The Potential Source of Unconventional Gas in the Baltic Basin (Lithuania)

BALTICA Volume 22 Number 2 December 2009 : 89-99

Organic matter of Early Silurian succession – the potential source of unconventional gas in the Baltic Basin (Lithuania)

Onytė Zdanavičiūtė, Jurga Lazauskienė

Zdanavičiūtė, O., Lazauskienė, J. 2009. Organic matter of Early Silurian succession – the potential source of unconventional gas in the Baltic Basin (Lithuania). Baltica, Vol. 22 (2), 89–98. Vilnius. ISSN 0067–3064.

Abstract Increasing demand for alternative energy sources leads to an interest in unconventional hydrocarbons, e.g. natural shale gas. Based on initial evaluation, the Early Silurian graptolite shales are studied as prospective shale gas sources in the Baltic region. This paper addresses the occurrence, generation potential and the volume of unconventional gas in place. New organic geochemistry data and vitrinite like macerals reflectance measurements, incorporated with well logs and core data were used to reveal the potential for shale gas. The Early Silurian shales are organically rich, type II, oil–prone marine sediments. The organic matter content ranges from 0.7 up to 19.2%. Hydrocarbon generation potential ranges from 7 up to 113 kg HC/t rock, and the Hydrogen Index ranges from 136 up to 571 mg HC/g TOC. Maturity of the organic matter increases southwestwards from

0.6 up to 1.94% (Ro). The Early Silurian shales in Western Lithuania generated large amounts of hydrocarbons with an estimated volume of expelled hydrocarbons of about 13.75 × 106 kg. This mass of hydrocarbons is equivalent to 389 × 105 cubic meters of methane for a section of 1 km2 and 150 m thickness.

Keywords Organic matter, graptolite shales, Early Silurian, Western Lithuania, shale gas potential.

Onytė Zdanavičiūtė [[email protected]], Institute of Geology and Geography, T. Ševčenkos 13, Vilnius 03223; Jurga Lazauskienė [[email protected]], Vilnius University, Čiurlionio 21/27, Vilnius, Lithuania. Manuscript submitted 7 May 2009; accepted 22 October 2009.

iNTRODUCT ION specific low–cost horizontal drilling. The worldwide demand, high energy prices and recent advances in The worldwide increasing of oil and natural gas horizontal drilling has enabled shale gas production consumption has given considerable interest in the new to become profitable (Horsfield, Schulz, GASH team alternative energy sources. So–called unconventional 2008). Shale gas is exploited in the United States of natural gas that includes coal bed methane, shale America and in 2006 production reached 1.1 tcf/year of gas and tight sand gas, is of great interest nowadays. unconventional gas that comprises 5.9% of annual gas Unconventional gas is the same substance as production (Jarvie et al. 2007). In that context, regions conventional natural gas, it just accumulates in of Europe, usually considered as unperspective in terms different than sandstone or carbonate reservoirs. The of conventional fossil fuels, might contain targets for shale gas is self–sourced thermogenic or biogenic shale gas exploration (Horsfield, Schulz, GASH team methane, stored in free, adsorbed and dissolved states 2008). Based on the preliminary evaluation of the shale in reservoirs, predominantly composed of fine grained gas potential in Europe, organic rich black shales of organic–rich low permeability matrix – mostly shales different age, e.g. Cambrian–Early Ordovician Alum (Jarvie et al. 2007). The gas shales are often both the shale, the Early Silurian Graptolite shale, Early Jurassic source rocks and the reservoir for the natural gas. The Posidonia shale, Carboniferous Namiurian and Early shale gas production in commercial quantities requires Cretaceous marine shales have been defined as the natural and artificial fractures to provide permeability, most perspective formations (Horsfield, Schulz, GASH by applying the hydraulic fracturing technologies and team 2008).

