Reflectance measurements of zooclasts and solid ...

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

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International Journal of Coal Geology journal homepage: www.elsevier.com/locate/ijcoalgeo

Reflectance measurements of zooclasts and solid bitumen in Lower Paleozoic shales, southern Scandinavia: Correlation to vitrinite reflectance Henrik I. Petersen a,⁎, Niels H. Schovsbo a, Arne T. Nielsen b a b

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

a r t i c l e

i n f o

Article history: Received 19 December 2012 Received in revised form 20 March 2013 Accepted 25 March 2013 Available online 8 April 2013 Keywords: Reflectance measurements Zooclasts Graptolite Vitrinite-like particles Chitinozoans Bitumen

a b s t r a c t Reflectance measurements have been carried out on zooclasts (graptolites, chitinozoans, and vase-shaped microfossils) and other organic particles (vitrinite-like particles, porous/granular vitrinite-like particles, and solid bitumen) in Middle Cambrian to Upper Silurian shales from central and southern Sweden and the Danish island of Bornholm (Baltic Sea). The most abundant organic components in all the shales are fragments of graptolites and vitrinite-like particles. The reflectance distribution of these two types of components is largely identical, and it is suggested that the vitrinite-like particles are fragments of graptolites without any recognizable morphology. Reflectance measurements of graptolites and vitrinite-like particles yield well-defined reflectance populations. In samples with average Rgraptolite and average Rvitrinite-like of > 0.75% Ro, the reflectance distributions are bimodal because of increasing bireflectance, and the average reflectance value of the well-defined lower reflecting population is arbitrarily used as maturity indicator. Our results suggest that with increasing thermal maturity the reflectance of graptolites increases faster than the predicted vitrinite reflectance. The relationship between graptolite reflectance and equivalent vitrinite reflectance can be expressed 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 rocks containing vitrinite, starts, accordingly, at 1.56% Ro graptolite reflectance. Porous/granular vitrinite-like particles occur in minor amounts and they may represent graptolite fragments with a non-smooth surface. They generally 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-shaped microfossils (VSM) that seem to follow the maturation trend of chitinozoans. In the present sample set, the reflectance of chitinozoans and VSM is comparable to that of graptolites at the same level of maturity. Reflectance measurements of solid bitumen are a poor maturity indicator, probably because bitumen can have various 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 sedimentary rocks, e.g. vitrinite particles, are a widely used and robust thermal maturity indicator (e.g. Hunt, 1996). Vitrinite is derived from partly decomposed and thermally matured ligno-cellulosic tissues of higher land plants, which appeared after the Late Silurian where the first vascular plants evolved. In the absence of vitrinite in Lower Paleozoic rocks, reflectance measurements have been carried out on zooclasts (graptolites, chitinozoans, and scolecodonts) and other organic particles (bitumen and vitrinite-like particles) (SuárezRuiz et al., 2012). In several studies the reflectance of various types of zooclasts and bitumen were measured in the same samples in order to establish reflectance correlations and to compare thermal evolution trends of the different components. Furthermore, it has been attempted ⁎ Corresponding author. Tel.: +45 38142455; fax: +45 3814 2050. E-mail address: [email protected] (H.I. Petersen). 0166-5162/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.coal.2013.03.013

to establish a calibration with other maturity indicators, such as vitrinite reflectance equivalent (VReqv), Rock-Eval Tmax values, conodont Color Alteration Index (CAI), Acritarch Alteration Index (AAI), Thermal Alteration Index (TAI) and atomic H/C ratios (e.g. Bertrand, 1990; Bertrand and Héroux, 1987; Buchardt and Lewan, 1990; Cole, 1994; Suchý et al., 2002; Tricker et al., 1992; Williams et al., 1998). Among zooclasts, graptolites are probably the most widely used group for reflectance measurements (Bustin et al., 1989; Christiansen et al., 1989; Clausen and 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 reflectance measurements (Goodarzi, 1985; Marshall, 1995; Obermajer et al., 1996; Tricker et al., 1992). The advantage of graptolites versus chitinozoans may be the much greater abundance of graptolites (Cole, 1994). Tricker et al. (1992) stated that the chance of obtaining sufficient chitinozoans for reflectance measurements in a whole rock sample is extremely low. Laufeld (1974) observed an average density of only about one chitinozoan vesicle per five gram of sediment from Gotland,

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

Sweden. Solid bitumen reflectance has been used in Lower Paleozoic rocks and in younger rocks containing scarce or non vitrinite (Gentzis and Goodarzi, 1993; Jacob, 1989; Landis and Castaño, 1995; Riediger, 1993; Schoenherr et al., 2007). Buchardt and Lewan (1990) and Xianming et al. (2000) measured the reflectance of so-called vitrinitelike macerals in the Cambrian–Ordovician Alum Shale from Scandinavia and in Lower Paleozoic shales from the Tarim Basin, China, respectively. The need for determining the thermal maturity of Lower Paleozoic rocks has increased with the intensified exploration for shale gas, an increasingly important unconventional gas resource that is related to the globally widespread occurrence of Cambrian–Silurian organic-rich shales (e.g. Jarvie, 2012; Pool et al., 2012; Schovsbo et al., 2011). Of particular importance is the correlation of zooclast reflectances to vitrinite reflectance equivalents (VReqv) as vitrinite reflectance values are calibrated to oil and gas generation and is the typical maturity indicator applied to model hydrocarbon generation. The present study examines the reflectance of graptolites, chitinozoans, vitrinite-like particles, porous/granular vitrinite-like particles, solid bitumen, and possible vase-shaped microfossils (VSM) in 16 Middle Cambrian to Upper Silurian shale samples from localities in Sweden and Denmark. The aims are (1) to investigate the applicability of reflectance measurements of various organic particle types to assess the thermal maturity of Lower Paleozoic shales in southern Scandinavia, and (2) to establish a correlation between the reflectance of these particle types and VReqv. The results are also compared with Rock-Eval Tmax values and a limited number of CAI values. 2. Samples and methods The shale samples include 13 outcrop samples and 3 core samples from shallow wells. They were collected in southern and central

