The petroleum potential of some Tertiary lignites from northern Greece as determined using pyrolysis and organic petrological techniques

Org. Geochem.Vol. 17, No. 6, pp. 805-826, 1991 Printed in Great Britain

0146-6380/91 $3.00+ 0.00 Pergamon Press plc

The petroleum potential of some Tertiary lignites from northern Greece as determined using pyrolysis and organic petrological techniques* MARTIN G. FOWLER,1 THOMASGENTZIS,2 FARIBORZGOODARZIl and ANTHONYE. FOSCOLOS3 qnstitute of Sedimentary and Petroleum Geology, Geological Survey of Canada, 3303-33rd St N.W., Calgary, Alberta, Canada T2L 2A7 2Coal Research Centre, Alberta Research Council, Devon, Alberta, Canada T0C 1E0 3Department of Mineral Resources Engineering, Technical University of Crete, 73133 Chania, Greece Abstract---Oil condensate and gas have recently been discovered in the Thermaikos Gulf area of northern Greece. The source of these hydrocarbons is not known, although there is a possibility that more mature equivalents of certain Tertiary brown coals and lignites may be responsible. A contribution from Tertiary terrestrial organic matter to the Epanomi oil is supported by its biomarker distributions, especially the predominance of triterpenoid compounds other than hopanes in the m/z 191 mass fragmentogram. In this study, the hydrocarbon potential of a suite of Greek Tertiary lignites is first examined using Rock-Eval analysis. Three samples are then further investigated using organic petrography, and gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) of the extracted hydrocarbon fractions, as well as by hydrous pyrolysis. An intermontane lignite (6OG-5) contains fluorescing huminite together with resinite. On extraction, this resinite is found to be of predominantly diterpenoid origin. Rock-Eval and hydrous pyrolysis support previous proposals that diterpenoid resinite needs a lower activation energy to generate hydrocarbons than most other types of organic matter. A sample of a lignite deposited in a deltaic environment (MM-3) contains a more diverse collection of liptinite macerals with resinite again predominating. The resinite appears to be mostly of triterpenoid and sesterterpenoid origin. The best hydrocarbon potential is shown by a sample deposited in a lagoonal environment (KAS-3). Little discrete resinite is observed in this sample but it does contain abundant fluorescing huminite. Although it has a low SJSt + $2 ratio, it gives a high yield on solvent extraction during which a significant fraction of the $2 is removed. The fluorescence of the huminite after extraction is greatly reduced. The hydrocarbons of the extract are believed to be predominantly derived from triterpenoid resinite. Unlike the pyrolysates of the other two lignites but in common with the Epanomi oil, the triterpenoids in the KAS-3 pyrolysate are dominated by resin-derived compounds rather than hopanes. This and other similarities between the composition of the pyrolysates of KAS-3 and the oil support the possibility that a more mature equivalent of this lignite could be the source of the Epanomi oil. Key words--Greece, hydrous pyrolysis, lignites, organic petrology, resinite, Rock-Eval

INTRODUCTION Oil, condensate and gas discoveries have recently been made in the Thermaikos G u l f area about 30 km southeast of Thessaloniki (Fig. 1). Oil was discovered by the Greek Public Petroleum Corporation at a depth of 2800.5-2838.5 m in Tertiary sediments during the drilling of the Epanomi-2 hole in 1989. Although the actual source rocks are not known, there is a strong suspicion that more mature equivalents of certain Tertiary brown coals could be the source of these hydrocarbons. There have been several reviews that have discussed the possibility that coals can be oilsource rocks (e.g. D u r a n d and Paratte, 1983; Saxby and Shibaoka, 1986; Murchison, 1987; Bertrand, 1989) but few actual proven examples. Low rank coals of Tertiary to Quaternary age are widespread in Greece. Sixty lignite-bearing basins containing reserves of 5.3 x 10 9 t of low rank coal have *Geological Survey of Canada No. 28190.

been discovered to date (Koukouzas, 1985). Many of these Tertiary coals are relatively rich in liptinite macerals, especially resinite, and contain fluorescing huminite (Cameron et al., 1984; Goodarzi et al., 1990) suggesting the possibility that they could have better hydrocarbon potential than is normally observed for coals. Most lignite deposits in Greece were formed in intermontane basinal settings in graben-fike structures, although some were deposited in near coastal settings such as deltas or alluvial plains (Foscolos et al., 1989). The diversity in the depositional environments is reflected in the suite of samples used in the present study, three of which were selected for more detailed investigations. The 6OG-5 sample comes from Varvoutis Mine in the Florina Basin, northwestern Greece. It is of Lower Pliocene age and was deposited in an intermontane basin. The MM-3 sample is of Miocene age and was deposited in an estuarine/delta setting near Moschopotamos in the Katerini Basin (Fig. 1). It is the lowermost of three thin coals (about 60 cm thick) 805

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that are each separated by about 1 m of sandstones. The KAS-3 sample originated from a depth of 1685.5 m in a well drilled on the Kassandra peninsula (Fig. 1). It is of Miocene age and was deposited in an environment influenced by marine (brackish) waters, probably a lagoon. In this paper we investigate the hydrocarbon potential of some Tertiary lignites from Greece, concentrating on the three samples described above and determine if a more mature equivalent of one of these coals could have been the source of the Epanomi oil. METHODS

Rock-Eval and C H N analysis Coals were pulverized to about 100 mesh in preparation for Rock-Eval, CHN analysis and extraction. Problems associated with Rock-Eval analysis of coals have been previously described (e.g. Peters, 1986; Law et al., 1989). An effort was made, during the initial stages of our studies on the hydrocarbon potential of coals, to arrive at a set of conditions that provided reasonably accurate and reproducible results from Rock-Eval analysis. When sample amounts of > 5 mg were used, TOC values from the Rock-Eval were usually much lower than those obtained from CHN analysis which led to erroneously high HI values.

