International Journal of Coal Geology 114 (2013) 1–18

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International Journal of Coal Geology

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Reflectance measurements of zooclasts and solid bitumen in LowerPaleozoic shales, southern Scandinavia: Correlation to vitrinite reflectance

Henrik I. Petersen a,⁎, Niels H. Schovsbo a, Arne T. Nielsen b

a Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmarkb Geological Museum, Natural History Museum of Denmark, University of Copenhagen, Øster Voldgade 5–7, DK-1350 Copenhagen K, Denmark

⁎ Corresponding author. Tel.: +45 38142455; fax: +E-mail address: hip@geus.dk (H.I. Petersen).

0166-5162/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.coal.2013.03.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 19 December 2012Received in revised form 20 March 2013Accepted 25 March 2013Available online 8 April 2013

Keywords:Reflectance measurementsZooclastsGraptoliteVitrinite-like particlesChitinozoansBitumen

Reflectance measurements have been carried out on zooclasts (graptolites, chitinozoans, and vase-shapedmicrofossils) and other organic particles (vitrinite-like particles, porous/granular vitrinite-like particles, andsolid bitumen) inMiddle Cambrian to Upper Silurian shales from central and southern Sweden and the Danishisland of Bornholm (Baltic Sea). The most abundant organic components in all the shales are fragments ofgraptolites and vitrinite-like particles. The reflectance distribution of these two types of components is largelyidentical, and it is suggested that the vitrinite-like particles are fragments of graptolites without any recogniz-able morphology. Reflectance measurements of graptolites and vitrinite-like particles yield well-definedreflectance populations. In samples with average Rgraptolite and average Rvitrinite-like of >0.75% Ro, the reflec-tance distributions are bimodal because of increasing bireflectance, and the average reflectance value of thewell-defined lower reflecting population is arbitrarily used as maturity indicator. Our results suggest thatwith increasing thermal maturity the reflectance of graptolites increases faster than the predicted vitrinitereflectance. The relationship between graptolite reflectance and equivalent vitrinite reflectance can beexpressed by the equation: VReqv = 0.73 R(graptolite + vitrinite-like)low + 0.16. The ‘gas generation window’,which normally is considered to begin at a vitrinite reflectance of 1.3% Ro in post-Lower Paleozoic rockscontaining vitrinite, starts, accordingly, at 1.56% Ro graptolite reflectance. Porous/granular vitrinite-like parti-cles occur in minor amounts and they may represent graptolite fragments with a non-smooth surface. Theygenerally yield slightly higher reflectance than non-granular vitrinite-like particles and graptolite fragments.The Middle Cambrian to Furongian (upper Cambrian) shales may contain sparse fragments of vase-shapedmicrofossils (VSM) that seem to follow the maturation trend of chitinozoans. In the present sample set, thereflectance of chitinozoans and VSM is comparable to that of graptolites at the same level of maturity. Reflec-tance measurements of solid bitumen are a poor maturity indicator, probably because bitumen can have var-ious origins and morphologies and it may not be indigenous to the host rock.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Reflectance measurements of organic matter dispersed in sedi-mentary rocks, e.g. vitrinite particles, are a widely used and robustthermal maturity indicator (e.g. Hunt, 1996). Vitrinite is derivedfrom partly decomposed and thermally matured ligno-cellulosic tis-sues of higher land plants, which appeared after the Late Silurianwhere the first vascular plants evolved. In the absence of vitrinite inLower Paleozoic rocks, reflectance measurements have been carriedout on zooclasts (graptolites, chitinozoans, and scolecodonts) andother organic particles (bitumen and vitrinite-like particles) (Suárez-Ruiz et al., 2012). In several studies the reflectance of various types ofzooclasts and bitumen were measured in the same samples in orderto establish reflectance correlations and to compare thermal evolutiontrends of the different components. Furthermore, it has been attempted

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to establish a calibrationwith othermaturity indicators, such as vitrinitereflectance equivalent (VReqv), Rock-Eval Tmax values, conodont ColorAlteration Index (CAI), Acritarch Alteration Index (AAI), Thermal Alter-ation Index (TAI) and atomic H/C ratios (e.g. Bertrand, 1990; Bertrandand Héroux, 1987; Buchardt and Lewan, 1990; Cole, 1994; Suchý etal., 2002; Tricker et al., 1992; Williams et al., 1998). Among zooclasts,graptolites are probably the most widely used group for reflectancemeasurements (Bustin et al., 1989; Christiansen et al., 1989; Clausenand Teichmüller, 1982; Goodarzi and Norford, 1985, 1987, 1989;Goodarzi et al, 1992; Link et al., 1990; Malinconico, 1993; Rantitsch,1995), but a number of authors have also applied chitinozoan reflec-tance measurements (Goodarzi, 1985; Marshall, 1995; Obermajer etal., 1996; Tricker et al., 1992). The advantage of graptolites versuschitinozoans may be the much greater abundance of graptolites (Cole,1994). Tricker et al. (1992) stated that the chance of obtaining sufficientchitinozoans for reflectance measurements in a whole rock sample isextremely low. Laufeld (1974) observed an average density of onlyabout one chitinozoan vesicle per five gram of sediment from Gotland,

2 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

Sweden. Solid bitumen reflectance has been used in Lower Paleozoicrocks and in younger rocks containing scarce or non vitrinite (Gentzisand Goodarzi, 1993; Jacob, 1989; Landis and Castaño, 1995; Riediger,1993; Schoenherr et al., 2007). Buchardt and Lewan (1990) andXianming et al. (2000) measured the reflectance of so-called vitrinite-likemacerals in the Cambrian–Ordovician Alum Shale from Scandinaviaand in Lower Paleozoic shales from the Tarim Basin, China, respectively.

The need for determining the thermal maturity of Lower Paleozoicrocks has increased with the intensified exploration for shale gas, anincreasingly important unconventional gas resource that is related tothe globally widespread occurrence of Cambrian–Silurian organic-richshales (e.g. Jarvie, 2012; Pool et al., 2012; Schovsbo et al., 2011). Ofparticular importance is the correlation of zooclast reflectances tovitrinite reflectance equivalents (VReqv) as vitrinite reflectance valuesare calibrated to oil and gas generation and is the typical maturity indi-cator applied to model hydrocarbon generation. The present studyexamines the reflectance of graptolites, chitinozoans, vitrinite-like par-ticles, porous/granular vitrinite-like particles, solid bitumen, and possi-ble vase-shaped microfossils (VSM) in 16 Middle Cambrian to UpperSilurian shale samples from localities in Sweden and Denmark. Theaims are (1) to investigate the applicability of reflectance measure-ments of various organic particle types to assess the thermal maturityof Lower Paleozoic shales in southern Scandinavia, and (2) to establisha correlation between the reflectance of these particle types and VReqv.The results are also compared with Rock-Eval Tmax values and a limitednumber of CAI values.

2. Samples and methods

The shale samples include 13 outcrop samples and 3 core sam-ples from shallow wells. They were collected in southern and central

Lower Palaeozoicdeposits

KH

Denmark

R

Fig. 1. Map showing locations of the studied samples from central and southern Sweden anPaleozoic strata.From Nielsen and Schovsbo (2011).

Sweden and on the Danish island of Bornholm in the Baltic Sea (Fig. 1;Table 1). The oldest samples are of Middle Cambrian age (Acerocarepisiformis Zone) and the youngest of Late Silurian age (Ludlow Stage).Nine of the samples represent the Middle Cambrian to Lower Ordovi-cian Alum Shale, and the remaining samples different Middle Ordovi-cian to Upper Silurian marine shales.