Sweden and on the Danish island of Bornholm in the Baltic Sea (Fig. 1; Table 1). The oldest samples are of Middle Cambrian age (Acerocare pisiformis Zone) and the youngest of Late Silurian age (Ludlow Stage). Nine of the samples represent the Middle Cambrian to Lower Ordovician Alum Shale, and the remaining samples different Middle Ordovician to Upper Silurian marine shales. Organic geochemical screening analyses were carried out in order to characterize the samples. The samples were analyzed for total organic 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 carbon before TOC determination. The samples were further pyrolyzed in a Source Rock Analyzer (SRA) system. Pellets of the shales suited for reflected light microscopy were prepared. The samples were lightly crushed and sieved between 63 μm and 1 mm. This fraction was embedded in epoxy, and the epoxy pellets were ground and polished to obtain a smooth surface. The embedding procedure takes into account density separation in the epoxy of the crushed shale material. Reflectance measurements (random, oil immersion) were conducted using a Leica DM4000M reflected light microscope equipped with a 25 × objective and the Diskus Fossil vitrinite reflectance system (Hilgers Technisches Buero, Germany). The reflectance measurements were taken at 546 nm (monochromatic light). Before measurement the microscope was calibrated against a YAG 0.903% Ro standard and an optical black (zero) standard. The Diskus Fossil system is software controlled and uses no photomultiplier which makes it very robust and linear to high reflectance values. It was therefore considered sufficient in this context only to use the YAG 0.903% Ro standard. Various studies have used random or maximum and minimum reflectance measurements, the latter because of graptolite anisotropy, in particular at graptolite

Lower Palaeozoic deposits

Kakeled Hällekis-1 RL-1

Sweden Grönvik

Öland

Ottenby

Denmark

Scania Röverakulan

Klinta

Kivik S. Sandby Bjärsjölagård Gislövshammar

Baltic Sea

Bornholm Cale do

Læså Øleå localities

nian

Def orm

ation Front

50 km

Fig. 1. Map showing locations of the studied samples from central and southern Sweden and the Danish island of Bornholm. Gray shading shows present day occurrence of Lower Paleozoic strata. From Nielsen and Schovsbo (2011).


3

Table 1 List of samples arranged according to age. Lab. no.

Locality

Type

Lithology

Formation

Stage

21074 21075 21076 21080 21079 21082 21084 21078 21083 21073 21418 21422 21427 22422 21077 21081

Röverakulan Klinta Bjärsjölagård Øleå at Slusegård Øleå at Billegrav Læså at Vasagård Kivik Ottenby Gislövhammar S. Sandby Hällekis-1 (26.75 m) Hällekis-1 (31.92 m) Kakeled RL-1 well (74.42 m) Grönvik Øleå at Borggård

Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Core Core Outcrop Core Outcrop Outcrop

Shale Shale Siltstone Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale Shale

Colonus Shale Öved-Ramsåsa Öved-Ramsåsa Cyrtograptus Shale Rastrites Shale Dicellograptus Shale Almelund Shale Alum Shale Alum Shale Alum Shale Alum Shale Alum Shale Alum Shale Alum Shale Alum Shale Alum Shale

Upper Silurian: Ludlow Upper Silurian: Ludlow Upper Silurian: Ludlow Lower Silurian: Wenlock Lower Silurian: Llandovery Upper Ordovician Middle Ordovician Lower Ordovician Lower Ordovician Furongian (u. Cambrian): Acerocare Zone Furongian (u. Cambrian) Furongian (u. Cambrian) Furongian (u. Cambrian): Acerocare Zone Furongian (u. Cambrian) Middle Cambrian: Acerocare pisiformis Zone Middle Cambrian: A. pisiformis Zone

reflectances above approximately 1% Ro (e.g. Goodarzi, 1984; Goodarzi and Norford, 1987; Link et al., 1990; Rantitsch, 1995). In this study random reflectance readings were taken, which is comparable to general practice for vitrinite reflectance measurements on dispersed organic matter DOM (ASTM D7708-11) and the ICCP DOMVR accreditation program. Readings were taken on different types of particles (graptolites, chitinozoans, vase-shaped microfossils(?), vitrinite-like particles, porous/granular vitrinite-like particles, and solid bitumen) providing the reflectance distribution of each individual particle type (Fig. 2). A total of 45 to 370 measurements were taken per sample, but in 12 of the 16 samples the number of readings was > 200. Average reflectance values of the different components were calculated, and for

the higher maturity samples yielding well-defined bimodal reflectance distributions, the average reflectance value of low- and highreflecting populations was calculated for graptolites and vitrinite-like particles. The conodont Color Alteration Index (CAI; Epstein et al., 1977; Rejebian et al., 1987) was determined for seven limestone samples associated 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 indicator in Lower Paleozoic rocks with no vitrinite, in Scandinavia for instance by Bergström (1980).

30

20

Total reflectance distribution

Graptolites

Number of measurements

25 15 20 10 15 5

10

5

0

0

0.5

1.0

1.5

2.0

2.5%

20 0 0

0.5

1.0 1.5 Reflectance

2.0

Vitrinite-like particles

2.5% 15

10

Porous/granular vitrinite-like particles

10

Chitinozoans

10

20

Bitumen

5

5 15 5

0

0

0.5

1.0

1.5

2.0

2.5%

10 0

0

0.5

1.0

1.5

2.0

2.5%

5 0 0

0

0.5

1.0

1.5

0

0.5 2.0

1.0

1.5

2.0

2.5%

2.5%

Fig. 2. Example of total reflectance histogram and individual histograms for different particle types measured in the Lower Ordovician Alum Shale from Ottenby, Öland, Sweden.

