Dispersed organic matter from Silurian shales of the Barrandian Basin, Czech Republic: optical properties, chemical composition and thermal maturity

International Journal of Coal Geology 53 (2002) 1 – 25 www.elsevier.com/locate/ijcoalgeo

Dispersed organic matter from Silurian shales of the Barrandian Basin, Czech Republic: optical properties, chemical composition and thermal maturity Va´clav Suchy´ a,1, Ivana Sy´korova´ b,*, Michal Stejskal c, Jan Sˇafanda d, Vladimı´r Machovicˇ c, Miroslava Novotna´ c b

a Institute of Geology AS CS, Rozvojova´ 135, 165 00 Prague, Czech Republic Institute of Rock Structure and Mechanics AS CR, V Holesˇovicˇka´ch 41, 182 09 Prague 8, Czech Republic c Institute of Chemical Technology, Technicka´ 5, 166 28 Prague, Czech Republic d Geophysical Institute AS CS, Bocˇnı´ II/1401, 141 31 Prague, Czech Republic

Received 21 December 2001; accepted 25 June 2002

Abstract Samples of lower Silurian (Liten and Kopanina formations) dark shales from the Barrandian basin, Czech Republic, were examined using reflected light microscopy, Fourier-Transform infrared spectroscopy (FT-IR) and gas chromatography-mass spectrometry (GC-MS). The samples contained uniform assemblage of organic material that included dominant graptolites, chitinozoa, bitumens and minor anthracite to meta-anthracite rank particle of unknown origin. Two types of graptolite morphologies were recognized: a nongranular blocky type and a granular type. Bitumen is also common in these rocks and forms discrete, oval bodies resembling droplets or angular particles with granular internal fabrics. The thermal maturity of the sediments was examined by measuring random reflectance of organic particles. The variations in graptolite reflectance in between three localities in the western part of the basin were relatively minor, suggesting a similar level of diagenetic transformation. The values range from 0.78% to 1.53% Rr. The reflectance of graptolites was correlated with the reflectance of chitinozoa and bitumen, and also indirectly with illite crystallinity data, which indicates thermal maturation levels in the hightemperature part of the oil window to the onset of wet gas/condensate zone. Mature petroleum that commonly impregnates veins crosscutting the sequence further supports a maturation level within the oil window. Graptolite and bitumen reflectance values markedly elevated above the regional diagenetic background were found in several centimeter-wide contact zones immediately adjacent to basalt sills that locally penetrate the Silurian strata. As shown by computer modeling, heating in the range of 600 – 800 jC, which lasted only several years, was sufficient to promote an increase in reflectance (up to 2 – 2.5% Rr) in the contact samples. With increasing thermal stress, the graptolite periderm undergoes progressive optical and structural changes being gradually converted into a highly condensed aromatic residuum structurally

* Corresponding author. Fax: +42-284-2134. E-mail address: [email protected] (I. Sy´korova´). 1 Present address: National Technical Museum, Kostelnı´ 42, 170 78 Prague 7, Czech Republic. 0166-5162/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 6 - 5 1 6 2 ( 0 2 ) 0 0 1 3 7 - 4

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and chemically similar to highly matured kerogen. On a basin-wide scale, however, the overall impact of basalt intrusions on organic maturity of enclosing shales was minimal. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Graptolite reflectance; Chitinozoan reflectance; Contact metamorphism; Hydrocarbon generation; Lower Paleozoic; Barrandian basin

1. Introduction Black shales are of enormous economic importance because they represent the bulk of the world’s hydrocarbon resource (Wignall, 1994). Many of the best known Mesozoic and Tertiary hydocarbon sources are transgressive black shales (Hallam, 1981; Creaney and Passey, 1993), and a number of studies have concentrated on their geochemical, sedimentological and paleogeographical aspects (Brooks and Fleet, 1987; Littke, 1993; see also Huc, 1995 and the references therein). Transgressive black shales of Lower Paleozoic age, however, are substantially less

understood, and systematic research in different parts of the world has only recently begun (Houseknecht et al., 1992; Cole, 1994; DaWang and Philp, 1997; Xiao et al., 2000; Lu¨ning et al., 2000a). In this study, we examine the organic petrology, geochemistry and maturity of lower Silurian transgressive black shales of the Barrandian basin in the central part of the Czech Republic (Fig. 1). Although no commercial sources of hydrocarbons have been found yet in the area, these sediments are of interest because of their specific organic matter assemblage, relatively low thermal maturity, and well-defined stratigraphical context. In order to evaluate the organic richness and better

Fig. 1. Simplified geological map of the Barrandian Basin.

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understand the depositional mechanism and thermal maturity of these shale facies, samples were collected from several representative outcrops and boreholes, and were subsequently analysed. Lower Silurian black shales, comparable to those of the Barrandian basin, are widespread in the United Kingdom (Leggett, 1980), Spain (Gutierrez-Marco and Sˇtorch, 1998), and Siberia (Makarov and Bazhenova, 1981). Age-correlative, organic matter-rich ‘‘hot’’ shales are also responsible for the origin of 80– 90% of Lower Paleozoic hydrocarbons in North Africa, and have played a major role in petroleum generation on the Arabian Peninsula (Jones and Stump, 1999; Lu¨ning et al., 2000a,b). Thus, the knowledge of Silurian black shales in the Barrandian basin can contribute to the understanding of regional and local factors that influenced lower Silurian organic matter accumulation worldwide.

2. Silurian black shales and their geological background During the Silurian time, the Barrandian basin occupied paleolatitudes between 30j and 40jS (Tait et al., 1995, 2000). The basin probably represented a narrow rift depression that developed over thinned continental crust (Havlı´cˇek, 1981). Significant Silurian extension was accompanied by the intrusions of numerous basalt (‘‘diabase’’) bodies along tectonic faults (Fiala, 1970). The sites chosen for this study belong stratigraphically to the Liten and Kopanina formations of lower to middle Silurian age (Fig. 2). On the basis of the Silurian paleogeography of Horny (1962) and Kukal (1985), the two organic matter-rich shaley formations are laterally correlative to deeper marine shelf environments and a marine slope environment partially influenced by active submarine volcanism (Fig. 3). The Liten Formation was deposited during a marine transgression (sea-level rise) following the late Ordovician (Hirnantian) fluvio-glacial deposits of the Kosov Formation (Brenchley and Sˇtorch, 1989). The lower part of the Formation, the Zˇelkovice Member, consists of dark fissile clay and siliceous shales that exhibit characteristic millimeter-scale lamination (Sˇtorch, 1986). The succeeding Litohlavy Member is a monotonous sequence of graptolitic shales interbed-

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ded with pale green, often calcareous claystones and mudstones. The deposition of the uppermost Motol Member was influenced by a distinct shallowing of the basin caused by syndepositional volcanic activity that affected both source rock quality and thickness regionally. Repeated volcanic activity generated a major, thick complex of basaltic lavas and volcanic tuffs that formed extensive submarine elevations along the northern rim of the basin (Fig. 3). Numerous sills of dolerite basalts (‘‘diabases’’) also intruded shallowly below the seafloor and thermally affected Silurian and Ordovician sediments (Sˇtorch, 1998). In the north and northeast, submarine volcanic seamounts were fringed by mixed volcanic-carbonate facies containing rich shallow-water benthic faunas (Krˇ´ızˇ, 1992). Graptolite-rich black shales originated in the relatively deeper south- and southwestern part of the basin. The shales probably accumulated on large, sediment-starved deeper shelves that formed following an early Silurian global rise in sea level (Sˇtorch, 1990). The depositional rate of the shales was slow, which resulted in a partially condensed stratigraphic sequence (Chlupa´cˇ, oral communication, 1996). The sedimentary environment was apparently quiet, hemipelagic, and completely or at least partially anoxic, with only periodical influence of high energy currents (Turek, 1983). Kukal (1985) and Sˇtorch and Pasˇava (1989) inferred, based on sedimentological evidence, that the depth at which the graptolite shales originated did not exceed some 150 –200 m. A similar paleogeographical situation also prevailed during the deposition of the overlying Kopanina Formation (Fig. 3). Sedimentation of dark graptolitic calcareous shales containing locally abundant tuffaceous material took place in a deep basin setting. In the north and northwest, shallow flats formed over redeposited volcaniclastic accumulations that provided a base for the local development of coral and stromatoporoid biostromes and bioherms. During the late Ludlow, a partial shallowing of the basin occurred, allowing for the deposition of a distinct cephalopod-rich limestone bed facies and the propagation of calciturbidite aprons into the basin (Suchy´ and Krhovsky´, 1996). The latest Silurian Prˇ´ıdolı´ Formation generally lacks any black shale lithologies and, therefore, was not a subject of the present study. This formation is characterized by uniform facies of rhythmically alter-

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Fig. 2. Silurian stratigraphy of the Barrandian Basin (modified from Chlupa´cˇ, 1988; Krˇ´ızˇ, 1998).

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Fig. 3. Idealized scheme of facies distribution of upper Liten and lower Kopanina formations, respectively (modified from Krˇ´ızˇ, 1998; Horny, 1962). The position of the localities mentioned in the text is also shown.

nating dark biomicrite and dark brown calcareous shale layers that are interpreted as distal calciturbidites and hemipelagic facies, respectively, deposited on a submarine slope (Suchy´, 1997). Throughout early Devonian time, the basin continued to develop with the deposition of shallow-water bioclastic and reef carbonate facies. During the Variscan orogeny (middle Devonian to late Carboniferous), the Lower Paleozoic sequences were eventually buried and exposed to temperatures in the range 90– 180 jC, which is in the oil and/or gas window (Suchy´ and Rozkosˇny´, 1996). The thickness of original post-Devonian overburden, now completely eroded, has been estimated as much as 1400 –3500 m (Suchy´ et al., 1996; Francu˚ et al., 1998).