Organic geochemical screening analyses were carried out in orderto characterize the samples. The samples were analyzed for total or-ganic carbon (TOC, wt.%), total carbon (TC, wt.%) and total sulfur (TS,wt.%) contents by combustion in a LECO CS-200 induction furnace,with HCl treatment of the samples to remove carbonate-bonded car-bon before TOC determination. The samples were further pyrolyzedin a Source Rock Analyzer (SRA) system.

Pellets of the shales suited for reflected light microscopy wereprepared. The samples were lightly crushed and sieved between63 μm and 1 mm. This fraction was embedded in epoxy, and theepoxy pellets were ground and polished to obtain a smooth surface.The embedding procedure takes into account density separation inthe epoxy of the crushed shale material. Reflectance measurements(random, oil immersion) were conducted using a Leica DM4000Mreflected light microscope equipped with a 25× objective and theDiskus Fossil vitrinite reflectance system (Hilgers Technisches Buero,Germany). The reflectance measurements were taken at 546 nm(monochromatic light). Before measurement the microscope was cal-ibrated against a YAG 0.903% Ro standard and an optical black (zero)standard. The Diskus Fossil system is software controlled and usesno photomultiplier which makes it very robust and linear to highreflectance values. It was therefore considered sufficient in this con-text only to use the YAG 0.903% Ro standard. Various studies haveused random or maximum and minimum reflectance measurements,the latter because of graptolite anisotropy, in particular at graptolite

Caledonian Deformation Front

RL-1

akeledällekis-1

GrönvikSweden

Bornholm

Öland

Scania

50 km

BalticSea

Ottenby

KlintaKivik

överakulan

S. Sandby BjärsjölagårdGislövshammar

Læså Øleå localities

d the Danish island of Bornholm. Gray shading shows present day occurrence of Lower

Table 1List of samples arranged according to age.

Lab. no. Locality Type Lithology Formation Stage

21074 Röverakulan Outcrop Shale Colonus Shale Upper Silurian: Ludlow21075 Klinta Outcrop Shale Öved-Ramsåsa Upper Silurian: Ludlow21076 Bjärsjölagård Outcrop Siltstone Öved-Ramsåsa Upper Silurian: Ludlow21080 Øleå at Slusegård Outcrop Shale Cyrtograptus Shale Lower Silurian: Wenlock21079 Øleå at Billegrav Outcrop Shale Rastrites Shale Lower Silurian: Llandovery21082 Læså at Vasagård Outcrop Shale Dicellograptus Shale Upper Ordovician21084 Kivik Outcrop Shale Almelund Shale Middle Ordovician21078 Ottenby Outcrop Shale Alum Shale Lower Ordovician21083 Gislövhammar Outcrop Shale Alum Shale Lower Ordovician21073 S. Sandby Outcrop Shale Alum Shale Furongian (u. Cambrian): Acerocare Zone21418 Hällekis-1 (26.75 m) Core Shale Alum Shale Furongian (u. Cambrian)21422 Hällekis-1 (31.92 m) Core Shale Alum Shale Furongian (u. Cambrian)21427 Kakeled Outcrop Shale Alum Shale Furongian (u. Cambrian): Acerocare Zone22422 RL-1 well (74.42 m) Core Shale Alum Shale Furongian (u. Cambrian)21077 Grönvik Outcrop Shale Alum Shale Middle Cambrian: Acerocare pisiformis Zone21081 Øleå at Borggård Outcrop Shale Alum Shale Middle Cambrian: A. pisiformis Zone

3H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

reflectances above approximately 1% Ro (e.g. Goodarzi, 1984; Goodarziand Norford, 1987; Link et al., 1990; Rantitsch, 1995). In this study ran-dom reflectance readings were taken, which is comparable to generalpractice for vitrinite reflectance measurements on dispersed organicmatter DOM (ASTM D7708-11) and the ICCP DOMVR accreditationprogram. Readings were taken on different types of particles (grapto-lites, chitinozoans, vase-shaped microfossils(?), vitrinite-like particles,porous/granular vitrinite-like particles, and solid bitumen) providingthe reflectance distribution of each individual particle type (Fig. 2).A total of 45 to 370 measurements were taken per sample, but in12 of the 16 samples the number of readings was >200. Average re-flectance values of the different components were calculated, and for

30

25

20

15

Num

ber

of m

easu

rem

ents

10

5

00 0.5 1.0

Reflectance1.5 2.0 2.5%

20

15

10

5

00 0.5 1.0 1.5 2.0 2.5%

Total reflectance distribution

10

5

00 0.5 1.0 1.5 2.0

10

5

00

Bitumen

Chitinozoans

Porouvitrinipartic

Fig. 2. Example of total reflectance histogram and individual histograms for different partic

the higher maturity samples yielding well-defined bimodal reflec-tance distributions, the average reflectance value of low- and high-reflecting populations was calculated for graptolites and vitrinite-likeparticles.

The conodont Color Alteration Index (CAI; Epstein et al., 1977;Rejebian et al., 1987) was determined for seven limestone samples as-sociated with the shale samples 21077, 21073, 21083, 21078, 21084,21082 and 21076. The color of conodonts changes from pale yellow(CAI = 1; immature to early mature) to black (CAI = 5; overmature)with increasing thermal maturity. CAI has been used as a maturity in-dicator in Lower Paleozoic rocks with no vitrinite, in Scandinavia forinstance by Bergström (1980).

20

15

10

5

00 0.5 1.0 1.5 2.0 2.5%

2.5%

0.5 1.0 1.5 2.0 2.5%

20

15

10

5

00 0.5 1.0 1.5 2.0 2.5%

s/granularte-likeles

Vitrinite-likeparticles

Graptolites

le types measured in the Lower Ordovician Alum Shale from Ottenby, Öland, Sweden.

4 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

3. Measured organic components

The different types of zooclasts and solid bitumen measured in thesamples comprise:

3.1. Graptolites

Fragments of graptolites were measured in all samples apart fromthe Upper Silurian Colonus Shale from Rövarekulan, Scania (Fig. 3;Table 2). Examples of graptolites in the Lower Ordovician AlumShale fromÖland (sample 21078, Ottenby) and southernmost Sweden(sample 21083, Gislövhammar) are shown in Fig. 3A and C. Graptolitefragments are in this study recognized by their lath-shape and occa-sional remnants of thecae. The presence of graptolites in the Lower Or-dovician (Tremadocian) Alum Shale from southernmost Sweden andÖland (Westergård, 1909, 1922), and in the Silurian (Llandoverianand Wenlockian) shales from the island of Bornholm have previouslybeen documented (Bjerreskov, 1975). Although dendroid graptolitesare known to occur from the Middle Cambrian, also in Baltoscandia(Bengtson andUrbanek, 1986; Öpik, 1933), the presence of graptolites

0.86%

A

1.84%

C

Fig. 3. Photomicrographs (white reflected light, oil immersion) of part of graptolite rhabd(B) Furongian Alum Shale, Sandby, Scania, Sweden (sample 21073). (C) Lower Ordovician

in the Furongian AlumShale in southern Sweden and on Bornholmhasnot previously been described. However, their occurrence in the AlumShale is strongly indicated by identification of rhabdosomes such asshown by the uppermost Furongian sample from southern Sweden(sample 21073, S. Sandby) (Fig. 3B). The graptolites in the MiddleCambrian to Furongian Alum Shale likely present benthic forms,whereas the graptolites in the Lower Ordovician shales were planktic(Cooper, 1999). The benthic forms may have been living at shallowerwater depth and during transportation to the depositional site indeeper water, the rhabdosomes were broken into smaller pieces.Under the microscope, the visible structures are tissues from the peri-derm (Link et al., 1990). The fragmentary graptolites show in reflectedlight granular or non-granular morphology, with the non-granularfragments showing higher reflectance and stronger bireflectancethan the granular fragments (Goodarzi, 1984). The non-granular frag-ments occur mainly in shales, whereas the granular fragments aremore common in rocks with a carbonate matrix. In agreement withthe shale matrix of the investigated samples, mainly non-granulargraptolite fragments were observed. Geochemical studies indicatethat the graptolite periderm has a more highly condensed aromatic

2.05%

B

osomes. (A) Lower Ordovician Alum Shale, Ottenby, Öland, Sweden (sample 21078).Alum Shale, Gislövhammar, Scania, Sweden (sample 21083).