4


3. Measured organic components The different types of zooclasts and solid bitumen measured in the samples comprise: 3.1. Graptolites Fragments of graptolites were measured in all samples apart from the Upper Silurian Colonus Shale from Rövarekulan, Scania (Fig. 3; Table 2). Examples of graptolites in the Lower Ordovician Alum Shale from Öland (sample 21078, Ottenby) and southernmost Sweden (sample 21083, Gislövhammar) are shown in Fig. 3A and C. Graptolite fragments are in this study recognized by their lath-shape and occasional remnants of thecae. The presence of graptolites in the Lower Ordovician (Tremadocian) Alum Shale from southernmost Sweden and Öland (Westergård, 1909, 1922), and in the Silurian (Llandoverian and Wenlockian) shales from the island of Bornholm have previously been documented (Bjerreskov, 1975). Although dendroid graptolites are known to occur from the Middle Cambrian, also in Baltoscandia (Bengtson and Urbanek, 1986; Öpik, 1933), the presence of graptolites

in the Furongian Alum Shale in southern Sweden and on Bornholm has not previously been described. However, their occurrence in the Alum Shale is strongly indicated by identification of rhabdosomes such as shown by the uppermost Furongian sample from southern Sweden (sample 21073, S. Sandby) (Fig. 3B). The graptolites in the Middle Cambrian 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 shallower water depth and during transportation to the depositional site in deeper water, the rhabdosomes were broken into smaller pieces. Under the microscope, the visible structures are tissues from the periderm (Link et al., 1990). The fragmentary graptolites show in reflected light granular or non-granular morphology, with the non-granular fragments showing higher reflectance and stronger bireflectance than the granular fragments (Goodarzi, 1984). The non-granular fragments occur mainly in shales, whereas the granular fragments are more common in rocks with a carbonate matrix. In agreement with the shale matrix of the investigated samples, mainly non-granular graptolite fragments were observed. Geochemical studies indicate that the graptolite periderm has a more highly condensed aromatic

2.05%

0.86%

A

B

1.84%

C Fig. 3. Photomicrographs (white reflected light, oil immersion) of part of graptolite rhabdosomes. (A) Lower Ordovician Alum Shale, Ottenby, Öland, Sweden (sample 21078). (B) Furongian Alum Shale, Sandby, Scania, Sweden (sample 21073). (C) Lower Ordovician Alum Shale, Gislövhammar, Scania, Sweden (sample 21083).


5

Table 2 Average reflectance values of organic components. Lab. no.

Vitrinite-like particles Total

High-refl.

0.70 n= 0.95 n= –

(0.069) 15 (0.081) 18

1.02 n= 1.29 n= –

(0.096) 29 (0.076) 16

2.16 n= 2.05 n= 2.04 n= 1.74 n= –

(0.126) 57 (0.161) 94 (0.152) 66 (0.130) 105

2.85 n= 2.62 n= 2.96 n= 2.18 n= –

(0.117) 43 (0.097) 21 (0.211) 44 (0.158) 32

1.61 n= 1.96 n= –

(0.065) 82 (0.112) 134

1.80 n= 2.37 n= –

(0.066) 126 (0.080) 26

% Ro (Std.) 21074 21075 21076 21080 21079 21082 21084 21078 21083 21073 21418 21422 21427 22422 21077 21081

0.92 n= 1.11 n= 0.75 n= 2.50 n= 2.21 n= 2.42 n= 1.85 n= 0.74 n= 1.74 n= 2.05 n= 0.54 n= 0.49 n= 0.46 n= 0.42 n= 0.33 n= 2.11 n=

(0.185) 45 (0.189) 34 (0.076) 46 (0.357) 158 (0.363) 148 (0.409) 161 (0.242) 139 (0.094) 142 (0.135) 121 (0.184) 183 (0.028) 96 (0.053) 170 (0.024) 145 (0.029) 159 (0.043) 167 (0.240) 265

Chitinozoans

?Vase-shaped microfossils

Porous ‘vitrinite’

Bitumen

% Ro (Std.)

% Ro (Std.)

% Ro (Std.)

% Ro (Std.)

Not detected

Not detected

Not detected

Not detected

?1.41 (0.045) n=3 –

?1.64 (0.335) n=3 Not detected

Not detected

Not detected

Not detected

Not detected

2.12 (0.118) n = 41 –

2.96 (0.183) n = 11 –

Not detected

2.12 n= 1.71 n= –

(0.110) 20 (0.113) 22

2.66 n= 2.05 n= –

(0.193) 9 (0.110) 12

Not detected

?2.73 (0.057) n=2 ?2.32 (0.166) n=4 Not detected

Not detected

Not detected

Not detected

1.51 n= 1.95 n= –

(0.115) 24 (0.119) 65

1.86 n= 2.31 n= –

(0.077) 42 (0.099) 20

?2.54 (0.315) n=4 1.78 (0.191) n=3 2.01 (0.056) n=3 1.85 (0.117) n = 16 0.54 (0.111) n=5 ?1.58 (0.243) n=2 Not detected

0.84 (0.087) n = 10 ?0.70 (0.119) n=6 ?2.01 (0.059) n=3 0.45 (0.053) n = 27 ?0.48 (0.074) n=4 Not detected

0.34 (0.062) n = 21 0.26 (0.038) n = 14 ?0.40 n=1 0.66 (0.281) n=3 0.31 (0.121) n = 19 0.35 (0.123) n = 13 0.23 (0.055) n = 67 0.41 (0.065) n = 20 0.64 (0.152) n = 25 0.28 (0.035) n = 52 ?0.27 (0.041) n=2 Not detected

Graptolites

Low-refl.

Total

Low-refl.

High-refl.

Not detected

Not detected

Not detected

1.36 n= 0.74 n= 2.33 n= 2.12 n= 2.30 n= 1.85 n= 0.78 n= 1.72 n= 2.04 n= 0.56 n= 0.51 n= 0.46 n= 0.43 n= 0.31 n= 2.12 n=

?1.08 (0.045) n=2 –

% Ro (Std.)