Sample sizes of 5 or 10 mg have been used in other studies of the hydrocarbon potential of coals using Rock-Eval (e.g. Teichmiiller and Durand, 1983; Durand and Paratte 1983; Littke et al., 1989). The results from 5 mg aliquots are those given in Table 2. Tma~ values from the 5 mg analyses tended to be significantly lower than expected for the rank of the coal. More accurate Tmax values, that were about 5°C higher than the 5 mg analysis results, were obtained when a 20 mg aliquot of sample was used. The Tmax values given in Table 2 for the original lignites are from 20 mg samples. Using such small sample amounts obviously can lead to errors because of sampling bias. Hence, all samples were run at least in duplicate and in some cases, where unexpected or widely diverging results were obtained, additional analyses were performed. A CEC elemental analyser with a combustion temperature of 970°C and a reduction temperature of 650°C was employed for CHN analysis. Tin capsules were used and the machine was calibrated using acetanilide. Samples were run in duplicate. Extraction and fractionation Samples were extracted using azeotropic chloroform:methanol (87:13) for 24 h. The lignite extracts were first treated with approx. 40 volumes of n-pentane to precipitate the asphaltenes. The deasphalted

Petroleum potential of Tertiary lignites

807

Table 1. Geochemicaldata on the Epanomi oil Table 1 %Total oil (% of Recovered Oil)

Gross Composition 1 Saturate HCs

13.5 (38.5)

Aromatic HCs

20.9 (58.5)

Resins + asphaltenes

1.5 (3.0)

Total recovered

35.8

Gasoline Range2

%Total oil

I: Isoheptane index

2.69

H: Heptane index

28.54

B: Toluene/nC7

0.83

F: nCT/Methylcyclohexane

0.86

GC and GC-MS pr/ph

2.73

pr/nCl7

0.61

nC17/nC27

0.34

rttt ~tS/,,,~tR 3

0.94

,~~ ~/,,,~,,+,~ ~ ~4

0.54

C291313(H),17a(H) 20S diasterane/ 5 • (H),14,* (H),17,, (H) 20R-ethyleholestane

1.35

1Determined after column chromatography 2Ratios given are those used by Thompson (1979, 1983, 1989). I= (2- + 3-methylhexanes)/(1C3-, lt3-, and lt2-dimethylcyelopentanes) H = 100 x n-heptane/(ll cyciohexane through methylcydohexane excluding lc-2-dimethylcyclopentane) 3,,, ~ t S / ct,,,a R = 5,,,(H),14tz(H),l Ta(H)20S/5a(H),14~t(H),17a(H)20R-C29

steranes 4a131~/"a,, = 5,~(H),141~(H),1713(H)/5a(H),14a(H),17a (H) + 5tt (H),141~(H) 1713(8) lignite extracts, lignite pyrolysates and Epanomi oil were fractionated using open column chromatography (3/4 activated alumina and 1/4 activated silica gel with an adsorbent:sample mass ratio of 100:l). Saturates were recovered by eluting with 3.5 ml of pentane/g of adsorbent. Aromatics were recovered by eluting with 4 m l of 50:50 pentane-dichloromethane/g of adsorbent and the resins were recovered with 4 ml/g of methanol. Hydrous pyrolysis

Two hydrous pyrolysis methods were used. One was a small-scale method, based on that used by Eglinton et aL 0986) which has previously been used to study coals in this laboratory (von der Dick et aL, 1989). This was employed to look at the hydrocarbon potential of three of the Greek coals. Experiments were carried out in purpose-built stainless steel "bomblets" (volume 35 ml). About 2 g of the coal sample (unextracted or pre-extracted) was used in

each experiment to which 20 ml of distilled water was added. The bomblet was then purged with nitrogen to remove air before being sealed. The bomblets were placed in a commercially available 1 litre pressure reactor (Parr Instrument Co.), also partly filled with water (to minimize the pressure differential across the homblet wall and to reduce temperature gradients in the system), which was in turn sealed and checked for leaks at between 1500 and 2000 psi helium. The temperature of the reactor was raised by a heating jacket with temperature accurate to about 5°C. Samples were either heated to 300 or 330°C and held isothermal for 72 h after which time the vessel was allowed to cool prior to opening. The gases produced during the experiment were not collected and were allowed to leak off. The products in the bomblets were extracted using dichloromethane. The pyrolysates were deasphalted and fractionated as described above. The extracted residues were analysed by Rock-Eval, CHN and optical techniques.

808

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Petroleum potential of Tertiary lignites The Kassandra coal sample was further investigated with a larger scale experiment. Two lumps of coal, both weighing around 10 g (total combined weight = 19.74 g) were placed in a 300 ml Parr Bomb with 100 ml of distilled water. The bomb was purged with helium and checked for leaks at between 1500 and 2000 psi helium. The bomb was first heated to 300°C and held isothermal for 72 h. At the end of this phase of the experiment the bomb was allowed to cool and the oil on the surface of the water was recovered as an "expelled oil" fraction. An additional 20 ml of distilled water was added to the bomb and the same sample was heated to 330°C and held isothermal for 72 h. The expelled oil from this phase of the experiment was also recovered. In addition, the coal residue was extracted ultrasonically with dichloromethane. These expelled oils, and the extract, were fractionated as above.

GC and GC-MS analysis Gas chromatograms of the saturate fractions were acquired on a Varian 3700 FID gas chromatograph (GC) using a 30 m DB-1 column with a temperature program of 60-300°C at 6°C/min. Gas chromatography-mass spectrometry (GC-MS) data were collected using a VG 70SQ hybrid MS-MS under the control of a VG 11-250 data system. Data were collected using a 100pA trap current and 70eV ionization voltage. The gas chromatograph was fitted with a 25 m DB-5 column which was coupled directly to the ion source and temperature programmed from 50 to 310°C at 4°C/min. The ions monitored in each experiment were rn/z 177.1638, 191.1794, 217.1950, 218.2028, 231.2106 and 259.2262. Full scan data, for peak identification by comparison of mass spectra, were obtained by scanning from m/z 650 to 50 and 1 s/decade. Gasoline-range hydrocarbons were analysed using the method of Snowdon and Osadetz (1988).

Microscopy Samples of the original coals, the extracted coals and the hydrous pyrolysis residues were prepared as pellets according to the method described by Mackowsky (1982). Reflectance measurements in oil (n o = 1.518 at 24°C) were determined using a Zeiss MPM II reflected light microscope fitted with halogen and fluorescent (HBO) light sources. The original and extracted coals were photographed using water immersion and fluorescent light (filters: excitation 450-490nm, beam splitter 510nm and barrier 520 nm). Maceral analysis was performed using a Swift Model F automatic point counter attached to the mechanical stage of the microscope. EPANOMI OIL

As indicated by the whole oil gas chromatogram [Fig. 2(a)], the Epanomi oil is light with only a minor portion (35.8%) recovered after fractionation.