Table 2Average reflectance values of organic components.

Lab. no. Vitrinite-like particles Graptolites Chitinozoans ?Vase-shapedmicrofossils

Porous ‘vitrinite’ Bitumen

Total Low-refl. High-refl. Total Low-refl. High-refl.

% Ro (Std.) % Ro (Std.) % Ro (Std.) % Ro (Std.) % Ro (Std.) % Ro (Std.)

21074 0.92 (0.185) 0.70 (0.069) 1.02 (0.096) Not detected Not detected Not detected Not detected Not detected Not detected Not detectedn = 45 n = 15 n = 29

21075 1.11 (0.189) 0.95 (0.081) 1.29 (0.076) 1.36 (0.234) ?1.08 (0.045) ?1.41 (0.045) ?1.64 (0.335) Not detected Not detected 0.34 (0.062)n = 34 n = 18 n = 16 n = 6 n = 2 n = 3 n = 3 n = 21

21076 0.75 (0.076) – – 0.74 (0.086) – – Not detected Not detected Not detected 0.26 (0.038)n = 46 n = 14 n = 14

21080 2.50 (0.357) 2.16 (0.126) 2.85 (0.117) 2.33 (0.337) 2.12 (0.118) 2.96 (0.183) ?2.54 (0.315) Not detected ?2.73 (0.057) ?0.40n = 158 n = 57 n = 43 n = 70 n = 41 n = 11 n = 4 n = 2 n = 1

21079 2.21 (0.363) 2.05 (0.161) 2.62 (0.097) 2.12 (0.182) – – 1.78 (0.191) Not detected ?2.32 (0.166) 0.66 (0.281)n = 148 n = 94 n = 21 n = 19 n = 3 n = 4 n = 3

21082 2.42 (0.409) 2.04 (0.152) 2.96 (0.211) 2.30 (0.334) 2.12 (0.110) 2.66 (0.193) 2.01 (0.056) Not detected Not detected 0.31 (0.121)n = 161 n = 66 n = 44 n = 31 n = 20 n = 9 n = 3 n = 19

21084 1.85 (0.242) 1.74 (0.130) 2.18 (0.158) 1.85 (0.233) 1.71 (0.113) 2.05 (0.110) 1.85 (0.117) Not detected Not detected 0.35 (0.123)n = 139 n = 105 n = 32 n = 35 n = 22 n = 12 n = 16 n = 13

21078 0.74 (0.094) – – 0.78 (0.061) – – 0.54 (0.111) Not detected 0.84 (0.087) 0.23 (0.055)n = 142 n = 71 n = 5 n = 10 n = 67

21083 1.74 (0.135) 1.61 (0.065) 1.80 (0.066) 1.72 (0.156) 1.51 (0.115) 1.86 (0.077) ?1.58 (0.243) Not detected ?0.70 (0.119) 0.41 (0.065)n = 121 n = 82 n = 126 n = 104 n = 24 n = 42 n = 2 n = 6 n = 20

21073 2.05 (0.184) 1.96 (0.112) 2.37 (0.080) 2.04 (0.190) 1.95 (0.119) 2.31 (0.099) Not detected ?2.18 (0.090) ?2.01 (0.059) 0.64 (0.152)n = 183 n = 134 n = 26 n = 85 n = 65 n = 20 n = 2 n = 3 n = 25

21418 0.54 (0.028) – – 0.56 (0.023) – – Not detected Not detected 0.45 (0.053) 0.28 (0.035)n = 96 n = 138 n = 27 n = 52

21422 0.49 (0.053) 0.47 (0.022) 0.60⁎ (0.029) 0.51 (0.068) 0.47 (0.018) 0.60⁎ (0.030) Not detected ?0.58 (0.008) ?0.48 (0.074) ?0.27 (0.041)n = 170 n = 144 n = 26 n = 175 n = 117 n = 58 n = 3 n = 4 n = 2

21427 0.46 (0.024) – – 0.46 (0.023) – – Not detected ?0.39 (0.071) Not detected Not detectedn = 145 n = 169 n = 2

22422 0.42 (0.029) – – 0.43 (0.033) – – Not detected Not detected 0.40 (0.083) 0.23 (0.043)n = 159 n = 115 n = 11 n = 15

21077 0.33 (0.043) – – 0.31 (0.047) – – Not detected 0.41 (0.080) 0.54 (0.065) 0.16 (0.043)n = 167 n = 110 n = 14 n = 69 n = 5

21081 2.11 (0.240) 2.07 (0.157) – 2.12 (0.148) – – Not detected Not detected Not detected 0.87 (0.088)n = 265 n = 263 n = 37 n = 8

Low-refl.: arbitrarily selected low-reflecting population in samples with bimodal reflectance distribution.High-refl.: arbitrarily selected high-reflecting population in samples with bimodal reflectance distribution.⁎ Possibly increased reflectance because of oxidation.

5H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

structure than vitrinite (Bustin et al., 1989). This inherited higher aro-maticity may cause a faster maturation rate.

3.2. Chitinozoans

Chitinozoans are of uncertain biological affinity and range strati-graphically from the Early Ordovician to the Late Devonian; for gen-eral descriptions see Goodarzi (1985), Tricker et al. (1992) andObermajer et al. (1996). They occur as single microfossils or as chainsof microfossils (Paris et al., 1999). Several species of chitinozoans havebeen recorded in Ordovician (including Tremadocian) strata in south-ern Sweden (Scania) and on Bornholm (e.g. Grahn and Nõlvak, 2007).A chitinozoan vesicle is flask- or bottle-shaped, 50–250 μm long, andconsists of an oral tube and a chamber with appendices (Goodarzi,1985; Tricker et al., 1992). Normally, only broken parts can be seenmicroscopically. Chitinozoans show isotropy in reflected light and oilimmersion (Obermajer et al., 1996). A few chitinozoans have beenrecorded in seven of the analyzed samples (Fig. 4; Table 2). Some ofthe particles identified as chitinozoans are flask- or bottle-shaped,whereas identification of smaller fragments without this charac-teristic shape remains uncertain. Pyrolysis and micro-FTIR data fromUpper Silurian chitinozoans from Turkey suggest that the vesiclesconsist of aliphatic compounds and a substantial proportion of aro-matic compounds that originated from a phenolic macromolecule(Dutta et al., 2007). A study on Lower Silurian chitinozoans fromSaudi Arabia likewise indicated a composition dominated by aromaticcompounds and with a low amount of aliphatic compounds (Jacob etal., 2007). None of these studies found chitin-related compounds inthe chitinozoans as suggested by some early studies (Goodarzi, 1985).