3. Samples and methods We examined representative samples of dark Silurian shales from three sites located in the southwestern part of the Barrandian Basin (Fig. 3). Approximately 20 samples from each locality were investigated. Most

of the samples from the Liten Formation were collected at the Kosov quarry, Beroun County that exposes the upper part of the formation (upper Sheinwoodian to Homerian; Sˇtorch, personal communication). The shales are uniformly black, fissile and locally exhibit millimeter-scale fine lamination. The Liten shale is locally bituminous; small droplets and impregnations of semiliquid yellow to green organic fluids resembling bitumen were occasionally observed during crushing. These organic materials occur within the larger fossils (e.g. cephalopod shells), in diagenetic concretions, and also in thin secondary calcite veins that crosscut the shale. The total organic carbon (TOC) content of the Liten shale is variable and ranges widely throughout the basin from about 0.2% to 8%, with most of the values between 2% and 4% (Sˇtorch and Pasˇava, 1989). The samples from the Kosov quarry contain between 2.0% and 2.4% TOC (Volk, 2000), and are commonly enriched with uranium as well, with average concentrations of 10.5 – 12.8 ppm (Maresˇova´, 1978). At many locations, the Liten shales were penetrated by numerous dolerite basalt sills of variable thickness that range from 40 –

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50 cm to 4 –10 m. In order to evaluate the influence of the intrusions on the maturity of adjacent shale and to discriminate these thermal effects from those associated with burial heating, a series of samples was examined at various distances from the contacts of a typical 4-m-thick basalt sill, both above and below the intrusive body. To avoid possible weathering effects (Leythaeuser, 1973; Clayton and Sweetlend, 1978), all surficial sample material was collected from more than 30 cm below the surface. The samples of shales from the overlying Kopanina Formation were mostly taken from the cores of a shallow research bore (Klonk-1), which pierced the

upper part of the formation (Fig. 3; Suchy´, 1999; Herten, 2000) and also from the cores of a deep well (Tobolka-1; depth interval 650 –798 m). Macroscopically, the samples were dark brown, often finely laminated calcareous shale that alternate with dark grey micritic limestone layers. Both shale and limestone layers appeared to be bituminous and produced a strong petroliferous odour upon crushing. Occasional calcite veinlets impregnated with semiliquid bitumens were also observed in some cores. The maximum total organic carbon (TOC) content of the Kopanina Formation is 2.08%, with a mean of 1.33% (Herten, 2000).

Fig. 4. Microphotographs under reflected light showing characteristic matter of the Silurian shales. Sections perpendicular to bedding, planepolarized incident light, oil immersion. All samples, unless otherwise stated, are from the Liten Formation, Kosov quarry. A – D: Graptolite fragments [A—fragment of graptolite showing the periderm wall (P), common canal (CC), and an apartural spine (As). A section of thecal branching from the upper part of the common canal. B—Nongranular graptolite fragments of a colony of ‘‘rooted dendroids’’. C—Graptolite fragment displaying cortical laminated tissue. D—Transverse section of a graptolite stipe showing fusellar tissue which has the form of complete tubes for thecae. Some tubes were filled with brightly reflecting sulfide minerals]. E – F: Natural bitumens [E—Completely rounded, highreflecting bitumen in shale matrix. Note numerous devolatization vacuoles inside the bitumen droplet. F—Subrounded fragment of lowreflecting granular bitumen]. G—Longitudal section of a chitinozoa test showing chamber (ch), neck (n) and aperture (a). H—Fragment of recycled organic matter. Note irregular shape and highly anisotropic ‘‘salt and pepper’’ internal fabrics characteristic of anthracite rank (Rr f 2.6%). Kopanina Formation, Tobolka-1 borehole, 650.3 m.

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Fig. 4 (continued ).

Organic matter reflectance data were collected on shale chips cut and polished perpendicular to the bedding. Such approach has a distinct advantage in that it enables the spatial and textural relationship between organic particles and mineral matrix to be observed. This, in turn, provides additional valuable paleoenvironmental and sedimentological information (Stasiuk et al., 1991). An Opton-Zeiss UMSP 30 microscope equipped with a monochromatic light (k = 546 nm) source and attached to a microcomputer was used for the measurement. Random, minimum and maximum reflectance values of individual organic fragments were determined using the immersion objectives under oil (RI = 1.518). Infrared spectra of individual graptolite fragments were acquired with a Nicolet Magna 700 FT-IR spectrometer fitted with a Spectra Tech infrared microscope accessory (MCD detector). First, a background spectrum was obtained from a gold-coated mirror. Then whole rock samples were cut perpendicular to the bedding and polished to produce a

smooth surface for Fourier-Transform infrared spectroscopy (FT-IR) measurements of the graptolite particles. Spectra were obtained from sample areas as small as 20  10 Am. In total, 1024 interferograms of 1660 data points were co-added, and then transformed, producing an infrared spectrum with 4 cm 1 resolution. The measurements were performed in reflectance mode and the resulting spectra were subsequently converted into the Kubelka – Munk scale. Common spectral manipulations (smoothing, curve fitting, and resolution) were performed with OMNIC (version 3) and GRAMS/386 (Galactic) software, respectively. Chemical composition of the organic fluids extracted from the sediments was investigated by gas chromatography-mass spectrometry (GC-MS). Cleaned rock chips were crushed in an annealed agate mortar and extracted with rectified and purified npentane. After multiple mechanical extraction, the extracts (20 ml), evaporated at atmospheric pressure and room temperature were analysed on GC-MS.

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Instrumental conditions of the GC 5890 Hewlett Packard 5671 and GC-MS Finigan Magnum mass spectrometers were as follows: capillary column DB5-30 m  0.32 mm, helium as a carrier gas, 20 cm/min. The thermal gradient was 1 min at 60 jC, then 14j/min to 320 jC. Computer-aided solving of the unsteady-state heat conduction equation (Cˇerma´k et al., 1996) for a 3-D cylindrically symmetric sill was applied to evaluate the thermal influence of the intrusions on adjacent shales. The ‘‘average’’ intrusive body was approximated by a vertical cylinder 50 m in diameter and 4 m in height. These dimensions seem to approximate fairly well the typical size of the intrusions present in the basin. Thermal effects from latent heat of fusion of a crystallizing magma were approximated by the value 3.8  105 J kg 1, released gradually from 1150 to 950 jC (Galushkin, 1997).

4. Organic constituents Silurian dark shales of the Barrandian basin contain specific assemblage of organic matter that consist primarily of fragments of graptolites and chitinozoans. In addition, two types of natural bitumen and highly reflecting, apparently recycled, organic matter have also been recognized (Fig. 4). Graptolites are by far the most common type of organic macerals observed in the Barrandian shales. Graptolites represent planktonic to epiplanktonic colonial animals that occur in Lower Paleozoic (Ordovician – Lower Devonian) marine sediments (Goodarzi, 1984, 1985). The graptolite skeleton (periderm) is composed of a chitinous substance (Kozlowski, 1949; Combaz, 1980), or a collagen-like protein (Towe and Urbanek, 1972; Crowther and Richards, 1977; Liu et al., 1995), that exhibits optical properties in reflected light similar to that of vitrinite (Goodarzi and Norford, 1987; Hoffknecht, 1991). Distinct morphological features of graptolite fragments as they appear under reflected light have been described in detail in a series of earlier studies (e.g. Teichmu¨ller, 1978; Link et al., 1990; Goodarzi, 1990; Hoffknecht, 1991). Graptolites settled on the sea bottom with their largest dimension horizontal and, therefore, the largest surface areas for the determination of optical properties are often present in sections parallel to bedding. In this orienta-

tion, graptolites show true maximum reflectance, while the true minimum reflectance and the strongest bireflectance are observed on sections perpendicular to bedding (Goodarzi and Norford, 1989; Hoffknecht, 1991). Graptolite microstructures visible in the Barrandian shales cut perpendicular to bedding most commonly include sections of graptolite cortical tissues, common canals, and/or layered walls of the periderms (Fig. 4A – D). If cortical tissues are thick enough, graptolites can be recognized by their finely laminated structure (Fig. 4C). The colours of graptolites, observed microscopically on polished surfaces, range from medium grey to light grey. Two distinct types of graptolite material are present in the shales studied: nongranular and granular (sensu Goodarzi, 1984). Based on relief in polished section, the granular graptolites appear to be softer, lower reflecting and show weaker anisotropy, whereas the nongranular graptolite fragments are hard and brittle, high reflecting and exhibit strong optical anisotropy. The relation between the granular and non-granular fragments is complex and different opinions exist (see Goodarzi and Norford, 1985; Link et al., 1990 for a review). Nongranular fragments may represent the remains of the graptolite exoskeleton, whereas the granular fragments may form parts of the soft body of the graptolite that once occupied the common canal (Goodarzi, 1984; Riediger et al., 1989). They may also represent the cortical tissue composing the outer layer of the periderm (Link et al., 1990). Of these two optical types of graptolite remains, the nongranular appears to be predominant in Barrandian shales and most of the measurements reported here were made on this type. In a few cases, however, where optical properties of both graptolite types were measured in identical samples, the reflectance of the nongranular type was found to be systematically higher than those of the granular type, a conclusion that corroborates earlier findings by Goodarzi (1984) (Fig. 5). In sections cut perpendicular to bedding, random reflectance of nongranular graptolites ranges from 0.78% to 2.04% Rr that corresponds to maximum reflectance of 1.80% and 3.60% Rmax, respectively. The relatively large discrepancy between random and maximum values of graptolite reflectance measured in some samples was probably due to a rapid heating of graptolite periderm that occurred near igneous intrusions (Bustin et al., 1989; Goodarzi, 1990).

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Fig. 5. Plot of nongranular versus granular graptolite reflectance. Note the almost perfect linear relationship between the two variables.