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Fig. 2. Gas chromatograms of the Epanomi oil: (a) whole oil; (b) gasoline range (C6, C7, Cs are n-hexane, n-heptane and n-octane respectively, B is benzene, MCYC6 is methylcyclohexane and T is toluene); and (c) C~0+saturate hydrocarbons. Pr is pristane and numbers refer to n-alkanes in (a) and (c). It consists almost entirely of hydrocarbons with a predominance of aromatics over saturates (Table 1). Gasoline-range analysis [Fig. 2(b)] suggests that the oil has not been biodegraded or water-washed. Using gasoline-range parameters proposed by Thompson (1979, 1983, 1987), given in Table 1, the oil is mature except in the case of the isoheptane value which indicates the oil is supermature. The whole oil chromatogram [Fig. 2(a)] indicates that the n-alkanes have a bimodal distribution. The lower molecular weight (C10---C12)n-alkanes are in much higher concentrations than their C~5+ homologues. A distinct second maximum occurs around nC27. This, and the high pristane to phytane ratio (2.7) are evidence for a contribution to this oil from source rocks containing terrestrially derived organic matter (Powell and McKirdy, 1973). The sterane distribution of the Epanomi oil [Fig. 3(a)] is dominated by C29 components with much lower amounts of C27 and C2s steranes, a distribution commonly reported in oils derived from source rocks containing predominantly terrestriallyderived organic matter (e.g. Hoffmann et al., 1984; Brooks, 1986; Philp and Gilbert, 1986). Sterane maturation ratios indicate the oil to be mature but they have not reached their endpoint values (Table 1). Terpanes are present in much higher concentrations than steranes. The m/z 191 fragmentogram [Fig. 3(b)] is unusual in that it is not dominated by hopanes.

810

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29S

27

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Fig. 3. (a) Mass fragmentograms of the Epanomi oil hydrocarbons: (a) m/z 217 showing sterane distribution (27DS and 29DS are 13fl(H), 17a(H) 20S C27 and C29 diasteranes, 29R and 29S are C29 5c~(H),14~(H), 17~(H) 20R and 20S steranes); (b) m/z 191 showing triterpane distributions (H is 17~(H), 21fl(H)-hopane and O is oleanane).

Petroleum potential of Tertiary lignites All the major peaks a r e C30 pentacyclic terpanes that have similar mass spectra. They all show a molecular ion of m/z 412, an M+-15 (m/z 397) fragment and a base fragment of m/z 191. The peak marked O in Fig. 3(b) co-elutes with, and shows the same mass spectrum as an 18~(H)-oleanane standard. The peak marked H is thought to be 17~(H)-hopane for similar reasons. Hopanes of other carbon numbers appear to be present in considerably lower abundance. A contribution to this oil from Tertiary terrestrial organic matter is supported by the presence of oleanane and several other non-hopanoid triterpenoids. HYDROCARBON P O T E N T I A L OF LIGNITES

Rock-Eval analysis Before looking in detail at three samples, a suite of Greek lignites was surveyed for their hydrocarbon potential using Rock-Eval analysis. The results of these analyses are shown as a plot of Hydrogen Index vs Oxygen Index in Fig. 4. As expected, many of the lignites plot near to the Type III curve. A few samples plot closer to the Type II curve and have higher HI values suggesting that they have better hydrocarbon potential. However, even these samples have HI values considerably lower than most conventional source rocks. The KAS-3 sample from the Kassandra well, and a sample from the nearby Pos-1 well, show the best hydrocarbon potential. Both of these coals are thought to have formed in near-marine lagoonal depositional settings. MM-3, the coal deposited in an estuarine environment, has significantly less potential. These samples have Tmax values approximately consistent with their maturity and low Production Indices (Table 2).

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811

Intermontane lignites from the Florina Basin such as 6OG-5 behave very differently from the above samples. They are very immature (around 0.25% Ro) but give much higher Sl values and Production Indices than the other samples (e.g. 6OG-5, Table 2). These samples are also characterized by their very low Tmaxvalues of between 365 and 368°C. Their $2 peaks are very jagged. Very low Tmax values have been previously observed in coals with a high resinite content (vonder Dick et al., 1989) although not always associated with high Production Indices. Because the Sl values are so high, it seems reasonable to consider the total hydrocarbon potential of these lignites and not just that represented by the $2 peak. HI values calculated the normal way (S2/TOC) and a "HI" value calculated by combining the S~ and $2 amounts (S, + S2/TOC) have both been plotted on Fig. 4. For sample 6OG-5, the addition of the S~ peak raises the HI value by around 25%, making it comparable to that of the Kassandra sample (Table 2). Atomic H/C ratios, derived from elemental analysis, indicate that 6OG-5 has the best hydrocarbon potential of the three coals followed by KAS-3 and then MM-3 (Table 2). The reason why elemental analysis shows 6OG-5 with better hydrocarbon potential than KAS-3, whilst Rock-Eval suggests the reverse, might be due to their differing methods of product detection (Peters, 1986). 6OG-5 is very immature and as indicated by its OI value, very oxygen rich. The FID of the Rock-Eval only responds to carbon mass and C - - H bonds whilst a common pyroproduct such as water (which would be expected to be a more important product from 6OG-5) is not included in the HI but is measured by elemental analysis (Peters, 1986).

Organic petrology Sample 6OG-5 from the intermontane Florina Basin is a typical immature Tertiary humic coal. It contains an abundance of woody tissues (huminite), fluorescing yellow-brown to dark brown and moderate amounts of humodetrinite and phlobaphinite [Plate l(a)]. The auto-fluorescence of the huminite in this very immature sample is probably primary fluorescence inherited from the original plant structures (Stout and Bensley, 1987). Phlobaphinite is a characteristic constituent of Tertiary coals found mainly in bark tissues, and is thought to be derived from tannins on the death of the plant (Teichmiiller, 1982). This sample was also rich in resinite which occurs as oval to rounded bodies filling cell tissues. They exhibit a yellow to light orange fluorescence. A small amount of microspores and liptodetrinite was also observed but no inertinite was evident [Plate l(a)]. The estuarine coal MM-3 from Moschopotamos is also a humic coal, consisting of humotelinite, liptinite, pyrite and very low amounts of inertinite (Table 4). The liptinite components, which are easily recognised under u.v. excitation, consist of resinite, exsudatinite, cutinite, sporinite, fluorinite (derived from essential plant oils) and liptodetrinite [Plate l(b)].