3.3. Vase-shaped microfossils (VSM)

Chitinozoan-like so-called vase-shaped microfossils (VSM) sensuPorter et al. (2003), have been found in the upper PrecambrianChuar Group of the Grand Canyon, Arizona, and in upper Precambrianrocks from Saudi Arabia (Binda and Bokhari, 1980; Bloeser et al., 1977;Porter et al., 2003). The microfossils in the Chuar Group were initiallydescribed as chitinozoans because of similarity in morphology, butPorter et al. (2003) suggested a relationship between VSM's and ex-tant testate amoebae. A few readings have been taken on particlesthat have been assigned as VSMs in four of the Middle Cambrian andFurongian samples (Table 2). The small particles resemble fragmentsof the chitinozoan neck or shoulder (Goodarzi, 1985; his Figs. 3a and5a) or prosome (Tricker et al., 1992; their Fig. 1), however, identifica-tion is uncertain.

3.4. Virtinite-like particles

Buchardt and Lewan (1990) described “kerogen macerals resem-bling vitrinite” in the Middle Cambrian to Lower Ordovician AlumShale in southern Scandinavia, and more recently vitrinite-like parti-cles were recorded in Cambrian to Ordovician rocks from the TarimBasin in China (Xianming et al., 2000). Schleicher et al. (1998) de-scribed vitrinite-like particles in upper Cambrian shales fromnorthernPoland, which they preferred to call bituminite because of the absenceof land-derived vitrinite in the late Cambrian. However, the photomi-crographs of bituminite shown by Schleicher et al. (1998; their Plate2a and b) resemble to a large extent graptolites. Vitrinite-like particlesdo not possess the diagnostic features of fragments from graptolites

2.03%

1.91%

A B

2.08%

C

Fig. 4. Photomicrographs (white reflected light, oil immersion) of chitinozoans. (A) Middle Ordovician Almelund Shale, Kivik, Scania, Sweden (sample 21084). (B) Lower SilurianRastrites Shale, Øleå at Billegrav, Bornholm (sample 21079). (C) Upper Ordovician Dicellograptus Shale, Læså at Vasagård, Bornholm (sample 21082).

6 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

(see above), and exhibit a more varied morphology (Fig. 5). Vitrinite-like particles have been measured in all the analyzed samples(Table 2). The vitrinite-like particles were originally reported to re-spond to maturation in a similar way as suppressed vitrinite, andgelification of polysaccharides (chitin) was suggested as their origin(Buchardt and Lewan, 1990).

3.5. Porous/granular vitrinite-like particles

In nine of the analyzed samples, vitrinite-like particles with aporous/granular surface were recognized (Fig. 6; Table 2), and theywere recorded as a separate group.

3.6. Bitumen

Solid bitumen is commonly observed in rocks where it occursamorphous within pores (Jacob, 1989). Solid bitumen is a so-called

secondary maceral and represents heavy petroleum, which havebeen generated within the host rock or has migrated into the rockfrom another source (Hunt, 1996; Landis and Castaño, 1995; Tayloret al., 1998). Migrated bitumen may thus occur in organic-lean rocks.It may, however, also have been formed by biodegradation of oil orfrom deasphalting. Dark to almost black (white reflected light) amor-phous bitumen was measured in most of the samples (Fig. 7; Table 2).In blue light illumination, the bitumen fluoresces orange.

4. Results and discussion

4.1. Organic geochemistry and comments on the source rock potential ofthe Alum Shale

TheMiddle Cambrian to Lower Ordovician Alum Shale samples areorganic-rich with TOC contents ranging from 5.95 to 18.59 wt.%(Table 3). The Middle Ordovician to Upper Silurian shales are

0.46%

A

0.88%

B

2.29%

C

0.44%

0.53%

A

0.69%

B

Fig. 6. Photomicrographs (white reflected light, oil immersion) of porous/granularvitrinite-like particles. (A) Furongian Alum Shale, Hällekis-1 well (26.75 m), Västergötland,Sweden (sample 21418). (B) LowerOrdovician Alum Shale, Gislövhammar, Scania, Sweden(sample 21083).

7H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

significantly leaner in organic matter (b1.90 wt.%), with the lowestcontent measured for the Colonus Shale from Rövarekulan (sample21074). The difference between TC and TOC is for most of the samplesinsignificant, but for the Lower Silurian Cyrtograptus Shale (sample21080) and the three Upper Silurian samples the difference clearlyindicates the presence of carbonate, in particular in sample 21074(Rövarekulan) (Table 3). In two of the samples, the TOC content isslightly higher than the TC content, but because of analytical uncer-tainty (±0.8%) this may happen for samples with almost identicalTOC and TC contents. TS varies from 0.01 to 10.07 wt.% with thehighest values recorded in the Alum Shale. The Tmax spans from415 °C to >600 °C, showing that the samples represent a widerange from thermally immature to overmature with regard to petro-leum generation. In the high mature samples no pyrolyzable kerogen

Fig. 5. Photomicrographs (white reflected light, oil immersion) of vitrinite-like parti-cles, likely representing fragments of graptolites (see text). (A) Furongian Alum Shale,Kakeled, Västergötland, Sweden (sample 21427). (B) Lower Ordovician Alum Shale,Ottenby, Scania, Sweden (sample 21078). (C) Furongian Alum Shale, Øleå at Borggård,Bornholm (sample 21081).

0.33%

0.21%

B

0.27%

A C

Fig. 7. Photomicrographs (white reflected light, oil immersion) of solid bitumen. (A) Middle Cambrian Alum Shale, Grönvik, Öland, Sweden (sample 21077). (B) Lower OrdovicianAlum Shale, Ottenby, Öland, Sweden (sample 21078). (C) Lower Silurian Rastrites Shale, Øleå at Billegrav, Bornholm (sample 21079).

8 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

is left as reflected by zero S1 and S2 yields and very low to zero HIvalues (Table 3). The Tmax values derived from samples with insignif-icant S2 yields is therefore in the best case uncertain, but more likelyunreliable.

Table 3Screening data and CAI.

Lab. no. Locality TOC TC TS

(wt.%)

21074 Röverakulan 0.07 6.99 0.0121075 Klinta 0.10 1.68 0.1121076 Bjärsjölagård 0.14 1.59 0.1921080 Øleå at Slusegård 0.68 1.17 0.4621079 Øleå at Billegrav 1.07 1.14 0.1521082 Læså at Vasagård 1.90 1.99 0.6121084 Kivik 1.08 1.10 1.1521078 Ottenby 7.31 7.45 1.1421083 Gislövhammar 7.44 7.89 3.6921073 S. Sandby 7.03 7.54 3.1621418 Hällekis-1 (26.75 m) 15.94 15.90 4.8721422 Hällekis-1 (31.92 m) 18.41 18.86 5.3321427 Kakeled 18.59 18.87 7.4522422 RL-1 well (74.42 m) 14.51 14.29 10.0721077 Grönvik 8.98 9.30 5.5121081 Øleå at Borggård 5.95 5.95 6.07

TOC: total organic carbon.TC: total carbon.TS: total sulfur.Tmax: temperature at maximum S2 generation.S1: free hydrocarbons.S2: hydrocarbons generated by decomposition of the kerogen during pyrolysis.HI: Hydrogen Index [HI = (S2 / TOC)100].PI: Production Index [PI = S1 / (S1 + S2)].CAI: Conodont Color Alteration Index.

The immature to mature samples yield HI values from 301 to395 mgHC/g TOC. These samples contain orange to yellow fluorescinglamalginite, telalginite and fluorescing amorphous organic matter(sapropelic kerogen) (Fig. 8). Conventionally, this would indicate an

Tmax S1 S2 HI PI CAI

(°C) (mg HC/g rock)

464 0 0 0 – –

438 0 0 0 – –

440 0 0 0 – 1.5594 0 0 0 – –

598 0 0 0 – –

603 0 0 0 – 3.5494 0 0 0 – 3440 0.77 28.87 395 0.03 1.5471 0.21 1.48 20 0.12 2601 0 0.23 3 0 3417 0.32 48.00 301 0.01 –

415 0.85 56.92 309 0.01 –

419 2.29 60.81 327 0.04 –

423 0.82 53.19 367 0.02 –

420 1.67 34.04 379 0.05 1485 0 0 0 – –

Graptolite Non-fluorescinggraptolite

A1 A2

Graptolite? Fluorescing graptolite?