Chitinozoans represent the second most common type of organic remains identified in the Silurian shales (Fig. 4G). Chitinozoans are an extinct group of marine microfossils whose systematic position remains unclear (Jansonius and Jenkins, 1978). They typically have hollow, organic-walled tests that are radially symmetrical about a central longitudinal axis and closed at one end, resembling a bottle. The tests of chitinozoans are probably made of polysaccharide – proteinic material, similar to that of graptolites (Mierzejewski, 1981). Optical reflectance of chitino-

zoan tests has been recently found as an excellent tool to establish thermal maturity of Lower Paleozoic rocks (Tricker, 1992; Obermajer et al., 1996). Reflectance of chitinozoan particles recognized in Barrandian shales ranges from 0.73% to 2.05% Rr and shows a good linear correlation with those of nongranular graptolites (Fig. 6). Besides abundant graptolite and chitinozoan fragments, two types of natural bitumens have been distinguished in the shales, based on their appearance and optical properties in reflected light: (1) a rounded,

Fig. 6. Crossplot of chitinozoan versus graptolite reflectance for samples of the Liten shale. See text for further details.

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nongranular solid bitumen (type A), and (2) an angular-to-rounded, granular solid bitumen (type B). The A-type bitumen forms discrete oval to circular bodies that resemble droplets, often with numerous internal pores (Fig. 4E). The latter type of bitumen has reflectance values that are remarkably close to those of nongranular graptolites in the same rocks, and range from 0.90% to 2.23% Rr (Fig. 7). The B-type bitumen (Fig. 4F) has a lower reflectance that varies from 0.30% to 0.60% Rr (Fig. 7). The reflectance heterogeneity of this granular variety appears to be due to small areas ( < 5 Am) of varying reflectance. Similar low reflectance bitumens are common in reservoir rocks and are referred to as reservoir bitumens (Gentzis and Goodarzi, 1990; Landis and Castan˜o, 1995). In general, the presence of solid bitumens in the shale suggests that hydrocarbons were generated and subsequently migrated. Both bitumen types exhibit distinct thermal overprints around the basalt sills that penetrate Silurian strata (see below). Therefore, the bitumens probably migrated into the rock unit prior to and/or during the intrusive thermal event(s). Highly reflecting, optically anisotropic fragments of organic matter of apparently allochtonous nature have been occasionally recognized only in the sediments of the Kopanina Formation. These distinctive organic particles occur preferentially in thin, isolated beds of dark laminated shale that appear to represent distal turbidite layers. They are characterized by a strong granular optical anisotropy (Khorasani et al., 1990) distinguishable under cross-polarized light (Fig. 4H). Under high magnification, some of the fragments

exhibit granular internal fabrics resembling the ‘‘pepper and salt’’ structure known from natural subgraphitic carbon (e.g. Diessel et al., 1978), and a reflectivity in the semi-anthracite to anthracite rank range ( f 2.5– 4.5% Rr). Isolated, kinked and eroded laths of a higher reflecting (>8 –10% Rr) semi-tographitic substance have also been recognized in some samples. Both anthracite and graphite particles are considered to represent fragments of thermally metamorphosed organic matter probably derived from metamorphic rocks and transported into deeper-water shale facies by turbidity currents.

5. Chemical composition of organic matter Rock– Eval pyrolysis data for dark shales of the Liten and Kopanina formations, extracted from several unpublished reports, are summarized in Fig. 8. The crossplot of hydrogen index (HI) against Tmax shows that the yields of hydrocarbon per unit mass organic matter were relatively good, and that the kerogen composition is intermediate between types II and III. Values of temperatures of maximum yield during pyrolysis (Tmax) indicate that the host shales attained thermal maturity corresponding to approximately 0.5 –1.0% Rr of vitrinite reflectance (Hunt, 1996). Upon extraction with n-pentane, samples of both Liten and Kopanina shales yielded comparable amounts of the extracts (0.029 and 0.037 wt.%, respectively). The gas chromatographic-mass spectro-

Fig. 7. Bitumen versus graptolite reflectance measured on samples of the Liten shale. See text for further details.

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Fig. 8. Crossplot of hydrogen index against temperature of maximum yield during pyrolysis (Tmax) for Silurian black shales, with maturation pathways for kerogen types and isoreflectance curves for vitrinite (Espitalie´, 1985) superimposed.

metric (GC-MS) analysis revealed that the extracts are geochemically very similar and consist of low molecular aliphatic hydrocarbons dominated by n-alkanes C13 – C34, which are typical of normal petroleum (Fig. 9A,B). Low Carbon Preference Index (CPI) values that range between 1.04 and 1.06 point to relatively advanced maturity of the petroleum, and corresponds to a vitrinite reflectance of about 1.2– 1.4% Rr (Hunt, 1996). The ratios between the isoprenoid hydrocarbons pristane, phytane and n-C17 and n-C18, which are frequently used as alternative temperature indicators of maturity (Tissot and Welte, 1984), are also low (0.14 –0.18 and 0.27– 0.51, respectively). This also broadly supports an advanced degree of thermal alteration of the petroleum. Comparable low values of pristane/n-C17 and phytane/nC18 have been shown as fairly typical for most of Barrandian Silurian and Ordovician organic matterrich sediments (Cˇejka et al., 1993). The pristane to phytane ratio that is commonly used to interpret the oil source-bed environment varies from 0.78 to 0.58 for the extracts of the Liten and Kopanina shales,

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respectively. A comparable low Pr/Ph ratio less than 1 typically occurs in some organic-rich anoxic carbonate sequences (Powell and McKirdy, 1973), or in some oils generated from Paleozoic carbonate source rocks (Illich and Grizzle, 1983). Nevertheless, for interpreting matured organic matter-rich sediments, such as those of the Silurian shales, the ratio should be used with care since it reaches a maximum at the beginning of catagenesis and then decreases with increasing thermal maturity (Boudou, 1984). We also analysed liquid to semiliquid yellow to brown-coloured hydrocarbon impregnations that occur within fractures crosscutting the shales of the Kopanina Formation (Fig. 10). The resulting chromatograms display a petroleum n-alkane composition that is similar to the whole rock extracts (Fig. 9C). Although no comparable geochemical characteristics of other Barrandian Lower Paleozoic formations are presently available, this compositional similarity may suggest that the petroleum impregnations within the Kopanina formation were probably generated within the sequence itself. The presence of terpane and sterane biomarkers that were identified in both the Kopanina shale impregnations, and whole rock extracts is characteristic and indicates that the maturity of the host rock probably has not reached a thermal maturity equivalent to 1.25% Rr vitrinite reflectance (Mackenzie, 1984). It is of interest to note that the petroleum-generating potential of the Kopanina formation has also been appreciated in an early study by Mala´n (1980), who classified these sediments as regional petroleum source rocks. In order to gain additional information on the chemical composition of Silurian organic matter, we investigated the maturity of the graptolite macerals that are the most ubiquitous and volumetrically predominant microscopic organic components of the shale. A series of graptolite samples, varying in maturity from 0.70% to about 2.0% Rr, was studied by micro Fourier-Transform infrared spectroscopy (FT-IR). The typical spectra of graptolite samples are shown in Fig. 11. The FT-IR spectra were obtained on the whole rock samples in which individual graptolite fragments were intimately mixed with the mineral matrix. Consequently, there is a contribution to the spectra from rock-forming minerals such as calcite, illite, and quartz, along with those of organic particles. In general, the typical FT-IR spectra of

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Fig. 9. Representative gas chromatography-mass spectrometry chromatograms of n-hexane extracts of Silurian shales of Liten (A) and Kopanina formations (B) and petroleum-like vein impregnations of the Kopanina Formation (C).

graptolite fragments appear rather similar to those of higher rank coals (Ibarra et al., 1996) and, to a lesser degree, of thermally matured kerogens (Rouxhet et al., 1980; Tissot and Welte, 1984; Rozkosˇny´ et al., 1994).

The graptolite material is characterized by: (1) a broad asymmetric peak between wavenumbers 3100– 3700 cm 1, which is assigned to OH hydroxyl groups; (2) a distinct peak centered around 1600 cm 1, which is

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Fig. 10. Field photograph showing the core of a dark micritic limestone of the Kopanina Formation crosscut by large, subvertical, petroleumfilled calcite vein. Note the dark fringe of the vein that is due to the petroleum staining. Arrow points to the top of the core. Klonk-1 borehole, 53.4 – 53.55 m.

interpreted as a skeletal aromatic CMC. This spectral region is, however, complicated by overlapping bands involving carbonyl stretching vibrations; (3) a broad and intense band at 1235 cm 1, which is considered to be associated with C – O bending vibrations, and O –H group deformation vibrations. In addition, the weak peaks between 600 and 900 cm 1 correspond to aromatic C – H out of plane deformation modes. These bands overlap with those of the mineral matrix of the

Fig. 11. Representative FT-IR spectra of two graptolite samples of contrasting thermal maturity. Liten shale, Kosov quarry. See text for more details.

host shale, particularly of clay minerals. Additional bands assigned to the vibration of esters (1740 cm 1), carboxyl groups (1709 cm 1), and the salts of carboxyl acids (1585 and 1550 cm 1) can also be distinguished in some samples in a broad region between wavenumbers 1550 and 1800 cm 1. The aliphatic C – H stretching and deformational vibrational bands in the IR spectra between approximately 2800 and 3000 cm 1 are of particular interest because of potential significant contributions to petroleum from these aliphatic structures (Tissot and Welte, 1984; Rouxhet et al., 1980). These vibrational bands can be used to evaluate chain length and the degree of branching that, in turn, reflect the degree to which a given organic matter has already produced petroleum hydrocarbons (Lopatin, 1983). The shorter the chain, the more branched aliphatic structures in the graptolite fragment are reflected in the IR spectra by a low CH2/CH3 band intensity ratio. A higher ratio indicates long and less branched aliphatic chains (Lin and Ritz, 1993). In this study, we applied a parameter based on the CH3 (2960 cm 1)/CH2 (2930 cm 1) intensity ratio to a series of graptolite fragments of variable thermal maturity collected around the igneous intrusions. We found that the intensity ratio I2960/I2930 correlates negatively with

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Fig. 12. CH3 (2960 cm 1)/CH2 (2930 cm 1) integrated intensity ratio versus optical reflectance for a series of graptolite fragments. The samples were collected at various distances from a 4-m-thick basalt intrusion. See text for the discussion.

optical reflectance of graptolite fragments (Fig. 12). This indicates that the aliphatic C – H bonds tend to diminish with an increasing degree of thermal transformation. This is similar to a decrease in aliphaticcontaining groups (CH, CH2, CH3) and depletion of aromatic C – H bonds in graptolite material with increasing thermal maturation found by Bustin et al. (1989).