812

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Plate 1. Petrographic characteristics of Greek lignite samples under u.v. light, water immersion (filters, excitation 450-490 nm, beam splitter 510 nm and barrier 420 nm). The long axis of each photograph is 230 #m. (a) Sample 6OG-5. Field of view consists mostly of fluorescing huminite (H) with some resinite (R) filling cavities. (b) Sample MM-3. Field of view shows bands of fluorescing resinite (R), exsudatinite (E) and fluorinite (F). Huminite (H) only weakly fluoresces. Note oil droplet (O) starting to form from exsudatinite. (c) Sample KAS-3. Shown here are bands of fluorescing humotelinite (HT) and humocollinite (HC). Also present is some more intensei~ tiuorescing liptinite (L). (d) Sample KAS-3. Bands of fluorescing humotelinite (HT) and humocollinite (HC) are again shown. Phlobaphinite (Ph) occurs within the humotelinite. Liptodetrinite (Ld) is also present. C is a carbonate clast. (e) Sample KAS-3 after extraction. Compared to (c) or (d) the fluorescence of the huminite is greatly reduced. Exsudatinite (E) and pyrite (P) now observable.

813

Plate 2. Residues of Greek lignites after hydrous pyrolysis at 300°C for 72 h under incident white light, oil immersion. Magnification same as Plate I. (a) KAS-3 original lignite residue after hydrous pyrolysis at 300°C for 72 h. Vitrinite-like (V) fragments containing devolatilization vacuoles (Va) are present indicating that sample was reactive during pyrolysis. (b) KAS-3 pre-extracted lignite residue after hydrous pyrolysis at 300°C for 72 h. Fused rounded vitrinite-like (V) fragments showing no devolatilization vacuoles. (c) KAS-3 original lignite residue after hydrous pyrolysis at 330°C for 72 h. Rounded vitrinite-like fragments (V) and some pyrite (P) are present. (d) 6OG-5 original coal residue after hydrous pyrolysis at 300°C for 72 h. Vitrinite-like fragments 0D containing some remnant morphology of a rounded resinite body (R). (e) MM-3 original lignite residue after hydrous pyrolysis at 300°C. Subangular vitrinite-like fragments (V) showing remnant of cutinite (Cu) morphology.

814

Petroleum potential of Tertiary lignites

815

Table 4. Maceralanalysisof originallignitesamplesbased on ~00 point-countsunder both white light and u.v. excitation. Numbers in parenthesesare calculatedon a mineral free basis Table 4 Maeerals

Samples KAS-3

60G-5

MM-3

Humotelinite

43.0

24.0

65.0

Humoeollinite

7.0

16.0

2.0

Humodetrinite

9.0

21.0

2.0

Total Huminlte

59.0 (84.3)

61.0 (65.6)

69.0

Sporinite

4.0

3.0

Resinite

7.0

15.0

Cutinlte

1.0

2.0

2.0

Liptodetrinite

5.0

9.0

9.0

Exsudatinite

1.0

Alginite

4.0

Total Liptinite

11.0 (15.7)

1.0

22.0 (23.7)

Semifusinite

1.0

Fusinite

10.0

Total Inertinite Mineral Matter

30.0

10.0 (10.7) 30.0

The huminite matrix also exhibits a weak brownish fluorescence. Resinite shows a bright yellow fluorescence and occurs in large ( > 200 p m length) masses parallel to the bedding plane rather than as the isolated ( ~ 2 0 p m length) bodies observed in the 6OG-5 sample. Brightly fluorescing exsudatinite occurs filling cracks and fissures perpendicular to the bedding in the huminite matrix. Exsudatinite is a secondary maceral which represents bitumen produced during late diagenesis and early catagenesis (Teichmiiller, 1982). In sample MM-3 exsudatinite can be observed exuding from both the resinite and fluorinite. In Plate l(b) an oil blob can be observed forming from the exsudatinite suggesting that this sample is at the initial stages of hydrocarbon generation. The KAS-3 sample from the Kassandra well has very different organic petrological characteristics from the other two samples. It shows distinct microstratification (microbanding) and virtually no inertinite [Plate l(c, d)]. This indicates that the lignite was deposited in a calm-water depositional environment where the organic matter was not exposed to the atmosphere (Goodarzi and Gentzis, 1987). Sample KAS-3 consists mainly of huminite (humotelinite and humocollinite, Table 4). The humotelinite occurs as thick bands without any liptinite inclusions, while the bulk of the liptinite macerals occur in the humocollinite matrix [Plate l(c)]. Also present is brownishfluorescing oval phlobaphinite [Plate 1(d)] within the humotelinite, carbonate clasts, small amounts of pyrite

1.0

7.0 and some alginite. The most striking characteristic of this sample is the intense fluorescence of the huminite group macerals. The vitrinite of a coal of this maturity should be showing a minimum of fluorescence intensity (TeichmiiUer and Durand, 1983; Lin and Davis, 1988). The perhydrous nature of the huminite in this coal is probably due to the incorporation of lipoidai (mostly resinite) substances, with those of the huminite precursors, during early diagenesis rather than the generation of bituminous substances from other macerals and their absorption by the huminites (Teichmtiller and Teichmiiller, 1982). Perhydrous coals are usually deposited in brackish-water environments and have a high bacterial input. EXTRACTION OF COALS Ten lignites were extracted by an azeotropic mixture of chloroform and methanol for 24 h. Only the results of those coals that were selected for the hydrous pyrolysis experiments will be discussed in any detail here. Sample MM-3 gives much the lowest yields of total extract and hydrocarbons of the three samples (Table 3). This sample also shows the least amount of change in its Rock-Eval results before and after extraction. There is only a small decrease in the combined SI + $2 amount. The lowering of HI appears to be because some material that was measured as S 2 is measured as Sl after extraction. The increase in

816

MARTING. FOWLERet al.