B1 B2

Non- Graptolite fragment

Vitrinite-likeparticle fluorescing

Fluorescing

C2C1

Fig. 8. Photomicrographs from the Furongian Alum Shale in the Hällekis-1 well showing graptolites and vitrinite-like particles in a strongly yellowish to orange fluorescingsapropelic groundmass composed of alginite (Type II kerogen). A1, B1 and C1 are in reflected white light (oil immersion), A2, B2 and C2 are in fluorescing-inducing blue light(oil immersion). (A1 and A2) Example of non-fluorescing graptolite. (B1 and B2) Dark orange fluorescing elongate particle with graptolite affinity. (C1 and C2) Non-fluorescinggraptolite fragment and orange fluorescing vitrinite-like particle. (For interpretation of the references to color in this figure legend, the reader is referred to the web version ofthis article.)

9H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

oil-prone source rock, but no economic oil occurrences sourced fromthe Alum Shale have been discovered. Wells on the Swedish islandof Gotland have producedminor amounts of oil from Ordovician lime-stones, but the Alum Shale has not been proven to be the source(Buchardt, 1999; Dahl et al., 1989). In northeast Poland and offshorein the Baltic Sea, small oil discoveries have been made in Middle Cam-brian sandstones, and the source of the oil is assumed to be the AlumShale (Kotarba, 2010). However, analyses of the kerogen in the AlumShale suggest an unusual composition (Bharati et al., 1992, 1995).The studies show that despite the Type II affinity of the Alum Shale

kerogen the organic matter appears to be a light oil or gas condensatesource producing unusual aromatic mixtures with a very low concen-tration of n-alkanes with more than 10 carbon atoms. The kerogenwas shown to have a higher degree of aromaticity than normalmarinekerogen and an unusual amount of carbon atoms bonded to oxygen.The aliphatic part of the kerogen consists of short straight andbranched alkyl chains typically with 1–6 carbon atoms. Such a compo-sition may have a limited capacity to generate liquid petroleum. Nev-ertheless, several of the investigated samples in this study containsolid bitumen (heavy petroleum) (see below). The structure of the

10 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

Alum Shale kerogen definitely calls for more investigations to clarifythe composition and petroleum generation potential of this 500 mil-lion year old organic matter.

4.2. Previous correlations to vitrinite reflectance equivalent (VReqv)

The increase in vitrinite reflectance with increasing burial temper-ature is caused by condensation and successive ordering of aromaticstructures in the degraded ligno-cellulosic material forming thevitrinitic organic matter (Carr and Williamson, 1990). Organic matterderived from true ligno-cellulosic precursor material is absent inLower Paleozoic rocks but the reflectance of zooclasts, vitrinite-likeparticles and bitumen also increases with increasing burial depthand temperature, although they may react at a different rate toheating than vitrinite derived from vascular plants (Clausen andTeichmüller, 1982; Goodarzi, 1984; Goodarzi and Norford, 1985,

Table 4Relation between vitrinite reflectance equivalents (VReqv) and the reflectance of zooclasts,

Publication Graptolites (Rgrap) Chitinozoans (VRchi)

Clausen and Teichmüller(1982)

Rgrap different from VReqv

Goodarzi (1984) Relation to VReqv not clearGoodarzi and Norford(1985)

Rather similar

Bertrand and Héroux(1987)

VReqv 0.4–0.8% lower thanRgrap and Rchi in the 1–2%VReqv range

Bustin et al. (1989) Rgrap similar to VReqv

Jacob (1989)Goodarzi and Norford(1989)

Rgrap(max) > VRmax(eqv)

0.2–0.5% VRmax(eqv) =0.6–1.2% Rgrap(max)

Oil window: 1.2–2.2% Rgrap(max)

(app. eqv. to 1.13–2.07%Rgrap(random), cf. Diessel andMcHugh, 1986)

Bertrand (1990) Rgrap slightly less than VReqv Rchi similar to VReqv

Buchardt and Lewan(1990)

Link et al. (1990) Lower levels of maturity:VRmax(eqv) > Rgrap(max)

Rgrap(max) increases faster thanVRmax(eqv) at high levels ofmaturity: 5.0–6.5% Rgrap(max)

corresponds to 4% VRmax(eqv)

Tricker et al. (1992) Rchi > VReqv

VReqv = (Rchi − 0.08Riediger (1993)

Cole (1994) Rgrap higher than VReqv. Immature:0.6% Rgrap = app. 0.5% VReqv

Mature: 1.1% Rgrap correspondingto app. 0.9% VReqv

Similar to graptolite/correlation

Landis and Castaño(1995)

Rantitsch (1995) Rgrap of graptolite fragmentsresembles VReqv at highmaturity (>3% VReqv(max))

Obermajer et al. (1996) Rchi 20–25% higher thVReqv. 0.65% Rchi =0.9% Rchi = 0.7% VRe

Xianming et al. (2000)

Schoenherr et al. (2007)

1989; Bertrand and Héroux, 1987; Bustin et al., 1989; Jacob, 1989;Bertrand, 1990; Buchardt and Lewan, 1990; Link et al., 1990; Trickeret al., 1992; Riediger, 1993; Cole, 1994; Landis and Castanõ, 1995;Rantitsch, 1995; Obermajer et al., 1996; Xianming et al., 2000;Schoenherr et al., 2007). A number of published relationships betweenvitrinite reflectance equivalents and the reflectance of graptolites,chitinozoans, vitrinite-like particles and solid bitumen are shown inTable 4. The published results show poor consistency and the grapto-lites in particular correlate inconsistently. Possible explanations forthis discrepancy may include difficulties in identifying graptolites,graptolite surface morphology (smooth or granular), and the stateof preservation. The four listed studies on chitinozoans show the re-flectance to be similar or higher than vitrinite reflectance (Table 4).Vitrinite-like particles have been reported to react as suppressedvitrinite (Buchardt and Lewan, 1990), or a more complex relationshiphas been proposed depending on the level of thermal maturity

vitrinite-like particles, and solid bitumen.

Vitrinite-like particles (Rvitr) Solid bitumen (RB)

VReqv = 0.618 RB + 0.4

Vitrinite-like macerals respondon heating as suppressedvitrinite, i.e. Rvitr lowerthan VReqv

) / 1.152VReqv b c. 0.72%: relationshipsimilar to Jacob (1989)VReqv > c. 0.72%:VReqv = 0.277 RB + 0.57

vitrinite

VReqv = (RB + 0.41) / 1.09

an0.5% VReqv.qv

Rvitr b VReqv up to 1.50% Rvitr

Rvitr > VReqv above 1.50% Rvitr

Rvitr b 0.75%: VReqv = 1.26Rvitr + 0.21. Rvitr = 0.75–1.50%:VReqv = 0.28 Rvitr + 1.03Rvitr > 1.50%:VReqv = 0.81 Rvitr + 0.18

VReqv = (RB + 0.2443) / 1.0495

11H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

(Xianming et al., 2000; Table 4). Solid bitumen has been reported tohave lower reflectance than vitrinite at lower maturities, whereasthe bitumen reflectance appears to exceed that of vitrinite at highermaturities (Table 4).

