1 Graptolites as fossil geo-thermometers and source material of
2 hydrocarbons: an overview of four decades of progress
3 Qingyong Luoa, b, Goodarzi Fariborzc, Ningning Zhonga, b*, Ye Wanga, b, Nansheng Qiua, b,
4 Christian B. Skovstedd, Václav Suchýe, Niels Hemmingsen Schovsbof, Rafał Morgag,
5 Jingyue Haoa, b, Anji Liua, b, Jin Wua, b, Xu Mina, b, Weixun Caoa, b, Jia Wua, b
6 a State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing 102249, China;
7 b College of Geoscience, China University of Petroleum, Beijing 102249, China;
8 c FG &Partner Ltd, Research Group, 29 Hawkside Mews NW., Calgary, Alberta, Canada;
9 d Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden;
10 e Nuclear Physics Institute, v. v. i.,Academy of Sciences of the Czech Republic, Na Truhlářce 39/64, 180 86 Prague 8, Czech
11 Republic;
12 f Geological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, DK-1350 Copenhagen K, Denmark;
13 g Silesian University of Technology, Faculty of Mining, Safety Engineering and Industrial Automation, Institute of Applied
14 Geology, Akademicka 2, 44-100 Gliwice, Poland.
15
16 *Corresponding author at: State Key Laboratory of Petroleum Resources and Prospecting, China
17 University of Petroleum, Changping, Beijing, 102249, China. Tel.: +86 10 89734548. E-mail address:
19
20 Abstract
21 The thermal maturity of Lower Paleozoic graptolite-bearing marine sediments, which host many
22 hydrocarbon deposits worldwide, has long been difficult to determine due to the absence of wood-derived
23 vitrinite particles for conventional vitrinite reflectance. In 1976, graptolite reflectance was introduced as
24 a new indicator for organic maturity of these deposits and has been used since in many regional studies.
25 The majority of these studies, however, were done on a limited sample set and a limited range of thermal
26 maturity, which resulted in a number of controversial views concerning the usefulness of graptolite
27 reflectance as an alternative paleothermal indicator and its correlation with vitrinite reflectance through
28 various proxies. In this paper, we review previous studies and combine those analyses with new data to 29 assess the physical and chemical characteristics of graptolite periderm with increasing thermal maturity.
30 We conclude that graptolite random reflectance (GRor) is a better parameter for the thermal maturity
31 assessment than graptolite maximum reflectance (GRomax) due to the better quality of available data and
32 time saving. Combining published data with results of our study of both natural and heat-treated
33 graptolites and vitrinite, we present a new correlation between GRor and equivalent vitrinite reflectance
34 (EqVRo), as EqVRo = 0.99GRor + 0.08. Chemical composition of graptolite periderm is similar to vitrinite;
35 graptolites are mainly kerogen Type II-III, are gas prone and have a substantial hydrocarbon potential.
36 Lower Paleozoic graptolite-bearing organic-rich sediments are important shale gas source rocks and
37 reservoirs globally and make a significant contribution to worldwide petroleum reserves.
38 Keywords: Optical characteristics; Graptolite reflectance; Chemical composition; Microstructure;
39 Wufeng–Longmaxi Formations; Alum Shale; Hot shale; Shale gas.
40
41 1. Introduction and previous studies
42 Lower Paleozoic graptolite-bearing rocks were mainly deposited in marine environments, and are
43 important source rocks globally, especially shale gas deposits in the Wufeng–Longmaxi (also known as
44 Wufeng–Lungmachi) sediments from China (Zou et al., 2010; Zou et al., 2012; Dai et al., 2014; Dai et
45 al., 2016; Luo et al., 2016; Zou et al., 2016; Luo et al., 2017; Luo et al., 2018). These organic matter
46 (OM)-rich facies were also identified as true or potential hydrocarbon source rocks in the Anadarko basin
47 in the USA (Wang and Philp, 1997), North Africa and Arabian Peninsula (Jones and Stump, 1999; Lüning
48 et al., 2000), Taurus region of Turkey (Varol et al., 2006), the Czech Republic (Suchý et al., 2002), and
49 the Siberian platform (Makarov and Bazhenova, 1981). More recently, such shales have been recognized
50 as potential targets for unconventional shale gas deposits in the Norwegian-Danish Basin (Schovsbo et al.
51 2011, 2014; Pool et al. 2012), and Baltic basin of Central Europe (Littke et al., 2011; Karcz et al., 2013;
52 Yang et al. 2017).
53 Vitrinite reflectance is a most commonly used indicator for the thermal maturity (Stach et al., 1982;
54 Taylor et al., 1998; Suárez-Ruiz et al., 2012; Hackley and Cardott, 2016). However, due to the lack of
55 vitrinite (coalified wood) in pre-Devonian rocks, the determination of thermal maturity of Lower
56 Paleozoic graptolite-bearing rocks is always a difficult topic and a hot debate for petroleum industry
57 (Goodarzi, 1984; Goodarzi, 1985a; Goodarzi and Norford, 1985, 1987; Bertrand and Heroux, 1987;
58 Bustin et al., 1989; Bertrand, 1990; Goodarzi et al., 1992a; Bertrand, 1993; Petersen et al., 2013; Luo et 59 al., 2016; Luo et al., 2017; Luo et al., 2018). Thus, the surrogate proxies, such as the reflectance of
60 zooclasts, vitrinite-like particles and solid bitumen, Tmax, and biomarkers, have been proposed to assess
61 organic maturity in Lower Paleozoic graptolite-bearing sediments (Teichmüller, 1978; Goodarzi and
62 Norford, 1987; Jacob, 1989; Schoenherr et al., 2007; Suárez-Ruiz et al., 2012; Schmidt et al., 2019).
63 Tmax may be unreliable due to low S2 in overmature sediments (Peters, 1986; Peters and Cassa, 1994).
64 The maturity-related biomarker ratios may be also influenced by depositional environments and biological
65 sources, which will increase the difficulty of data interpretation (Radke and Welte, 1983; Radke et al.,
66 1986; Radke, 1988; George and Ahmed, 2002; Peters et al., 2005). In addition, the biomarker ratios may
67 be invalid to assess thermal maturity of overmature sediments (Peters et al., 2005). Graptolite-bearing
68 rocks often contain bitumen and other zooclasts (Teichmüller, 1978; Goodarzi and Norford, 1987; Jacob,
69 1989; Schoenherr et al., 2007; Suárez-Ruiz et al., 2012; Schmidt et al., 2019). The difficulty with using
70 of solid bitumen reflectance is due to large variation in most samples, and their origin, e.g., by thermal
71 cracking, biodegradation and deasphalting (George et al., 1994; Hwang et al., 1998; Mastalerz et al., 2018),
72 all of which increase difficulty in data interpretation (e.g., Gonçalves et al., 2014; Fink et al., 2016). The
73 origin of discrete vitrinite-like particles remains controversial, and possible explanations includes:
74 migrated bitumen, either indigenous or exogenous to the host rock (Bertrand and Heroux, 1987);
75 gelification of polysaccharides (Buchardt and Lewan, 1990); residues of algae after maturation (Wang et
76 al., 1994); biodegraded zooclasts that are the product of a reducing to strongly reducing environment
77 (Xiao et al., 1997); marine humification of planktonic and benthic organisms (Romankevich, 1984), the
78 so called “marine vitrinite group” (Zhong and Qin, 1995); and fragments of graptolites (Petersen et al.,
79 2013).
80 Zooclasts have clear advantage in reflectance studies due to their specific biological sources
81 compared to that of solid bitumen and vitrinite-like particles, and thus their reflectance was naturally
82 regarded as having a superior potential as a thermal maturity proxy. In general, graptolites are more
83 common than other zooclasts (e.g., chitinozoans and scolecodonts) in Lower Paleozoic marine rocks, and
84 as a result, the nature of graptolite reflectance has been a “hot topic” for organic petrologists over several
85 decades (Kurylowicz et al., 1976; Teichmüller, 1978; Goodarzi, 1984, 1985a; Bertrand and Heroux, 1987;
86 Bertrand, 1990; Link et al., 1990; Cardott and Kidwai, 1991; Hoffknecht, 1991; Goodarzi et al., 1992b;
87 Malinconico, 1992; Tricker et al., 1992; Malinconico, 1993; Wang et al., 1993; Cole, 1994; Gentzis et al.,
88 1996; Liu et al., 2001; Bertrand et al., 2003; Petersen et al., 2013; İnan et al., 2016; Lavoie et al., 2016; 89 Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Synnott et al., 2018; Wang et al., 2019a).
90 Graptolite reflectance is a useful proxy of thermal maturity (Goodarzi, 1984; Goodarzi and Norford,
91 1985), and has been used to assess eroded thicknesses when used in conjunction with reflectance of
92 bitumen and sedimentological and tectonic evidences (Goodarzi et al., 1992b; Gentzis et al., 1996). The
93 graptolite maximum reflectance (GRomax) or graptolite random reflectance (GRor) was adopted to estimate
94 thermal maturity in worldwide sediments (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi et al.,
95 1985; Goodarzi and Norford, 1989; Goodarzi et al., 1992b; Malinconico, 1992, 1993; Rantitsch, 1995;
96 Gentzis et al., 1996; Bertrand et al., 2003; Petersen et al., 2013; İnan et al., 2016; Lavoie et al., 2016; Luo
97 et al., 2016; Luo et al., 2017; Luo et al., 2018). Some researchers have established correlations among
98 reflectances of graptolite and other zooclasts, pyrobitumen and vitrinite (Bertrand and Heroux, 1987;
99 Bertrand, 1990; Bertrand, 1993; Yang and Hesse, 1993; Bertrand and Malo, 2001; Bertrand et al., 2003),
100 and Conodont Alteration Index (CAI; Goodarzi and Norford, 1985; Hoffknecht, 1991; Gentzis et al.,
101 1996). Moreover, the graptolite reflectance has also been successfully correlated with some inorganic
102 paleothermal indices, including the illite “crystallinity” index (e.g. Kemp et al., 1985; Oliver, 1988;
103 Hoffknecht, 1991; Rantitsch, 1995, 1997; Suchý et al., 2015), and metamorphic index minerals in pre-
104 greenschist meta-sedimentary rocks (e.g. Malinconico, 1992). Graptolite reflectance is commonly related
105 to the lithology, and it will be higher in shales than in limestones (Link et al., 1990). Redox conditions
106 may have an impact on graptolite reflectance, and higher reflectance was observed in graptolites deposited
107 under oxic environments in comparison with those from anoxic environments (Cole, 1994). Weathering
108 may affect graptolite reflectance (Goodarzi and Norford, 1985; Hoffknecht, 1991; Goodarzi et al., 1992a).
109 Raman spectroscopy of graptolites was widely studied and used to assess organic maturity (Suchý et al.,
110 2004; Liu et al., 2013; İnan et al., 2016; Mumm and İnan, 2016; Morga and Pawlyta, 2018; Wang et al.,
111 2019a). However, these studies were mostly focused on a small number of samples representing a very
112 limited range of thermal maturity. Thus, the physical and chemical characteristics of graptolites in wider
113 thermal maturity settings is reviewed herein for Lower Paleozoic sediments from a number of localities
114 based on the literature and new unpublished data is presented. In reviewing the optical characteristics,
115 chemical composition, and microstructure of the graptolites, their implications for the global petroleum
116 industry were also discussed.
117 2. The biological structure and composition of graptolites
118 Graptolites are extinct colonial planktonic hemichordate invertebrates that lived mainly in the early 119 Paleozoic ocean (Clarkson, 1981). Their structure has been widely studied and is illustrated in Fig. 1
120 (Moore, 1955; Clarkson, 1981; Crowther, 1981). The rhabdosome (colony) may have more than one
121 branch (stipe) (Fig. 1a). Along a stipe, thecae house the individual zooids (Fig. 1b, 1c), which were
122 connected by a stolon to other individuals through the common canal (Fig. 1c). The periderm material of
123 the rhabdosome is generally composed of two layers, fusellar tissue and cortical tissue (Fig. 1e)
124 (Crowther, 1981). The periderm with fusellar layers, thecae and common canal may be observed in
125 reflected light under the optical microscope as discussed below.