0.60⁎ (0.029)

0.47 (0.022) n = 144 –

n = 26 –

–

–

–

–

2.07 (0.157) n = 263

–

(0.234) 6 (0.086) 14 (0.337) 70 (0.182) 19 (0.334) 31 (0.233) 35 (0.061) 71 (0.156) 104 (0.190) 85 (0.023) 138 (0.068) 175 (0.023) 169 (0.033) 115 (0.047) 110 (0.148) 37

Not detected

0.47 (0.018) n = 117 –

0.60⁎ (0.030)

Not detected

n = 58 –

Not detected

–

–

Not detected

–

–

Not detected

–

–

Not detected

Not detected

Not detected ?2.18 (0.090) n=2 Not detected ?0.58 (0.008) n=3 ?0.39 (0.071) n=2 Not detected 0.41 (0.080) n = 14 Not detected

0.40 (0.083) n = 11 0.54 (0.065) n = 69 Not detected

0.23 n= 0.16 n= 0.87 n=

(0.043) 15 (0.043) 5 (0.088) 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.

structure than vitrinite (Bustin et al., 1989). This inherited higher aromaticity may cause a faster maturation rate. 3.2. Chitinozoans Chitinozoans are of uncertain biological affinity and range stratigraphically from the Early Ordovician to the Late Devonian; for general descriptions see Goodarzi (1985), Tricker et al. (1992) and Obermajer et al. (1996). They occur as single microfossils or as chains of microfossils (Paris et al., 1999). Several species of chitinozoans have been recorded in Ordovician (including Tremadocian) strata in southern Sweden (Scania) and on Bornholm (e.g. Grahn and Nõlvak, 2007). A chitinozoan vesicle is flask- or bottle-shaped, 50–250 μm long, and consists of an oral tube and a chamber with appendices (Goodarzi, 1985; Tricker et al., 1992). Normally, only broken parts can be seen microscopically. Chitinozoans show isotropy in reflected light and oil immersion (Obermajer et al., 1996). A few chitinozoans have been recorded in seven of the analyzed samples (Fig. 4; Table 2). Some of the particles identified as chitinozoans are flask- or bottle-shaped, whereas identification of smaller fragments without this characteristic shape remains uncertain. Pyrolysis and micro-FTIR data from Upper Silurian chitinozoans from Turkey suggest that the vesicles consist of aliphatic compounds and a substantial proportion of aromatic compounds that originated from a phenolic macromolecule (Dutta et al., 2007). A study on Lower Silurian chitinozoans from Saudi Arabia likewise indicated a composition dominated by aromatic compounds and with a low amount of aliphatic compounds (Jacob et al., 2007). None of these studies found chitin-related compounds in the chitinozoans as suggested by some early studies (Goodarzi, 1985).

3.3. Vase-shaped microfossils (VSM) Chitinozoan-like so-called vase-shaped microfossils (VSM) sensu Porter et al. (2003), have been found in the upper Precambrian Chuar Group of the Grand Canyon, Arizona, and in upper Precambrian rocks from Saudi Arabia (Binda and Bokhari, 1980; Bloeser et al., 1977; Porter et al., 2003). The microfossils in the Chuar Group were initially described as chitinozoans because of similarity in morphology, but Porter et al. (2003) suggested a relationship between VSM's and extant testate amoebae. A few readings have been taken on particles that have been assigned as VSMs in four of the Middle Cambrian and Furongian samples (Table 2). The small particles resemble fragments of the chitinozoan neck or shoulder (Goodarzi, 1985; his Figs. 3a and 5a) or prosome (Tricker et al., 1992; their Fig. 1), however, identification is uncertain. 3.4. Virtinite-like particles Buchardt and Lewan (1990) described “kerogen macerals resembling vitrinite” in the Middle Cambrian to Lower Ordovician Alum Shale in southern Scandinavia, and more recently vitrinite-like particles were recorded in Cambrian to Ordovician rocks from the Tarim Basin in China (Xianming et al., 2000). Schleicher et al. (1998) described vitrinite-like particles in upper Cambrian shales from northern Poland, which they preferred to call bituminite because of the absence of land-derived vitrinite in the late Cambrian. However, the photomicrographs of bituminite shown by Schleicher et al. (1998; their Plate 2a and b) resemble to a large extent graptolites. Vitrinite-like particles do not possess the diagnostic features of fragments from graptolites

6


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 Silurian Rastrites Shale, Øleå at Billegrav, Bornholm (sample 21079). (C) Upper Ordovician Dicellograptus Shale, Læså at Vasagård, Bornholm (sample 21082).

(see above), and exhibit a more varied morphology (Fig. 5). Vitrinitelike particles have been measured in all the analyzed samples (Table 2). The vitrinite-like particles were originally reported to respond to maturation in a similar way as suppressed vitrinite, and gelification 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 a porous/granular surface were recognized (Fig. 6; Table 2), and they were recorded as a separate group.

secondary maceral and represents heavy petroleum, which have been generated within the host rock or has migrated into the rock from another source (Hunt, 1996; Landis and Castaño, 1995; Taylor et al., 1998). Migrated bitumen may thus occur in organic-lean rocks. It may, however, also have been formed by biodegradation of oil or from deasphalting. Dark to almost black (white reflected light) amorphous 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 of the Alum Shale

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

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


7

0.44%

0.46%

0.53%

A

A

0.88% 0.69%

B

B

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

2.29%

significantly leaner in organic matter (b1.90 wt.%), with the lowest content measured for the Colonus Shale from Rövarekulan (sample 21074). The difference between TC and TOC is for most of the samples insignificant, but for the Lower Silurian Cyrtograptus Shale (sample 21080) and the three Upper Silurian samples the difference clearly indicates the presence of carbonate, in particular in sample 21074 (Rövarekulan) (Table 3). In two of the samples, the TOC content is slightly higher than the TC content, but because of analytical uncertainty (±0.8%) this may happen for samples with almost identical TOC and TC contents. TS varies from 0.01 to 10.07 wt.% with the highest values recorded in the Alum Shale. The Tmax spans from 415 °C to > 600 °C, showing that the samples represent a wide range from thermally immature to overmature with regard to petroleum generation. In the high mature samples no pyrolyzable kerogen

C Fig. 5. Photomicrographs (white reflected light, oil immersion) of vitrinite-like particles, 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).

8


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 Ordovician Alum Shale, Ottenby, Öland, Sweden (sample 21078). (C) Lower Silurian Rastrites Shale, Øleå at Billegrav, Bornholm (sample 21079).

is left as reflected by zero S1 and S2 yields and very low to zero HI values (Table 3). The Tmax values derived from samples with insignificant S2 yields is therefore in the best case uncertain, but more likely unreliable.