$1 could also be due to retention of some solvenf prior to the analysis of the extracted sample. This is considered unlikely as the extracted samples were left under partial vacuum in a desiccator for 1 week to remove solvent prior to analysis. The Tm~xshows an increase of 2°C. The vitrinite reflectance increased by 0.1-0.45% R 0 which is the value obtained for the other two coal seams at Moschopotamos which are not as rich in liptinites. Hence, the original vitrinite reflectance of this sample was suppressed by the extracted material. From the Rock-Eval analysis of the original coals, sample 6OG-5 might have been predicted to have given the greatest amount of extract because of its high $1 value. Although it gave substantially more extract than MM-3, it was much less than the KAS-3 sample (Table 3). The decrease in the S~ + $2 total is approximately equal to that of the S] value of the original 6OG-5 sample (Table 2). However, the extracted sample still shows a substantial S~ peak which may have come from the $2 or be residual S~ or a mixture of both. The HI decreases because the $2 is lower. The Tm~x of the extracted sample is the same as that of the original sample suggesting it still contains resinite. This retention of resinite after extraction was confirmed optically. The only change in the appearance of the sample was an increase in fluorescence intensity and a slight increase in the reflectance of the huminite (Table 2). The KAS-3 sample had a low Production Index both before and after extraction (Table 2). However, extraction causes a large reduction in the $2 and this sample provided the greatest yield of extract and hydrocarbons (Table 3). Associated with these changes was an increase of 8°C in the Tmax. Extraction causes a substantial reduction of the fluorescence intensity of the huminite which now has a greenish-brown low intensity fluorescence [Plate l(e)], thus making the huminite cell structure more visible. The reflectance of the huminite increases from 0.44 to 0.48% Ro suggesting that the material that was causing the huminite to fluoresce could also be responsible for suppressing the original reflectance. Yellow fluorescing tiny blobs of exsudatinite can be observed filling cavities in the phlobaphinite after extraction. Whilst extraction caused a large decrease in HI, in common with the other two samples, the atomic H/C ratio did not change. Gas chromatograms o f original coal extracts

The proportion of hydrocarbons in the extracts of all three samples is low and that of the saturate hydrocarbons even lower (Table 3). However, the C,5+ saturate fraction gas chromatograms (sfgc) (Fig. 5) illustrate some very interesting differences between the three samples. The sfgc of the KAS-3 sample is dominated by a mostly unresolved hump of terpanes around C30 [Fig. 5(a)]. There are smaller amounts of sesquiterpanes, diterpanes, isoprenoids (especially pristane),

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60

Fig. 5. Saturate fraction gas chromatograms of lignite extracts; (a) KAS-3, (b) 6OG-5 and (c) MM-3.

and n-alkanes. These latter compounds are dominated by C23-C31 members with a pronounced odd carbon number preference. No attempt has yet been made to identify individual components of the triterpane hump, but compounds other than hopanes predominate. The aromatic gas chromatogram is dominated by a hump of compounds which are probably C30-derived aromatic triterpenoid compounds. The abundance of triterpenoids suggests that angiosperms were the major contributors to this coal. The sfgc of the 6OG-5 sample is dominated by diterpanes [Fig. 5(b)]. There are minor amounts of sesquiterpanes and n-alkanes. The gas chromatogram of aromatic hydrocarbons is dominated by aromatic diterpenoid compounds and compounds with similar elution times to the compounds of triterpenoid origin that predominate in the KAS-3 aromatic gas chromatogram. Hence, most of the material extracted from this coal has been derived from diterpenoid resinite with some triterpenoid contribution. As there are still rounded bodies of resinite present in the coal after extraction, and the huminite fluorescence intensity has greatly decreased, it is probable that resinite dispersed throughout the huminite is what has been extracted. The huminite fluoresence intensity increases because at the very low maturity of 6OG-5, the primary fluoresence of the huminite is greater than that of the resinite (Teichmiiller and Durand, 1983) and is consequently suppressed. The larger pieces of resinite that were observed parallel to the bedding may also be

Petroleum potential of Tertiary lignites extractable but because of their mass this may take longer than the 24 h used in this work. The largest peaks in the saturate fraction gas chromatogram of MM-3 are odd numbered C23--C31 n-alkanes. These are possibly derived from cutinous materials such as the cutinite that was microscopically observed in this sample. There is also a hump of triterpanes and a second hump of compounds that elute around n C22, possibly C24 terpanes derived from sesterterpenoids (cf. Wang and Simoneit, 1990). Diterpanes are not present in large amounts indicating that the resinite is different from the Florina sample. This is also suggested by the aromatic gas chromatogram which shows a predominance of compounds that elute earlier than the triterpenoid compounds and later than diterpenoids. These could be derived from sesterterpenoids. HYDROUS PYROLYSIS EXPERIMENTS

K A S - 3 sample Figure 6 shows the effects of hydrous pyrolysis on sample KAS-3 as indicated by Rock-Eval analyses of the original coal, the extracted coal and the pyrolysis residues. Heating the original coal (i.e. coal not extracted) at 300°C causes a large decrease in the HI (Table 2) and produces the largest extract yield (Table 3). As discussed above, extraction removes much of the $2 peak in the original KAS-3 lignite. Therefore, the high extract yields and large drop in $2 after pyrolysis of the original sample at 300°C could be misleading since we extracted the whole sample after each of the small-scale pyrolysis experiments. The Rock-Eval results from both the original and preextracted 300°C residues are very similar (Table 2). It

600

II

I

OXYGEN INDEX

Fig. 6. HI vs OI plot of KAS-3 and its pyrolysis residues. 0 , Original lignite; O, extracted lignite; A, the 300°C residues; and II, 330°C residues.

