotirov and Sokolov (1998). The coal bed is 80% durain and
15– 20% clarain with rare occurrences of other lithotypes. The coal samples examined average (f 78%) huminite, (f 10%) liptinite, ( f 2%) inertinite and (f 10%) minerals. Humotelinite is a major humite maceral subgroup (f 44%) and is almost entirely made of the maceral ulminite; both texto-ulminite and euulminite types are observed (Fig. 1), with the euulminite B variety the most common. Humodetrinite is also a major subgroup (f 27%), and is made of approximately equal amounts of attrinite and densinite (Fig. 2). Humocollinite is a relatively minor subgroup (f 7%) and is composed of about 4%
Fig. 6. Resinite (R), fluorescent light, oil immersion, magnification 2500.
Fig. 8. Fluorinite (F), fluorescent light, oil immersion, magnification 1200.
3. Results and discussion 3.1. Maceral composition of the coal
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levigelinite (eugelinite type) and 3% corpohuminite. The corpohuminite is both the phlobaphinite type shown in Figs. 1 and 3, and the pseudo-phlobaphinite type. As noted above, the liptinite group macerals are minor components (f 10%). Sporinite is common (f 3%) and fluoresces light yellow to orange (Fig. 4). Cutinite is not common (< 1%) but is easily identified by its diagnostic shape and light yellow to yellow – greenish fluorescence (Fig. 5). Resinite is slightly more common (up to 2%) and appears brown in white light and has low relief; it is more easily identified using the fluorescence mode of illumina-
tion where it appears light yellow to dark orange (Fig. 6). Alginite is the most abundant liptinite group maceral (f 6%) and shows variable shapes and fluorescence colors (Fig. 7). Suberinite (Fig. 3) is weakly to non-fluorescent, which is consistent with the subbituminous rank of the coal (Taylor et al., 1998). Small amounts of fluorinite (0% to f 1.5%) are sometimes observed (Fig. 8). The fluorinite can only be positively identified using the fluorescence mode of illumination where it shows its diagnostic bright fluorescence (yellow to yellow green). It commonly occurs in layers.
Table 1 Average petrographic composition of 137 coal samples examined in this study Maceral group
Ave. vol.%
Maceral subgroup
Ave. vol.%
Maceral submaceral or variety observed
vol.%
Huminite (vitrinite)
f78
Humotelinite (telovitrinite)
f44
textinite
0–1 19 – 97
Liptinite
~10
Inertinite
~2
Humodetrinite (detrovitrinite)
~27
ulminite texto-ulminite eu-ulminite B attrinite
Humocollinite (gelovitrinite)
~7
densinite corpohuminite
0 – 27 0–6
phlobaphinite pseudo-phlobaphinite gelinite levigelinite porigelinite
0–8
~10
sporinite cutinite resinite liptodetrinite alginite suberinite fluorinite exsudatinite bituminite
0.5 – 6 <1 0–2 0.5 – 6 1 – 13 0–2 0 – 1.5 0 0
Teloinertinite
~1
Detroinertinite
~1
Geloinertinite
0
fusinite semifusinite funginite inertodetrinite micrinite macrinite
0 0 1 1 0 0
0 – 26
Minerals
~10
~10
0 – 10
Total
100
100
100
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Inertinite group macerals are rarely observed. The absence of fusinite, semifusinite and macrinite is noteable. The absence of fusinite is also observed by Kamenov et al. (1965) and Siskov (1988). Only small amounts (1%) of funginite (Fig. 2) and inertodetrinite are observed. The funginite includes single to multichambered spores as well as a massive plechtenchyminite variety. 3.2. Indices of the coal facies Average maceral contents expressed on a wholecoal basis are used for calculation of the indices. The Groundwater Influence Index (GWI) by Calder et al. (1991) is calculated as: GWI ¼ ¼
ðgelinite þ corpohuminite þ mineralsÞ ðtexinite þ ulminite þ densiniteÞ ð4 þ 3 þ 10Þ ¼ 0:3 ð0 þ 44 þ 13Þ
The Vegetation Index (VI) by Calder et al. (1991) is calculated as: VI ¼
ðtextinite þ ulminite þ fusinite þ semifusinite þ suberinite þ resiniteÞ ð0 þ 44 þ 1 þ 1Þ ¼ ðdensinite þ inertodetrinite þ alginite þ liptodetrinite þ sporinite þ cutiniteÞ ð13 þ 1 þ 6 þ 3 þ 3Þ
The Tissue Preservation Index (TPI) by Diessel (1986) is calculated as:
TPI ¼ ¼
ðtextinite þ ulminite þ semifusinite þ fusiniteÞ ðdensinite þ macrinite þ inertodetriniteÞ ð0 þ 44 þ 0Þ ¼ 3:1 ð13 þ 0 þ 1Þ
The Gelification Index (GI) by Diessel (1986) is calculated as:
GI ¼ ¼
ðvitrinite þ macriniteÞ ðsemifusinite þ fusinite þ inertodetriniteÞ ð78 þ 0Þ ¼ 78 ð0 þ 0 þ 1Þ
According to the GWI and VI values, the type of the mire is an Ombrotrophic bog (Calder et al., 1991). According to the TPI and GI values, and the low mineral content, the peat bog originated in a forested, continuously wet raised bog where, according to Diessel (1992), the mild humification and strong gelification of the plant tissues are attributed to a high rate of subsidence (Table 1).