89 voir properties (determined by porosity, permeability, fracturing and adsorption potential) of the shales. As a result of recent hydrocarbon exploration activi- ties, the Early Silurian succession in Lithuania has been quite intensively studied, but only in terms of conventional source rocks potential, mostly focusing on the lithofacies and diagenetic aspects, reservoir properties of the succession (Lapinskas 1996, 2000; Musteikis 1993; Paškevičius 1997; Lazauskiene et al. 2002, 2003; Zdanaviciute, Lazauskiene 2004, 2007). Until now no studies have been carried on of the Early Silurian succession in terms of the distribution, generation potential and the prospectivity of shale gas (neither on any of the other unconventional hydrocarbon types). Thus, this paper brings forward an analysis of the Early Silurian shaley succession in the Western Lithuania with aims to predict the generation, occurrence and the most probable potential of the shale gas. Evaluation of the vertical and lateral occurrence including heterogeneities in content and composition of the organic mater together with estimation of the methane gas volume generated throughout evolution of the Baltic Sedimentary Basin (as a function of maturity Fig. 1. Location of the Baltic Silurian Basin showing the and organic mater type) and prediction of the shale gas distribution of Silurian lithofacies (modified after Lazau resources will also be dealt within in this study. skiene et al. 2003). A–B – line of analyzed cross–section presented in Fig. 2. 1–North German–Polish Caledonides; The distribution of shales, prospective in terms of 2–non–deposition craton area; 3–deep shelf facies: 3a–black shale gas, is determined based on existing well log, core graptolite shales; 3b–shaley facies; 4–open shelf facies and seismic data, while the evaluation of the generation (marlstones and limestones); 5–lagoonal facies (limestones, potential and volumes of the methane gas is based on dolomites); 6–Caledonian deformation front; 7–location of cross–section A–B; 8–recent erosional limit of Silurian the results of the analytic studies of the organic matter strata; 9–boundaries of facial zones. and vitrinite reflectance.

The Early Palaeozoic organic rich Upper Cambrian–Tremadoc black shales (Alum shales) and the Early Silurian graptolite shales are considered as the most prospective for the shale gas potential in the Baltic Sea region. While the Cambrian–Tremadocian Alum shales occur mostly in Scandina- via (Buchardt, Lewan 1990), the Silurian shales are distributed in the territory of Lithuania (Figs 1, 2). Within the Silurian succession in Lithuania, the oil source rocks comprise the complex of Llan- dovery, Wenlock and the Early Ludlow shaley strata. For the evaluation of the shale gas potential the key issues are to predict the volume of gas in place Fig. 2. Lithofacial cross–section throughout the Silurian succession in Lithuania. and reservoir qualities of shaley Line of cross–section A–B is shown in Fig. 1. 1–black shales with graptolites; layers. Gas generation potential of 2–carbonaceous claystones; 3–clayey marlstones; 4–carbonaceous marlstones; 5–clayey limestones; 6–limestones; 7–organogenicdetrital limestones; 8–clayey gas shales is mostly controlled by limestones and marlstones; 9–dolomites; 10–dolomites and gypsum; 11–clayey the TOC content, type and maturity dolomites; 12–dolomites interbedded with marlstones; 13–reefogenic buildups; of the organic matter and the reser- 14–boundaries of Silurian stages and/or formations.