817

is further supported by the amount of extract obtained from the 300°C experiment on the original coal (202.4 mg/g org C) being similar to the sum of the amounts of extracts obtained from the original coal (116.2 mg/g org C) and the pre-extracted sample after heating at 300°C (101.63 mg/g org C). However, the optical evidence discussed below suggests that the original coal does react in a different way to the pre-extracted sample during pyrolysis. The amount of extract obtained after the 330°C experiments was less for the original coal sample, but slightly greater for the pre-extracted sample, compared to their 300°C equivalents (Table 3). The hydrocarbon yields, and proportion of hydrocarbons in the extract, are higher than from the 300°C experiments for both samples. The decrease in the amount of extract obtained from the original coal at 330°C, compared to the equivalent sample at 300°C, is probably due to the production of a greater proportion of lighter products that were not measured in these experiments. In all extracts there is a predominance of aromatics over saturates. Neither of the 300°C residues show fluorescence when examined under u.v. light. They do show some important differences under reflected light. The 300°C residue of the original coal is composed of vitrinitelike fragments that are rounded to sub-angular and contain large (up to 100#m in dia) devolatization vacuoles [Plate 2(a)]. The presence of the vacuoles suggests the coal was extremely reactive during pyrolysis and produced large amounts of volatiles. The residue of the pre-extracted lignite consists of angular to well-rounded vitroplasts (Davis et al., 1976; Steller, 1981) which have fused together to form chains but which do not show any devolatization vacuoles [Plate 2(b)]. Hence, it appears that extraction greatly reduces the reactivity of KAS-3 as suggested by the Rock-Eval results. The reflectance of the vitrinite-like material in both 300°C residues is about the same (Table 2). Both of the 330°C residues have a similar appearance when examined microscopically [Plate 2(c)]. They show the development of subangular to rounded vitrinite-like fragments and devolatilization vacuoles. The latter are larger in the residue of the original lignite than the pre-extracted sample suggesting again that extraction reduces the reactivity of KAS-3. The reflectance of the vitrinite-like material differed between the two samples with the residue of the pre-extracted lignite showing a somewhat higher reflectance. The Cls+ sfgc of the pyrolysates (Fig. 7), unlike the original extract [Fig. 5(a)], are dominated by n-alkanes. In the chromatogram of the pyrolysate from the 300°C original coal, the most abundant terpanes elute around nC17 and around nC~4 with the triterpenoid hump being relatively small. The lower molecular weight terpanes may be derived by cracking of triterpenoid compounds observed in the original coal extract. Cyclic compounds are in very low abundance, relative to the n-alkanes and C15-C20 acyclic

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Fig. 7. Saturate fraction gas chromatograms of KAS-3 pyrolysates: (a) original lignite 300°C; (b) original lignite 330°C; (c) pre-extracted lignite 300°C; (d) pre-extracted lignite 330°C. Numbers refer to n-alkanes and pr is pristane.

isoprenoids in the 330°C sample. In the gas chromatograms of the pre-extracted coal pyrolysates, the terpane humps are considerably smaller compared to n-alkane peaks than they are in the original coal pyrolysates. This suggests that most of the triterpenoids present in the orginal lignite were removed by extraction. The pyrolysates of the original sample contain n-alkanes from C23 to C3~ that show an odd

carbon number preference and a high ratio of pristane to phytane. There is no significant odd n-alkane carbon number preference in either of the pyrolysates from the pre-extracted lignite but they do show high pr/ph ratios. The gas chromatograms of the aromatic hydrocarbons are dominated by lower molecular weight compounds. Only the pyrolysates of the unextracted

Petroleum potential of Tertiary lignites

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Fig. 8. m/z 191 mass fragmentograms of 330°C lignite pyrolysates: (a) KAS-3 original lignite; (b) KAS-3 pre-extracted lignite; (c) 6OG-5 original lignite; (d) MM-3 original lignite. 29, 30 and 31 are C29-C31 17~t(H),21fl(H)-hopanes, Tm is 17:t(H)-trisnorhopane, M is 17fl(H),21ct(H)-moretane and O is oleanane.

sample show the presence of the higher molecular weight compounds observed in the original extract and these are in low concentrations (especially in the 330°C pyrolysate). The proportion of hydrocarbons in the expelled oil fraction,• obtained from the large-scale experiment

using unextracted pieces of KAS-3, is much higher than that extracted from the small-scale experiments, up to 46.7% after heating at 330°C. The gas chromatograms of the expelled saturate hydrocarbons are similar to those of the pyrolysates obtained after the small-scale experiments with the original coal.

820

MARTIN G. FOWLERet al.

The m/z 191 mass fragmentograms of the KAS-3 330°C pyrolysates from the small-scale experiments are shown in Fig. 8. The pyrolysate of the unextracted sample is dominated by C30 terpanes. Identification of 18ct(H)-oleanane and 17~(H)-hopane was made as for the Epanomi oil. The pyrolysate of the preo extracted sample shows a very different distribution with the resin-derived compounds in lower abundance and a higher concentration of hopanes. 18ct(H)oleanane shows a similar abundance to 17~t-(H)hopane in both pyrolysates suggesting its precursors were not extracted with the resin-derived compounds. 6OG-5 As indicated by Fig. 9 and Table 2, the results obtained by Rock-Eval analysis of the residues, after heating sample 6OG-5 at 300 and 330°C, were similar for both the original and the pre-extracted sample. The yields of extract obtained from the experiments with the pre-extracted sample are extremely low (Table 3), even if the amount of extract removed from the original lignite is taken into account. As there is a substantial decrease in HI during these experiments, some compounds must have been generated. These are presumably light compounds that were not measured in these experiments. Hence, the extraction of the diterpenoid resinite seems to have enabled a greater amount of cracking to have occurred either during or after generation of hydrocarbons from this sample. The HI and H/C values of the 300°C residues are lower than those obtained from the 330°C experiments using the other two coals suggesting that 6OG-5 may be more reactive at lower temperatures. Tmax and "vitrinite" reflectance results of the 300°C residues,