4. Conclusion The present paper is the first source of complete data for the petrography of the coal from the Oranovo – Simitli basin, Bulgaria. Fifteen macerals are described using the classification of the Stopes – Heerlen system. Fluorinite is found for the first time in Bulgarian coal. The indices of the coal facies are used for the first time to determine the origin of Bulgarian coal.
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Acknowledgements The authors wish to thank Prof. Dr. Reinhard Sachsenhofer from the Montanuniversita¨t of Leoben – ¨ AD scholarship for help with these Austria and the O investigations.
References Calder, J., Gibbling, M., Mukhopadhyay, P., 1991. Peat formation in a Westphalian B piedmont setting, Cumberland basin, Nova Scotia: implications for the maceral-based interpretation of rheotrophic and raised paleomires. Bull. Soc. Geol. Fr. 162, 283 – 298. Diessel, C.F.K., 1986. On the correlation between coal facies and depositional environments. Proc. 20th Symp., Dep. Geol., Univ. Newcastle, N.S.W., 19 – 22. Diessel, C.F.K., 1992. Coal-Bearing Depositional Systems. Springer, Berlin. 721 pp. Kamenov, B., Konstantinova, V., Yordanov, B., 1965. Geology and coal-bearing of the Oranovo – Simitli basin. Rev. Bulg. Geol. Soc. 1, 1 – 19 (in Bulgarian, with English Abstr.).
Kortenski, J., Sotirov, A., 2000. Mineral content, feeding provinces and genesis of the minerals in the coal from the Oranovo – Simitli basin, Bulgaria. Rep. Bulg. Acad. Sci. 53, 37 – 40. Lyons, P.C., 2000. Funginite and sclerotinite—two new macerals of the inertinite group. Int. J. Coal Geol. 44, 95 – 98. Marinova, R., Zagorchev, I., 1993. Geological Map of Bulgaria M 1:100 000, Map ‘‘Razlog’’. Explanation Notice. Geology and Geophysics Ltd., Sofia. 62 pp. (in Bulgarian). Siskov, G., 1988. Theoretical Basic of the Biochemical Coalification. Sofia University St. Kliment Ohridski, Sofia. 181 pp. (in Bulgarian). Siskov, G., 1997. Bulgarian low rank coals: geology and petrology. European Geology and Technology. Geological Society Special Publication, vol. 125, pp. 141 – 148. Sotirov, A., Sokolov, D., 1998. Major elements in the coal from the Oranovo – Simitli basin. Rev. Bulg. Geol. Soc. 59, 165 – 175 (in Bulgarian, with English Abstr.). Taylor, G.H., Teichmu¨ller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic Petrology. Gebru¨der Borntraeger, Berlin, Stuttgart. 704 pp. Vatsev, M., 1991. Lithostratigraphy of the Neogene from the Simitli and Brejani Graben. Ann. Univ. Min. Geol. St. Ivan Rilski 37, 23 – 38 (in Bulgarian, with English Abstr.).