6. Organic maturity and petroleum potential Since marine deposits of pre-Devonian age often contain little or no vitrinite, which is the most common maceral used in thermal maturity studies, precise evaluation of the thermal maturity of Lower Paleozoic sediments is problematic (Bertrand and He´ roux, 1987). Some workers have reported the occurrence of vitrinite-like materials in Cambrian –Ordovician sediments and employed these macerals to evaluate thermal maturation (e.g. Kisch, 1980; Buchardt and Lewan, 1990). Many other researchers have attempted to use colour changes of microfossils such as acritarchs

(Dorning, 1986; Williams et al., 1998) or conodonts (Epstein et al., 1977; Deaton et al., 1996). More recently, reflectance methods on materials such as graptolites (Goodarzi, 1990; Hoffknecht, 1991; Goodarzi et al., 1992; Cole, 1994), dispersed bitumens (Gentzis and Goodarzi, 1990; Landis and Castan˜o, 1995), and chitinozoans (Tricker et al., 1992; Obermajer et al., 1996) have also proved useful where no alternative maturation data can be gained from other techniques. In the Barrandian shales, graptolites and natural bitumens are the most common organic constituents and correlations between graptolite reflectance and traditional indices of organic maturation have already been published (Bertrand and He´roux, 1987; Goodarzi and Norford, 1989; Bertrand, 1990; Hoffknecht, 1991; Cole, 1994; Gentzis et al., 1996). Thus, in the present study, we use the reflectance of graptolite and bitumen particles to assess geothermal history and provide insight into the maturation level of the Silurian sediments. 6.1. The effect of igneous intrusions The most obvious thermal phenomena that apparently affected the maturity of Silurian shales were those associated with igneous sills. Numerous sills of doleritic basalts, the thickness of which ranges from several centimeters to several tens of meters, intersected Silurian strata at many localities throughout the basin. When the sills emplaced during late Wenlockian to early Ludlow time, the organic matter of adjacent shales underwent rapid carbonization, being exposed to a temperature of about 800– 900 jC (i.e. Hanson and Barton, 1989). We examined in detail the thermal alteration of Silurian graptolite-rich shale enclosing a 4-m-thick basalt sill in the Kosov quarry (Figs. 13 and 14). Graptolite remains, affected by an intense and relatively short-term heating close to the igneous body, show a range of interesting optical and micromorphological features (Fig. 13). Besides elevated reflectance (Rr) values, the samples exhibit significant optical anisotropy and therefore higher

Fig. 13. Reflected light microphotographs showing characteristic microscopic features (left) and respective optical reflectance patterns (right) of four graptolite fragments collected at various distances from a 4-m-thick basalt intrusion. Note that the graptolite fragments tend to develop diffuse margins (A, B, C, D) and small internal devolatization pores (D) as the thermal alteration increases. Intense sulfide mineralization of shale matrix, seen as minute brightly reflecting spots, is especially developed in the samples close to the contacts (C, D). See text for more details.

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Fig. 14. Diagram showing graptolite and bitumen reflectance values at various distances from a 4-m-thick basal sill. Note that the reflectivity of all organic macerals follow essentially the same metamorphic trend.

bireflectance. These changes are commonly accompanied by the development of small devolatization pores within the graptolite fragments that tend to become smaller and diffuse near the intrusive contacts (Fig. 13C,D). Similar changes in optical properties of organic fragments around magmatic intrusions have also been mentioned in some previous studies and were ascribed to a rapid reordering of the molecular structure of organic matter due to fast heating (Goodarzi et al., 1988; Khorasani et al., 1990). Contact metamorphism usually affects the maturity of the intruded rocks to an extent which depends on the thickness of the intrusion, the depth of emplacement, the rate of cooling, and/or the extent of hydrothermal convection around the igneous body (Peters et al., 1978). As demonstrated by Bostick (1973) and Dow (1977a,b), the width of the alteration zone varies between about one and two times the thickness of the intrusive body. In the present case, however, graptolite and bitumen reflectances show a sudden increase from a minimum outside the aureole, which corresponds to the regional ‘‘diagenetic background’’ ( f 0.78 – 0.98% Rr of graptolite reflectance), to more than 2%

Rr near the sill contacts (Fig. 14). Thus, elevated values of optical reflectance are, in fact, restricted to a narrow zone immediately adjacent to the intrusion contacts, with traceable alteration starting to take effect at about 70 –80% of the sill thickness. These values appear to be surprisingly low, given the initial temperature of basalt magma of at least 900 jC (Hanson and Barton, 1989). In order to provide a more detailed insight into the thermal alteration of Silurian sediments by igneous intrusives, we further applied the unsteady-state heat conduction equation (C`erma´k et al., 1996) for a cylindrically symmetric model of the sill. This algorithm simulates the thermal adjustment between an instantaneously emplaced two-dimensional intrusion and its host rocks. The input parameters are listed in Table 1. Fig. 15 illustrates the thermal history of the complete igneous/sedimentary column at different times after the igneous event. Each curve corresponds to the temperature as a function of the depth, at indicated times after intrusion. One month after the emplacement of the sill, the thermal signature of the igneous body can still be clearly recognized as a high-temperature spike. Peak sediment temperatures around the sill-intruded depth

V. Suchy´ et al. / International Journal of Coal Geology 53 (2002) 1–25 Table 1 Parameters adopted in the thermal model of the basalt intrusion Half width of intrusion Initial temperature of intrusion Surface temperature Heat of crystallization Depth below the surface Terrestrial heat flow Thermal conductivity

2 900 – 1200 10 3.8  105 5 – 180 60 1

m jC jC J kg 1 m mW m 2 Wm 1K

1

interval range from the initial intrusion temperature at contact to 80– 100 jC a few tens of meters away from the sill. These temperatures were attained during approximately 100 days to 4 –6 years after the intrusive event. The 1-year curve shows that peak temperatures at the sill site decrease rapidly and that the heat is transferred to the sediments in the vicinity of the sill, smoothing out the spike. Thereafter, the rocks cool down uniformly, and at about the 6-year stage, the whole section falls below 100 jC. Calculations show that the temperature perturbation declines to a few jC above the background temperature of 20 jC within 500 years. Fig. 16 shows the time – temperature curves of the sediment enclosing the sill at various times after

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the emplacement of the intrusion. Each curve of this diagram displays the time –temperature path of individual rock samples (Fig. 14) that have been examined for their organic matter reflectance. The convective circulation system that often develops above the intrusion (i.e. Peters et al., 1983; Krynauw et al., 1994) would somewhat lower the maximum temperatures and reduce the effective heating time experienced by the sediments close to the upper igneous contact. The diagram shows that the maximum temperatures in a given section generally did not exceed 800 jC. A noteworthy aspect is that the duration of the heating event was relatively short. Most intense heating lasted only for some 200– 250 days following the emplacement of the sill. Then, during a longer period of several years, the temperature perturbation gradually declined to a value near the geothermal gradient. Thus, the mathematical modeling essentially supports the data of organic matter reflectance measurements, in that the thermal influence of basalt intrusions on adjacent Silurian black shale was a limited and relatively short-lived event, the narrow contact zones immediately adjacent to the igneous contacts being affected most.

Fig. 15. Calculated thermal profiles across the basalt intrusion and its host sediment. The initial temperature of the intrusion was 1100 jC. Each curve is designated with respective time after intrusion (days).

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Fig. 16. Calculated time – temperature curves at various distances from the sill. The numbers of individual T – t curves relate to sample numbers as shown in Fig. 14. The initial temperature at the upper and lower contacts was 1100 jC. The somewhat different pattern of the curves above and below the sill is due to the influence of hydrothermal convection that dissipated the heat above the igneous body.

6.2. Regional maturity of Silurian shale Most of the examined graptolites in the Kosov section that were apparently unaffected by igneous intrusions possess Rr values that range from 0.78% to 0.98%. These reflectance values would correspond to a vitrinite reflectance equivalent of approximately 0.7 – 0.9% Rr when Cole’s (1994) conversion chart is used (Fig. 17) and can be considered as ‘‘diagenetic background’’ (Suchy´ et al., 2002). The chitinozoan reflectances measured in that section are approximately between 0.73% and 1.02%, which also points to a vitrinite reflectance equivalent of about 0.56 –0.82% Rr, using Tricker’s formula [Rv=(Rchit 0.08)/1.152] (Tricker, 1992). The reflectance of type II granular bitumen in the Kosov section ranges between 0.30% and 0.45%, and this would similarly translate to a vitrinite reflectance equivalent of 0.58 – 0.79% Rr, applying the formulas Rv = 0.618Rbit + 0.4 (Jacob, 1985), Rv=(Rbit + 0.41)/1.09 (Landis and Castan˜o, 1995) and Cole’s (1994) chart, respectively. That means that the organic matter in the Kosov section is within the oil window, or a mature stage in terms of hydrocarbon generation. Silurian shales in the Klonk

area, located in the southernmost corner of the basin, also appear to be within the oil window. Reflectance of vitrinite-like macerals with an average value of 1.02% Rr, which are consistent with the oil window maturation conditions, has been reported from the immediately overlying upper Silurian Prˇ´ıdolı´ Formation (Suchy´ and Rozkosˇny´, 1996). Cumulative Rock– Eval Pyrolysis data on the Lochkov, Prˇ´ıdolı´, and Kopanina formations in the Klonk-1 borehole fall between 435 and 450 jC of Tmax (Herten, 2000; Kranendonck, 2000), indicating a vitrinite reflectance level of about 0.71 – 0.92% Rr, through application of the conversion curves of Espitalie´ et al. (1984) and Peters (1986). Somewhat higher levels of organic maturity of Silurian shales were encountered in the Tobolka-1 borehole, about 2 km southeast from the Kosov quarry. At this site, Mala´n (1980) determined the reflectance of the rather vaguely defined ‘‘vitrinitic dispersinites’’ of the Kopanina Formation at about 1.20% Rr, while Rozkosˇny´ (personal communication; 1994) recorded the reflectance of primary bitumen in the immediately overlying Prˇ´ıdolı´ Formation at a level of 1.29% Rr (i.e. about 1.19% Rr of vitrinite, according to Jacob’s formula). Comparable values were recently confirmed

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Fig. 17. Chart showing the correlation between vitrinite, graptolite and bitumen reflectance and hydrocarbon generation and preservation zones (modified from Gentzis et al., 1996; Cole, 1994).

by E. Francu˚ (personal communication; 2000), who has reported graptolite reflectance values ranging from 1.15% to 1.53% Rr in dark shales of the Kopanina and Liten formations, respectively, corresponding to approximately 1.0 – 1.30% Rr of vitrinite, based on Cole’s (1994) chart. Therefore, most of the reflectance data presently available consistently show that in the study area the Silurian shales are within the oil window or at the transition between the oil and wet gas/ condensate zones.