126 Initially, studies of the chemical structure of graptolites were related to the periderm based on the
127 textures and phylogeny, and these studies assumed that graptolite periderm is chitinous (Eisenack, 1932;
128 Kozlowski, 1949). Later, a collagen-like structure for the cortical and fusellar tissues was proposed for
129 graptolites (Fig. 1) (Towe and Urbanek, 1972; Crowther, 1981; Bates and Kirk, 1986; Urbanek and
130 Mierzejewski, 1986; Liu et al., 1996). Both collagen and chitin are supportive tissues, which mainly serve
131 to support cellular structures (Tasch, 1980). Collagen is a glycoprotein, and chitin with a molecular
132 formula of C32H54O21N4 is a nitrogenous carbohydrate forming a N-acetyl glucosamine groups polymer
133 (Leninger, 1975; Bustin et al., 1989). More recent studies indicate, however, that the residual graptolite
134 periderms are composed of complex aliphatic polymers that are immune to base hydrolysis rather than
135 collagen (e.g. Briggs et al., 1995; Gupta et al., 2006). Aliphatic components of graptolite periderms are
136 probably derived from the graptolite itself via in situ polymerization (e.g. Gupta et al., 2006).
137 138
139 Fig. 1. The biological structure of graptolites showing their theca, aperture, common canal, virgella and periderm (Modified after Moore,
140 1955; Clarkson, 1981). A) Rhabdosome (colony) with two stipes (branches); B) close-up of rhabdosome structures; C) Close-up of two
141 thecae which house individual zooids; D) transverse cross-section of rhabdosome; E) periderm structure.
142
143 3. Sampling and methods 144 The graptolite-bearing sediments examined for this study included the Wufeng–Longmaxi
145 Formations, Pingliang Formation, China (Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a); Liteň
146 Formation from Czech (Suchý et al., 2002); and Alum Shales from Estonia and Sweden (Petersen et al.,
147 2013; Sanei et al., 2014; Luo et al., 2018). Detailed geological information can be found in these above
148 references. Two graptolite-bearing Wufeng–Longmaxi samples (sample ID: CKMB-Y1 (GRor = 1.32%)
149 and CKMB-Y2 CKMB-Y1 (GRor = 1.28%)) (Wang et al., 2019a) and an upper Carboniferous coal (VRor
150 = 1.07%) were selected to conduct artificial maturation at 350℃, 400℃, 450℃ and 500℃ for three days,
151 respectively. The natural and heated samples were cut parallel or perpendicular to bedding in order to
152 make polished sections on EcoMet 250 with AutoMet 250 for the maceral observation and reflectance
153 measurement. Organic petrography and reflectance were done on a Leica 4500P microscope with 154 CRAIC/MPS 200 microscope photometer. The maximum-minimum and random reflectances were
155 measured under polarized and non-polarized light, respectively, as described in Luo et al. (2016); Luo et
156 al. (2017); Luo et al. (2018).
157 For scanning electron microscopy (SEM), sediment was cut, polished using emery paper, and milled
158 using Ar ion milling. For study of pores, graptolites were found and then marked using a diamond lens
159 under an optical microscope. The SEM study was on a Zeiss Crossbeam 540 Focused Ion Beam-SEM
160 (FIB-SEM).
161 4. Optical characteristics of graptolite periderm
162 Kurylowicz et al. (1976) reported the reflectance characteristics of graptolites and proposed that their
163 optical properties are similar to vitrinite. Clausen and Teichmüller (1982) described the morphology and
164 reflectance of graptolites from Germany and Sweden, with maximum reflectances ranging from 0.8% to
165 10.0%. In 1980–1990s, Goodarzi and colleagues extensively studied the optical properties of Middle
166 Ordovician to Upper Silurian graptolites from Poland, Sweden, Turkey and Canada (Goodarzi, 1984,
167 1985a; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1987; Bustin et al.,
168 1989; Goodarzi and Norford, 1989; Riediger et al., 1989; Link et al., 1990; Goodarzi et al., 1992a;
169 Goodarzi et al., 1992b; Gentzis et al., 1996), which advanced the understanding of the petrographic
170 characteristics of graptolites. In recent years, graptolite-bearing sediments have become significant targets
171 of shale gas exploration, and a resurgence of work has been conducted on the organic petrology of
172 graptolites (Petersen et al., 2013; Sanei et al., 2014; Haeri-Ardakani et al., 2015; Caricchi et al., 2016;
173 İnan et al., 2016; Lavoie et al., 2016; Luo et al., 2016; Ma et al., 2016; Mumm and İnan, 2016; Cheshire
174 et al., 2017; Luo et al., 2017; Cardott and Curtis, 2018; Luo et al., 2018; Morga and Kamińska, 2018;
175 Morga and Pawlyta, 2018; Reyes et al., 2018; Synnott et al., 2018; Wang et al., 2019a).
176 Microscopically, graptolites can be identified by their morphology, e.g., fusellar layer, granularity,
177 other visible structures as well as anisotropy (Kurylowicz et al., 1976; Teichmüller, 1978; Clausen and
178 Teichmüller, 1982; Goodarzi, 1984, 1985a; Bertrand and Heroux, 1987; Goodarzi and Norford, 1987,
179 1989; Bustin et al., 1989; Riediger et al., 1989; Bertrand, 1990; Petersen et al., 2013; Luo et al., 2016;
180 Luo et al., 2017; Luo et al., 2018). Two textures are apparent in graptolites, i.e., non-granular (NGG) and
181 granular (GG), in Paleozoic sediments worldwide, e.g., Poland, Sweden, Turkey and Canada (Goodarzi,
182 1984, 1985a; Goodarzi and Norford, 1985; Goodarzi et al., 1985; Goodarzi and Norford, 1987, 1989;
183 Goodarzi et al., 1992a; Goodarzi et al., 1992b), the Alum Shale from Europe (Petersen et al., 2013; Sanei 184 et al., 2014; Luo et al., 2018), and the Wufeng–Longmaxi Formations and Pingliang Formation from
185 China (Fig. 2) (Wang et al., 1993; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a).
186 In general, GG and NGG are found in carbonates and mudstones, respectively (Goodarzi and Norford,
187 1985; Goodarzi and Norford, 1987; Petersen et al., 2013). NGG and GG were thought to be derived from
188 different sections of the rhabdosome. While the former may be part of the wall with fusellar layers, the
189 latter may be derived from the common canal (Goodarzi, 1984). However, according to recent artificial
190 maturation studies on Estonian Alum Shale with GG, this texture is completely altered to NGG when
191 GRor reached up to 1.24% after heating for 3 days at 400 ℃. This indicates that pristine GG may be altered
192 to NGG due to thermal stress (Luo et al., 2018).
193 GG has weaker anisotropy and lower reflectance than NGG (Figs. 2b and 3) (Goodarzi, 1984;
194 Goodarzi and Norford, 1987; Petersen et al., 2013; Luo et al., 2016; Luo et al., 2017; Luo et al., 2018).
195 Suchý et al. (2002) found that NGG random reflectance has a positive correlation with GG random
196 reflectance in Lower Silurian sediments from the Barrandian Basin, Czech Republic. In rotation of the
197 objective stage under polarized light, NGG will display extinction twice, and thus two GRomax and two
198 GRomin can be measured (Figs. 3a, b and 4) (Luo et al., 2016; Luo et al., 2018), however, GG does not
199 show such clear patterns (Fig. 3c and d). 200
201 Fig. 2 Granular-textured graptolites (GG) and non-granular-textured graptolites (NGG) from Chinese and Estonian sediments. (a) GG in
202 low-maturity Alum Shale from Estonia (GRor = 0.65%), non-polarized light; (b) GG and NGG in high-maturity shale from Chongqing,
203 China (GRomax = 2.57%), non-polarized light (Luo et al., 2018); (c) grey black NGG in low-maturity shale from Gansu, China (GRor =
204 0.48%), non-polarized light; (d) NGG in overmature shale from Chongqing, China (GRor = 3.85%), showing strong anisotropy, polarized
205 light; (e) NGG in overmature shale from Chongqing, China (GRor = 3.77%), showing strong anisotropy, polarized light; (f) NGG in
206 overmature shale from Chongqing, China (GRomax = 4.47%), showing fusellar layers, polarized light (modified from Luo et al., 2018).
207 (a-e) sections perpendicular to bedding; (f) sections parallel to bedding.
208 5 6 (a) (b) 5 4
4
3 (%)
(%) 3
o o R R 2 2
1 1
0 0 0 40 80 120 160 200 240 280 320 360 0 40 80 120 160 200 240 280 320 360 209 Degree Degree
2.5 2.2 (c) (d)
2.1
2.2 (%)
(%) 2
o o
R R 1.9 1.9
1.6 1.8 0 40 80 120 160 200 240 280 320 360 0 40 80 120 160 200 240 280 320 360 210 Degree Degree
211 Fig. 3. Reflectance measured on NGG (a and b) and GG (c and d) under polarized light in a circular rotation of the microscope stage.
212
213 Fig. 4. Photomicrographs displaying extinction characteristics of the same NGG from the Wufeng–Longmaxi shales in sections
214 perpendicular to bedding with 360° rotation of the microscope stage, oil immersion, polarized light. The angle between each picture is 215 90 degrees. The graptolite reflectance (%) is given beside the measurement site (red square).
216
217 In general, NGG is more frequently found than GG in most Paleozoic sediments (Goodarzi and
218 Norford, 1987; Petersen et al., 2013; Luo et al., 2016), thus, only NGG will be discussed in the main
219 discussion of this paper. In polished blocks, low-maturity graptolites are often similar to natural bitumen
220 stringers, but they are more angular than bitumen and often show typical graptolitic features (Figs. 2, 4
221 and 5), and they display a plate-like appearance at high maturity (Fig. 2). NGG is blocky in sections
222 parallel to bedding, but may display a long stipe-like morphology in sections normal to bedding (Figs. 2
223 and 5) (Link et al., 1990). The fusellar layers are observed in sections parallel to bedding (Fig. 2). NGG
224 was divided into two classes, lath- and blocky-shaped (Riediger et al., 1989). Blocky-shaped graptolites
225 show lower maximum reflectance and bireflectance (BRo) than lath-shaped graptolites. Some NGG of the
226 Liteň Formation from Czech Republic displays weak brown fluorescence (Fig. 5), and Hoffknecht (1991)
227 and Wang et al. (2019a) also found similar phenomena in low-maturity graptolites. 228
229 Fig. 5. NGG from the Czech Republic (Liteň Formation, Barrandian area) with weak brown fluorescence of the Liteň Formation in
230 sections perpendicular to bedding. (a, c, e) under reflected light; (b, d, f) the same field as (a, c, e), under fluorescence light. (a-b) GRor =
231 0.85%; (c-f) GRor = 0.83%.
232 233 5. Physical and chemical properties of graptolites as thermal maturity indicators 234 5.1 Chemical structure changes of graptolites with maturation 235 OM is sensitive to pressure and temperature (Khavari-Khorasani, 1975; Teichmüller, 1982; Goodarzi
236 and Norford, 1985). Optical properties of OM include reflectances, refractive and absorptive index (van
237 Krevelen, 1961). Reflectance of OM is related to the aromatic structure of organic molecules; reflectance 238 and aromaticity increase continuously with increase in maturity. The trends of optical properties of OM
239 over the visible spectrum (400–700nm) are used to determine molecular structural changes occurring
240 during maturation (Khavari-Khorasani, 1975; Goodarzi, 1985a; Goodarzi and Macqueen, 1990).