The immature to mature samples yield HI values from 301 to 395 mg HC/g TOC. These samples contain orange to yellow fluorescing lamalginite, telalginite and fluorescing amorphous organic matter (sapropelic kerogen) (Fig. 8). Conventionally, this would indicate an

Table 3 Screening data and CAI. Lab. no.

Locality

21074 21075 21076 21080 21079 21082 21084 21078 21083 21073 21418 21422 21427 22422 21077 21081

Röverakulan Klinta Bjärsjölagård Øleå at Slusegård Øleå at Billegrav Læså at Vasagård Kivik Ottenby Gislövhammar S. Sandby Hällekis-1 (26.75 m) Hällekis-1 (31.92 m) Kakeled RL-1 well (74.42 m) Grönvik Øleå at Borggård

TOC

TC

TS

6.99 1.68 1.59 1.17 1.14 1.99 1.10 7.45 7.89 7.54 15.90 18.86 18.87 14.29 9.30 5.95

0.01 0.11 0.19 0.46 0.15 0.61 1.15 1.14 3.69 3.16 4.87 5.33 7.45 10.07 5.51 6.07

(wt.%) 0.07 0.10 0.14 0.68 1.07 1.90 1.08 7.31 7.44 7.03 15.94 18.41 18.59 14.51 8.98 5.95

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.

Tmax

S1

(°C)

(mg HC/g rock)

464 438 440 594 598 603 494 440 471 601 417 415 419 423 420 485

0 0 0 0 0 0 0 0.77 0.21 0 0.32 0.85 2.29 0.82 1.67 0

S2

0 0 0 0 0 0 0 28.87 1.48 0.23 48.00 56.92 60.81 53.19 34.04 0

HI

PI

CAI

0 0 0 0 0 0 0 395 20 3 301 309 327 367 379 0

– – – – – – – 0.03 0.12 0 0.01 0.01 0.04 0.02 0.05 –

– – 1.5 – – 3.5 3 1.5 2 3 – – – – 1 –


Graptolite

A1

Non-fluorescing graptolite

A2

Graptolite?

Fluorescing graptolite?

B1 Graptolite fragment

9

B2

Vitrinite-like particle

Nonfluorescing

Fluorescing

C1

C2

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 fluorescing sapropelic 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-fluorescing graptolite 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 of this article.)

oil-prone source rock, but no economic oil occurrences sourced from the Alum Shale have been discovered. Wells on the Swedish island of Gotland have produced minor amounts of oil from Ordovician limestones, but the Alum Shale has not been proven to be the source (Buchardt, 1999; Dahl et al., 1989). In northeast Poland and offshore in the Baltic Sea, small oil discoveries have been made in Middle Cambrian sandstones, and the source of the oil is assumed to be the Alum Shale (Kotarba, 2010). However, analyses of the kerogen in the Alum Shale 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 condensate source producing unusual aromatic mixtures with a very low concentration of n-alkanes with more than 10 carbon atoms. The kerogen was shown to have a higher degree of aromaticity than normal marine kerogen and an unusual amount of carbon atoms bonded to oxygen. The aliphatic part of the kerogen consists of short straight and branched alkyl chains typically with 1–6 carbon atoms. Such a composition may have a limited capacity to generate liquid petroleum. Nevertheless, several of the investigated samples in this study contain solid bitumen (heavy petroleum) (see below). The structure of the

10


Alum Shale kerogen definitely calls for more investigations to clarify the composition and petroleum generation potential of this 500 million year old organic matter. 4.2. Previous correlations to vitrinite reflectance equivalent (VReqv) The increase in vitrinite reflectance with increasing burial temperature is caused by condensation and successive ordering of aromatic structures in the degraded ligno-cellulosic material forming the vitrinitic organic matter (Carr and Williamson, 1990). Organic matter derived from true ligno-cellulosic precursor material is absent in Lower Paleozoic rocks but the reflectance of zooclasts, vitrinite-like particles and bitumen also increases with increasing burial depth and temperature, although they may react at a different rate to heating than vitrinite derived from vascular plants (Clausen and Teichmüller, 1982; Goodarzi, 1984; Goodarzi and Norford, 1985,

1989; Bertrand and Héroux, 1987; Bustin et al., 1989; Jacob, 1989; Bertrand, 1990; Buchardt and Lewan, 1990; Link et al., 1990; Tricker et 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 between vitrinite reflectance equivalents and the reflectance of graptolites, chitinozoans, vitrinite-like particles and solid bitumen are shown in Table 4. The published results show poor consistency and the graptolites in particular correlate inconsistently. Possible explanations for this discrepancy may include difficulties in identifying graptolites, graptolite surface morphology (smooth or granular), and the state of preservation. The four listed studies on chitinozoans show the reflectance to be similar or higher than vitrinite reflectance (Table 4). Vitrinite-like particles have been reported to react as suppressed vitrinite (Buchardt and Lewan, 1990), or a more complex relationship has been proposed depending on the level of thermal maturity

Table 4 Relation between vitrinite reflectance equivalents (VReqv) and the reflectance of zooclasts, vitrinite-like particles, and solid bitumen. Publication

Graptolites (Rgrap)

Clausen and Teichmüller (1982) Goodarzi (1984) Goodarzi and Norford (1985) Bertrand and Héroux (1987)

Rgrap different from VReqv

Bustin et al. (1989) Jacob (1989) Goodarzi and Norford (1989)

Bertrand (1990) Buchardt and Lewan (1990)

Link et al. (1990)

Chitinozoans (VRchi)

Vitrinite-like particles (Rvitr)

Relation to VReqv not clear Rather similar VReqv 0.4–0.8% lower than Rgrap and Rchi in the 1–2% VReqv range Rgrap similar to VReqv VReqv = 0.618 RB + 0.4 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 and McHugh, 1986) Rgrap slightly less than VReqv

Rchi similar to VReqv Vitrinite-like macerals respond on heating as suppressed vitrinite, i.e. Rvitr lower than VReqv