90 Geological setting deepen south–westward: in the east the depth is ~200 m whereas in the west – 2050 m (Fig. 3). The thickness of The Baltic Sedimentary Basin (BSB) developed as a the Early Silurian strata varies in a range of 40 meters, passive margin basin during the latest Precambrian– the thinnest section of 120 m recorded in the central Middle Ordovician (Poprawa et al. 1999). The change and south–westernmost part of the area (adjacent to to convergent margin setting related to the collision wells Ramučiai–1, Vabalai–1), whereas the thickness processes along the south–western rim of the Baltica increases towards the north and southwest (adjacent tectonic plate is recorded during the Late Ordovician– to wells Mikoliškės–1) or show the locally restricted Early Silurian (Lazauskiene et al. 2002). The western increase in thickness adjacent to some wells (e.g. and central parts of the basin have subsided throughout Stoniškiai–1; Fig. 4). the Silurian time, while the easternmost part shows the The facial composition of the Silurian succession gradual uplift since the Late Silurian. The sedimentary in the BSB shows considerable heterogeneity, being evolution of the BSB during the Silurian is recorded related to changes in basin architecture during the Silu- in a more than 1 km thick succession of shales and rian and different sedimentary palaeoenvironments. carbonates in the Lithuania offshore (Lapinskas Mostly terrigenous facies (graptolite shales and clay- 2000; Lazauskiene et al. 2003) and thins eastwards stones) dominate in the western and central parts of the to 50–100 m (Fig. 2). The structural evolution of the Lithuania, grading into marlstones in the transitional area has had a significant impact on formation of the zone and to the limestone and dolomite facies, form- petroleum play of the BSB. Western Lithuania area is ing the carbonate platform in the East (Figs 1, 2). The strongly dissected by faults, penetrating the crystalline Early Llandovery terrigenous–calcareous succession basement and the Early Palaeozoic succession. The was formed in response to the major sea level fall at the faults, dissecting the sedimentary cover are oriented Ordovician–Silurian boundary. Up to 40 m thick Early N–S, W–E, NW–SE and NE–SW predominantly. Two Llandovery basal shallow water organogenic–detrital major systems of late Caledonian reverse faults, carbonates, accumulated in the eastern marginal zone, oriented WSW–ENE and SSW–NNE prevail in the pass to the south–west into greenish dark claystones studied area (Fig. 3; Sliaupa et al. 2002). The local and shales (Lapinskas 2000; Lazauskiene et al. 2003). uplifts are confined to SSW–NNE trending faults, Middle–Late Llandovery in the Western Lithuania showing rather regular spacing. The Silurian strata is predominantly composed of thin laminated black and dark grey graptolite shales, cyclically interbedded with grey and red– colored claystones and marlstones (see Fig. 2). High percentage of mudstones presented within the Llandovery section indicates that they were presumably deposited within a deep–shelf setting under dysaerobic to anaerobic conditions (Lapinskas 2000; Lazauskiene et al. 2003). The Early Silurian transgression reached a maximum at the end of Llandovery and the deposition of Early Wenlock dark graptolite shales occurred within the generally regressive sea–level mode. The Wenlock section in the western part is represented by dark grey graptolite shales and clayey marlstones with rare interlayers of clayey limestones, deposited within the deep–marine environments, while dolomitic marlstones with very fine–grained and organogenic–detrital limestones prevail in the eastern and central facial zones. Early Ludlow dolomitic marlstones Fig. 3. The depth of occurrence of the base of the Early Silurian in Western Lithuania of eastern part pass westwards into (modified after Čyžienėet al. 2006). 1–faults; 2–isohipses; 3–the shore line; 4–the state boundaries of the Republic of Lithuania; 5–well. dark grey and black claystones,