241 Structurally, OM is composed of ordered and amorphous carbons (Cartz and Hirsch, 1960; van
242 Krevelen, 1961; Goodarzi, 1985a). The ratio of ordered (aromatic) carbons to amorphous (alicyclic side
243 chains, aliphatic) carbons increases with maturation/heat treatment (Cartz and Hirsch, 1960; van Krevelen,
244 1961) in two phases:
245 ① Devolatilization of amorphous carbon increase in aromaticity and size of the polynuclear systems
246 during early stage of maturation and low temperature heat treatment;
247 ② Transformation of 2D (turbostratic) to 3D (graphitic) ordering, which occurs at high maturity
248 and at temperature >600 ℃, results in increase and better ordering of the aromatic structure (Cartz
249 and Hirsch, 1960; van Krevelen, 1961; Goodarzi and Murchison, 1972; Goodarzi, 1985a).
250 Dispersion of optical properties of graptolite periderm is similar to that of bitumen and vitrinite in
251 the visible spectrum and three spectral patterns are documented:
252 ① Low-maturity (CAI = 1) graptolite behaves similarly to low rank vitrinite and bitumen with
253 curves of all optical parameters decreasing with increasing wavelength from blue to red (Figs. 6 and
254 7), indicating low content of aromatic carbon (Marshall and Murchison, 1971; Khavari-Khorasani,
255 1975);
256 ② Moderately-mature (CAI = 4) graptolite behaves similarly to semi-anthracite and impsonite
257 bitumen. The optical parameters are nearly flat, from blue to red (Figs. 6 and 7), indicating gradual
258 molecular variations, e.g., greater condensed aromatic carbon (Marshall and Murchison, 1971;
259 Khavari-Khorasani, 1975);
260 ③ Highly mature graptolite (CAI = 3.5–5) behaves similarly to anthracite and pyrobitumens. The
261 trends of optical indices rise continuously from blue to red (Figs. 6 and 7), similar to the pattern from
262 a highly aromatic, condensed molecular structure (Marshall and Murchison, 1971; Cook et al., 1972;
263 Khavari-Khorasani, 1975; Goodarzi, 1985a). Graptolites at this stage develop macro-properties
264 similar to graphite (Teichmüller et al., 1979). 265
266 Fig. 6. Dispersion of maximum reflectance (a) in air (%Ramax) and (b) in oil (%Romax) of graptolite fragments with increased maturity
267 based on their CAI (conodont alteration index), shown as number beside each spectrum for graptolite in limestone (L), argillaceous
268 limestone (A) and shale (S) matrices (after Goodarzi, 1985a).
269
270 Fig. 7. Dispersion of refractive (a) and (b) absorptive indices of graptolite fragments with increased maturity based on their CAI, shown
271 as number beside each spectrum for graptolites in limestone (L), argillaceous limestone (A) and shale (S) matrices (after Goodarzi,
272 1985a). 273
274 5.2 Graptolite reflectance 275 5.2.1 Graptolite maximum, minimum and random reflectance
276 Similar to vitrinite (Davis, 1978), GRomax and graptolite minimum reflectance (GRomin) are observed
277 in sections parallel and normal to bedding, respectively (Goodarzi, 1984; Link et al., 1990; Goodarzi et
278 al., 1992a). However, in some samples, GRomax in sections parallel to bedding is lower than that in sections
279 normal to bedding (Table 1 and Fig. 8), which may be because the graptolites in these samples are not
280 truly parallel to bedding or normal to bedding due to an unusual burial process and/or sample preparation
281 (Malinconico, 1992, 1993; Luo et al., 2017). In sections parallel to bedding, GRomax positively correlates
282 with GRomin (Fig. 9), whereas in sections perpendicular to bedding, such relationship cannot be observed
283 (Hoffknecht, 1991; Luo et al., 2017). This is because GRomin in sections parallel to bedding is generally
284 equal to the graptolite intermediate reflectance (GRoint) (Luo et al., 2017).
285
286 Table 1. Comparison of graptolites reflectance (including maximum, minimum and bireflectance) in sections parallel and normal to
287 bedding. Part of the data for the Wufeng–Longmaxi graptolites are derived from Luo et al. (2017), and the data of the Upper Silurian
288 graptolites of Turkey are from Goodarzi (1984).
Sections parallel to bedding Sections normal to bedding Depth Sample no. Location Formation Lithology GRomax GRomin BRo GRomax GRomin BRo (m) (%) (%) (%) (%) (%) (%)
DTB-6-9 Datianba, China Outcrop Shale 3.41 1.92 1.49 3.68 0.99 2.69 Wufeng- JX-4 Jinxi, China Outcrop Shale 5.48 2.61 2.87 5.61 0.64 4.98 Longmaxi QQ1-3 Qianjiang, China 794.00 Shale 4.76 3.14 1.62 5.64 0.75 4.90 Upper No. 2 Tufanbeyli, Turkey Outcrop Shale 4.61 4.17 0.44 4.42 1.08 3.34 Silurian
289
290 Fig. 10 displays a comparison of GRor in sections parallel and perpendicular to bedding. In contrast
291 to GRomax, GRor in sections perpendicular to bedding, especially in samples with higher thermal maturity,
292 is much lower than that in sections parallel to bedding, similar to the results in the Qusaiba Hot Shales
293 from Saudi Arabia (İnan et al., 2016), and is much more concentrated than that in sections parallel to
294 bedding as indicated by their lower SD values (Fig. 10). In general, unimodal histograms of GRomax and
295 GRor were found in investigated samples, especially in sections perpendicular to bedding (Figs. 8 and 10)
296 (Cole, 1994; Haeri-Ardakani et al., 2015; Lavoie et al., 2016; Luo et al., 2018; Reyes et al., 2018). In 297 addition, NGG reflectance in carbonate rocks is lower than that of shales at similar depths, which may be
298 due to oxidation caused by carbonate dissolution (Goodarzi and Norford, 1985; Link et al., 1990). This is
299 similar to that of VRor in different lithologies (Goodarzi et al., 1988; Goodarzi et al., 1993).
300
301
302 Fig. 8. Comparison of GRomax of the Wufeng–Longmaxi sediments in (a, c and e) sections parallel to bedding and (b, d and f) sections
303 perpendicular to bedding from Chongqing.
304 305
306 Fig. 9. The linear coalification trend for graptolites based on the plot of GRomax VS. GRomin (after Goodarzi, 1984).
307
308
309 Fig. 10. Comparison of GRor of the Wufeng–Longmaxi sediments in (a, c, e) sections parallel to bedding and (b, d, f) sections
310 perpendicular to bedding from Chongqing (cited from Luo et al., 2019). 311
312 From the methodological point of view, the main difference between the measurement technique of
313 vitrinite and graptolite reflectance has to do with the sample preparation. Graptolite maturity
314 determination was generally on the basis of whole-rock polished blocks in sections parallel to bedding
315 and using of GRomax (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987, 1989;
316 Gentzis et al., 1996). GRomax will increase with burial depth and is a useful tool to unravel the thermal
317 maturity of Lower Paleozoic deposits (Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a; Goodarzi
318 and Norford, 1987, 1989; Riediger et al., 1989; Link et al., 1990; Gentzis et al., 1996; Luo et al., 2016;
319 Luo et al., 2017). However, other researchers have used GRor to assess maturity levels (Bertrand and
320 Heroux, 1987; Bertrand, 1990; Bertrand, 1993; Cole, 1994; Bertrand and Malo, 2001; Bertrand et al.,
321 2003; Petersen et al., 2013; Sanei et al., 2014; İnan et al., 2016; Yang, 2016; Luo et al., 2018; Reyes et al.,
322 2018; Synnott et al., 2018; Wang et al., 2019a).
323 Based on data from published literature, GRomax shows a strong positive correlation with GRor in
324 sections perpendicular to bedding, and the former is around twice as large as the latter (Fig. 11). The
325 comparison of SD for GRomax and GRor is shown in Fig. 12. In general, SD is greater for GRomax than that
326 of GRor. Therefore, for routine maturation study, measurements of GRor is as accurate and as useful as
327 those of GRomax. 328
6 Wang et al., 2019a y = 0.52x + 0.43 Luo et al., 2018 r = 0.95 Malinconico, 1993 4 Link et al., 1990
2 Mean random Mean random reflectance graptoliteof(%)
0 0 3 6 9 Mean maximum reflectance of graptolite (%) 329
330 Fig. 11. Mean GRomax vs. mean GRor in sections perpendicular to bedding (Link et al., 1990; Malinconico, 1993; Luo et al., 2018; Wang
331 et al., 2019a).
332 0.9 0.9 (a) Malinconico, 1992 (b) Luo et al., 2018 Luo et al., 2018 Petersen et al., 2013 Wang et al., 2019a Bertrand et al., 2003 Lavoie et al., 2016 0.6 0.6 Williams et al. 1998 Wang et al., 2019a y = 0.03x + 0.07
r = 0.51
SD SD (%) SD SD (%)
0.3 0.3
y = 0.07x + 0.07 r = 0.75 0 0 0 3 6 9 0 2 4 6 333 Mean maximum reflectance of graptolite (%) Mean random reflectance of graptolite (%)
334 Fig. 12. The relationship between (a) mean GRomax in sections perpendicular to bedding, (b) mean GRor and SD (Malinconico, 1992;
335 Williams et al., 1998; Bertrand et al., 2003; Petersen et al., 2013; Lavoie et al., 2016; Luo et al., 2018; Wang et al., 2019a).
336
337 5.2.2 Graptolite bireflectance (BRo)
338 BRo in sections parallel to bedding is much lower than that in sections perpendicular to bedding,
339 especially in overmature sediments (Table 1) (Goodarzi, 1984). The anisotropy of graptolites will increase
340 with increasing thermal maturity as suggested by positive correlation between GRomax and BRo (Luo et
341 al., 2017). Vitrinite displays lower bireflectance than that of graptolite at similar maturation levels because
342 of the biaxial negative property of graptolites (Bustin et al., 1989; Hoffknecht, 1991; Luo et al., 2017).
343 5.2.3 The coalification path of the graptolites
344 The plot of GRomax vs. GRomin can be used to assess the coalification path of the graptolites (Fig. 9).
345 However, it should be noted that GRomin in the Fig. 9 was achieved in section parallel to bedding, and is
346 nearly equal to GRoint (Goodarzi, 1984; Goodarzi and Norford, 1985; Goodarzi and Norford, 1987;
347 Goodarzi et al., 1992a; Malinconico, 1993; Luo et al., 2017; Luo et al., 2018). When all the graptolite
348 reflectance data from the published literature are plotted in Fig. 9, a linear coalification trend can be
349 determined for the graptolites measured parallel to bedding (Fig. 9) (Goodarzi, 1984; Goodarzi and
350 Norford, 1985; Goodarzi and Norford, 1987; Goodarzi et al., 1992a; Malinconico, 1993; Luo et al., 2017;
351 Luo et al., 2018).
352 5.2.4 The relationship between graptolite reflectance and other paleothermal indices
353 5.2.4.1 Comparison with other organoclasts reflectance
354 Similar to vitrinite and solid bitumen (see also Ferreiro Mählmann and Le Bayon, 2016 for a recent 355 review on vitrinite and solid bitumen thermal maturity studies), graptolite reflectance displays an
356 increasing trend with the increasing burial depth, and can determine organic maturity in Lower Paleozoic
357 sediments without vitrinite, although no consensus of the exact relationship between these indices has yet
358 been reached (Table 2 and Fig. 13) (Clausen and Teichmüller, 1982; Goodarzi, 1984, 1985a, b; Bertrand
359 and Heroux, 1987; Goodarzi and Norford, 1987; Teichmüller, 1987; Bustin et al., 1989; Goodarzi and
360 Norford, 1989; Bertrand, 1990; Goodarzi et al., 1992a; Bertrand, 1993; Cole, 1994; Gentzis et al., 1996;
361 Bertrand and Malo, 2001; Bertrand et al., 2003; Petersen et al., 2013; Luo et al., 2018; Synnott et al.,
362 2018). Goodarzi (1985a) found that the optical dispersion of graptolites is similar to vitrinite and bitumen,
363 indicating that their physical and chemical nature is sensitive to temperature and can be used as a proxy
364 for organic maturity in Lower Paleozoic deposits. The graptolites show higher reflectance and stronger
365 anisotropy than that of solid bitumen, chitinozoans and scolecodonts (Goodarzi, 1984, 1985b; Goodarzi
366 and Norford, 1987, Bertrand et al., 2003; Petersen et al., 2013). According to Yang and Hesse (1993), the
367 bireflectance of graptolites displays a similar trend to that of pyrobitumen when Ro < 4.5–5.0 %. Other
368 researchers suggested that graptolite reflectance is similar to that of chitinozoan for both natural and
369 artificially matured samples (Bertrand and Heroux, 1987; Bertrand, 1990; Cole, 1994; Reyes et al., 2018).