Lower levels of maturity: VRmax(eqv) > Rgrap(max) Rgrap(max) increases faster than VRmax(eqv) at high levels of maturity: 5.0–6.5% Rgrap(max) corresponds to 4% VRmax(eqv)

Tricker et al. (1992)

Rchi > VReqv VReqv = (Rchi − 0.08) / 1.152 VReqv b c. 0.72%: relationship similar to Jacob (1989) VReqv > c. 0.72%: VReqv = 0.277 RB + 0.57

Riediger (1993)

Cole (1994)

Landis and Castaño (1995) Rantitsch (1995)

Obermajer et al. (1996)

Xianming et al. (2000)

Schoenherr et al. (2007)

Solid bitumen (RB)

Rgrap higher than VReqv. Immature: 0.6% Rgrap = app. 0.5% VReqv Mature: 1.1% Rgrap corresponding to app. 0.9% VReqv

Similar to graptolite/vitrinite correlation

VReqv = (RB + 0.41) / 1.09 Rgrap of graptolite fragments resembles VReqv at high maturity (>3% VReqv(max)) Rchi 20–25% higher than VReqv. 0.65% Rchi = 0.5% VReqv. 0.9% Rchi = 0.7% VReqv Rvitr b VReqv up to 1.50% Rvitr Rvitr > VReqv above 1.50% Rvitr Rvitr b 0.75%: VReqv = 1.26 Rvitr + 0.21. Rvitr = 0.75–1.50%: VReqv = 0.28 Rvitr + 1.03 Rvitr > 1.50%: VReqv = 0.81 Rvitr + 0.18 VReqv = (RB + 0.2443) / 1.0495


(Xianming et al., 2000; Table 4). Solid bitumen has been reported to have lower reflectance than vitrinite at lower maturities, whereas the bitumen reflectance appears to exceed that of vitrinite at higher maturities (Table 4). 4.3. Reflectance measurements: vitrinite-like particles and graptolites Most reflectance measurements were taken on vitrinite-like particles (Rvitr) and graptolites (Rgrap) with the total amount of readings in

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

0.32% 0.46%

0.45%

A

B

0.57%

0.78%

C

D

2.12%

1.76%

E

11

F

Fig. 9. Photomicrographs (white reflected light, oil immersion) of graptolites showing increasing reflectance (A → F) related to increasing thermal stress.

12


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

particles form a unimodal distribution with an average reflectance of b0.75% Ro. A unimodal reflectance distribution with comparable reflectance values is also observed for graptolites in the same sample as illustrated by the almost perfect overlap between the reflectance distributions of vitrinite-like particles and graptolites in sample 21418

0.32% 0.47%

A

B

0.88%

0.56%

C

D

2.29%

1.88%

E

F

Fig. 10. Photomicrographs (white reflected light, oil immersion) of vitrinite-like particles, likely representing fragments of graptolites, showing increasing reflectance (A → F) with increasing thermal stress.


13

Average VR(Graptolite): 0.56 (n=138) Average VR(Vitrinite-like): 0.54 (n=96) 80

60

Graptolites and Vitrinite-like particles

70

50 40

Number of measurements

60

30 20

50 10 40

0 0 30

30

25

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 1.0%

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 1.0%

20 20 15 10

10 5

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7% 0 0

Reflectance

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.

(Table 2). Average values were calculated for arbitrarily selected normal distributed low- and high-reflecting populations. Fig. 12 shows crossplots of Rvitr(low) versus Rgrap(low) and Rvitr(high) versus Rgrap(high), where (i) ‘low’ designates the average reflectance value of the unimodal reflectance distribution of the immature to early mature samples and the average reflectance value of the normal distributed low-reflecting population of the higher maturity samples with a bimodal reflectance distribution and (ii) ‘high’ designates the average reflectance value of the unimodal reflectance distribution of the immature to early mature samples and the average reflectance value of the normal distributed high-reflecting population of the higher maturity samples with a bimodal reflectance distribution. The relationship between graptolites and vitrinite-like particles can be expressed by the equations RgrapðlowÞ ¼ 0:97 RvitrðlowÞ þ 0:03

ð1Þ

RgrapðhighÞ ¼ 0:91 RvitrðhighÞ þ 0:09:

ð2Þ

The correlations are robust with calculated regression lines having correlation coefficients of r 2 = 0.99 (low) and r 2 = 0.97 (high), respectively. The 1:1 correlation indicates a similar thermal evolution of the vitrinite-like particles and graptolites during maturation. This may indicate that the vitrinite-like particles consist of non-descript fragments of graptolites. Alternatively, the vitrinite-like particles originate from an unknown source with an organic structure that matures identically to graptolites during progressive burial and increasing heating. Two Middle Cambrian samples are included in the trend (Fig. 12A). The particles in these samples and in the Furongian samples may have been derived from benthic graptolites as the first planktic graptolites appeared at the Cambrian/Ordovician boundary (Cooper, 1999). Buchardt and Lewan (1990) reported that vitrinite-

like particles respond to maturation in a similar manner as suppressed vitrinite, but the identical thermal evolution of graptolites and vitrinite-like particles shown in the present study does not corroborate this observation. Suppression of vitrinite reflectance because of bitumen impregnation must always be considered as a possible complicating factor in highly sapropelic rocks (Carr, 2000; Lo, 1993; Petersen et al., 2006; Price and Barker, 1985). The Alum Shale is characterized by a strongly fluorescing algal-derived groundmass, but as shown in Fig. 8A2, relatively high-reflecting and non-fluorescing graptolites show no signs of suppression in that sample. However, it is also possible to find graptolites and vitrinite-like particles showing fluorescence in blue light irradiation, which could suggest bitumen impregnation (Fig. 8B2, C2). Reflectance suppression may thus in theory be a problem in the sapropelic Alum Shale, but as discussed in Section 4.1, the kerogen in the Alum Shale seems to have an unusual composition (Bharati et al., 1992, 1995) and maybe the problem is smaller than expected because of a limited capacity to generate liquid petroleum, including bitumen. Because vitrinite-like particles are interpreted as fragments of graptolites their well-constrained reflectance distributions are merged (Table 5). For the bimodal distributions, the low-reflecting population is arbitrarily selected as it overall is better constrained by a higher number of readings (Fig. 12; Table 5). The relationship between the highand low-reflecting populations is expressed by the equation (Fig. 13) 2

RðgrapþvitrÞhigh ¼ 1:28 RðgrapþvitrÞlow þ 0:03; r ¼ 0:96:

ð3Þ

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

14

A


2.4

Table 5 Reflectance of graptolites + vitrinite-like particles.