91 The location map of the distribution of the sediments, that could be considered as „black shales” and the lithological profile are shown in Fig. 2. In this paper results of such the geochemical studies are sum- marized: vitrinite reflectance data, total organic carbon (TOC) analysis data, Rock Eval pyrolysis, determi- nation of the composition of saturated hydrocarbons by means of gas chromatography (GC–12 samples in total), and biomarker analysis by means of gas chromatography– mass spectrometry (GC–MS–12 samples in total). The analyses were carried out at the laboratories of Polish Geological Institute, Denmark and Greenland Geologi- cal Survey and SINTEF Petroleum Research Institute (Norway). Preliminary information about content of total organic carbon (TOC) in the rocks was determined by combustion LECO IR212 induc- tion furnace after elimination of carbonates and silicates by solving Fig. 4. Thicknesses of the Early Silurian in Western Lithuania. 1–faults; 2–isopahs; them in hydrochloric acid. Informa- 3–the shore line; 4–the state boundaries of the Republic of Lithuania; 5–wells. tion about the type of organic matter and degree of the maturity was obtained adopting a relatively simple, deposited within the regressive environments. Late inexpensive and efficient method of Ludlow is represented by grey calcareous marlstones pyrolysis – so–called Rock Eval analysis, performed interbedded with light organogenic–detrital limestones using a Delsi Model II Rock Eval instrument. in the west and organogenic–detrital limestones and dolomites in the eastern and central parts of the basin The gas chromatography was performed on (Lazauskiene et al. 2003). The Pridolian sedimentation Hewlett–Packard 5890 gas chromatographers. Gas has been marked by the general transgressive trend – chromatography and mass spectrometry of saturated tidal flat and lagoonal dolomites, dolomitic marlstones hydrocarbons were performed with the HP5971A and fragmental limestones gave way to open shelf spectrometer coupled to an above mentioned gas biomicritic marlstones in the central part and, to the chromatographer. west, to deeper water shales in the Early Pridolian sec- The maturity of the organic matter was determined tion. Light grey limestones with marlstone interbeds by measuring reflectance indices of vitrinite like mac- comprise the Late Pridoli in the east, passing westwards erals. The measurements were performed on polished into grey marlstones and calcareous claystones (Fig. thin sections (1–2 mm) or on concentrated indigenous 2; Lazauskiene et al. 2003). kerogen (isolated from rock matrix) by means of po- larised microscope of Aksioskop type (Zeiss). Meas- MATERIAL and METHODS urements were performed in monochromatic light at a wavelength 546 nm through immersion oil. The mean The data describing the general structure, occurrence reflectance value (R %) has been derived for the every and composition of the Silurian sequence came o from well, core, well log, seismic data and results thin section to average the measurements of the differ- of analytic studies. The analytic studies were ent vitrinite like macerals. performed on selected samples of organic matter All these standard geochemical methods and equip- dispersed in the clayey–muddy Silurian deposits. In ment are described in detail in the previous publica- total 80 core samples from of 20 boreholes from the tions, related to the geochemical studies of the organic Western Lithuania were analyzed. Part of the data matter and petroleum potential in the Baltic Sedimen- has previously been published by the senior author tary Basin (Zdanaviciute, Lazauskiene 2004, 2007). (Zdanaviciute, Lazauskiene 2004, 2007) with emphasis In order to unravel the occurrence, the structural of the petroleum potential. framework and the thickness distribution of the Early

92 Silurian shales, a wide set of well, 2D reflection seis- claystones (Kadūnienė 2001). Still, taking into account mic and core data from nearly 200 wells, located in the lithological composition of the Early Silurian shale the Western Lithuania, have been employed (Figs 2, strata only the Middle Llandovery–Wenlock organic 3, 4). rich graptolite shales would be considered in terms of the „black shales” in this paper. Kerogen type, organic richness and The results of the analysis showed the syngenetic, thermal maturity sapropelic, marine origin organic matter together with the vitrinite like macerals and abundant charred The most favourable conditions for the accumulation remains prevailing. Liptinite comprises the major part of the organic matter in the Baltic Sedimentary Basin of the studied organic matter; its content reaches or during Silurian time were in the deep–shelf setting. Here even exceeds 20%. Liptinite and dispersed liptodentrite successions of black and dark–grey–colored graptolite particles have been found in sapropelic organic matter shales, interbedded with claystones, marlstones and and, more rarely, as the scattered particles. Fragments dark–colored microcristaline limestones were formed. and well–preserved particles of tasmanites, quite The organic matter content within the sediments of this particular for the sapropelic marine sediments, were complex can reach up to 9–16.5% or more (Kadūnienė also frequently observed. Sporinite, cutinite and the 2001). This organic rich complex comprises the organic mater originated from resins or wax, are quite Middle Llandovery–Early Ludlow strata, and is rare (Zdanavičiūtė, Swadowska 2002). regarded as one of three major source rock intervals Based on RockEval screening pyrolysis data, the within the Baltic Sedimentary Basin (Zdanaviciute, organic matter has been attributed to sapropelic organic Lazauskiene 2004, 2007). The thickness of the Middle matter of type II kerogen, as classified by Espitalie Llandovery–Early Ludlow succession reaches up et al., 1985. Type II kerogen originates from mixed to 360 m in the southwestern part of Lithuania. The phytoplankton, zooplankton, and bacterial debris usu- content of the organic matter within the section varies ally in marine sediments (Peters et al. 2005). Thus, the quite considerably both vertically and laterally – the organic matter of type II comprises the major source most organic rich rocks are recorded in the lowermost rocks’ levels in Lithuania and Baltic Sedimentary Basin part of the interval (within the Middle Llandovery (Zdanavičiūtė, Bojesen-Koefoed 1997). shaley strata) and gradually decrease upwards. The The total organic carbon (TOC) ranges up to 19.2% average content of the organic matter (Corg) in the (wt.) with the highest values recorded predominantly in Middle Llandoverian graptolite shales reaches up to black shales of the Early Silurian (Table 1). Hydrogen