7. Discussion The organic materials encountered in Barrandian Silurian shales include mainly graptolite particles, with minor chitonozoan fragments. In some samples, rounded particles of natural bitumens and highly reflecting grains of redeposited organic matter were also observed. This uniform assemblage of organic

macerals identified in Barrandian shales is comparable to those of other Lower Paleozoic sediments, particularly of Scandinavia (Goodarzi et al., 1988), Canada (Goodarzi et al., 1985; Bertrand and He´roux, 1987; Riediger et al., 1989), and China (Wang et al., 1993). Essentially all of the organic macerals recognized in Barrandian samples appear to be intrabasinal, with only a very small part being classified as redeposited, or of extrabasinal in origin. Rounded and/or highly coalified organic particles were recognized only in some isolated beds of the upper part of the Kopanina Formation and interpreted as turbidite deposits. An important paleogeographic implication for the depositional environment of Silurian shales is that deposition probably occurred at a considerable depth and/or relatively far from a land source that would supply many of the detrital components and recycled organic grains into the sediments. This, in turn, suggests that during the early Paleozoic time, the Barrandian basin probably represented a laterally extensive depositional

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system that spread far outside its present-day erosional margins (see Havlı´cˇek, 1981 for details of early paleogeographic models of the Barrandian). Infrared spectroscopy demonstrates that the most common microscopic organic material recognizable in the shale—the graptolites—comprise principally an aromatic structure with aliphatic groups that, at a higher level of thermal maturation, undergoes transformation towards a more highly conjugated and condensed aromatic structure. The latter appears to be rather similar to those of thermally mature kerogens, or high rank coals. This conclusion is somewhat surprising since some earlier workers have suggested that the graptolite periderm is constructed of a chitinous substance (Kozlowski, 1949) or, alternatively, of a collagen-like proteinic matter (Towe and Urbanek, 1972; Urbanek and Mierzejewski, 1986; Liu et al., 1995). Perhaps, thermally labile chitinous and/or protein materials may have comprised the fresh (i.e. thermally immature) graptolite exoskeleton that was not preserved at higher levels of thermal maturity (Link et al., 1990; Stankiewicz et al., 1998). Very early diagenetic replacement of original graptolite periderm by components probably derived from algal cell walls has been recently proposed by Briggs et al. (1995). Two populations of natural bitumens were found intimately associated with graptolite remains in many samples, indicating multiple paths of hydrocarbon generation – migration in these formations. Petrographical evidence shows that the rounded, nongranular bitumens (type A bitumen) may have been generated from the graptolites themselves as a result of increased thermal maturity. Moreover, the fact that this bitumen type develops similar optical properties to graptolites, when exposed to heat flow around the intrusions, suggests that there may be a similarity in chemical composition between the organic matter of bitumens and that of graptolites (e.g. Fig. 7). A certain similarity in optical properties of bitumens and graptolites has already been pointed out by Goodarzi (1984) and Gentzis and Goodarzi (1990), who attributed this optical convergence to comparable molecular structural changes, for example, an increase in aromaticity and ordering of aromatic carbon that develops in both types of organic materials with increased thermal maturity. Rock –Eval pyrolysis data indicate that the Barrandian shales contain kerogen which is geochemically

intermediate between types II and III. According to Tissot and Welte (1984), type II kerogen, which is common in many petroleum source rocks, is related to marine sediments with autochthonous organic matter. Kerogen type III is derived mainly from land plants, and is generally less favourable with respect to petroleum generation. Thus, the kerogen of the Barrandian Silurian shales appears to possess some potential with respect to both petroleum and/or gas generation. Similar judgements about the hydrocarbon generation potential of comparable Lower Paleozoic graptolitebearing sediments occurring elsewhere has also been proposed by Hoffknecht (1991) and Liu et al. (1995). Basalt intrusions that penetrate Silurian strata, though very abundant in the basin, had only limited thermal effect on the enclosing sediments. Organic matter reflectance measurements show that an appreciable increase of organic maturity was confined only to a ‘‘zone of influence’’, which was immediately next to the intrusions having an extent comparable to those of the intrusive bodies themselves. Nevertheless, unusually low reflectance values measured at the igneous contacts (i.e. graptolite reflectance f 2% Rr) are surprising and require some specific comments. We think that a likely explanation for these reduced thermal effects is that the sills probably intruded into soft, water-saturated claystones, located just below the seafloor, which has been demonstrated by independent sedimentological evidence (Sˇtorch, 1998). In this environment, the circulation of surface water probably cooled the initial magma temperature quickly, thus allowing only for a limited increase of organic matter reflectance in adjacent sediments. Low values of organic matter reflectance, generally not exceeding 2.0 – 2.5% Rr of graptolite, have also been recorded along intrusive contacts in other Barrandian localities where Silurian sequences were invaded by basalt sills (Mala´n, 1980; Hrabal, 1989), suggesting that the suppression of thermal influence may have operated on a basin-wide scale. In the Silurian strata that apparently were not affected by igneous contact metamorphism, the reflectance of graptolites vary between f 0.80% and 1.50% Rr. These reflectance levels, which are believed to represent a regional diagenetic background, are commonly associated with low illite crystallinity values that range between 1.1 and 0.65 Dj2h (Suchy´ and Rozkosˇny´, 1996; Suchy´ et al., 2002). Comparable

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values of organic and mineral metamorphism indicate mid- to high-grade diagenetic paleotemperatures, generally below 120 jC (see also Kosakowski et al., 1999; Ma¨hlmann, 2001). Collectively, this suggests oil window maturation conditions (T f 80 – 170 jC; Tissot and Welte, 1984; Quigley and Mackenzie, 1988) for Silurian rocks in the study area. As indicated by recent paleothermal studies (Francu˚ et al., 1998; Suchy´ et al., 2001), fluid inclusion (Dobesˇ et al., 1997; Volk et al., 2000) and apatite fission track analysis data (Filip and Suchy´, 1999), the Lower Paleozoic Barrandian sequences experienced burial to the depth of oil window during late Devonian to early Carboniferous (Mississippian) time. A 3-kmthick stratigraphic or tectonic overburden, now almost completely eroded, was probably responsible for producing temperatures in the range of 80– 120 jC in the western part of the basin. Further to the east and northeast, somewhat higher maximum burial paleotemperatures up to 140 –180 jC occurred, which are indicative of wet gas and gas condensates (Suchy´ and Rozkosˇny´, 1996; Suchy´ et al., 1996). The reason for these lateral paleothermal variations is not fully understood. The present data, nevertheless, clearly point to the potential petroleum perspectives of the Silurian shales, at least in the western, less thermally matured part of the basin. This interpretation disputes the previous conclusion of Mala´n (1980), who considered the Lower Paleozoic sediments of the whole Barrandian basin as overmature with respect to liquid hydrocarbons. We believe that much of this discrepancy was probably due to the fact that Mala´n (1980) used uncorrected graptolite reflectance values as being equal to vitrinite reflectance, thus shifting his measurements substantially higher. Petroleum perspectives of the western Barrandian are additionally supported by numerous small-scale occurrences of liquid or semiliquid hydrocarbons that occur in many shale samples. In particular, petroleumlike liquids were encountered inside calcite-filled fractures that cut through the Kopanina Formation in the Klonk-1 borehole. Similar findings, though not always precisely documented, were also noted elsewhere in literature. Sˇima´nek (1970), for instance, reported on the occurrence of a ‘‘petroleum-like slurry’’ in the Bykos-30 shallow uranium-exploration borehole that penetrated Liten black shale in the southermost part of the area studied (Fig. 3). More

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recently, Volk et al. (1998, 2000) described the occurrence of green and green-yellow hydrocarbon liquids in carbonate concretions in Silurian shales at the Kosov quarry, and elsewhere in the Barrandian basin. These hydrocarbons, when analysed, were identified as mature petroleums (CPI around 1.0) that were generally similar to those extracted from the adjacent Liten and Kopanina formations. Geochemical uniformity of hydrocarbons extracted from Silurian shales is not surprising in view of similar depositional conditions and uniform organic constituents present in the sequence. Also, small variations in thermal maturity of the extracts (as expressed by CPI and isoprenoid hydrocarbons ratios) between individual localities in the western Barrandian are in accord with the relatively uniform thermal maturity of dispersed organic matter. Thus, petroleum present within the Silurian strata offers another independent piece of evidence that during the geological past, strata in the Barrandian Basin generated hydrocarbons, a conclusion consistent with the results of recent time –temperature modeling (Francu˚ et al., 1998).