./ m z H z ¢r

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especially of the pre-extracted sample, also indicate that the residues of 6OG-5 are more "mature" than those of the other coals. It should be emphasized that these experiments were run at the same time and under exactly the same conditions as the KAS-3 experiments. It is possible that diterpenoid resinite generates hydrocarbons at lower temperatures than other macerals (Snowdon and Powell, 1982). The very low Tmaxvalue obtained for this sample, and the other lignites from the Florina Basin, suggests that the activation energy needed to generate hydrocarbons from diterpenoid resinite may be significantly lower than that for other types of organic matter. Alternatively, the greater reactivity of 6OG-5 may be related to the lower initial maturity of this sample and the greater amounts of other products generated such as carbon dioxide. No fluorescing material was observed in the residues from the 300°C experiments. Under reflected light the organic matter shows a morphology typical of vitfinite (telinite). Remnant morphology of the original rounded resinite/phlobaphinite bodies can still be observed within the telinite cell cavities [Plate 2(d)]. The only morphology present in the 330°C residues is subangular homogeneous fragments of vitrinite-like material. This material shows only a slight increase in reflectance from the 300°C residues. The C~5+ saturate fraction gas chromatograms of the pyrolysates (Fig. 10) are dominated by n-alkanes. In all the pyrolysates, except that from the 300°C preextracted sample, the n-alkanes show an odd carbon number preference. The 300°C pyrolysate of the pre-extracted sample shows a similar carbon number distribution but with an even carbon number preference over the C22-C26 range. This is thought to be related to the extremely small amount of extract that was obtained from this experiment. It is evident that diterpanes are still important components of the pyrolysates of the unextracted sample. There are also other terpane peaks around nCl4 and nC~7, especially in the 300°C chromatogram. Terpanes are present in much lower amounts in the pyrolysates of the pre-extracted sample presumably because of the removal of some of their precursors by extraction. Acyclic isoprenoids, especially pristane have also been generated and all the pyrolysates show high pr/ph ratios. The m/z 191 mass fragmentograms of the pyrolysates of 6OG-5 are dominated by hopanes [e.g. Fig. 8(c)]. Other triterpanes were not detected. The steranes show an extreme predominance of C29 members.

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Fig. 9. HI vs OI plot of 6OG-5 and its pyrolysis residues. Annotation as for Fig. 6.

Hydrous pyrolysis at 300°C did not reduce the HI values of the MM-3 sample significantly although there was a large reduction in the OI value (Fig. 11, Table 2). However, considerably higher extract and hydrocarbon yields are obtained after the 300°C experiments than from the original sample extraction. Heating at 330°C increased the yield of extract obtained and caused the HI and H/C values to decrease

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(Fig. 12) which for each temperature are similar for both the original and pre-extracted samples. These chromatograms differ from the original extract in showing a broader range of n-alkanes and an increased abundance of isoprenoids, especially pristane. Both the unextracted and the pre-extracted 300°C gas chromatograms show a hump of compounds eluting

822

MARTIN G. FOWLERet al.

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OXYGEN INDEX

Fig. 11. HI vs OI plot of MM-3 and its pyrolysis residues. Annotation as for Fig. 6. around the nCl7 peak. These compounds are greatly diminished in the 330°C chromatograms where the Cl~C20 acyclic isoprenoids dominate the branched/ cyclic fraction. The m / z 191 mass fragmentograms [e.g. Fig. 8(d)] of pre-extracted and unextracted samples are dominated by hopanes with other compounds such as the tricyclic terpanes in very low abundance. The C29 steranes predominate over the C27 and C2s steranes. The dominant material in the residues is angular "vitrinite-like" material [Plate 2(e)]. No devolatization vacuoles such as those in the KAS-3 residue were observed suggesting that this sample did not react in the same way. DISCUSSION

KAS-3 is indicated by Rock-Eval analysis to have the best hydrocarbon potential, but compared to "normal" source rocks the quality of the organic matter as measured by the Hydrogen Index is not particularly good. However, all three samples do have a better potential to source hydrocarbons than is generally assumed for coals. This is supported by the amount of fluorescing material that can be observed under u.v. light. The proportion of liptinites compared to vitrinites and inertinites estimated from pointcounting under both white light and u.v. light, on a mineral-free basis, is 30% for MM-3, 24% for 6OG-5 and 16% for KAS-3. This apparently suggests that MM-3 has better hydrocarbon potential than KAS-3 in contrast to the geochemical data. However, as described below a more detailed visual examination of KAS-3, and the geochemical results, suggest that much of the hydrogen-rich material in KAS-3 is within

the fluorescing huminite. Additionally, this sample contains a significant (4%) amount of alginite. Hence, quantitative maceral analysis of coals containing perhydrous vitrinite does not truly indicate their true hydrocarbon potential. This is in agreement with Khavari-Khorasani (1987) who found that the oilproneness of Walloon Coals from the Surat Basin was governed not only by the different liptinite macerals but also by the perhydrous nature of the vitrinite and the lack of inertinite macerals. Bertrand (1989) came to similar conclusions in a study of North Sea Jurassic coals. The unusually high fluorescence displayed by huminite in the KAS-3 sample, for its level of maturity, can be explained using the fluorogeochemical model of Lin and Davis (1988). According to these authors, as coalification proceeds up to the subbituminous stage the primary fluorescence of huminite is lost because an increase in aromaticity causes a greater delocalization of the u-electrons. Development of the mobile phase, during the bituminous stage of coalification, results in secondary fluorescence of vitrinite, although the vitfinite network itself is too condensed to fluoresce. The resinous material in the KAS-3 sample, being easily extracted, is obviously not strongly bound within the huminite structure. It therefore may be acting in a similar fashion to the mobile phase in bituminous coals by containing fluorophores with less delocalised electrons than the huminite. Once the huminite is extracted and the resinous material removed, its fluorescence intensity decreases to the low intensity expected for its level of maturity. All three coals give much higher extract yields than expected from their Rock-Eval S~ values. The lack of correlation between extract yields and S~ values has been commented upon by several workers (e.g. Clementz, 1979; Snowdon, 1984; Espitali6 et al., 1985; Peters, 1986) and occurs because the solvent extract includes heavy compounds such as resins and asphaltenes that are cracked in the $2 temperature range and, therefore, not included in the S l peak. Sample KAS-3 differs from the other two samples by showing a much larger drop in $2 with extraction. Moreover, the amount of hydrocarbons alone (9.41 mg/g rock) extracted from KAS-3 is higher than would be expected from its S~ value (6.38 mg/g rock). Gas chromatography of the saturate and aromatic hydrocarbon fractions of this sample are dominated by triterpenoid compounds. Tarafa et al. (1983) reported that compounds up to nC32 were volatile enough to be measured as part of the S~ peak. Hence, it is probable that not all of the triterpenoid compounds in KAS-3 are being measured in the S~ peak. This, especially, might be the case for aromatics which are usually less volatile than n-alkanes of the same carbon number. Resinous materials either occurring as discernible resinite bodies (as in MM-3 and 6OG-5) or dispersed within the huminite matrix (as in KAS-3) are the most important contributors to the hydrocarbon potential

Petroleum potential of Tertiary lignites 75s4-~

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of the coals. While the alginite observed in KAS-3 would also be expected to contribute to the hydrocarbon potential of this sample, it cannot be a major factor. This is because, as discussed previously, a major part of the hydrocarbon potential of KAS-3 can be removed by extraction, which has no apparent effect on the alginite.