4.3. Reflectance measurements: vitrinite-like particles and graptolites

Most reflectance measurements were taken on vitrinite-like parti-cles (Rvitr) and graptolites (Rgrap) with the total amount of readings in

0.32%

A

0.57%

C

1.76%

E F

Fig. 9. Photomicrographs (white reflected light, oil immersion) of graptolites sh

individual samples ranging from 34 to 265 and 6–169, respectively(Table 2). The calculated average reflectance values show that thethermal maturity of the investigated samples range from immatureto highly mature (Table 2). Hence, both vitrinite-like particles andgraptolites reveal an increase in reflectance with progressive thermalmaturation (Figs. 9 and 10; Table 2), a characteristic of graptolites al-ready noted e.g. by Clausen and Teichmüller (1982) and Goodarzi andNorford (1985). In most of the immature to early mature samples(based on Tmax; Table 3) reflectance measurements on vitrinite-like

0.46%

0.45%

B

0.78%

D

2.12%

owing increasing reflectance (A → F) related to increasing thermal stress.

12 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

particles form a unimodal distribution with an average reflectance ofb0.75% Ro. A unimodal reflectance distribution with comparable re-flectance values is also observed for graptolites in the same sampleas illustrated by the almost perfect overlap between the reflectancedistributions of vitrinite-like particles and graptolites in sample 21418

0.32%

A B

0.56%

C D

1.88%

E F

Fig. 10. Photomicrographs (white reflected light, oil immersion) of vitrinite-like particles, likincreasing thermal stress.

(Hällekis-1 well, 26.75 m) (Fig. 11). At higher maturity (average totalreflectance values of >0.75% Ro) the distributions for both vitrinite-like particles and graptolites show a more or less well-defined bimodalreflectance distribution, likely because of increased bireflectance causedby higher maximum burial temperature (Goodarzi and Norford, 1987)

0.47%

0.88%

2.29%

ely representing fragments of graptolites, showing increasing reflectance (A → F) with

60

50

40

30

20

10

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0%

Average VR(Graptolite): 0.56 (n=138)Average VR(Vitrinite-like): 0.54 (n=96)

30

25

20

15

10

5

0

60

70

80

50

40

30

20

10

0

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0%

0 0.1 0.2 0.3 0.4

Reflectance

Num

ber

of m

easu

rem

ents

0.5 0.6 0.7%

GraptolitesandVitrinite-likeparticles

Fig. 11. Example of overlapping reflectance distribution of graptolites and vitrinite-like particles, suggesting that the latter are fragments of graptolites. Upper right: graptolites;lower right: vitrinite-like particles. Sample 21418, Hällekis-1 well (26.75 m), Västergötland, Sweden.

13H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

(Table 2). Average valueswere calculated for arbitrarily selected normaldistributed low- and high-reflecting populations. Fig. 12 shows cross-plots of Rvitr(low) versus Rgrap(low) and Rvitr(high) versus Rgrap(high),where (i) ‘low’ designates the average reflectance value of the unimodalreflectance distribution of the immature to early mature samples andthe average reflectance value of the normal distributed low-reflectingpopulation of the higher maturity samples with a bimodal reflectancedistribution and (ii) ‘high’ designates the average reflectance value ofthe unimodal reflectance distribution of the immature to early maturesamples and the average reflectance value of the normal distributedhigh-reflecting population of the higher maturity samples with a bi-modal reflectance distribution. The relationship between graptolitesand vitrinite-like particles can be expressed by the equations

Rgrap lowð Þ ¼ 0:97 Rvitr lowð Þ þ 0:03 ð1Þ

Rgrap highð Þ ¼ 0:91 Rvitr highð Þ þ 0:09: ð2Þ

The correlations are robust with calculated regression lines havingcorrelation coefficients of r2 = 0.99 (low) and r2 = 0.97 (high), re-spectively. The 1:1 correlation indicates a similar thermal evolutionof the vitrinite-like particles and graptolites during maturation. Thismay indicate that the vitrinite-like particles consist of non-descriptfragments of graptolites. Alternatively, the vitrinite-like particles orig-inate from an unknown source with an organic structure that maturesidentically to graptolites during progressive burial and increasingheating. Two Middle Cambrian samples are included in the trend(Fig. 12A). The particles in these samples and in the Furongian sam-ples may have been derived from benthic graptolites as the firstplanktic graptolites appeared at the Cambrian/Ordovician boundary(Cooper, 1999). Buchardt and Lewan (1990) reported that vitrinite-

like particles respond tomaturation in a similar manner as suppressedvitrinite, but the identical thermal evolution of graptolites andvitrinite-like particles shown in the present study does not corrobo-rate this observation. Suppression of vitrinite reflectance because ofbitumen impregnation must always be considered as a possible com-plicating factor in highly sapropelic rocks (Carr, 2000; Lo, 1993;Petersen et al., 2006; Price and Barker, 1985). The Alum Shale is char-acterized by a strongly fluorescing algal-derived groundmass, but asshown in Fig. 8A2, relatively high-reflecting and non-fluorescing grap-tolites show no signs of suppression in that sample. However, it isalso possible to find graptolites and vitrinite-like particles showingfluorescence in blue light irradiation, which could suggest bitumenimpregnation (Fig. 8B2, C2). Reflectance suppression may thus intheory be a problem in the sapropelic Alum Shale, but as discussedin Section 4.1, the kerogen in the Alum Shale seems to have an unusualcomposition (Bharati et al., 1992, 1995) and maybe the problem issmaller than expected because of a limited capacity to generate liquidpetroleum, including bitumen.

Because vitrinite-like particles are interpreted as fragments ofgraptolites their well-constrained reflectance distributions are merged(Table 5). For the bimodal distributions, the low-reflecting populationis arbitrarily selected as it overall is better constrained by a higher num-ber of readings (Fig. 12; Table 5). The relationship between the high-and low-reflecting populations is expressed by the equation (Fig. 13)

R grapþvitrð Þhigh ¼ 1:28 R grapþvitrð Þlow þ 0:03; r2 ¼ 0:96: ð3Þ

Porous/granular vitrinite-like particles have been observed in somesamples (Table 2). In themajority of the samples, a few porous/granularvitrinite-like particles have been identified with uncertainty. Goodarzi

0 0.4 0.8 1.2 1.6 2 2.4

Vitrinite-like particles (%Ro; total or low-reflecting popu.)

0

0.4

0.8

1.2

1.6

2

2.4A

B

Gra

ptol

ites

(%R

o; to

tal o

r lo

w-r

efle

ctin

g po

pu.)

Rgrap(low)=0.97Rvitr(low)+0.03

0 1 2 3 40.5 1.5 2.5 3.5

Vitrinite-like particles (%Ro; total or high-reflecting popu.)

0

1

2

3

0.5

1.5

2.5

Gra

ptol

ites

(%R

o; to

tal o

r hi

gh-r

efle

ctin

g po

pu.)

r2 = 0.97

r2 = 0.99

Rgrap(high)=0.91Rvitr(high)+0.09

M. Cambrian

M. Cambrian

Fig. 12. Cross-plots of the reflectance of graptolites and vitrinite-like particles. (A) Totalpopulation for lower maturity samples with a unimodal reflectance distribution com-bined with the low-reflecting population of higher maturity samples with a bimodalreflectance distribution. (B) Total population for lower maturity samples with aunimodal reflectance distribution combined with the high-reflecting population ofhigher maturity samples with a bimodal reflectance distribution. Very good correlationcoefficients and a nearly perfect 1:1 correlation suggest that the vitrinite-like particlesare fragments of graptolites.

Table 5Reflectance of graptolites + vitrinite-like particles.

Lab. no. Locality Graptolites + vitrinite-like particles

Total Low-refl. High-refl.