8. Summary and conclusions A regional study of the optical and geochemical properties of organic matter dispersed in dark Silurian shales of the Barrandian basin indicates the following. (1) Organic materials in Silurian shales (Liten and Kopanina formations) include nongranular graptolites (blocky and lath-shaped varieties), granular graptolites, chitinozoans, rounded nongranular bitumen, angular granular bitumen, and highly reflecting recycled organic particles. (2) In the western part of the Barrandian basin, graptolite reflectance values range from f 0.78% to 1.5% Rr. These relatively small lateral variations suggest a similar level of thermal maturity over the area. Associated reflectances of chitinozoans and bitumens that display a strong correlation with the graptolite reflectances point to a maturation level corresponding to about 0.80 –1.30% Rr of vitrinite reflectance equivalent, which is characteristic of the high-temperature part of the oil window zone. Mature petroleum impregnations are commonly associated with these sediments. A significant point resulting from this study is that the Silurian strata are not

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overmature with respect to hydrocarbon preservation, as has been indicated by some previous studies (e.g. Mala´n, 1980). The area should, therefore, be explored for potential oil/gas deposits. Organic metamorphism of the Silurian strata probably occurred during burial of the Lower Paleozoic sequence during late Devonian – early Carboniferous time. (3) Graptolite and bitumen reflectance levels significantly above the regional diagenetic background (up to 2 – 2.5% Rr) were found in narrow contact aureoles around mid- to late Silurian basalt sills that locally penetrate the sediments. Elevated values of organic matter reflectance were caused by geologically short (about several years) periods of high temperature (600 – 800 jC), which converted the graptolite organic matrix into a conjugated and condensed aromatic structure, similar to that of highly matured coals or kerogens. However, the basin-wide impact of basalt magmatism on the maturity of Silurian shales was probably negligible due to a limited extent of contact aureoles, as well as the apparently fast cooling rate of the intrusives. Acknowledgements We wish to express our gratitude to Cement Bohemia Praha and Geofond Prague, who provided us permission to collect rock samples from quarries and cored boreholes, respectively. Special thanks are due to Ms. J. Rajlichova, who prepared most of the figures and diagrams. Financial support provided by the Research Grants No. A30127703/1997 (GA AS CR) and No. 205/99/0594 (GA CR) and funding from the Key Research Projects (research area ‘‘Dynamics of lithospheric processes’’) of the Academy of Science of the Czech Republic, respectively, is gratefully acknowledged. The paper has benefited from constructive reviews of Dr. C.F. Eble (Kentucky Geological Survey) and Dr. F. Goodarzi (Geological Survey of Canada). References Bertrand, R., 1990. Correlations among the reflectances of vitrinite, chitinozoans, graptolites and scolodonts. Org. Geochem. 15, 565 – 574. Bertrand, R., He´roux, Y., 1987. Chitinozoan, graptolite and scolecodont reflectance as an alternative to vitrinite and pyrobitumen

reflectance in Ordovician and Silurian strata, Anticosti Island, Quebec, Canada. Am. Assoc. Pet. Geol. Bull. 71, 951 – 957. Bostick, N.H., 1973. Time as a factor in thermal metamorphism of phytoclasts (coaly particles). Septie`me Congre`s Internat. de Stratigraphie et de Ge´ologie du Carbonife`re, Krefeld, 1971; Compte Rendu 2, 183 – 192. Boudou, J.P., 1984. Chlorofom extracts of a series of coals from Mahakam Delta. Org. Geochem. 6, 431 – 437. Brenchley, P.J., Sˇtorch, P., 1989. Environmental changes in the Hirnantian (upper Ordovician) of the Prague Basin, Czechoslovakia. Geol. J. 24, 165 – 181. Briggs, D.E.G., Kear, A.J., Baas, M., Deleeuw, J.W., Rigby, S., 1995. Decay and composition of the hemichordate rhabdopleura—implications for the taphonomy of graptolites. Lethaia 28, 15 – 23. Brooks, J., Fleet, A.J., 1987. Marine petroleum source rocks. Geol. Soc. Spec. Publ. 26. Blackwell, Oxford, 444 pp. Buchardt, B., Lewan, M.D., 1990. Reflectance of vitrinite-like macerals as a thermal maturity index for Cambrian – Ordovician alum shale, southern Scandinavia. Am. Assoc. Pet. Geol. Bull. 74, 394 – 406. Bustin, R.M., Link, C., Goodarzi, F., 1989. Optical properties and chemistry of graptolite periderms following laboratory simulated maturation. Org. Geochem. 14, 355 – 364. Cˇejka, J., Holy´, L., Krtil, J., Krˇ´ıbek, B., Sedla´cˇek, V., 1993. Sources and thermal maturation of organic matter in sediments and kaustobiolites of the Bohemian Massif: Part I. Extractable compounds and vacuum-pyrolysates. Bull. Czech Geol. Surv. 68, 25 – 35. Cˇerma´k, V., Sˇafanda, J., Kresl, M., Kucˇerova´, L., 1996. Heat flow studies in Central Europe with special emphasis on data from former Czechoslovakia. Tecton. Metallog. 5, 109 – 123. Chlupa´cˇ, I., 1988. Sites of Geological Interests Near Prague. Academia, Praha, pp. 1 – 249 (in Czech). Clayton, J.L., Sweetlend, P.J., 1978. Subaerial weathering of sedimentary organic matter. Geochim. Cosmochim. Acta 42, 305 – 312. Cole, G.A., 1994. Graptolite – Chitinozoan reflectance and its relationship to other geochemical maturity indicators in the Silurian Qusaiba Shale, Saudi Arabia. Energy Fuels 8, 1443 – 1459. Combaz, A., 1980. Les ke´roge`nes vus au micrscope. In: Durand, B. (Ed.), Kerogen-Insoluble Organic Matter from Sedimentary Rocks. E´ditions Technip, Paris, pp. 55 – 111 (in French). Creaney, S., Passey, Q.R., 1993. Recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework. Am. Assoc. Pet. Geol. Bull. 77, 386 – 401. Crowther, P.R., Richards, R.B., 1977. Cortical bandages and the graptolite zooid. Geol. Palaeontol. 11, 9 – 46. DaWang, H., Philp, R.P., 1997. Geochemical study of potential source rocks and crude oils in the Anadarko basin, Oklahoma. Am. Assoc. Pet. Geol. Bull. 81, 249 – 275. Deaton, B.C., Nestell, M., Balsam, W.L., 1996. Spectral reflectance of conodonts: a step toward quantitative color alteration and thermal maturity indexes. Am. Assoc. Pet. Geol. Bull. 80, 999 – 1007. Diessel, C.F.K., Brothers, R.N., Black, P.M., 1978. Coalification and graphitization in high-pressure schists in New Caldonia. Contrib. Mineral. Petrol. 68, 63 – 78.

V. Suchy´ et al. / International Journal of Coal Geology 53 (2002) 1–25 Dobesˇ, P., Suchy´, V., Sedla´cˇkova´, V., Stanisova´, N., 1997. Hydrocarbon fluid inclusions from fissure quartz: a case study from the Barrandian basin (Lower Palaeozoic), Czech Republic. In: Boiron, M.C., Pironon, J. (Eds.), Proceedings of the XIVth European Current Research on Fluid Inclusions, Volume de Resumes. CNRS, Nancy, pp. 86 – 87. Dorning, K.J., 1986. Organic microfossil geothermal alteration and interpretation of regional tectonic provinces. J. Geol. Soc. (Lond.) 143, 219 – 220. Dow, W.G., 1977a. Kerogen studies and geological interpretations. J. Geochem. Explor. 7, 79 – 99. Dow, W.G., 1977b. Contact metamorphism of kerogen in sediments from Leg 41 Cape Verde rise and basin. Int. Rep. DSDP 41, 839 – 847. Epstein, A.G., Epstein, J.B., Harris, L.D., 1977. Conodont color alteration—an index to organic metamorphism. Geol. Surv. Prof. Pap. 995, Washington, 27 pp. Espitalie´, R., 1985. Use of Tmax as a maturation index for different types of organic matter. In: Burrus, J. (Ed.), Thermal Modelling in Sedimentary Basins. Editions Technip, Paris, pp. 475 – 496. Espitalie´, R., Marquis, F., Barsony, I., 1984. Geochemical logging. In: Voorhees, K.J. (Ed.), Analytical Pyrolysis. Techniques and Applications. Butterworth, London, pp. 276 – 304. Fiala, F., 1970. Silurian and Devonian Diabases of the Barrandian. Sb. Geol. Ved, Geol. 17, 7 – 89 (in Czech). Filip, J., Suchy´, V., 1999. Fission-track analysis (FTA) as a tool to reveal thermal evolution of rocks: first application in the Czech Republic. Geolines 8, 21 (abstract). Francu˚, E., Mann, U., Suchy´, V., Volk, H., 1998. Model of burial and thermal history of the Tobolka-1 borehole profile in the Prague basin. Acta Univ. Carol., Geol. 42, 248 – 249. Galushkin, Yu.I., 1997. Thermal effects of igneous intrusions on maturity of organic matter: a possible mechanism of intrusion. Org. Geochem. 26, 645 – 658. Gentzis, T., Goodarzi, F., 1990. A review of the use of bitumen reflectance in hydrocarbon exploration with examples from Melville Island, Arctic Canada. In: Nuccio, V.F., Barker, C.E. (Eds.), Applications of Thermal Maturity Studies to Energy Exploration. Soc. Econ. Paleont. Mineral. Spec. Publ. 26, pp. 23 – 36. Gentzis, T., deFreitas, T., Goodarzi, F., Melchin, M., Lenz, A., 1996. Thermal maturity of Lower Paleozoic successions in Arctic Canada. Am. Assoc. Pet. Geol. Bull. 80, 1065 – 1084. Goodarzi, F., 1984. Organic petrography of graptolite fragments from Turkey. Mar. Pet. Geol. 1, 202 – 210. Goodarzi, F., 1985. Dispersion of optical properties of graptolite epiderms with increased maturity in early Paleozoic organic sediments. Fuel 64, 1735 – 1740. Goodarzi, F., 1990. Graptolite reflectance and thermal maturity of Lower Paleozoic rocks. In: Nuccio, V.F., Barker, Ch.E. (Eds.), Applications of Thermal Maturity Studies to Energy Exploration. Soc. Econ. Paleont. Mineral. Spec. Publ., pp. 19 – 22. Goodarzi, F., Norford, B.S., 1985. Graptolites as indicator of the temperature histories of rocks. J. Geol. Soc. (Lond.) 142, 1089 – 1099. Goodarzi, F., Norford, B.S., 1987. Optical properties of graptolite periderm—a review. Bull. Geol. Soc. Den. 35, 141 – 147.