Petrography of the coal from the Oranovo–Simitli basin, Bulgaria and indices of the coal facies Anton Sotirov a,*, Jordan Kortenski b b
a The Carel Corporation, Keller, TX 76248, USA Department of Economic Geology, University of Mining and Geology ‘‘St. Ivan Rilski’’, Sofia 1700, Bulgaria
Accepted 8 August 2003
Abstract Petrographic analyses of 137 Bulgarian coal samples collected from an underground mine in the Oranovo – Simitli basin show vitrinite (huminite) reflectance values near 0.34%. This middle Miocene age coal is mostly made of vitrinite (huminite), with about 10% liptinite and lesser amounts of inertinite. The inertinite group macerals are entirely funginite (sclerotinite) and inertodetrinite. The liptinite group macerals include fluorinite, which is identified for the first time in a Bulgarian coal. The abundant, degraded vitrinite group macerals result in a high gelification index and a high tissue preservation index, which are consistent with a forested, continuously wet raised-bog depositional environment. D 2003 Published by Elsevier B.V. Keywords: Petrography; Paleo-environment; Subbituminous coal; Bulgaria
1. Introduction The Oranovo– Simitli coal basin is in southwest Bulgaria, about 100 km south of the capital city of Sofia, and belongs to the Strouma –Mesta Province (Siskov, 1997). Archaic amphibolites and gneisses occur in the coastal zone of the basin (Marinova and Zagorchev, 1993). Coal occurs in the 150 –200-mthick, middle Miocene age Oranovo Formation. Up to 16 coal beds, ranging from 1 to 53 m thick, are
* Corresponding author. 253 Tzar Ozvoboditel Str., P.O. Box 7, Kyustendil 2500, Bulgaria. E-mail address: [email protected] (A. Sotirov). 0166-5162/$ - see front matter D 2003 Published by Elsevier B.V. doi:10.1016/j.coal.2003.08.004
present in the Oranovo Formation. Other sediments in the Oranovo Formation include alternating layers of sand, sandstone and aleurolite (silt) with sandy clay enriched in bitumen and plant detritus (Vatsev, 1991). Kortenski and Sotirov (2000) examined the minerals in coal from the Oranovo –Simitli basin and observe pyrite, marcasite, chalcopyrite, galena, albite, anorthite, analcime, kaolinite, sericite, illite, kammererite (chlorite), montmorillonite, epidote, titanite, calcite, siderite, aragonite, witherite, dolomite, hematite, braunite, rutile, gypsum and apatite. Other than a few limited studies (Kamenov et al., 1965; Siskov, 1988), the petrographic composition of coal in the Oranovo– Simitli basin is not well known. The purpose of the present investigation is to more completely
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Fig. 1. Texto-ulminite (TU), Eu-ulminite (EU), phlobaphinite (Ph), reflected light, oil immersion, magnification 1500.
Fig. 3. Gelinite (G), phlobaphinite (Ph), suberinite (SB), reflected light, oil immersion, magnification 1500.
examine the maceral composition of coal in the Oranovo –Simitli basin and to use the resulting data to infer the depositional environment of the ancient peat bog.
Polished coal blocks were examined using a Leitz – Wetzlar microscope at 384 white-light magnification, and 300 fluorescence-mode magnification. Vitrinite reflectance (random) was measured on huminite using 546 nm monochromatic light, an oil immersion objective and a sapphire calibration standard (Ro = 0.58%). A minimum of 500 macerals were counted for each specimen. The vitrinite group macerals are described using Stopes – Heerlen brown coal nomenclature (International Committee for Coal and Organic Petrology) described by Taylor et al. (1998). Although the coal is of subbituminous rank, brown
2. Materials and methods The 137 coal specimens examined in this study were collected as channel samples from an underground coal mine within the Oranovo– Simitli basin.
Fig. 2. Attrinite (AT), densinite (D), sclerotinite (SC, now called funginite). Note: the term sclerotinite is obsolete; these macerals are now called either funginite or sclerotinite (Lyons, 2000). Reflected light, oil immersion, magnification 1500.
Fig. 4. Sporinite (S), liptodetrinite (Ld), fluorescent light, oil immersion, magnification 1200.
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Fig. 5. Cutinite (C), fluorescent light, oil immersion, magnification 2500.
Fig. 7. Alginite (A), fluorescent light, oil immersion, magnification 2500.
coal maceral nomenclature is used because it allows easy comparison with results from other Bulgarian coal.