370 Zooclast reflectance (including chitinozoan, graptolite, and scolecodonts) was also compared with
371 vitrinite and solid bitumen reflectances in Lower Paleozoic deposits from Canada (Bertrand and Heroux,
372 1987; Bertrand, 1990; Bertrand, 1993; Bertrand and Malo, 2001; Bertrand et al., 2003). In these Canadian
373 studies, graptolite reflectance was slightly lower than, or similar to, vitrinite reflectance (Bertrand, 1990)
374 (Table 2 and Fig. 13). In a similar way, Yang and Hesse (1993) established that GRor is lower than VRor
375 when GRor < 2.6%, but graptolite has higher reflectance than vitrinite when GRor > 2.6%. The Silurian
376 Qusaiba Shale from Saudi Arabia deposited under anoxic environments has lower graptolite random
377 reflectance than that deposited in oxic environments (Cole, 1994). Zhong and Qin (1995) proposed a
378 formula between EqVRo and GRor based on vitrinite-like particle reflectance (Rvl) and GRor in the
379 sediments from Tarim Basin and South China. In recent years, various new equations have been proposed
380 to illustrate the relationship between EqVRo and GRor or GRomax (Petersen et al., 2013; Colţoi et
381 al., 2016; Synnott et al., 2018; Luo et al., 2018; Wang et al., 2019a) (Table 2 and Fig. 13).
382 The equations proposed by Cole (1994) (anoxic); Petersen el al., (2013) and Coltoi et al., (2016) are very
383 close to each other; whereas the formulas presented by Bertrand (1990) and Luo et al. (2018) are
384 essentially coincident and close to the 1:1 line (Fig. 13). 385 Table 2. Various equations describing the relationship between EqVRo and GRor or GRomax.
GRor data Equations Source Formation range
Log10 GRor = -0.04+1.10×Log10 EqVRo (Bertrand, 1990) 0.5–3.0% Upper Gaspe Limestone Group and Chaleurs Group, Canada
EqVRo = 0.8 ×GRor (anoxic) (Cole, 1994) 0.62–2.05% Silurian Qusaiba Shale, Saudi Arabia
EqVRo = 0.65 ×GRor (oxic)
EqVRo = 0.882 ×GRor－0.366 (Zhong and Qin, 1.8–4.9% Cambrian-Ordovician, China 1995)
EqVRo = 0.73 ×GRor + 0.16 (Petersen et al., 0.47–2.14% Alum Shale, Scandinavia 2013)
EqVRo = 0.785×GRor + 0.05 (Colţoi et al., 2016) No data Silurian intervals, Romania
EqVRo = 0.232+ 0.499×GRor (Synnott et al., 0.59–1.02% Cape Phillips Formation, Canada 2018)
EqVRo = 1.055×GRor－0.053 (Luo et al., 2018) 0.65–4.03% Wufeng–Longmaxi Formation
EqVRo = 0.546×GRomax +0.35
EqVRo = 0.97×GRor－0.2 (2.19% < (Wang et al., 2019a) 1.21–4.91% Wufeng–Longmaxi Formation
GRor < 3.5%);
EqVRo = 0.22×GRor +2.55(GRor > 3.5%)
EqVRo = 0.99×GRor + 0.08; This study
EqVRo = 0.515×GRomax + 0.506
386
6 Bertrand (1990)
Cole (1994) (anoxic)
Cole (1994) (oxic)
Zhong and Qin (1995)
4 Petersen et al. (2013)
Colţoi et al. (2016)
(%) Synnott et al. (2018) o
Luo et al. (2018) (GRo) EqvR Luo et al. (2018) (GRomax) 2 Wang et al. (2019a) (2.19% < GRor < 3.5%)
Wang et al. (2019a) (3.5% < GRor < 4.91%)
This study (GRo)
This study (GRomax)
1:1 line 0 0 2 4 6 387 GRo (%) (except where noted)
388 Fig. 13. The various relationships between GRo or GRomax and EqvRo.
389 390 In China, graptolites were usually interpreted as solid bitumen or vitrinite-like particles in the
391 Wufeng–Longmaxi Formations, due to the misidentification on OM, leading to a long-term debate on
392 their thermal maturity. Luo et al. (2018) have described how to discriminate between graptolite and
393 bitumen in those formations. NGG displays a smoother surface, stronger anisotropy, and higher random
394 and maximum reflectance than solid bitumen based on observation and measurement under polarized and
395 non-polarized light (Luo et al., 2018; Wang et al., 2019a). In the Wufeng–Longmaxi Formations, the solid
396 bitumen random reflectance (SBor) has strong positive correlations with GRor and GRomax in sections
397 perpendicular to bedding (Fig. 14a and b) (Luo et al., 2018; Wang et al., 2019a). Data from other
398 graptolite-bearing sediments (Yang and Hesse, 1993; Bertrand et al., 2003; Yang, 2016; Reyes et al., 2018)
399 are plotted with the Wufeng–Longmaxi data in Fig. 14c and d, which indicates that SBor and GRor are
400 nearly equivalent, very different from their relationship in Fig. 14a. This is because the reflectances from
401 Yang and Hesse (1993), Bertrand et al. (2003), Yang, (2016), and Reyes et al. (2018) were measured in
402 random orientation whereas the data in the two Wufeng-Longmaxi studies were measured on sections
403 perpendicular to bedding (Luo et al., 2018; Wang et al., 2019a).
9 12 (a) (b) Wang et al., 2019a
Luo et al., 2018 y = 2.12x - 0.57 r = 0.92 6 8
3 4
y = 1.19x - 0.09 Wang et al., 2019a
r = 0.95 (%) graptolite reflectance of maximum Mean
Mean random Mean random reflectance graptoliteof(%) Luo et al., 2018
0 0 0 2 4 6 0 2 4 6 Mean random reflectance of solid bitumen (%) 404 Mean random reflectance of solid bitumen (%) 9 9 (c) Wang et al., 2019a (d) Reyes et al., 2018 Luo et al., 2018 Yang et al., 2016 Reyes et al., 2018 outlier outlier Bertrand et al., 2003 Yang et al., 2016 Yang and Hesse, 1993 6 Bertrand et al., 2003 6 Yang and Hesse, 1993
3 3
y = 1.05x + 0.20 y = 0.98x + 0.32
r = 0.94 Mean random Mean random reflectance graptoliteof(%) Mean random Mean random reflectance graptoliteof(%) r = 0.95
0 0 0 2 4 6 0 2 4 6 Mean random reflectance of solid bitumen (%) Mean random reflectance of solid bitumen (%) 405
406 Fig. 14. The relationship between mean SBor and (a, c and d) mean GRor, and (b) mean GRomax (Yang and Hesse, 1993; Bertrand et al.,
407 2003; Yang, 2016; Luo et al., 2018; Reyes et al., 2018; Wang et al., 2019a).
408
409 5.2.4.2 Comparison with CAI
410 Some studies have correlated the relationship between GRomax and VRor through CAI. When
411 sediments have GRomax of 0.6–1.2%, it means that they are immature (~0.2–0.5% VRor, CAI 1.5); when
412 GRomax is 1.2–2.2%, sediments are in the oil window (~0.5–1.30% VRor, CAI: 1.5–2.5) (Fig. 15)
413 (Goodarzi and Norford, 1989; Goodarzi et al., 1992a; Gentzis et al., 1996). However, these results should
414 be used very carefully, because a CAI generally implies a range of thermal maturity (Epstein et al., 1977).
415 Graptolite and vitrinite reflectances are more sensitive to thermal maturity than CAI, especially in
416 overmature sediments (Goodarzi and Norford, 1985). While graptolite reflectance displays an excellent
417 correlation with CAI at low thermal maturity (CAI: 1–4), their correlation is not clear in sediments with
418 higher CAI (Goodarzi and Norford, 1985; Goodarzi et al., 1992a). 419
420 Fig. 15. The relationship between CAI and GRomax (Goodarzi and Norford, 1989; Goodarzi et al., 1992a; Gentzis et al., 1996).
421
422 5.2.4.3 Comparison with Tmax
423 Graptolite reflectance has also been compared to Tmax for the same samples (Cole, 1994; Petersen et
424 al., 2013; İnan et al., 2016; Synnott et al., 2018). Cole (1994) studied organic maturity of Silurian Qusaiba
425 deposits, Saudi Arabia, using zooclasts reflectance and conventional maturity indicators (e.g., Tmax), and
426 proposed that GRor will increase with the increasing oxygen contents of the depositional environments.
427 He found that EqVRo of the sediments deposited under anoxic environments is equal to 80% of GRor,
428 whereas the EqVRo of the sediments deposited under oxic environments is equal to 65% of GRor (Table
429 2) (Cole, 1994). Petersen et al. (2013) have determined the relationship between GRor and EqVRo on the
430 basis of Tmax, which can be expressed as: EqVRo = 0.73×GRor + 0.16 (Table 2). It should be noted that
431 GRor displays a bimodal distribution due to the random orientation of the polished blocks in their study.
432 İnan et al. (2016) determined the thermal maturity of the hot shales from Saudi Arabia based on graptolite
433 reflectance, organic geochemical and spectroscopic methods. Other studies on Tmax and graptolite
434 reflectance are conflicting and inconclusive (Table 2) (e.g., Petersen et al., 2013; Synnott et al., 2018).
435 The difficulty in using Tmax is mostly due to unreliable values of Tmax, the influence of kerogen type, and
436 low S2 values (e.g., Petersen et al., 2013; Synnott et al., 2018). 437 5.2.4.4 Comparison of artificial thermal-treatment of graptolites and coals/vitrinite
438 Laboratory experiments where graptolites and vitrinite with similar maturity were heat treated (220–
439 600 ℃) under similar conditions have shown that graptolites develop high reflectance at 600 ℃ (Bustin
440 et al., 1989). Reyes et al. (2018) used hydrous pyrolysis on immature graptolite-bearing sediments
441 (GRor=0.55%) of the Boas River Formation treated to temperatures of 310 to 350℃, and proposed the
442 relationship between GRor and Rvl as the following equation: Rvl = 0.79×GRor. In contrast, Luo et al. (2018)
443 heated graptolite-bearing sediment, with wider maturity (GRor = 0.65 to 4.03%), through a temperature
444 range of 350 ℃ and 550 ℃ and found that their reflectance is similar to that of the vitrinite.
445 Comparison of all published data and data from this study on heat–treated and natural graptolites
446 and vitrinite (or vitrinite-like particles) (Bustin et al., 1989; Bertrand, 1990; Luo et al., 2018; Reyes et al.,
447 2018), regardless of the difference of the experimental methods, indicates a strong and positive
448 relationship (Fig. 16). This relationship can be expressed as:
449 EqVRo = 0.99×GRor + 0.08 (1)
450 This equation is essentially coincident with Bertrand (1990) and Luo et al. (2018), and close to the
451 1:1 line (Fig. 9).
452 Equation (1) combined with Fig. 11 can correlate GRomax to that of EqVRo and it can be expressed
453 as:
454 EqVRo = 0.515×GRomax + 0.506 (2)
455
6
y = 0.99x + 0.08 r = 0.99
4
Luo et al., 2018 2 Bertrand, 1990
Vitrinite random Vitrinite random reflectance (%) Bustin et al., 1989 Reyes et al., 2018 This study 0 0 2 4 6 Graptolite random reflectance (%) 456
457 Fig. 16. The relationship between VRor and GRor. The Rvl (reflectance of vitrinite-like particles) values adopted from Reyes et al. (2018) 458 were converted to EqVRo according to the equation proposed by Xiao et al. (2000).