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

M. Cambrian

Graptolites (%Ro; total or low-reflecting popu.)

Lab. no.

Locality

Graptolites + vitrinite-like particles Total

2

1.6

21074

Röverakulana

21075

Klinta

21076

Bjärsjölagård

21080

Øleå at Slusegård

21079

Øleå at Billegrav

21082

Læså at Vasagård

21084

Kivik

21078

Ottenby

21083

Gislövhammar

21073

S. Sandby

21418

Hällekis-1 (26.75 m)

21422

Hällekis-1 (31.92 m)

21427

Kakeled

22422

RL-1 well (74.42 m)

21077

Grönvik

21081

Øleå at Borggård

r2 = 0.99

1.2

0.8

0.4 M. Cambrian

0

0.4

0.8

1.2

1.6

2

2.4

Vitrinite-like particles (%Ro; total or low-reflecting popu.) 3

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

2.5 r2 = 0.97

2

a b

0.92 n= 1.12 n= 0.75 n= 2.45 n= 2.21 n= 2.41 n= 1.85 n= 0.76 n= 1.73 n= 2.05 n= 0.55 n= 0.50 n= 0.46 n= 0.43 n= 0.32 n= 2.11 n=

(0.185) 45 (0.189) 38 (0.079) 60 (0.359) 228 (0.352) 169 (0.405) 193 (0.241) 174 (0.076) 204 (0.146) 225 (0.186) 268 (0.025) 231 (0.062) 341 (0.023) 314 (0.031) 274 (0.045) 277 (0.231) 302

0.70 n= 0.95 n= –

(0.069) 15 (0.081) 19

1.02 n= 1.34 n= –

(0.096) 29 (0.119) 25

2.14 n= 2.05 n= 2.06 n= 1.74 n= –

(0.119) 95 (0.161) 111 (0.145) 86 (0.130) 131

2.85 n= 2.63 n= 3.00 n= 2.19 n= –

(0.113) 49 (0.107) 23 (0.198) 43 (0.173) 42

1.61 n= 1.97 n= –

(0.065) 82 (0.120) 210

1.80 n= 2.38 n= –

(0.066) 126 (0.080) 36

0.47 (0.019) n = 258 –

0.60b (0.029) n = 83 –

–

–

–

–

2.07 (0.157) n = 263

2.53 (0.146) n = 34

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

1.5

1

0.5

0 0

0.5

1

1.5

2

2.5

3

3.5

4

Vitrinite-like particles (%Ro; total or high-reflecting popu.) Fig. 12. Cross-plots of the reflectance of graptolites and vitrinite-like particles. (A) Total population for lower maturity samples with a unimodal reflectance distribution combined with the low-reflecting population of higher maturity samples with a bimodal reflectance distribution. (B) Total population for lower maturity samples with a unimodal reflectance distribution combined with the high-reflecting population of higher maturity samples with a bimodal reflectance distribution. Very good correlation coefficients and a nearly perfect 1:1 correlation suggest that the vitrinite-like particles are fragments of graptolites.

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

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

Graptolites (%Ro; total or high-reflecting popu.)

High-refl.

% Rgrap + vitr (Std.)

0

B

Low-refl.

3

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

2.5

2

1.5 r2 = 0.96

1

0.5

0 0

0.5

1

1.5

2

2.5

Graptolites+Vitrinite-like particles (%Ro; low-reflecting popu.) Fig. 13. Cross-plot of the ‘total population of lower maturity samples + low-reflecting population of higher maturity samples’ versus the ‘total population of lower maturity samples + high-reflecting population of higher maturity samples’ for graptolites + vitrinite-like particles.


3

A

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

Rchi+VSM=1.05R(grap+vitr)low+0.04 Chitinozoans Outlier Vase-shaped microfossils (VSM)

2.5

Chitinozoans and VSM (%Ro)

Porous vitrinite-like particles (%Ro)

2.5

3

2

1.5 r2 = 0.97 1

15

2

1.5 r2 = 0.94

1

0.5 0.5 0 0

0.4

0.8

1.2

1.6

2

2.4

Graptolites+Vitrinite-like particles (%Ro)

0 0

0.4

0.8

1.2

1.6

2

2.4


B

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

Rporous vitr ¼ 1:17 RðgrapþvitrÞlow –0:05:

0.8

Bitumen (%Ro)

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

1

0.6

0.4 r2= 0.54

ð4Þ 0.2

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

0 0

C

4

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

0.8

1.2

1.6

2

2.4

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

CAI (Conodont Alteration Index)

4.4. Correlation of graptolite reflectance to chitinozoans, solid bitumen and CAI A few reflectance measurements were taken on chitinozoans in the Lower Ordovician to Upper Silurian shales and on vase-shaped microfossils (VSM) in the Cambrian shales (Table 2). A plot of these measurements versus the reflectance of graptolites + vitrinite-like particles show a good correlation (r 2 = 0.95) (Fig. 15A). It is notable that the reflectance values of chitinozoans and VSM form a welldefined trend, which suggest that these zooclasts are genetically related as proposed by Bloeser et al. (1977) and Binda and Bokhari (1980), or that they at least have a similar geochemical structure that reacts in the same way during heating. The established correlation shows only

0.4


3 r2 = 0.90

2

1

0 0

0.4

0.8

1.2

1.6

2


2.4


minor difference between the reflectance of chitinozoans/VSM and graptolites + vitrinite-like particles RchiþVSM ¼ 1:05 RðgrapþvitrÞlow þ 0:04:

Tmax = 63.03VR + 389.11 n = 621 r2 = 0.88

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 orange fluorescence. The correlation of solid bitumen reflectance to the reflectance of graptolite + vitrinite-like particles is poor and has no statistical significance (Fig. 15B). A plausible explanation could be that the bitumen is not genetically related to the shale in which it occurs. In south-central Sweden marginally mature to mature bitumen occurs in immature Alum Shale and mature bitumen is also found in overmature Alum Shale (Dahl et al., 1989). This was taken as evidence for migration of bitumen from sites where the Alum Shale had been locally heated by intrusions. It has also been suggested that different types of solid bitumen have different optical properties because of textural and morphological differences (see discussion in Suárez-Ruiz et al., 2012). This limits the value of using reflectance measurements of bitumen as a maturity indicator. The CAI values have been plotted against the reflectance of graptolites + vitrinite-like particles (Fig. 15C; Table 3). The correlation coefficient is r 2 = 0.90 and the relationship can be expressed by the equation CAI ¼ 1:31 RðgrapþvitrÞlow þ 0:49:

A 600

Tmax (°C)

16

500

400

0

1

B

500

The gradual increase in reflectance of graptolites with increasing thermal maturity suggests a general maturation path corresponding to that of vitrinite, and there is also an overall similarity in the chemistry of graptolites and vitrinite at all maturation levels (Bustin et al., 1989). However, it has always been an issue to calibrate the reflectance of zooclasts with vitrinite reflectance. It is attempted to use Tmax as an independent maturity indicator in order to ‘link’ Lower Paleozoic rocks lacking vitrinite with vitrinite-containing post-Silurian rocks. Petersen (2006) published a correlation between Tmax and VR of vitrinite-rich (humic) coals. This correlation has in the present study been updated with data from more than hundred new humic coal samples and now includes more than 600 samples (Fig. 16A). The relationship between Tmax and VR has a good correlation (r 2 = 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 a good correlation (r 2 = 0.93) and can be described by the equation (Fig. 16B) Tmax ¼ 45:48 RðgrapþvitrÞlow þ 400:16:

ð8Þ

Samples with extremely low S2 pyrolysis yields may result in high and unreliable Tmax values, and these assumed unrealistic values have arbitrarily been omitted from the correlation. The correlation confidence is thus less for high Tmax values. Furthermore, the correlation is hampered by the small number of samples and the relatively limited temperature range covered by reliable Tmax values. Hence, the

4

Upper Silurian Middle Ordovician Lower Ordovician Upper Cambrian Middle Cambrian

Tmax (°C)

480

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

3

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

ð6Þ

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

2

Humic coals (%Ro)

460

r2 = 0.93

440

420

400 0

0.4

0.8

1.2

1.6

2

2.4

Graptolites+Vitrinite-like particles (%Ro) 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 reflectance of graptolites + vitrinite-like particles using the low-reflecting population for higher maturity samples.

correlation may be improved when more data becomes available and 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 RðgrapþvitrÞlow þ 0:16:

ð9Þ

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


2.5

VReqv=0.73R(grap+vitr) low+0.16 1:1 line

Vitrinite reflectance eqvivalent (%Ro)

2.25

2

1.75

1.5

1.25

1

0.75

0.5 0.5

0.75

1

1.25

1.5

1.75

2

2.25

2.5

2.75

3

Graptolites+Vitrinite-like particles (%Ro) Fig. 17. Correlation between the reflectance of graptolites + vitrinite-like particles (using the low-reflecting population of higher maturity samples) and vitrinite reflectance 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 reflectance of vitrinite at the same maturity level.

more condensed aromatic structure than vitrinite (Bustin et al., 1989), which likely causes the faster increase in reflectance. According to the present correlation the onset of the gas window (vitrinite reflectance: 1.3% Ro) corresponds to a graptolite reflectance of 1.56% Ro. A similar result was published for Cambrian shales from northern Poland by Schleicher et al. (1998). 5. Conclusions The correlation between various maturity indicators are shown in Table 6. The following conclusions can be drawn: (1) The investigated Middle Cambrian to Upper Silurian shale samples contain various zooclasts, the most abundant being graptolites (graptolites + vitrinite-like particles). This suggests in turn that graptolites lived somewhere in Baltoscandia during deposition of the Alum Shale; no macro-fossils of graptolites have been found in pre-Ordovician Alum Shale.

Table 6 Correlation of maturity indicators. Rgrap + vitr (% Ro)

VReqv (% Ro)

Tmax (°C)

0.50 0.60 0.75 1.00 1.25 1.50 1.56 1.75 2.00 2.25 2.50 2.75 3.00

0.54 0.60 0.72 0.90 1.09 1.27 1.30 1.45 1.63 1.81 2.00 2.18 2.36

423 428 435 446 458 469 472 481 492 504 515 527 538

Shale gas

Shale gas, Rgrap + vitr > 1.56%

17

(2) The reflectance of graptolites and vitrinite-like particles in the same sample is identical. This is taken to suggest that the vitrinite-like particles in fact may be fragments of graptolites. (3) The combination of measurements made on graptolites and vitrinite-like particles provides a well-defined reflectance population 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 bimodal; the lower population is arbitrarily selected to determine the average reflectance. (5) In southern Scandinavia, the relationship between graptolite reflectance and the equivalent vitrinite reflectance is suggested to follow the correlation: VReqv = 0.73 R(grap + vitr)low + 0.16. This implies that the reflectance of graptolites increases faster than the reflectance of vitrinite. In the context of shale gas, this indicates that the gas window starts at 1.56% Ro graptolite reflectance (corresponding to 1.3% Ro vitrinite reflectance equivalent). (6) The Middle Cambrian to Furongian shales contain sparse small particles that resemble vase-shaped microfossils (VSM); the reflectance of these particles seems to fall on the maturation trend (reflectance) of chitinozoans. The maturation (reflectance) trend of chitinozoans and VSM is comparable to that of graptolites. (7) Solid bitumen occurs in most of the analyzed samples, but in this sample set reflectance measurements of bitumen cannot be used to determine the thermal maturity. (8) Porous/granular vitrinite-like particles occur in minor amounts; they may represent graptolite fragments with a non-smooth surface.

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