1.6%, in the Wenlock section the Corg values is 1.2% Index (HI) and Production Index (PI) are parameters and decreases to 0.8% in the Early Ludlow shales and that describe the hydrogen content of the organic matter

Table 1. Rock Eval analyses data (selected samples with TOC >1%) and calculated „vitrinite“ reflectance data

(Ro) for Lower Silurian deposits.

Well name Depth, m Lithology TOC Tmax S1 S2 HI PP PI Ro Akmenė 71 1367 Claystone 7.34 432 1.46 37.35 509 38.81 0.04 0.616 Baubliai 1 1813 Claystone 19.2 442 10.06 103.76 540 113.82 0.09 0.796 Geniai 1 1754 Claystone 6.72 443 2.88 19.77 294 22.65 0.13 0.814 Girdžiai 48 1373.5 Claystone 3.74 437 0.82 15.61 417 16.43 0.05 0.706 Klaipėda 1 2040.3 Claystone 5.58 448 3.69 16.24 291 19.93 0.19 0.904 Kuliai 1 2031.6 Claystone 3.78 442 3.02 15.11 400 18.13 0.17 0.796 Lauksargiai 1 1671.1 Claystone 17.2 442 6.98 76.17 443 83.15 0.08 0.796 Lauksargiai 3 1688.8 Claystone 3.52 446 3.87 14.3 406 18.17 0.21 0.868 Laužai 1 1962.5 Claystone 9.03 445 3.07 41.03 454 44.1 0.07 0.85 Malukai 1 1612 Claystone 9.72 425 2.25 55.53 571 57.78 0.04 0.49 Mamiai 1 1686.2 Claystone 1.16 443 0.17 1.65 142 1.82 0.09 0.814 Mikoliškės 1 2031 Claystone 11.19 442 4.58 37.57 336 42.15 0.11 0.796 Nida 1 2044.2 Claystone 11.1 443 4.38 42.52 383 46.9 0.09 0.814 Paluknė 1 1662 Claystone 2.03 437 0.33 6.38 315 6.71 0.05 0.708 Pociai 3 1891 Claystone 6.87 443 3.64 19.75 287 23.39 0.16 0.814 Ramučiai 1 2009.1 Claystone 4.49 446 3.89 6.01 134 9.9 0.39 0.868 Rukai 1 1894.1 Claystone 10.8 440 5.64 35.23 326 40.87 0.14 0.76 Šatrija 1 1727.6 Claystone 14.1 440 6.27 68.82 488 75.09 0.08 0.76 Vilkaviškis-131 1192 Claystone 4.07 429 0.7 13.75 338 14.45 0.05 0.562

TOC – total organic carbon (weight %); Tmax – max temperature of pyrolysis, °C; S1 – amount of free hydrocarbons (kg HC/t rock);

S2 – amount of pyrolizable hydrocarbons (kg HC/t rock); HI – Hydrogen index (100 × S2/TOC); PP – petroleum potential S1+S2

(kg HC/t rock); PI – Production Index (S1 / S1+S2); Ro – „vitrinite“ reflectance data calculated from maxT .