459
460 5.3. The chemical composition of graptolite periderm 461 The electron microprobe, FTIR and Raman spectrum have been widely used to study the chemical
462 composition of various organic macerals including graptolite fragments (Zerda et al., 1981; Green et al.,
463 1983; Jehlička and Bény, 1992; Wopenka and Pasteris, 1993; Bustin et al., 1993; Mastalerz and Bustin,
464 1993b, a, 1995, 1996, 1997; Ward and Gurba, 1999; Kelemen and Fang, 2001; Ward et al., 2005, 2007,
465 2008; Chen et al., 2012; Chen et al., 2014; Wang et al., 2014; Wilkins et al., 2014; Wilkins et al., 2015;
466 Chen et al., 2015b; Lünsdorf, 2016; Cheshire et al., 2017; Henry et al., 2018).
467 5.3.1 Electron microprobe
468 Recently, the electron microprobe has been used to identify elemental compositions of graptolites in
469 the Silurian Llandovery–Ludlow shales from northern Poland (Morga and Kamińska, 2018). GRor of these
470 shales ranges between 1.30% and 1.83%. Carbon is the predominant element in graptolite periderm;
471 carbon, oxygen, nitrogen and sulfur contents range from 84.92 to 91.45 %, 2.56 to 8.43 %, 1.43 to 2.89 %
472 and 0.02 to 0.90 %, respectively (Morga and Kamińska, 2018). In order to compare the elemental
473 composition of graptolite and vitrinite (or telocollinite), data for vitrinite were collected from the literature
474 (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). O content negatively correlates with C content
475 (Fig. 17a), similar to vitrinite (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008). For elemental
476 comparison with reflectance in Fig. 17, the vitrinite maximum reflectance reported by Ward et al. (2005,
477 2008) was converted to VRor based on the equation proposed by Komorek and Morga (2002), and GRor
478 in Morga and Kamińska (2018) was calculated to EqVRo based on equation (1). C and O content of
479 graptolites is similar to that of vitrinite with similar thermal maturity (Fig. 17b and c) (Mastalerz and
480 Bustin, 1997; Ward et al., 2005, 2008). Similar to the vitrinite, C content of graptolites in shales from
481 Poland increases as organic maturity increases, while O content decreases as organic maturity increases
482 (Fig. 17b and c). The O/C ratio in the graptolites displays a negative correlation with EqVRo (Morga and
483 Kamińska, 2018), comparable to that of vitrinite (Mastalerz and Bustin, 1997; Ward et al., 2005, 2008)
484 (Fig. 17d). These variations of the elemental composition in graptolites are consistent with the maturation
485 process and/or the loss of the O-bearing functional groups from organic macromolecules during thermal
486 evolution (Taylor et al., 1998; Bustin and Guo, 1999). 30 4 (a) Vitrinite (Ward et al., 2008) (b) Vitrinite (Ward et al., 2005) Vitrinite (Mastalerz and Bustin, 1997) Graptolite (Morga and Kamińska, 2018) 3
20 (%)
o 2
O O (%) EqVR 10 1
0 0 60 70 80 90 100 60 70 80 90 100 487 C (%) C (%)
4 4 (c) (d)
3 3
(%) (%) o
o 2 2
EqVR EqVR
1 1
0 0 0 10 20 30 0 0.1 0.2 0.3 0.4 488 O (%) O/C (%)
489 Fig. 17. Comparison of the chemical composition and reflectance of graptolites and vitrinite. The data for the graptolites are from Morga
490 and Kamińska (2018); data on vitrinite from Mastalerz and Bustin (1997) and Ward et al. (2005, 2008). (a) The negative correlation
491 between O and C; (b) the positive correlation between EqVRo and C; (c) the negative correlation between EqVRo and O; (d) the negative
492 correlation between EqVRo and O/C. The legends in b–d are the same as in a.
493
494 5.3.2 Fourier Transform Infrared Spectroscopy (FTIR)
495 FTIR can provide fundamental information not only about the molecular structure of macerals but
496 also about the thermal maturity of the hydrocarbon source rocks, and has been widely used to study the
497 functional groups of the macerals in coals with variable rank (Mastalerz and Bustin, 1993a, b, 1995, 1996;
498 Chen et al., 2012; Wang et al., 2014; Chen et al., 2015b; Wang et al., 2017b). However, several studies
499 have also been conducted on graptolites using FTIR (Bustin et al., 1989; Liu et al., 1996; Suchý et al., 500 2002; Suchý et al., 2004; Caricchi et al., 2016; İnan et al., 2016; Morga and Kamińska, 2018). Bustin et
501 al. (1989) used FTIR to study evolution of graptolites during artificial thermal-treatment, and found that
502 graptolites display a decrease of aliphatic chains and a depletion of aromatic C–H with increase in thermal
503 maturity/heat treatment due to dealkylation and aromatization. Bustin et al. (1989) also found that
504 graptolites are highly aromatic and have fewer aliphatic structures than vitrinite, whereas Hoffknecht
505 (1991) proposed that the percent of aliphatic compounds in graptolites is greater than that in vitrinite.
506 The parameter CH2/CH3 band intensity ratio can provide information about the length of aliphatic
507 chains (Lin and Ritz, 1993), and Suchý et al. (2002) found that the CH3/CH2 ratio increases with
508 increasing GRor. The CH3/CH2 ratio reported by Suchý et al. (2002) were converted to the CH2/CH3 ratio,
509 plotted their data in Fig. 18 and compared it with the results of Morga and Kamińska (2018). The data
510 from these two studies displays opposite correlations of CH2/CH3 ratio with GRor (Fig. 18). The data of
511 the Silurian deposits in the Polish Baltic Basin from Caricchi et al. (2016), however, did not show any
512 obvious correlation (Fig. 18), which may be because in that study, FTIR was conducted on the kerogen
513 concentrate rather than pure graptolites, and the reflectance data was very scattered in a short interval of
514 the same well. In low rank coals (Ro < 1.5%), the CH2/CH3 ratio shows a strong negative correlation with
515 vitrinite reflectance, similar to the results of Morga and Kamińska (2018). This indicates that the aliphatic
516 chains become shorter with maturation. The CHar/CH2+CH3 ratio increases with burial depth and GRor,
517 suggesting an increase in the aromatization of graptolite periderm (Morga and Kamińska, 2018).
518 Graptolites have been thought to be primarily type II kerogen and less commonly type III according to
519 FTIR parameters of ‘A2’ and ‘C2’ factors (Morga and Kamińska, 2018 and references therein).
3
2.5
2 (%)
or 1.5 GR
1
0.5 Suchý et al., 2002 Caricchi et al., 2016 Morga and Kamińska, 2018 0 0.4 0.8 1.2 1.6 2 520 CH2/CH3 521 Fig. 18. The relationship between CH2/CH3 and GRor.
522
523 5.3.3 Raman spectroscopy
524 Raman spectrum is a non-destructive and rapid microstructure analysis technique, and has been used
525 widely as an indicator for thermal maturity of sediments in last several decades (Zerda et al., 1981; Green
526 et al., 1983; Jehlička and Bény, 1992; Wopenka and Pasteris, 1993; Kelemen and Fang, 2001; Romero-
527 Sarmiento et al., 2014; Wilkins et al., 2014; Wilkins et al., 2015; Lünsdorf, 2016; Cheshire et al., 2017;
528 Henry et al., 2018; Jubb et al., 2018; Wilkins et al., 2018; Khatibi et al., 2018a, b). The Raman spectrum
529 of OM is generally composed of two broad peaks, the graphite band (G band) and the disordered band (D
530 band). In the recent decade, this technique has also been used to study graptolites (Suchý et al., 2004; Liu
531 et al., 2013; İnan et al., 2016; Mumm and İnan, 2016; Cheshire et al., 2017; Morga and Pawlyta, 2018;
532 Wang et al., 2019a). Suchý et al. (2004) used micro-Raman spectroscopy to determine the microstructure
533 of the graptolite materials located around an igneous sill, and proposed that the parameter AD/AG
534 (1350/1600 cm–1) peak area can be used to infer organic maturity. Liu et al. (2013) analysed the Raman
535 spectrum from a Silurian graptolite-bearing sediment from Qiaokou, Sichuan, China. Mumm and İnan
536 (2016) and İnan et al. (2016) have conducted Raman spectroscopy on Silurian Qusaiba samples with GRor
537 ranging from 0.76% to 2.20%, and found that both G band position and Raman band separation (RBS;
538 between G peak shift and D peak shift) display strong positive correlations with GRor. Cheshire et al.
539 (2017) have also used RBS to estimate the thermal maturity of the Silurian Qusaiba samples with EqVRo
540 ranging from 0.9% to 2.1% according to the relationship between RBS and VRor proposed by Sauerer et
541 al. (2017), which is consistent with the results from other proxies, (e.g., kerogen elemental composition,
542 kerogen skeletal density and BET specific surface area). Morga and Pawlyta (2018) found that Raman
543 band intensity ratio (ID1/IG) shows a robust correlation with GRor in the Polish Silurian shales, and can be
544 used to calculate EqVRo values. The relationship between ID1/IG and EqVRo has been determined by
545 Morga and Pawlyta (2018) as equation (3):
546 EqVRo = 1.7319 (ID1/IG) – 0.3295 (3)
547 Wang et al. (2019a) studied Raman spectra of graptolites in Chinese sediments from a wider thermal
548 maturity range (GRor ranging from 1.21 to 4.91%). They found that D1 peak position decreases with
549 increasing GRor when GRor < 4.0–4.5%, whereas D1 peak position increases with increasing GRor when
550 GRor > 4.0–4.5%. Both G peak position and RBS display a reverse trend with increasing GRor in 551 comparison with D1 peak position (Wang et al., 2019a). However, it is worth noting that samples with
552 GRor > 4.5% are very limited and the maximum of GRor in this sample set is only 4.91%, thus, more data
553 are needed to illustrate their variation tendency at the high-maturity end. Wang et al. (2019a) established
554 the relationship between GRor and EqVRo as equations (4) and (5), respectively, through the intermediate
555 conversion of RBS (2.1% < GRor < 3.5%) and ID1/IG (GRor > 3.5%) based on the relationship between
556 VRor and RBS (2.1% < GRor < 3.5%), ID1/IG (GRor > 3.5%) proposed by Liu et al. (2013).
557 EqVRo = 0.97 GRor– 0.2 (2.1%＜ GRor＜ 3.5%) (4)
558 EqVRo = 0.22 GRor＋ 2.55 (GRor＞ 3.5%) (5)
559 Recently, Hao et al. (2019) compared the Raman spectral parameters of the naturally and artificially
560 matured samples, and the latter has a lower degree of coalification than the former due to insufficient
561 structural transformation of artificially matured samples. They found that GRor displays positive
562 correlations with the full width at half maximum of the D1 band and the G band (FWHM-D/FWHM-G),
563 AD/AG and RBS, and negative correlations with D1 peak position and FWHM-G, and established the
564 relationship between EqVRo and RBS as follows:
565 EqVRo = 0.089 RBS – 19.937 (6)
566 In order to compare the relationship between RBS and reflectance among various types of OM, the
567 data for vitrinite, inertinite, bitumen and graptolite were collected from several publications noted in Fig.