% Rgrap + vitr (Std.)

21074 Röverakulana 0.92 (0.185)n = 45

0.70 (0.069)n = 15

1.02 (0.096)n = 29

21075 Klinta 1.12 (0.189)n = 38

0.95 (0.081)n = 19

1.34 (0.119)n = 25

21076 Bjärsjölagård 0.75 (0.079)n = 60

– –

21080 Øleå at Slusegård 2.45 (0.359)n = 228

2.14 (0.119)n = 95

2.85 (0.113)n = 49

21079 Øleå at Billegrav 2.21 (0.352)n = 169

2.05 (0.161)n = 111

2.63 (0.107)n = 23

21082 Læså at Vasagård 2.41 (0.405)n = 193

2.06 (0.145)n = 86

3.00 (0.198)n = 43

21084 Kivik 1.85 (0.241)n = 174

1.74 (0.130)n = 131

2.19 (0.173)n = 42

21078 Ottenby 0.76 (0.076)n = 204

– –

21083 Gislövhammar 1.73 (0.146)n = 225

1.61 (0.065)n = 82

1.80 (0.066)n = 126

21073 S. Sandby 2.05 (0.186)n = 268

1.97 (0.120)n = 210

2.38 (0.080)n = 36

21418 Hällekis-1 (26.75 m) 0.55 (0.025)n = 231

– –

21422 Hällekis-1 (31.92 m) 0.50 (0.062)n = 341

0.47 (0.019)n = 258

0.60b (0.029)n = 83

21427 Kakeled 0.46 (0.023)n = 314

– –

22422 RL-1 well (74.42 m) 0.43 (0.031)n = 274

– –

21077 Grönvik 0.32 (0.045)n = 277

– –

21081 Øleå at Borggård 2.11 (0.231)n = 302

2.07 (0.157)n = 263

2.53 (0.146)n = 34

a Only vitrinite-like particles; graptolites not detected.b Possibly increased reflectance because of oxidation.

r2 = 0.96

0 0.5 1 1.5 2 2.5

Graptolites+Vitrinite-like particles (%Ro; low-reflecting popu.)

0

1

2

3

0.5

1.5

2.5

Gra

ptol

ites+

Vitr

inite

-like

par

ticle

s (%

Ro;

hig

h-re

flect

ing

popu

.) R(grap+vitr) high=1.28R(grap+vitr) low+0.03

Fig. 13. Cross-plot of the ‘total population of lower maturity samples + low-reflectingpopulation of higher maturity samples’ versus the ‘total population of lower maturitysamples + high-reflecting population of higher maturity samples’ for graptolites +vitrinite-like particles.

14 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

(1984) and Goodarzi and Norford (1985, 1987) recognized two types ofgraptolite fragments, non-granular and granular. The non-granular (togranular) type was mainly found in shaly rocks and the granular typein carbonate rocks. The investigated samples in the present study areshales, which may explain the relative rarity of porous/granularvitrinite-like particles. The porous/granular vitrinite-like particles maythus be fragments of graptolites, and for the majority of the samples a

r2 = 0.97

0 0.4 0.8 1.2 1.6 2 2.4

Graptolites+Vitrinite-like particles (%Ro)

0

0.5

1

1.5

2

2.5

3P

orou

s vi

trin

ite-li

ke p

artic

les

(%R

o)

Rporous vitr=1.17R(grap+vitr)low-0.05

Fig. 14. Relationship between the reflectance of porous/granular vitrinite-like particlesand the reflectance of graptolites + vitrinite-like particles using the low-reflectingpopulation of higher maturity samples. The reflectance of the porous/granular particlesis generally slightly higher. An ‘outlier’ with a seemingly abnormal low reflectance ofthe porous/granular vitrinite-like particles has been omitted from the correlation.

A

B

C

r2 = 0.90

r2 = 0.94

r2= 0.54

0 0.4 0.8 1.2 1.6 2 2.4

Graptolites+Vitrinite-like particles (%Ro)

0

0.5

1

1.5

2

2.5

3

Chi

tinoz

oans

and

VS

M (

%R

o)

ChitinozoansOutlierVase-shaped microfossils (VSM)

Rchi+VSM=1.05R(grap+vitr)low+0.04

0 0.4 0.8 1.2 1.6 2 2.4

Graptolites+Vitrinite-like particles (%Ro)

0

0.2

0.4

0.6

0.8

1

Bitu

men

(%

Ro)

1

2

3

4

AI (

Con

odon

t Alte

ratio

n In

dex)

CAI=1.31R(grap+vitr) low+0.49

15H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

plot against the reflectance of graptolites + vitrinite-like particlesyields a good correlation coefficient of r2 = 0.97 and follows the equa-tion (Fig. 14)

Rporous vitr ¼ 1:17 R grapþvitrð Þlow–0:05: ð4Þ

However, a single sample (21083, Gislövhammar) does not fit thiscorrelation and has been omitted (outlier), which introduces uncertain-ty about the robustness of Eq. 4. The reflectance of the porous/granularvitrinite-like particles is higher than for graptolites + vitrinite-like par-ticles which contrasts with the results of Goodarzi (1984) who foundthat the non-granular graptolite fragments had a higher reflectancethan the granular ones.

4.4. Correlation of graptolite reflectance to chitinozoans, solid bitumenand CAI

A few reflectance measurements were taken on chitinozoans inthe Lower Ordovician to Upper Silurian shales and on vase-shapedmicrofossils (VSM) in the Cambrian shales (Table 2). A plot of thesemeasurements versus the reflectance of graptolites + vitrinite-likeparticles show a good correlation (r2 = 0.95) (Fig. 15A). It is notablethat the reflectance values of chitinozoans and VSM form a well-defined trend, which suggest that these zooclasts are genetically relat-ed as proposed by Bloeser et al. (1977) and Binda and Bokhari (1980),or that they at least have a similar geochemical structure that reacts inthe same way during heating. The established correlation shows only

0 0.4 0.8 1.2 1.6 2 2.4

Graptolites+Vitrinite-like particles (%Ro)

0

C

Fig. 15. Correlation of the reflectance of graptolites + vitrinite-like particles usingthe low-reflecting population for higher maturity samples versus (A) chitinozoans,(B) solid bitumen, (C) conodont Color Alteration Index (CAI). In the present sampleset, the reflectance of chitinozoans is almost similar to that of graptolites at the samematurity level. The reflectance of bitumen shows no statistically significant correlationto the reflectance of graptolites. CAI yields a good correlation to graptolite reflectance,however, the relationship is based on a small number of samples.

A

B

r2 = 0.93

n = 621r2 = 0.88

0 1 2 3 4

Humic coals (%Ro)

400

500

600

Tmax = 63.03VR + 389.11

0 0.4 0.8 1.2 1.6 2 2.4

Graptolites+Vitrinite-like particles (%Ro)

400

420

440

460

480

500T

max

(°C

)T

max

(°C

)

Upper SilurianMiddle OrdovicianLower OrdovicianUpper CambrianMiddle Cambrian

Tmax=45.48R(grap+vitr)low+400.16

Fig. 16. (A) Correlation between Tmax and the reflectance of vitrinite-rich humic coals;plot updated from Petersen (2006). (B) Correlation between Tmax and the reflectanceof graptolites + vitrinite-like particles using the low-reflecting population for highermaturity samples.

16 H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

minor difference between the reflectance of chitinozoans/VSM andgraptolites + vitrinite-like particles

RchiþVSM ¼ 1:05 R grapþvitrð Þlow þ 0:04: ð5Þ

The small difference in reflectance agrees with the results of Cole(1994).