Vitrinite (huminite) reflectance ranged from 0.33% to 0.34%, which is consistent with the subbituminous C rank of these coals determined by Sotirov and Sokolov (1998). The coal bed is 80% durain and
15– 20% clarain with rare occurrences of other lithotypes. The coal samples examined average (f 78%) huminite, (f 10%) liptinite, ( f 2%) inertinite and (f 10%) minerals. Humotelinite is a major humite maceral subgroup (f 44%) and is almost entirely made of the maceral ulminite; both texto-ulminite and euulminite types are observed (Fig. 1), with the euulminite B variety the most common. Humodetrinite is also a major subgroup (f 27%), and is made of approximately equal amounts of attrinite and densinite (Fig. 2). Humocollinite is a relatively minor subgroup (f 7%) and is composed of about 4%
Fig. 6. Resinite (R), fluorescent light, oil immersion, magnification 2500.
Fig. 8. Fluorinite (F), fluorescent light, oil immersion, magnification 1200.
3. Results and discussion 3.1. Maceral composition of the coal
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levigelinite (eugelinite type) and 3% corpohuminite. The corpohuminite is both the phlobaphinite type shown in Figs. 1 and 3, and the pseudo-phlobaphinite type. As noted above, the liptinite group macerals are minor components (f 10%). Sporinite is common (f 3%) and fluoresces light yellow to orange (Fig. 4). Cutinite is not common (< 1%) but is easily identified by its diagnostic shape and light yellow to yellow – greenish fluorescence (Fig. 5). Resinite is slightly more common (up to 2%) and appears brown in white light and has low relief; it is more easily identified using the fluorescence mode of illumina-
tion where it appears light yellow to dark orange (Fig. 6). Alginite is the most abundant liptinite group maceral (f 6%) and shows variable shapes and fluorescence colors (Fig. 7). Suberinite (Fig. 3) is weakly to non-fluorescent, which is consistent with the subbituminous rank of the coal (Taylor et al., 1998). Small amounts of fluorinite (0% to f 1.5%) are sometimes observed (Fig. 8). The fluorinite can only be positively identified using the fluorescence mode of illumination where it shows its diagnostic bright fluorescence (yellow to yellow green). It commonly occurs in layers.
Table 1 Average petrographic composition of 137 coal samples examined in this study Maceral group
Ave. vol.%
Maceral subgroup
Ave. vol.%
Maceral submaceral or variety observed
vol.%
Huminite (vitrinite)
f78
Humotelinite (telovitrinite)
f44
textinite
0–1 19 – 97
Liptinite
~10
Inertinite
~2
Humodetrinite (detrovitrinite)
~27
ulminite texto-ulminite eu-ulminite B attrinite
Humocollinite (gelovitrinite)
~7
densinite corpohuminite
0 – 27 0–6
phlobaphinite pseudo-phlobaphinite gelinite levigelinite porigelinite
0–8
~10
sporinite cutinite resinite liptodetrinite alginite suberinite fluorinite exsudatinite bituminite
0.5 – 6 <1 0–2 0.5 – 6 1 – 13 0–2 0 – 1.5 0 0
Teloinertinite
~1
Detroinertinite
~1
Geloinertinite
0
fusinite semifusinite funginite inertodetrinite micrinite macrinite
0 0 1 1 0 0
0 – 26
Minerals
~10
~10
0 – 10
Total
100
100
100
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Inertinite group macerals are rarely observed. The absence of fusinite, semifusinite and macrinite is noteable. The absence of fusinite is also observed by Kamenov et al. (1965) and Siskov (1988). Only small amounts (1%) of funginite (Fig. 2) and inertodetrinite are observed. The funginite includes single to multichambered spores as well as a massive plechtenchyminite variety. 3.2. Indices of the coal facies Average maceral contents expressed on a wholecoal basis are used for calculation of the indices. The Groundwater Influence Index (GWI) by Calder et al. (1991) is calculated as: GWI ¼ ¼
ðgelinite þ corpohuminite þ mineralsÞ ðtexinite þ ulminite þ densiniteÞ ð4 þ 3 þ 10Þ ¼ 0:3 ð0 þ 44 þ 13Þ
The Vegetation Index (VI) by Calder et al. (1991) is calculated as: VI ¼
ðtextinite þ ulminite þ fusinite þ semifusinite þ suberinite þ resiniteÞ ð0 þ 44 þ 1 þ 1Þ ¼ ðdensinite þ inertodetrinite þ alginite þ liptodetrinite þ sporinite þ cutiniteÞ ð13 þ 1 þ 6 þ 3 þ 3Þ
The Tissue Preservation Index (TPI) by Diessel (1986) is calculated as:
TPI ¼ ¼
ðtextinite þ ulminite þ semifusinite þ fusiniteÞ ðdensinite þ macrinite þ inertodetriniteÞ ð0 þ 44 þ 0Þ ¼ 3:1 ð13 þ 0 þ 1Þ
The Gelification Index (GI) by Diessel (1986) is calculated as:
GI ¼ ¼
ðvitrinite þ macriniteÞ ðsemifusinite þ fusinite þ inertodetriniteÞ ð78 þ 0Þ ¼ 78 ð0 þ 0 þ 1Þ
According to the GWI and VI values, the type of the mire is an Ombrotrophic bog (Calder et al., 1991). According to the TPI and GI values, and the low mineral content, the peat bog originated in a forested, continuously wet raised bog where, according to Diessel (1992), the mild humification and strong gelification of the plant tissues are attributed to a high rate of subsidence (Table 1).