568 19 (Kelemen and Fang, 2001; Suchý et al., 2004; Wilkins et al., 2014; Mumm and İnan, 2016; Sauerer et
569 al., 2017; Morga and Pawlyta, 2018; Wang et al., 2019a; Hao et al., 2019). The RBS of vitrinite, inertinite,
570 bitumen and graptolites increases as thermal maturity increases when the reflectance is lower than 4%,
571 with the exception of the data from Morga and Pawlyta (2018). However, these data become significantly
572 scattered when the reflectance is higher than 4%, especially for the coal/vitrinite data from Liu et al. (2013)
573 and Kelemen and Fang (2001). In addition, G peak position is negatively correlated with GRor in samples
574 from Poland (Morga and Pawlyta, 2018), which opposites to results from the Saudi Arabian Qusaiba
575 samples and the Chinese Wufeng–Longmaxi shales (İnan et al., 2016; Mumm and İnan, 2016; Wang et
576 al., 2019a). These inconsistencies may be due to differences in conditions as follows: (1) sample
577 preparation; (2) experimental conditions (e.g., laser wavelength); (3) data processing; (4) analysis of
578 various OM, and intra-particle chemical heterogeneity (Lünsdorf, 2016; Sauerer et al., 2017; Henry et al.,
579 2018; Jubb et al., 2018; Hao et al., 2019). Thus, uniform conditions are required to better determine the
580 correlation between Raman spectral parameters and graptolite reflectance in future studies. 290
260
)
1
- RBS(cm
230 Coal (Kelemen and Fang, 2001) Vitrinite & inertinite (Wilkins et al., 2014) Vitrinite (Liu et al., 2013) Type III kerogen (Kelemen and Fang, 2001) Type II kerogen (Sauerer et al., 2017) Type II kerogen (Kelemen and Fang, 2001) Graptolite (Suchý et al., 2004) Graptolite (Mumm and İnan, 2016) Graptolite (Morga and Pawlyta, 2018) Graptolite (Wang et al., 2019a) Graptolite (Hao et al., 2019) Bitumen (Liu et al., 2013) 200 0 1 2 3 4 5 6 7 8 581 Ro (%)
582 Fig. 19. The relationship between RBS and reflectance for the vitrinite, inertinite, bitumen and graptolite. The minimum and maximum
583 values of vitrinite and bitumen random reflectance have been given by Liu et al. (2013) rather than the average, thus, half of the total
584 minimum and maximum values were used as Ro in this figure.
585
586 6. The microstructure of graptolites 587 6.1 Scanning Electron Microscopy (SEM) 588 Loucks et al. (2012) classified three types of shale pore systems based on SEM: OM, interparticle,
589 and non-OM intraparticle pores. Although OM pores are within OM, they can form an organic
590 interconnected pore network due to OM connectivity, thus making a significant contribution to the overall
591 porosity (Passey et al., 2010; Loucks et al., 2012; Mastalerz et al., 2013; Milliken et al., 2013). OM pores
592 in the size range from 5 to 750 nm can adsorb and store methane simultaneously (Loucks et al., 2009),
593 and shales with higher OM content display stronger capacity of methane adsorption because of larger
594 surface area supported by OM micropores (Hickey and Henk, 2007; Ross and Bustin, 2007). OM pores
595 are widely identified in North American and Chinese gas-bearing shales, e.g., the Barnett, Ohio, Antrim,
596 Marcellus, Woodford, Wufeng–Longmaxi and Longtan shales (Hill and Nelson, 2000; Curtis, 2002; Jarvie
597 et al., 2007; Chalmers et al., 2012; Curtis et al., 2012; Loucks et al., 2012; Mastalerz et al., 2013; Milliken 598 et al., 2013; Tian et al., 2013; Cardott et al., 2015; Hackley and Cardott, 2016; Luo et al., 2016; Ma et al.,
599 2016; Gentzis et al., 2017; Zhang et al., 2017). Pores within graptolites have been studied by SEM (Luo
600 et al., 2016; Ma et al., 2016; Cardott and Curtis, 2018). While many micro-nanopores are widely found
601 in the overmature Wufeng–Longmaxi Formations (Fig. 20) (Luo et al., 2016; Ma et al., 2016), scarce
602 nanopores are present in immature graptolites (%GRor=0.66) from the Lower Ordovician Polk Creek
603 Shale (Cardott and Curtis, 2018). Micro-nano pores in Chinese graptolites generally display irregular or
604 elliptical or nearly spherical shapes, and can be more easily observed in sections parallel to bedding than
605 in sections perpendicular to bedding (Luo et al., 2016; Ma et al., 2016).
606 Graptolites fragments with pores were recently thought to be solid bitumen due to their presence
607 within interparticle pores (İnan et al., 2018). However, Luo et al. (2018) found some graptolites within
608 the interparticle pore space (Fig. 5 in Luo et al., 2018), which look like solid bitumen, but morphologically
609 display graptolitic fusellar layers under polarized light. In order to resolve this conflict, graptolites were
610 marked using diamond lenses during observation under reflected light and then observed under SEM.
611 Many pores were found in the graptolites, although they were smaller and less numerous than those within
612 solid bitumen (Fig. 20).
613 The development and distribution of OM pores in some graptolites are controlled by their fine
614 biological structures (Luo et al., 2016; Ma et al., 2016). Graptolites are generally thought to have low
615 porosity, ranging from 1.62% to 4.23% and averaging 2.51% (Ma et al., 2016). Pores in graptolites can
616 further make up an interconnected system due to their alignment along the fusellar layers, and may
617 connect with discrete porous OM detritus and/or other pore systems (Luo et al., 2016; Ma et al., 2016),
618 which is beneficial to the storage and the exploitation of shale gas in graptolite-bearing sediments. OM
619 pores are also regarded as an important pore system in other gas bearing shales, e.g., Barnett shale (Loucks
620 et al., 2009; Chalmers et al., 2012).
621 622
623 Fig. 20. OM Pores in the Wufeng–Longmaxi Formations. (a) Solid bitumen and graptolite under dry objective; (b) same graptolite under
624 SEM as (a); (c) SEM image showing of location of images d-f; (d-f) images show the pores in solid bitumen are slightly larger and more
625 numerous than that in the graptolite (marked by arrows).
626
627 6.2 High resolution transmission electron microscopy (HRTEM) 628 HRTEM can study OM microstructure at nm scale (Krzesińska et al., 2009; Chalmers et al., 2012;
629 Pawlyta, 2013; Romero-Sarmiento et al., 2014; Morga and Pawlyta, 2018). Morga and Pawlyta (2018)
630 studied the nanostructure of graptolites with GRor of 1.83% using HRTEM. The parallel alignment of
631 carbon layers in the graptolite can be observed in their HRTEM photos, and the dimension of the basic 632 structural units is around 1–2 nm, indicating high molecular ordering, in agreement with the study of
633 Goodarzi (1984).
634 7. The regional maturation and hydrocarbon generation potential of the graptolite-
635 bearing sediments 636 7.1 The hydrocarbon generation potential of graptolites
637 Wang et al. (2017a) concentrated mature (EqVRo around 1.10%) graptolite fragments from two
638 Wufeng–Longmaxi shales. The TOC in the two isolated graptolites is 42.93% and 71.34%, S1 is 6.55 and
639 15.18 mg/g, S2 is 31.71 and 60.36 mg/g, and HI is 74 mg/g TOC and 85 mg/g TOC, respectively, which
640 is much higher than that in the whole rocks (Table 3). İnan et al. (2016) found that an immature isolated
641 graptolite graptolite had HI of 200 mg/g TOC and OI of 30 mg/g TOC, much higher than that of mature
642 graptolites (Wang et al., 2017a). These data indicate that the immature graptolites are hydrogen rich,
643 similar to kerogen type II-III, and have good hydrocarbon potential which is consistent with finding of
644 Hoffknecht (1991), Bustin et al. (1989) and Liu et al. (1996) based on FTIR. Pyrolysis chromatography
645 of the two Chinese graptolies found that the CH4 yields are 12.35 mg/g and 23.61 mg/g, respectively
646 (Wang et al., 2017a). The weak brown fluorescence in the graptolites of the Liteň Formation from the
647 Czech Republic also indicates that the graptolites have some hydrocarbon generation potential (Fig. 5),
648 which means that the graptolite can not be classified as “inertinite-like” macerals (Hoffknecht, 1991).
649 Further, the formation of the OM pores is related with hydrocarbon generation (Loucks et al., 2009),
650 which can explain the rare nanopores in low-maturity graptolites from the Lower Ordovician Polk Creek
651 Shale (Cardott and Curtis, 2018). On the other hand, the abundant pores in the graptolites with high
652 maturity also supports their gas generation and storage potential (Fig. 20). These factors suggest that these
653 graptolites are mainly gas prone, which is consistent with the results of Hoffknecht (1991), and that
654 graptolites possibly make a significant contribution to the contents of shale gas in graptolite-bearing
655 sediments.
656 Table 3. Comparison of the Rock-Eval data of whole rock versus graptolites (MB-2 and MB-3 from Wang et al., 2017a; K1 from İnan et
657 al., 2016).
HI OI Sample ID Lithology Formation S1 (mg/g) S2 (mg/g) Tmax (℃) TOC (%) (mg/g TOC) (mg/g TOC)
MB-2 (whole rock) Shale O3w 0.54 1.77 455 3.04 58 8
MB-3 (whole rock) Shale S1l 0.69 2.88 458 3.88 74 5
MB-2 (graptolite) Graptolite O3w 6.55 31.71 464 42.93 74 43 MB-3 (graptolite) Graptolite S1l 15.18 60.36 456 71.34 85 2
K1 (graptolite) Graptolite S1l 200 30
658
659 Following are examples of worldwide regional maturation and hydrocarbon generation potential
660 studies on graptolite-bearing sedimentary strata. 661 7.2 Canada 662 Graptolite reflectance was used in several regional thermal maturation studies in Canada (Riediger
663 et al., 1989; Link et al., 1990; Goodarzi et al., 1992c; Gentzis et al.,1996). Most of these regional studies
664 were carried out in Canadian Arctic (Goodarzi et al.,1992c; Gentzis et al.,1996) on graptolitic shale and
665 equivalent late Ordovician-early Middle Devonian strata over a distance of over 600 km to determine
666 horizontal variation of thermal maturity and depositional history from Melville Island in southwest to
667 Ellesmere Island in northeast (Fig. 21). The Middle Devonian is up to 3000 m thick, and extends from
668 central Ellesmere Island to Melville Island (Fig. 21) (Trettin, 1989, 1990).
669 Based on graptolite reflectance, the sedimentary succession is overmature (3.5–4.2% GRomax) in
670 western Melville Island, mature in central Melville island (1.75–2.1% GRomax), and immature (0.60–0.80%
671 GRomax) in the Prince of Wales islands, eastern Bathurst, Cornwallis, Baillie Hamilton, Dundas, and
672 Devon Island (Fig. 21). Therefore, the maturity of graptolitic shale increases from mature (1.32% GRomax)
673 in south of Ellesmere Islands to overmature (4.7% GRomax) in north (Fig. 21).
674 Variations in thermal maturity may be due to the areal difference in geothermal gradient and
675 overburden thicknesses. Since regional geothermal gradient has been determined to be ~25 °C/Km in
676 Melville Island (Gentzis, 1991), and this value is considered within conventional scope of geothermal
677 gradient from other North American basins (Gretener, 1982), the difference in thermal maturation is
678 probably due to the thicknesses of the original overburden (Fig. 22). 679
680 Fig 21. Map of the Canadian Arctic Islands showing graptolite maximum reflectance (%GRomax) and maturity of the graptolitic-bearing 681 strata (modified after Gentzis et al., 1996). The green area on the map are designated national park and wilderness. 682
GRomax (%) 0 1 2 3 4 5 0
1000
2000
3000
4000
5000 Erodedsection (m)
6000
7000
683 8000
684 Fig. 22. Variation of graptolite (%GRomax) and overburden strata (eroded sedimentary loading) in Arctic Canada (after Gentzis et al., 685 1996). 686 687 7.3 China 688 The Wufeng–Longmaxi graptolite-bearing sediments in China have various thermal maturities (Luo
689 et al., 2016; Luo et al., 2017; Luo et al., 2018; Wang et al., 2019a), and have the focus of interest due to
690 both the increased importance of shale gas in the global petroleum industry (Curtis, 2002; Bowker, 2007; 691 Jarvie et al., 2007; Ross and Bustin, 2009; Loucks et al., 2012; Dai et al., 2014; Hackley and Cardott,
692 2016), and the successful exploration and exploitation of shale gas in the area (Dai et al., 2014; Dai et al.,
693 2016). The Wufeng–Longmaxi sediments are characterized by abundant OM (TOC: 0.51–25.73%,
694 average 2.59%), overmature levels (mostly EqVRo > 2%), thick beds (30–130 m), abundant OM pores
695 and strong gas generation intensity (Table 4) (Zou et al., 2011; Dai et al., 2014; Dai et al., 2016; Luo et
696 al., 2016; Ma et al., 2016; Luo et al., 2017; Luo et al., 2018), important for the generation and
697 accumulation of shale gas. The first large shale gas field was discovered in the Wufeng–Longmaxi
698 Formations, Fuling, Chongqing, with gas reserves over 100×109 m3 (Dai et al., 2014; Dai et al., 2016).
699 PetroChina developed the Changning-Weiyuan shale gas field in the Wufeng–Longmaxi sediments in
700 2014, and the gas production reached 28×108 m3 in 2016 (Ma and Xie, 2018).