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Goodarzi, F., Norford, B.S., 1989. Variation of graptolite reflectance with depth of burial. Int. J. Coal Geol. 11, 127 – 141. Goodarzi, F., Snowdon, L.R., Gunther, P.R., Jenkins, W.A.M., 1985. Preliminary organic petrography of Palaeozoic from the Grand Banks, Newfoundland. Mar. Pet. Geol. 2, 254 – 259. Goodarzi, F., Stasiuk, L.D., Lindholm, K., 1988. Graptolite reflectance and thermal maturity of Lower and Middle Ordovician shales from Scania, Sweden. Geol. Fo¨ren. Stockh. Fo¨rh. 110, 225 – 236. Goodarzi, F., Gentzis, T., Harrison, Ch., Thorsteinsson, R., 1992. The significance of graptolite reflectance in regional thermal maturity studies, Queen Elizabeth Islands, Arctic Canada. Org. Geochem. 18, 347 – 357. Gutierrez-Marco, J.C., Sˇtorch, P., 1998. Graptolite biostratigraphy of the Lower Silurian (Llandovery) shelf deposits of the Western Iberian Cordillera, Spain. Geol. Mag. 135, 71 – 92. Hallam, A., 1981. Facies Interpretation and Stratigraphic Record. Freeman, Oxford. Hanson, R.B., Barton, M.D., 1989. Thermal development of lowpressure metamorphic belts: results from two-dimensional models. J. Geophys. Res. 94 (B8), 10363 – 10377. Havlı´cˇek, V., 1981. Development of a linear sedimentary depression exemplified by the Prague Basin (Ordovician – Middle Devonian; Barrandian area—central Bohemia). Sbor. Geol. Ved, Geol. 35, 7 – 48. Herten, U., 2000. Petrographische und geochemische Charakterisierung der Pelit-Lagen aus der Forschungsbohrung Knonk-1 (Suchomasty/Tchechische Republic). Internal Research Report 3751, Forschungszentrum Ju¨lich, 78 pp. Hoffknecht, A., 1991. Mikropetrographische, organisch-geochemische, Mikrothermometrische und mineralogische Untersuchungen zur Bestimmung der organischen Reife von Graptolithen – Periderm. Go¨tt. Arb. Geol. Pala¨ontol. 48, 98 pp. Horny, R., 1962. Das mittelbo¨hmische Silur. Geologie 11, 843 – 916. Houseknecht, D.W., Hathon, L.A., McGilverty, T.A., 1992. Thermal Maturity of Paleozoic Strata in the Arkoma Basin. Okla. Geol. Surv. Circ. 93, 122 – 132. Hrabal, J., 1989. A study on the reflectance of dispersed organic matter in selected samples of clayey shales of the Barrandian. Geol. Hydrometall. Uranu 13, 3 – 17 (in Czech). Huc, A.Y. (Ed.), 1995. Paleogeography, Paleoclimate, and Source Rocks. AAPG Studies in Geology, vol. 40. Tulsa, 347 pp. Hunt, J.M., 1996. Petroleum Geochemistry and Geology, 2nd ed. Freeman, New York, 744 pp. Ibarra, J.V., Mun˜oz, E., Moliner, R., 1996. FTIR study of the evolution of coal structure during the coalification process. Org. Geochem. 24, 725 – 735. Illich, H.H., Grizzle, P.L., 1983. Comment on ‘‘Comparison of Michigan basin crude oils’’ by Volger et al. Geochim. Cosmochim. Acta 47, 1151 – 1155. Jacob, H., 1985. Disperse solid bitumens as an indicator for migration and maturity in prospecting for oil and gas. Erdo¨l Kohle 38, 365. Jansonius, J., Jenkins, W.A.M., 1978. Chitinozoan. In: Haq, B.U., Boersma, A. (Eds.), Introduction to Marine Micropaleontology. Elsevier Biomedical, New York, pp. 341 – 357. Jones, P.J., Stump, T.E., 1999. Depositional and tectonic setting of

24

V. Suchy´ et al. / International Journal of Coal Geology 53 (2002) 1–25

the lower Silurian hydrocarbon source rock facies, central Saudi Arabia. Am. Assoc. Pet. Geol. Bull. 83, 314 – 332. Khorasani, G.K., Murchison, D.G., Raymond, A.C., 1990. Molecular disordering in natural cokes approaching dyke and sill contacts. Fuel 69, 1037 – 1046. Kisch, H.J., 1980. Incipient metamorphism of Cambro – Silurian clastic rocks from the Ja¨mtland Supergoup, central Scandinavian Caledonides, western Sweden: illite crystallinity and ‘‘vitrinite’’ reflectance. J. Geol. Soc. (Lond.) 137, 271 – 288. Kosakowski, G., Kunert, V., Clauser, Ch., Franke, W., Neugebauer, H.J., 1999. Hydrothermal transients in Variscan crust: paleotemperature mapping and hydrothermal models. Tectonophysics 306, 325 – 344. Kozlowski, R., 1949. Les graptolites et quelques nouveaux groups d’animaux due Tremadoc de la Pologne. Palaeontol. Pol. 31, 1 – 235. Kranendonck, O., 2000. Petrographische und geochemische Charakterisierung der Karbonatba¨nke aus der Forschungsbohrung Knonk-1 (Suchomasty/Tchechische Republic). Internal Research Report 3750, Forschungszentrum Ju¨lich, 115 pp. Krˇ´ızˇ, J., 1992. Silurian Field Excursions: Prague Basin (Barrandian), Bohemia. National Museum of Wales, Geological Series no. 13, 111 pp., Cardiff. Krˇ´ızˇ, J., 1998. Silurian. In: Chlupa´cˇ, I., Havlı´cˇek, V., Krˇ´ızˇ, J., Kukal, Z., Sˇtorch, P. (Eds.), Palaeozoic of the Barrandian (Cambrian to Devonian). Czech Geological Survey, Prague, pp. 79 – 101. Krynauw, J.R., Behr, H.J., Van Den Kerhof, A.M., 1994. Sill emplacement in wet sediments: fluid inclusion and cathodoluminescence studies at Crunehoga, western Drowning Maud Land, Antarctica. J. Geol. Soc. (Lond.) 151, 777 – 794. Kukal, Z., 1985. The Evolution of Sediments of the Bohemian Massif Czech Geological Survey, Prague, 223 pp. (in Czech). Landis, Ch.R., Castan˜o, J.R., 1995. Maturation and bulk chemical properties of a suite of solid hydrocarbons. Org. Geochem. 22, 137 – 149. Leggett, J.K., 1980. British Lower Palaeozoic black shales and their palaeoceanographic significance. J. Geol. Soc. (Lond.) 137, 139 – 156. Leythaeuser, D., 1973. Effects of weathering on organic matter in shales. Geochim. Cosmochim. Acta 37, 113 – 120. Lin, R., Ritz, G.P., 1993. Reflectance FT-IR microspectrometry of fossil algae contained in organic-rich shales. Appl. Spectrosc. 47, 265 – 271. Link, C.M., Bustin, R.M., Goodarzi, F., 1990. Petrology of graptolites and their utility as indices of thermal maturity in Lower Paleozoic strata in northern Yukon, Canada. Int. J. Coal Geol. 15, 113 – 135. Littke, R., 1993. Deposition, Diagenesis and Weathering of Organic Matter-Rich Sediments. Lecture Notes in Earth Sciences, vol. 47. Springer-Verlag, Berlin. Liu, D., Hou, X., Jiang, J., 1995. A study on composition and structure of graptolites using micro-FT-IR and TOF-SIMS. Sci. Geol. Sin. 4, 105 – 110. Lopatin, N.V., 1983. Obrazovanie goruitkikh iskopemykh (The Origin of Fossil Fuels). Nedra Publ. House, Moscow, 193 pp. (in Russian). Lu¨ning, S., Craig, L.S., Loydell, D.K., Sˇtorch, P., Fitches, B.,

2000a. Lower Silurian ‘‘hot shales’’ in North Africa and Arabia: regional distribution and depositional model. Earth-Sci. Rev. 49, 121 – 200. Lu¨ning, S., Loydell, D.K., Sutcliffe, O., Salem, A.A., Zanella, E., Craig, J., Harper, D.A.T., 2000b. Silurian – Lower Devonian black shales in Morocco: which are the organically richest horizons. J. Pet. Geol. 23, 293 – 311. Mackenzie, A.S., 1984. Application of biological markers in petroleum geochemistry. In: Brooks, J., Welte, D.H. (Eds.), Advances in Petroleum Organic Geochemistry. Academic Press, London, pp. 115 – 214. Ma¨hlmann, R.F., 2001. Correlation of very low grade data to calibrate a thermal maturity model in a nappe tectonic setting, a case study from the Alps. Tectonophysics 334, 1 – 33. Makarov, K.K., Bazhenova, T.K., 1981. Organic Geochemistry of the Paleozoic and Pre-Paleozoic of the Siberian Platform and the Perspectives of Oil and Gas. Nedra, Leningrad, 209 pp. (in Russian). Mala´n, O., 1980. Petrological investigation of dispersed organic matter (MOD) in the deep bore Tobolka 1. Fol. Mus. Rerum Nat. Bohem. Occident., C (Plzenˇ), 3 – 51. Maresˇova´, Z., 1978. Trace elements in Silurian sediments of the Barrandian. Ces. Kras (Beroun) 3, 7 – 22 (in Czech with German abstract). Mierzejewski, P., 1981. TEM study on ultrastructure of chitinozoan vesicle wall. Cah. Micropaleontol. 1, 59 – 70. Obermajer, M., Fowler, M.G., Goodarzi, F., Snowdon, L.R., 1996. Assessing thermal maturity of Palaeozoic rocks from reflectance of chitinozoa as constrained by geochemical indicators: an example from southern Ontario, Canada. Mar. Pet. Geol. 13, 907 – 919. Peters, K.E., 1986. Guidelines for evaluating petroleum source rocks using programmed pyrolysis. Am. Assoc. Pet. Geol. Bull. 70, 318 – 329. Peters, K.E., Simoneit, B.R.T., Brenner, S., Kaplan, I.R., 1978. Vitrinite reflectance – temperature determinations for intruded Cretaceous black shales in the eastern Atlantic. In: Oltz, D.F. (Ed.), Low Temperature Metamorphism of Kerogen and Clay Minerals, Symposium in Geochemistry. The Pacific Section of the SEPM, Los Angeles, CA, pp. 53 – 58. Peters, K.E., Whelan, J.K., Hunt, J.M., Tarafa, H.F., 1983. Programmed pyrolysis of organic matter from thermally altered Cretaceous black shales. Am. Assoc. Pet. Geol. Bull. 67, 2137 – 2149. Powell, T.G., McKirdy, D.M., 1973. The effect of source material, rock type and diagenesis on the n-alkane content of sediments. Geochim. Cosmochim. Acta 37, 523 – 633. Quigley, T.M., Mackenzie, A.S., 1988. The temperature of oil and gas formation in the subsurface. Nature 333, 549 – 552. Riediger, C., Goodarzi, F., Macqueen, R.W., 1989. Graptolites as indicators of regional maturity in Lower Paleozoic sediments, Selwyn Basin, Yukon and Northwest Territories, Canada. Can. J. Earth Sci. 26, 2003 – 2015. Rouxhet, P.G., Robin, P.L., Nicaise, G., 1980. Characterization of kerogens and their evolution by infrared spectroscopy. In: Durand, B. (Ed.), Kerogen. Insoluble Organic Matter From Sedimentary Rocks. Edition Technip, Paris, pp. 163 – 190. Rozkosˇny´, I., Machovicˇ, V., Pavlı´kova´, H., Hemelı´kova´, B., 1994. Chemical structure of migrabitumens from Silurian Crinoidea,