The bitumen observed in the samples is generally low-reflecting(Fig. 7; Table 2), and in blue light illumination it often displays orangefluorescence. The correlation of solid bitumen reflectance to the re-flectance of graptolite + vitrinite-like particles is poor and has nostatistical significance (Fig. 15B). A plausible explanation could bethat the bitumen is not genetically related to the shale in which it oc-curs. In south-central Sweden marginally mature to mature bitumenoccurs in immature Alum Shale and mature bitumen is also found inovermature Alum Shale (Dahl et al., 1989). This was taken as evidenceformigration of bitumen from siteswhere the Alum Shale had been lo-cally heated by intrusions. It has also been suggested that differenttypes of solid bitumen have different optical properties because of tex-tural and morphological differences (see discussion in Suárez-Ruizet al., 2012). This limits the value of using reflectance measurementsof bitumen as a maturity indicator.

The CAI values have been plotted against the reflectance ofgraptolites + vitrinite-like particles (Fig. 15C; Table 3). The correla-tion coefficient is r2 = 0.90 and the relationship can be expressedby the equation

CAI ¼ 1:31 R grapþvitrð Þlow þ 0:49: ð6Þ

However, the correlation should be treated with caution becauseof the low number of CAI values. The onset of the gas window hasbeen determined to approximately 1.6% Ro graptolite reflectance,thus corresponding to a CAI of approximately 2.5.

4.5. Correlation of graptolite reflectance to vitrinite reflectanceequivalent and comments on the gas window

The gradual increase in reflectance of graptolites with increasingthermal maturity suggests a general maturation path correspondingto that of vitrinite, and there is also an overall similarity in the chem-istry of graptolites and vitrinite at all maturation levels (Bustin et al.,1989). However, it has always been an issue to calibrate the reflec-tance of zooclasts with vitrinite reflectance. It is attempted to useTmax as an independent maturity indicator in order to ‘link’ Lower Pa-leozoic rocks lacking vitrinite with vitrinite-containing post-Silurianrocks. Petersen (2006) published a correlation between Tmax and VRof vitrinite-rich (humic) coals. This correlation has in the presentstudy been updated with data from more than hundred new humiccoal samples and now includes more than 600 samples (Fig. 16A).The relationship between Tmax and VR has a good correlation (r2 =0.88) and can be described by the equation

Tmax ¼ 63:03 VR þ 389:11: ð7Þ

The relationship between Tmax and R(grap + vitr)low has likewise agood correlation (r2 = 0.93) and can be described by the equation(Fig. 16B)

Tmax ¼ 45:48 R grapþvitrð Þlow þ 400:16: ð8Þ

Samples with extremely low S2 pyrolysis yields may result in highand unreliable Tmax values, and these assumed unrealistic values havearbitrarily been omitted from the correlation. The correlation confi-dence is thus less for high Tmax values. Furthermore, the correlationis hampered by the small number of samples and the relatively limit-ed temperature range covered by reliable Tmax values. Hence, the

correlation may be improved when more data becomes availableand it should thus be used with these limitations in mind.

Combining Eqs. (7) and (8) provides the correlation between VReqv

and R(grap + vitr)low

VReqv ¼ 0:73 R grapþvitrð Þlow þ 0:16: ð9Þ

This equation translates reflectance values measured on grapto-lites and vitrinite-like particles to equivalent vitrinite reflectancevalues, as shown in Fig. 17. R(grap + vitr)low is higher than VReqv andthe difference between the two sets of measurements increases withmaturity. Reflectance measurements of graptolites and vitrinite-likeparticles will thus overestimate thermal maturity. The graptolite peri-derm consists of an aromatic structure with aliphatic groups with a

0.5 1 1.5 2 2.5 30.75 1.25 1.75 2.25 2.75

Graptolites+Vitrinite-like particles (%Ro)

0.5

1

1.5

2

2.5

0.75

1.25

1.75

2.25

Vitr

inite

ref

lect

ance

eqv

ival

ent (

%R

o)

VReqv=0.73R(grap+vitr) low+0.16

1:1 line

Fig. 17. Correlation between the reflectance of graptolites + vitrinite-like particles(using the low-reflecting population of higher maturity samples) and vitrinite re-flectance equivalents. The relationship can be expressed by the equation VReqv =0.73 R(grap + vitr)low + 0.16. The reflectance of graptolites is higher than the reflectanceof vitrinite at the same maturity level.

17H.I. Petersen et al. / International Journal of Coal Geology 114 (2013) 1–18

more condensed aromatic structure than vitrinite (Bustin et al., 1989),which likely causes the faster increase in reflectance. According to thepresent correlation the onset of the gas window (vitrinite reflectance:1.3% Ro) corresponds to a graptolite reflectance of 1.56% Ro. A similarresult was published for Cambrian shales from northern Poland bySchleicher et al. (1998).

5. Conclusions

The correlation between various maturity indicators are shown inTable 6. The following conclusions can be drawn:

(1) The investigated Middle Cambrian to Upper Silurian shale sam-ples contain various zooclasts, the most abundant being grap-tolites (graptolites + vitrinite-like particles). This suggests inturn that graptolites lived somewhere in Baltoscandia duringdeposition of the Alum Shale; no macro-fossils of graptoliteshave been found in pre-Ordovician Alum Shale.

Table 6Correlation of maturity indicators.

Rgrap + vitr

(% Ro)VReqv

(% Ro)Tmax

(°C)Shale gas

0.50 0.54 4230.60 0.60 4280.75 0.72 4351.00 0.90 4461.25 1.09 4581.50 1.27 4691.56 1.30 472 Shale gas, Rgrap + vitr > 1.56%1.75 1.45 4812.00 1.63 4922.25 1.81 5042.50 2.00 5152.75 2.18 5273.00 2.36 538

(2) The reflectance of graptolites and vitrinite-like particles in thesame sample is identical. This is taken to suggest that thevitrinite-like particles in fact may be fragments of graptolites.

(3) The combination of measurements made on graptolites andvitrinite-like particles provides a well-defined reflectance pop-ulation that can be used to assess the thermal maturity.

(4) In mature samples with high-reflecting particles (average Rgrap

and average Rvitr > 0.75%) the reflectance distribution is bi-modal; the lower population is arbitrarily selected to determinethe average reflectance.

(5) In southern Scandinavia, the relationship between graptolitereflectance and the equivalent vitrinite reflectance is suggestedto follow the correlation: VReqv = 0.73 R(grap + vitr)low + 0.16.This implies that the reflectance of graptolites increases fasterthan the reflectance of vitrinite. In the context of shale gas, thisindicates that the gaswindow starts at 1.56% Ro graptolite reflec-tance (corresponding to 1.3% Ro vitrinite reflectance equivalent).

(6) The Middle Cambrian to Furongian shales contain sparse smallparticles that resemble vase-shaped microfossils (VSM); thereflectance of these particles seems to fall on the maturationtrend (reflectance) of chitinozoans. The maturation (reflec-tance) trend of chitinozoans and VSM is comparable to that ofgraptolites.

(7) Solid bitumen occurs inmost of the analyzed samples, but in thissample set reflectance measurements of bitumen cannot beused to determine the thermal maturity.

(8) Porous/granular vitrinite-like particles occur in minor amounts;they may represent graptolite fragments with a non-smoothsurface.

Acknowledgments

J. Halskov (GEUS) is thanked for drafting the figures. S. Stouge isthanked for the CAI determinations. Sample material from the RL-1well was kindly provided by Gripengas. We thank two anonymous re-viewers for their constructive comments. The present study is pub-lished with the permission of the Geological Survey of Denmark andGreenland (GEUS).

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