701 In the Wufeng–Longmaxi Formations, graptolite-derived OM accounts for 20–93 vol.% of total OM
702 (Luo et al., 2016). The abundance of graptolites displays a positive correlation with TOC in both Wufeng–
703 Longmaxi Formation from Sichuan Basin and Pingliang Formation from Erdos Basin (Zhu et al., 2015;
704 Deng et al., 2016; Borjigin et al., 2017). Based on the comparison of natural and artificial maturation
705 samples of Wufeng–Longmaxi Formations and Alum Shales (Sweden) (Luo et al., 2018), OM in the high-
706 maturity Wufeng–Longmaxi shales is very similar to the artificially-matured Alum Shales (Fig. 23). This
707 suggests that OM in immature Wufeng–Longmaxi shales is similar to OM in the immature Alum Shales.
708 The Wufeng–Longmaxi Formations have been subdivided and correlated on the basis of lithology
709 or logging curves, however, it has not been accepted widely due to the lower resolution and precision of
710 these methods (Zou et al., 2015; Chen et al., 2017). As important biostratigraphic indicator, graptolite
711 species have been widely used for the stratigraphic correlation (Chen, 1984; Bergström and Mitchell,
712 1986; Chen et al., 1987; Cooper, 1999; Chen et al., 2000; Chen et al., 2004; Chen et al., 2005; Loydell,
713 2011; Chen et al., 2017). Chen et al. (2015a) proposed a new subdivision and correlation for the Wufeng–
714 Longmaxi Formations based on the graptolite biostratigraphy. According to this revised stratigraphy,
715 intervals (WF2–WF3 and LM2–LM6) were regarded as the best targets for shale gas (Chen et al., 2015a;
716 Chen et al., 2017), and they generally contain abundant graptolites (> 30%) (Qiu et al., 2018). The pores
717 in these graptolites also act as a reservoir and/or migration pathway of the shale gas (Ma et al., 2016; Luo
718 et al., 2018; Qiu et al., 2018).
719 720 721 Fig. 23. Comparison of OM in natural Wufeng–Longmaxi shales (a) and heat-treated Alum Shale heated at 550 ℃ for 3 days (b).
722
723 A regional maturation pattern based on GRor converted to vitrinite (Yang, 2016; Luo et al., 2018;
724 Wang et al., 2019a) for the Wufeng–Longmaxi Formations from Chongqing and surrounding areas
725 indicates that graptolite-bearing strata are mostly overmature, with maturity increasing from southwest
726 towards northeast with an EqVRo of 1.17–4.93%, the maximum in sample TT from Pingshan, Sichuan
727 province, and minimum in well YC1 from Chengkou, Chongqing (Fig. 24). 728 Table 4. Geological and geochemical data of selected global graptolite-bearing sediments.
Country/Continent Area/Basin Formation Age TOC (%) EqVRo (%) Depth (m) Thickness (m) Porosity (%) Gas content (m3/t)
China Chongqing Wufeng–Longmaxi Upper Ordovician–Lower Silurian 0.51–25.73 1.17–4.93 900–4500 30–130 3–10 1.7–4.5 Norwegian– Denmark Danish Basin Alum Shale Miaolingian–Tremadocian 3.00–9.00 1.8–2.5 2000–4000 30–180 3–6 avg. 0.85 (onshore) Sweden Colonus Trough Alum Shale Miaolingian–Tremadocian 3.00–16.00 1.7–2.0 700–1000 70–90 avg. 6.5% avg. 0.85
Swedena Central Sweden Alum Shale Miaolingian–Tremadocian 1.00–21.00 0.5–1.1 20–100 20–35 / max. 7.6
Poland BPLBb Piaśnica Upper Furongian–Tremadocian 3–12 0.55–2.40c /d 10–34 3.17–6.89 / Poland BPLB Sasino Caradoc 1–7 0.5–4.2c / 1.5–70 2.24–10.94 / Poland BPLB Pasłęk Llandovery 1–6 0.7–4.0c / 20–80 3.95–5.56 (Jantar Member) / Poland BPLB Pelplin–lower part Wenlock 0.5–1.7 0.6–2.5c / 30–1000 5.25–7.88 / North Africa Ghadames Basin Tanezzuft Llandovery–Wenlock 2–16 0.7–2.0 2000–5000 20–100 / 2.4–8.5
729 a Biogenic gas;
730 b BPLB = Baltic–Podlasie–Lublin Basin;
c 731 Mean random reflectance of the vitrinite–like macerals (Rvl);
732 d “/” means no data.
733 734
735 Fig. 24. The EqVRo distribution of the Wufeng–Longmaxi shales in Chongqing and surrounding regions. Data at each sample location is
736 well/outcrop name/EqVRo. Data are from Yang (2016); Luo et al. (2018); Wang et al. (2019a).
737
738 7.4 Denmark and Sweden 739 Lower Palaeozoic strata is widely distributed in Denmark, Sweden, Norway, Poland and the Baltic
740 States (Fig. 25). In this area, terrestrial to shallow marine sandy deposition commenced in lower Cambrian
741 (Serie 2) following an overall transgression of the Baltic continent (Nielsen and Schovsbo, 2011, 2015).
742 From the middle Cambrian (Miaolingian) to the Lower Ordovician (Tremadocian), organic rich mud
743 deposition known as the Alum Shale Formation in Denmark, Sweden and Norway, the Türisalu Formation
744 in Estonia, the Kaporye Formation in the St. Petersburg region and the Slowinski Formation as well as
745 Piaśnica Formation in NE Poland completely dominated (Nielsen and Schovsbo, 2007). The shale is now
746 present in the deeply buried strata towards the Polish-German and Swedish-Norwegian Caledonian fronts, 747 in the Baltic area and as erosional outliers in Sweden (Fig. 25), representing only a minor part of the
748 original huge deposition area that its maximum at the Lower Ordovician may have extended for more
749 than 800 000 km2 (Schovsbo et al., 2018).
750
751
752 Fig. 25. Outline of approximate original distribution of the Alum Shale Formation and present-day occurrence of the Lower Paleozoic
753 strata in southern Scandinavia (modified from Schovsbo et al., 2018).
754
755 Thermogenic gas trapped in the Alum Shales has been explored in Denmark (Schovsbo and Jakobsen,
756 2019) and Sweden (Pool et al., 2012) (Table 4). In Denmark and Sweden, a considerable resource has
757 been estimated by the U.S. Geological Survey (mean = 67 × 109 m3 gas; Gautier et al., 2013) and by the
758 European Geological Surveys (Zijp et al., 2017) to be present mostly in the Cambrian (Miaolingian) to
759 Ordovician (Tremadocian) Alum Shale Formation. Typical Alum Shale has TOC values from 5–10%
760 (even up to 25% TOC in some samples) (Fig. 26), and their thermal maturity ranges from immature
761 (EqVRo<0.5%) in Central Sweden and Estonia, dry gas mature (EqVRo: 1.6–2.5%) in most of Denmark
762 and southern Sweden, and to post-mature to low-grade metamorphic in Norway (EqVRo >3%; Buchardt
763 et al., 1997; Petersen et al., 2013) (Table 4). The shale is graptolitic with macroscopic remains of typical
764 the pelagic Rhapdinopora occurring abundantly in the Tremadocian shale. In the Cambrian, microscopic
765 remains of graptolites also occur presumably from benthic species (Petersen et al., 2013) - an 766 interpretation that has been supported by the rediscovery of Miaolingian benthic graptolites in the Alum
767 Shale from Norway (Wolvers and Maletz, 2016). At this time, no quantification of the contribution of
768 graptolite carbon to the total TOC content have been made, and the only published study of the Alum
769 Shale nanopore system by Henningsen et al., 2018 did not specifically investigate the graptolite
770 contribution to the total porosity.
771 The Lower Paleozoic shale gas prospective areas in Denmark largely follow the margins of the
772 Norwegian–Danish Basin (Schovsbo et al., 2014). Exploration of this play is still limited, and
773 representative well data and production test data for the shales are lacking. The only exploration well in
774 Denmark revealed gas in the Alum Shale Formation, however, no test production was carried out, so the
775 commercial potential is still unknown (Schovsbo and Jacobsen, 2019). The well was drilled within a so-
776 called ‘sweet spot’, defined as an area with expected highest gas content (c.f. Schovsbo et al., 2014).
777 Results from this well revealed the Alum Shale to be 40 m thick, compared to up to 180 m in the
778 depocenter offshore Denmark. In Sweden, thermogenic gas has been explored in Scania, within the
779 Colonus Trough, which is a fault bounded graben system in southern Sweden. Exploration indicated that
780 the Alum Shale Formation was gas mature (EqVRo: 1.7–2%), located at 700–1000 m depth (Table 4), but
781 did not contain gas in economically producible quantities, possibly due to gas leakage as a result of uplift
782 (Pool et al., 2012). In south-central Sweden, gas-bearing immature to low-maturity Alum Shale with
783 depths < 150 m have been known for several decades and been under exploration for commercial shale
784 gas production (Table 4); they are thought to be composed of mixtures of thermogenic and bacterially-
785 derived gas (Schultz et al., 2015; Schovsbo and Nielsen 2017). 786
787 Fig. 26. Composite profile of TOC content in the Alum Shale in Scania, southern Sweden (modified from Schovsbo, 2003).
788 7.5 Poland 789 The highest shale gas exploration potential in Poland is related to the Cambrian–Lower Ordovician
790 (upper Furongian–Tremadocian), Upper Ordovician (Caradoc) and Silurian (Llandovery–Wenlock) black
791 and dark grey shales with abundant graptolite remains within the Baltic-Podlasie-Lublin Basin (BPLB) at
792 the slope of the East European Craton (EEC) (Table 4, Figs. 27 and 28) (Poprawa, 2010; Karcz et al.,
793 2013; Podhalańska, 2013). Due to lateral facies variability, hydrocarbon potential of these rocks varies
794 throughout the area. The Lower Paleozoic deposits were buried to significantly different depths in the
795 individual parts of the BPLB. Tectonic processes resulted in a break-up of the Basin into segments (Karcz
796 et al., 2013). The current burial depth within the significant part of BPLB is shallow enough for
797 commercial shale gas exploration, and geological structure is simple, especially within the Baltic Basin
798 and Podlasie Basin (Poprawa, 2010). 799
800 Fig. 27 The distribution of Upper Ordovician–Lower Silurian sediments in the Baltic-Podlasie-Lublin Basin (BPLB) at the slope of the
801 East European Craton (EEC) (modified after Poprawa, 2010). Abbreviations: SPW =Płock-Warsaw zone; SBN = Biłgoraj-Narol zone;
802 TESZ = Trans-European Suture zone.
803
804 The upper Furongian–Tremadocian Alum-Shale-equivalent black bituminous shales belong to the
805 Piaśnica Formation, which occurs only within the Baltic Basin (Fig. 28). Their thickness is from 10 m (in
806 the onshore part) to 34 m (in the offshore part) (Szymański, 2008; Więcław et al., 2010; Podhalańska et
807 al., 2016a), thinner than the Alum Shale in Denmark (30–180 m) and southern Sweden (70–90 m); and
808 average TOC content ranges between 3% and 12.0% (Table 4). Thermal maturity of OM increases with
809 the increasing depth from NE to SW from the main phase of oil generation through the condensate and
810 wet gas phase to the dry gas generation phase. The Rock-Eval Tmax temperature and Rvl vary from 430 °C 811 to 505 °C, and from 0.55% to 2.4%, respectively (Poprawa, 2010; Więcław et al., 2010; Karcz and Janas,
812 2016; Podhalańska et al., 2016a) (Table 4). The effective porosity ranges between 3.17% and 6.89%
813 (Table 4) (Dyrka, 2016). S2 is up to 72 mg HC/g rock, while HI varies from 6 to 484 mg HC/ TOC
814 (Więcław et al., 2010). The gas content in the Piaśnica Formation reaches 7.6 m3/t in the Lębork S-1 well
815 (Lehr and Keeley, 2016) (Table 4), comparable to that in North American gas-bearing sediments (e.g.