Gas chromatographic evidence indicates that differing types of plants contributed the resins to the coals. The 6OG-5 sample is dominated by diterpenoidderived compounds suggesting that gymnosperms were the major contributors to this intermontane lignite. The hydrocarbons of the lagoonal lignite KAS-3 are dominated by triterpenoid compounds,

824

MARTIN G. FOWLERet al.

probably derived from angiosperms. A more diverse compounds with minor amounts of higher molecular set of compounds are present in the extract of the weight n-alkanes. Although the m / z 191 mass fragrnentograms of the deltaic sample MM-3, which may be a function of this depositional environment, with the predominant Epanomi oil [Fig. 3(b)] and the Kassandra 330°C resinite-derived compounds being different to those of unextracted pyrolysate [Fig. 8(a)] are not identical the other two samples. The high proportion of resinite- they do show important similarities, principally the derived cyclic compounds in the extracts of all three presence of the resin-derived triterpanes in concensamples indicates that their biochemical precursors are trations greater than the hopanes. Both samples also not strongly bound within the coal matrix (Lu and have a predominance of C29 over C27 and C28 steranes. Kaplan, 1990). The different ways in which the three However, this characteristic was shown by pyrolysates samples behave upon extraction and hydrous pyro- of all three lignite samples. The Epanomi oil is light lysis is because they contain different resinites with with a predominance of aromatics over saturates. For reasons discussed above this is also consistent with it different properties. Horsfield et al. (1988) have questioned the use of being sourced by a lignite similar to KAS-3. Hence, the HI as a parameter in assessing the hydrocarbon it is possible that the Epanomi oil has been sourced potential of a coal containing a high abundance of from a more mature equivalent of the Kassandra coal resinite or fluorescing vitrinite. They found that the or coal that contains angiosperm-derived organic HI of Talang Akar coals correlated positively with matter. Other explanations are possible. For example, the amount of resinite and fluorescing vitrinite yet the the oil which is fairly light, may have been sourced in C~0÷ fraction of the Ardjuna Basin oils (thought to be a deeper more conventional source rock and picked sourced from the coals), based on the results of up the easily extractable resin C30 triterpanes during pyrolysis-gas chromatography, was predominately migration. Because the biomarker distributions of derived from "matrix liptinite". Horsfield et al. (1988) the oil are mature, the lignite which the oil would suggested this because pyrolysis-gas chromatograms have to come into contact with in this scenario would of resinite and fluorescing vitrinite from the Talang have to be of much higher rank than the KAS-3 Akar coals were dominated by cyclic compounds sample. Further geochemical study and especially whilst the Ardjuna Basin oils, believed to be sourced a better knowledge of the subsurface geology of the from these coals, are dominated by n-alkanes thought Thermaikos Gulf area is needed to resolve which of to be derived principally from "matrix liptinite" the two above hypotheses for the Epanomi oil is the (bituminite + liptodetrinite). These authors thought more plausible. that resinite-derived compounds made only a minor contribution to the oil but were important in the CONCLUSIONS expulsion process. Liptodetrinite does occur in significant quantities in KAS-3 (about 7% of the organic The results of this study suggest that some Tertiary matter) and could be the major source of the higher lignites from northern Greece have better hydrocarbon molecular weight n-alkanes, but unlike the Ardjuna potential than is normally attributed to coals. The Basin oils, the C~0+ fraction constitutes only a minor Greek lignites that show the better hydrocarbon portion, and long chain paraffins even less, of the potential are those that contain a greater amount Epanomi oil. The low proportion of resin-derived of resinite-derived material, either as discrete bodies compounds in the hydrocarbon fractions of the or dispersed within perhydrous huminite. For sample KAS-3 pyrolysates is in agreement with the results of KAS-3, the dispersed resinite is the cause of the Horsfield et al. (1988) that, when mature, resinite intense huminite fluorescence. Although not indicated would not be a major contributor to the C~0+ by Rock-Eval analysis [Sl/(Sl + $2) is very low], it is hydrocarbons in an oil. Mass balance considerations easily extractable indicating that it is not strongly indicate that most of the pyrolysis products were not bound to the huminite. Other lipinite macerals also recovered because they are of low molecular weight. make a lesser contribution to the hydrocarbon Diterpenoid resinite (such as that in 6OG-5) breaks potential of these coals. From a very limited data down to form mostly low molecular weight aromatic set, it appears that the type of resinite correlates products on hydrous pyrolysis (Lewan and Williams, with depositional environment. Intermontane lignites 1987; Hwang and Teerman, 1988). The triterpenoid (e.g. 6OG-5) are dominated by diterpenoid resinite resin in KAS-3 may also give similar products al- and lignites deposited on a delta plain contain other though at higher temperatures than those proposed by types such as triterpenoid resinite in the case of Snowdon and Powell (1982) for diterpenoid resinite. KAS-3. A source of the Epanomi oil from a coal similar to The much lower concentrations of triterpenoids compared to lower molecular weight terpanes in the 300°C the KAS-3 that contains predominately angiospermpyrolysate of the original coal (compared to the derived organic matter is supported by the geooriginal extract) suggests that the former compounds chemical evidence. This is best indicated by the are easily cracked. Hence, from the results of this predominance of non-hopanoid triterpenoids in both work, one might predict that an oil sourced from the the oil and the KAS-3 pyrolysates. However, the KAS-3 coal would be dominated by light aromatic evidence is not unequivocal and a more detailed

Petroleum potential of Tertiary lignites geological investigation of the Thermaikos G u l f area will be needed before this can be confirmed. Acknowledgements--The Public Petroleum Corporation of Greece (D.E.P.) is thanked for providing the sample of Epanomi oil and the lignite samples from the KAS-3, POS-I and NR-I wells. We also acknowledge Amy Paget and our colleagues at the ISPG (Sneh Achal, Paul Brooks, Marg Northcott and Lloyd Snowdon) for technical assistance and advice. We thank Rui Lin and Erdem Iziz for their thoughtful reviews. REFERENCES

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