4. Conclusion The present paper is the first source of complete data for the petrography of the coal from the Oranovo – Simitli basin, Bulgaria. Fifteen macerals are described using the classification of the Stopes – Heerlen system. Fluorinite is found for the first time in Bulgarian coal. The indices of the coal facies are used for the first time to determine the origin of Bulgarian coal.
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A. Sotirov, J. Kortenski / International Journal of Coal Geology 57 (2004) 71–76
Acknowledgements The authors wish to thank Prof. Dr. Reinhard Sachsenhofer from the Montanuniversita¨t of Leoben – ¨ AD scholarship for help with these Austria and the O investigations.
References Calder, J., Gibbling, M., Mukhopadhyay, P., 1991. Peat formation in a Westphalian B piedmont setting, Cumberland basin, Nova Scotia: implications for the maceral-based interpretation of rheotrophic and raised paleomires. Bull. Soc. Geol. Fr. 162, 283 – 298. Diessel, C.F.K., 1986. On the correlation between coal facies and depositional environments. Proc. 20th Symp., Dep. Geol., Univ. Newcastle, N.S.W., 19 – 22. Diessel, C.F.K., 1992. Coal-Bearing Depositional Systems. Springer, Berlin. 721 pp. Kamenov, B., Konstantinova, V., Yordanov, B., 1965. Geology and coal-bearing of the Oranovo – Simitli basin. Rev. Bulg. Geol. Soc. 1, 1 – 19 (in Bulgarian, with English Abstr.).
Kortenski, J., Sotirov, A., 2000. Mineral content, feeding provinces and genesis of the minerals in the coal from the Oranovo – Simitli basin, Bulgaria. Rep. Bulg. Acad. Sci. 53, 37 – 40. Lyons, P.C., 2000. Funginite and sclerotinite—two new macerals of the inertinite group. Int. J. Coal Geol. 44, 95 – 98. Marinova, R., Zagorchev, I., 1993. Geological Map of Bulgaria M 1:100 000, Map ‘‘Razlog’’. Explanation Notice. Geology and Geophysics Ltd., Sofia. 62 pp. (in Bulgarian). Siskov, G., 1988. Theoretical Basic of the Biochemical Coalification. Sofia University St. Kliment Ohridski, Sofia. 181 pp. (in Bulgarian). Siskov, G., 1997. Bulgarian low rank coals: geology and petrology. European Geology and Technology. Geological Society Special Publication, vol. 125, pp. 141 – 148. Sotirov, A., Sokolov, D., 1998. Major elements in the coal from the Oranovo – Simitli basin. Rev. Bulg. Geol. Soc. 59, 165 – 175 (in Bulgarian, with English Abstr.). Taylor, G.H., Teichmu¨ller, M., Davis, A., Diessel, C.F.K., Littke, R., Robert, P., 1998. Organic Petrology. Gebru¨der Borntraeger, Berlin, Stuttgart. 704 pp. Vatsev, M., 1991. Lithostratigraphy of the Neogene from the Simitli and Brejani Graben. Ann. Univ. Min. Geol. St. Ivan Rilski 37, 23 – 38 (in Bulgarian, with English Abstr.).