93 Table 2. The organic matter composition of hydrocarbons and biomarker parameters of thermal maturity for Lower Silurian (selected samples).

homoho- Asphal Aro- hopane/ sterane Well Saturate, Polar, Pr/ Ph/ hopane/ pane tene, matic, CPI Pr/Ph moreta- izom. name % % n-C n-C sterane izom. % % 17 18 ne ratio ratio Akmenė-71 50.9 7.6 6.8 85.6 1.00 0.93 0.47 1.74 0.94 0.85 0.54 0.23 Baubliai-1 37.6 9.4 21.6 69.0 1.00 0.52 0.27 2.42 1.93 0.83 0.56 0.35 Geniai-1 14.6 30.8 13.3 55.9 1.10 0.76 0.51 1.89 1.03 0.89 0.61 0.49 Geniai-1 11.3 34.1 15.0 50.9 1.10 0.68 0.39 2.23 0.98 0.89 0.60 0.46 Mikoliškės-1 18.9 31.7 14.6 53.7 1.02 0.42 0.28 2.06 1.0 0.88 0.58 0.49 Ramučiai-1 4.13 77.8 5.8 16.4 1.02 0.19 0.18 1.24 bd bd bd bd

CPI – Carbon Preference Index calculated as 2×sum (n-C 23–31)/2×sum (n-C24–30) + n-C22 + n-C32 v; Pr/n-C 17 – pristane/n-heptadecane ratio; Ph/n-C 18 – phytane/n-octodecane ratio; Pr/Ph – pristane/phytane ratio; bd – component is below detection. and the amount of hydrocarbons generated from the the Silurian shales that contain predominantly marine, organic matter. The highest HI values of 540 mg HC/g algal–derived organic matter in the Baltic Silurian Ba-

TOC are were recorded in the Baubliai–1 well. The sin. Tmax has been compared with other thermal maturity majority of the organic rich samples (TOC > 1.0%) data, e.g. vitrinite reflectance and biomarker data. have HI values of 300–400 mg HC/g TOC. The reflectance of vitrinite is one of the standard The thermal maturity of the organic matter in the optical thermal maturity indices. The vitrinite com- studied area was defined integrating a wide set of data prises macerals derived from land plants and is one

– Rock Eval Tmax and production index data (Table 1); of the primary components of most types of kerogens the reflectance measurements obtained from vitrinite (Peters et al. 2005). However, the majority of the suc- like macerals and the biomarker analysis data obtained cession of the Early and Middle Palaeozoic age does from indigenous rock extracts (Table 2). not contain vitrinite marcels, as the land–plants had

Rock Eval Tmax values of the Early Silurian shales in not yet evolved in pre–Devonian times. Therefore, in Western Lithuania range from 425 to 448oC, gradually order to determine the thermal evolution for the Early increasing with depth (Table 1, Fig. 5). PI values have and Middle Palaeozoic sediments often are used the been defined in a range of 0.04–0.39. The distribution reflectance of the vitrinite like macerals. Buchardt and of Tmax values imply that the maturity of the organic Lewan (1990) suggested that vitrinite like macerals matter increases to the west. The peak oil generation from the Middle Cambrian to Lower Ordovician Alum („oil window”) was recorded in southWestern Lithua- Shales section in South Scandinavia respond to heating nia. It is worthwhile to mention that the extremely high in a similar way to that of what they call „suppressed maturity values are locally recorded within the area vitrinite” in shales of the younger age (in contrast to adjacent to Ramučiai and Rukai wells (Fig. 6). „coal vitrinite”). The authors described this for natural The values of Tmax and PI also are partly dependant samples and thermally immature samples which were on the type of the organic matter. Thus, Tmax can not be artificially matured using hydrous pyrolysis. regarded as a direct indicator of thermal maturity for The measured data of vitrinite like macerals reflect-

ance and data of Tmax, converted to vitrinte reflectance equivalent, are presented in Table 1 and Fig. 5. For the conversion the following equation has been adopted:

Ro (calculated from Tmax) = 0.0180×Tmax – 7.16 As the Early Palaeozoic oils in the Baltic basin are described as low–sulphur (Zdanaviciute, Lazauskiene, 2004, 2007), such a conversion is applicable also for shales containing low–sulfur type II and type III kerogen (Jarvie et al. 2001), thus, could be successfully adopted to study the Early Palaeozoic succession on Lithuania. Still, the equation is of limited use to analyze o samples with very low maturity (where Tmax is < 420 C) or extremely high maturity, where S2 (second peak of Rock Eval pyrogram) is less than 0.50 mg HC/g rock Fig. 5. Tmax and „vitrinite” reflectance (measured and calculated from Tmax) versus depth. (Peters et al. 2005).