V. Suchy´ et al. / International Journal of Coal Geology 53 (2002) 1–25 Prague Basin, Barrandian (Bohemia). Org. Geochem. 21, 1131 – 1140. Sˇima´nek, V., 1970. Naftove´ geochemicke´ posouzenı´ silursky´ch hornin z jv. cˇa´sti Barrandienu [Petroleum-geochemical evaluation of Silurian rocks from the souteastern part of the Barran´ strˇednı´ u´stav geologicky´ Praha, pobocˇka Brno, 5 pp. dian]. U (unpublished internal report no. P 22150, Geofond, Prague, in Czech). Stankiewicz, B.A., Mastalerz, M., Hof, C.H.J., Bierstedt, A., Flannery, B., Briggs, E.G., Evershed, R.P., 1998. Biodegradation of the chitin – protein complex in crustacean cuticle. Org. Geochem. 28, 67 – 76. Stasiuk, L.D., Osadetz, K.G., Goodarzi, F., Gentzis, T., 1991. Organic microfacies and basinal tectonic control on source rock accumulations: a microscopic approach with examples from an intracratonic and extensional basins. Int. J. Coal Geol. 19, 457 – 481. Sˇtorch, P., 1986. Ordovician – Silurian boundary in the Prague Basin (Barrandian area, Bohemia). Sbor. Geol. Ved, Geol. 41, 69 – 103. Sˇtorch, P., 1990. Upper Ordovician – lower Silurian sequences of the Bohemian Massif, central Europe. Geol. Mag. 127, 225 – 239. Sˇtorch, P., 1998. VIII. Volcanism. In: Chlupa´cˇ, I., Havlı´cˇek, V., Krˇ´ızˇ, P., Sˇtorch, P. (Eds.), Palaeozoic of the Barrandian (Cambrian to Devonian). Czech Geological Survey, Prague, pp. 149 – 164. Sˇtorch, P., Pasˇava, J., 1989. Stratigraphy, chemistry and origin of the Lower Silurian black graptolitic shales of the Prague Basin (Barrandian, Bohemia). Vestn. Ustred. Ust. Geol. 64, 143 – 162. Suchy´, V., 1997. Sedimentological research of the Prˇ´ıdolı´ Formation (Upper Silurian), the Barrandian area: preliminary results. Zpravy o geologickych vyzkumech v roce 1996. Czech Geological Survey, Prague, pp. 131 – 132 (in Czech). Suchy´, V., 1999. Stratigraphical Research on the Silurian – Devonian Boundary Beds at Klonk Section, Barrandian Basin (Czech Republic), Progress Report I: Preliminary Results of the Klonk-1 Borehole Internal Research Report of the Institute of Geology CAS, Prague, 29 pp. Suchy´, V., Krhovsky´, J., 1996. Cephalopod limestones of the Barrandian Basin (Silurian), Czech Republic: sedimentary environments and stratigraphic significance. In: Mutti, M., Simo, T., Weissert, H., Baker, P. (Eds.), Carbonates and Global Change: An interdisciplinary Approach, SEPM/IAS Research Conference, June 22 – 27, 1996. Wildhaus, Switzerland, pp. 131 – 132 Abstract Book. Suchy´, V., Rozkosˇny´, I., 1996. Diagenesis of clay minerals and organic matter in the Prˇ´ıdolı´ Formation (Upper Silurian), the Barrandian Basin, Czech republic: first systematic survey. Acta Univ. Carol., Geol. 38, 401 – 409. Suchy´, V., Rozkosˇny´, I., Zˇa´k, K., Francu˚, J., 1996. Epigenetic dolomitization of the Prˇ´ıdolı´ formation (Upper Silurian), the Barrandian basin, Czech Republic: implications for burial history of Lower Paleozoic strata. Geol. Rundsch. 85, 264 – 277. Suchy´, V., Filip, J., Sˇafanda, J., Sy´korova´, I., Stejskal, M., 2001. Thermal history of sedimentary basins of central Bohemia as evidenced by organic matter reflectance, fission-track analysis and clay mineral diagenesis: current progress. 9th Coal Geology Conference, Prague 2001, Book of Abstracts. Charles University, Prague, p. 41.

25

Suchy´, V., Dobesˇ, P., Filip, J., Stejskal, M., Zeman, A., 2002. Conditions for veining in the Barrandian Basin (Lower Palaeozoic), Czech Republic: evidence from fluid inclusion and apatite fission track analysis. Tectonophysics 348, 25 – 50. Tait, J., Bachtadse, V., Soffel, H., 1995. Upper Ordovicina paleogeography of the Bohemian Massif—implications for Armorica. Geophys. J. Int. 122, 211 – 218. Tait, J., Bachtadse, V., Dinares-Turell, J., 2000. Paleomagnetism of Siluro – Devonian sequences, NE Spain. J. Geophys. Res. Solid Earth 105, 23595 – 23603. Teichmu¨ller, M., 1978. Nachweis von Graptolithen – Periderm in geschieferten Gesteinen mit Hilfe kohlenpetrologischer Methoden. Neues Jahrb. Geol. Pala¨ontol., Monatsh. 7, 430 – 447. Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence, 2nd ed. Springer, Berlin, pp. 1 – 699. Towe, K.M., Urbanek, A., 1972. Collagen-like structures in Ordovician graptolite periderm. Science 277, 443 – 445. Tricker, P.M., 1992. Chitinozoan reflectance in the Lower Palaeozoic of the Welsh Basin. Terra Nova 4, 231 – 237. Tricker, P.M., Marshall, J.E.A., Badman, T.D., 1992. Chitinozoan reflectance: a Lower Palaeozoic thermal maturity indicator. Mar. Pet. Geol. 9, 302 – 307. Turek, V., 1983. Hydrodynamic conditions and the benthic community of upper Wenlockian calcareous shales in the western part of the Barrandian (Kosov quarry). Cˇas. Mineral. Geol. 28, 245 – 260. Urbanek, A., Mierzejewski, P., 1986. A possible new pattern of cortical deposits in Tremadoc dendroid graptolites from chert nodules. In: Hughes, C.P., Rickards, R.B. (Eds.), Palaeoecology and Biostratigraphy of Graptolites. Geol. Soc. Amer. Spec. Paper 20, pp. 13 – 19. Volk, H., 2000. Source rocks, bitumens and petroleum inclusions from the Prague Basin (Barrandian, Czech Republic)—constraints for petroleum generation and migration from petrology, organic geochemistry and basin modelling. PhD Thesis, Der Fakulta¨t fu¨r Bergbau, Hu¨ttenwesen und Geowissenschaften der Rhenish-Westfa¨lisch-Technisches-Hochschule (RWTH) Aachen, 337 pp. Volk, H., Suchy´, V., Sy´korova´, I., Francu˚, E., Mann, U., Wilkes, H., 1998. Bitumens and Fluid Inclusions from the Barrandian Basin (Czech Republic). Erlanger Geol. Abh., Sonderbd. 2, 104 – 105. Volk, H., Mann, U., Burde, O., Horsfield, B., Suchy´, V., 2000. Petroleum inclusions and residual oils: constraints for deciphering petroleum migration. J. Geochem. Explor. 69 – 70, 595 – 599. Wang, X., Hoffknecht, A., Jianxin, X., Li, Z., Chen, S., Brocke, R., Erdtmann, B.H., 1993. Thermal maturity of the Sinian and early Paleozoic in West Hubei, China, assessed by CAI, reflectance and geochemical studies. Stratigr. Paleontol. China 2, 19 – 45. Wignall, P.B., 1994. Black Shales. Clarendon Press, Oxford, 127 pp. Williams, S.H., Burden, E.T., Mukhopadhyay, P.K., 1998. Thermal maturity and burial history of Paleozoic rocks in western Newfoundland. Can. J. Earth Sci. 35, 1307 – 1322. Xiao, X.M., Wilkins, R.W.T., Liu, D.H., Liu, Z.F., Fu, J.M., 2000. Investigation of thermal maturity of Lower Palaeozoic hydrocarbon source rocks by means of vitrinite-like maceral reflectance—a Tarim Basin case study. Org. Geochem. 31, 1052 – 1941.