816 Jarvie, 2012). 817
818 Fig. 28 Generalized lithostratigraphy of the Upper Cambrian-Silurian deposits in the Polish part of the Baltic-Podlasie-Lublin Basin
819 (after Podhalańska 2016b). 820 The Caradoc (Sandbian–lower Katian) rocks (i.e., Sasino Formation) are most fully developed in the
821 northern and western parts of the Baltic Basin, mainly composed of dark grey and black shales with rich
822 graptolite fauna (Fig. 28) (Modliński and Szymański, 1997; Podhalańska, 2013; Podhalańska et al.,
823 2016a). Similar rocks but with lower thickness and stratigraphic extent stretch to the southeast in the
824 Podlasie and Lublin areas (different regional lithostratigraphic equivalents). Thickness of the Sasino
825 Formation and its equivalents in other parts of the BPLB varies from 1.5 m in the eastern onshore part of
826 the Basin to 70 m in its offshore part (Table 4) (Modliński and Szymański, 1997, 2008; Poprawa, 2010;
827 Więcław et al., 2010; Podhalańska et al., 2016a; Podhalańska et al., 2016b). The average TOC content in
828 Sasino Formation sediments ranges from 1% to 4%, and even up to 7%, in the offshore part (Table 4)
829 (Poprawa, 2010; Więcław et al., 2010; Karcz and Janas 2016; Podhalańska et al., 2016a). Thermal
830 maturity increases from NE to SW with increasing depth from the main phase of oil generation (0.6–1.1%
831 Rvl) through condensate and wet gas generation phase (1.1–1.4% Rvl) to the dry gas generation phase
832 (>1.4% Rvl), reaching 4.2% within the Baltic Basin (Table 4) (Więcław et al., 2010; Podhalańska et al.,
o 833 2016a). Tmax values reported for the BPLB are 425–474 C (Więcław et al., 2010; Karcz and Janas 2016).
834 The effective porosity ranges between 2.24% and 10.94% (Table 4) (Dyrka, 2016). S2 reported for the
835 Baltic Basin varies from 0.05 to 10.6 mg HC/g rock, and HI ranges from 11 to 359 HC/g TOC.
836 Llandovery (Rhuddanian-Telychian) shales make up the Pasłęk Formation, whereas the Wenlock
837 (Sheinwoodian-Homerian) rocks belong to the lower part of the Pelplin Formation (Sheinwoodian-
838 Ludfordian as a whole) (Fig. 28) (Podhalańska et al., 2016b). They contain rich and variable graptolite
839 fauna, which made it possible to determine biostratigraphic levels and limits of chronostratigraphic units
840 (Podhalańska, 2013). The Llandovery shales of the Pasłęk Formation occur throughout vast parts of the
841 western slope of EEC. Their thickness generally increases from the east to the west, reaching a maximum
842 of 80 m. However, in the major part of the BPLB, it ranges from 20 to 40 m (Table 4). Average TOC
843 values are usually 1% to 2.5%, except for the Podlasie Basin, where they reach 6% (and even > 15% in
844 individual layers) (Table 4). High TOC content was particularly documented in the lower part of the
845 Pasłęk Formation (the Jantar member) (Modliński et al., 2006; Poprawa 2010; Więcław et al., 2010; Karcz
846 et al., 2013). Rvl in the Baltic Basin varies from 0.7% to 2.3%, and rises to 4.0% in the Podlasie Basin,
847 showing the NE-SW trend (Table 4) (Poprawa, 2010; Więcław et al., 2010; Karcz et al., 2013). Tmax
o 848 temperature reported for the Baltic Basin falls within the range of 435–534 C. S2 varies from 0.12 to 40.6
849 mg HC/g rock, while HI varies from 8 to 436 mg HC/g TOC (Więcław et al., 2010). 850 The Jantar member (Rhuddanian–lower Aeronian), composed of bituminous shales, is the most
851 perspective unit of the Pasłęk Formation within the onshore and offshore areas of the Baltic Basin (Fig.
852 28) (Podhalańska et al., 2016a). Its maximum thickness is 18 m (Modliński et al., 2006; Podhalańska et
853 al., 2016a). The average TOC content varies from 2% to 5%, and the thermal maturity increases from NE
854 to SW, starting with the oil generation window and passing through condensate and wet gas phase to dry
855 gas generation phase (Poprawa 2010; Karcz and Janas, 2016; Podhalańska et al., 2016a; Podhalańska et
856 al., 2016b). Rock-Eval Tmax values range between 438 and 485 °C (Karcz and Janas, 2016). Average
857 effective porosity is 3.95–5.56% (Table 4) (Dyrka, 2016).
858 The Wenlock shales have the largest extent of all stratigraphic units. They have been documented
859 within the whole BPLB (Fig. 28) (Podhalańska et al., 2016a). Their thickness is highly variable, from 30
860 m in SE part of the Lublin Basin to over 1000 m in western part of the Baltic Basin (Table 4) (Poprawa,
861 2010; PIG, 2012; Karcz et al., 2013; Podhalańska et al., 2016a). Average content of OM in individual
862 Wenlock sections in central and western parts of the Baltic Basin and the Podlasie Basin usually ranges
863 from 0.5% to 1.4% TOC. In eastern part of the Baltic Basin and in the Lublin Basin, it is higher, rising to
864 about 1–1.7% TOC (Poprawa, 2010, Więcław et al., 2010; Karcz and Janas, 2016; Podhalańska et al.,
865 2016a). Thermal maturity of OM increases from NE to SW, from the oil generation phase through the
866 condensate and wet gas phases to dry gas generation phase (Podhalańska et al., 2016a). Rvl equals 0.6–
o o 867 2.5% (Table 4), and Tmax values vary from 425 C to 509 C (Więcław et al., 2010; Karcz and Janas 2016;
868 Podhalańska et al., 2016a). Average effective porosity is 5.25–7.88% (Table 4) (Dyrka, 2016). S2 in the
869 Baltic Basin ranges between 0.07 and 3.5 mg HC/g rock. HI is 7–335 mg HC/g TOC (Więcław et al.,
870 2010). 871 7.6 Arabia and North Africa 872 The Lower Silurian organic-rich shales (hot shales) were widely deposited in the Arabia and North
873 Africa due to a strong rise of sea level during the latest Ordovician to Early Silurian, including e.g.,
874 Tanezzuft Formation in Libya, Tunisia and Algeria, Tanf Formation in Syria, Akkas Formation in Iraq,
875 Qusaiba Formation in Saudi Arabia, and Batra or Mudawwara Formation in Jordan (Loydell, 1998;
876 Lüning et al., 2003; Soua, 2014; İnan et al., 2016). They are characterized by type II kerogen, TOC up to
877 20%, and thickness is generally less than 100 m (Lüning et al., 2000; Belaid et al., 2010; Soua, 2014;
878 Wang et al., 2019b). Their thermal maturity varies significantly due to variable burial history, ranging
879 from immature to post mature (Lüning et al., 2005; Belaid et al., 2010; Soua, 2014; İnan et al., 2016; 880 Wang et al., 2019b). In North Africa, 80–90% of Paleozoic-sourced petroleum are derived from these
881 Silurian hot shales, and in Arabia, some oil and sweet gas in Paleozoic-Mesozoic reservoirs are regarded
882 to be sourced from these organic-rich sediments (Lüning et al., 2000; Al-Juboury and Al-Hadidy, 2009;
883 İnan et al., 2016). In recent years, the hot shales were thought to contain shale oil/gas potential due to
884 their wide distribution, thickness, abundant OM and variable thermal maturity (Soua, 2014; İnan et al.,
885 2016; Wang et al., 2019b). For example, the gas content of the hot shales falls between 2.4 and 8.5 m3/t
886 in the Ghadames Basin, and their geochemical characteristics and petroleum accumulation conditions are
887 comparable to the Wufeng–Longmaxi Formations (Table 4) (Wang et al., 2019b). 888 8. Conclusions 889 Based on a review on the optical characteristics, chemical composition, and microstructure of the
890 graptolites, the following conclusions can be reached:
891 (1) There are two types of graptolites: granular (GG) and non-granular (NGG). GG have lower
892 reflectance and weaker anisotropy than NGG. GG can be altered to NGG with increasing thermal maturity.
893 (2) The aromaticity and ordering of the aromatic structure of graptolite increasing with increasing
894 maturity is similar to that of vitrinite and bitumen, illustrating that their physical and chemical parameters
895 are reliable geo-thermometers. There is a positive correlation between GRor and GRomax, allowing GRor
896 to be used as a measure of thermal maturation for the Lower Paleozoic sediments. The relationship
897 between GRor and EqVRo can be expressed as: EqVRo = 0.99GRor + 0.08.
898 (3) Electron microprobe and FTIR analyses are very useful tools to study chemical compositions of
899 graptolites, indicating that their compositions are similar to vitrinite. Standard measurement and data
900 processing are required for the improved analysis of Raman spectrum for graptolites. Graptolites contain
901 abundant pores, which are less abundant and smaller than those in solid bitumen.
902 (4) Based on the chemical composition and organic geochemical characteristics of the graptolites,
903 they are mainly gas prone, similar to vitrinite, and have a significant hydrocarbon generation potential.
904 (5) OM in the Wufeng–Longmaxi sediments is dominated by graptolites and solid bitumen. Their
905 thermal maturity was determined by GRor, in the range of 1.17–4.93%, indicating marginally mature to
906 overmature.
907 (6) The graptolite-bearing shales are worldwide hydrocarbon source rocks and contribute
908 significantly to global petroleum reserves. Graptolite serve both as source material and, due to nano-
909 microporosity, as reservoir and are important to the accumulation of shale gas in these sediments. 910 911 Acknowledgments 912 This work was supported by National Natural Science Foundation of China (No. 41503028,
913 41773031 and 41830424). Field sampling in the Czech Republic was partly supported by OP RDE, MEYS,
914 under the project “Ultra-trace isotope research in social and environmental studies using accelerator mass
915 spectrometry” (Reg. No. CZ.02.1.01/0.0/0.0/16_019/0000728). We thank Dr. Roger MacQueen of the
916 Geological Survey of Canada for reviewing an early version of this manuscript and for his valuable
917 suggestion. We also thank Zhongliang Ma at the Wuxi Institute of Petroleum Geology, Sinopec Petroleum
918 Exploration and Development Research Institute for conducting the artificial maturation experiments in
919 this study. We are very grateful to Dr. MaryAnn Love Malinconico and one anonymous reviewer for their
920 critical but constructive and valuable comments that significantly improved the quality and language of
921 this paper. The authors also appreciate Managing Editor Dr. Shuhab Khan for his precious time and energy
922 to handle this paper.
923
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