94 The data, presented in Fig 6, show the mean reflectance values

ranging from 0.6 to 0.7% Ro in the north–eastern part of Lithuania and

gradually increasing to 0.8–0.9%Ro in the southwestern, the deepest buried part of the basin. The reflectance values, implying thermal maturities beyond the peak of liquid hydrocarbons generation were

recorded in Klaipėda–1 (0.87% Ro) and Rukai–1 (1.01% Ro) wells. The measurements in Ramučiai–1 well show extremely high mean reflect-

ance values of 1.94% Ro, implying the late gas generation phase. Such a locally increased thermal maturity of the organic matter can probably be explained in terms of magmatic Fig. 6. Sketch map of the distribution of mean „vitrinite” reflectance values in intrusions in this area (Kepežinskas the Silurian source rocks in Lithuania (modified after Zdanaviciute, Lazauskiene 2007). et al. 1996), or geothermal anomalies of different origin, rather than by the depth of occurrence of sediments. Such a palaeothermal anomalies also are supported by di- abase intrusions identified by well, seismic, gravity and magnetic data in the Late Silurian and, rarely, in the Cambrian–Early Silurian succession in the Baltic Sea (Motuza et al. 1994; Sliaupa et al. 2002). Moreover, the radiogenic heating, related to the emplacement of anarogenic granites that contain increased amounts of K, U, Th, also must be considered (Sliaupa et al. 2005).

The composition of the organic matter

The group composition of the extractable organic matter (mostly depending on the biological origin of the organic matter) indicates the polar (S+N+O) and saturated hydrocarbons dominating over the aromatic hydrocarbons. Polar hydrocarbons prevail in the composition of the organic matter, usually comprising from 16.4 up to 85.6% of the total amount, while the content of asphaltenes varies in the rage of 11.3–50.9% (Table 2). The depositional environments and the level of the thermal maturity of the organic matter have Fig. 7. Plot of the distribution of n-alkanes in the Early Silurian organic matter. been assessed by the analysis of (a)–well Ramučiai–1, depth 2009.1 m; (b)–well Nida–1, depth 2044.2 m. the saturate hydrocarbon fractions

95 by gas chromatography–mass spectrometry methods, of high molecular weight. As the maturity of the or- with the use of different biomarker parameters that are ganic matter of source rocks is directly related to the commonly applied in the geochemical studies. Analy- depth of occurrence, the maximum of the maturity is sis of GC chromatograms shows that most of saturate recorded in the southwestern part of Lithuania. The fractions compounds are dominated by n–alkanes in samples from this area typically are characterized by the range from n–C11 to n–C18 carbon atoms in the high hopane/moretane ratio (mostly is > 0.8), high molecule with one maximum (Fig. 7a) Therefore, the homohopane isomerisation ratio (0.54–0.61) and mod- results of the analysis of the shale extract from well erate to high sterane isomerisation ratio (0.23–0.49).

Nida–1 (2044.2 m depth) showed a bimodal, smooth Moreover, relatively high content of the C30 diahopane n–alkane distribution with modes at n–C15 and n–C25, was recorded in some of wells in the western part of as well as moderate amount of acyclic isoprenoids Lithuania. The organic matter in well Ramučiai–1 is (Fig. 7b). It could be tentatively suggested that hy- over mature, thus, it was not possible to estimate the drocarbons from this sample were derived from two maturity in this well by using biomarker of saturate different sources, one of which had relatively „waxy” hydrocarbon parameters. composition. Presumably, the shales with the high C20+ n–alkanes content in extractable organic matter (such Organic matter transformation n–alkanes distribution is recorded in the Early Silurian and volumes of hydrocarbons samples of Western Lithuania) will show lower gas– generated to–oil ratio (GOR) values that is consistent with the oil maturity window; whereas samples with low C20+ hydrocarbons content (< 5%) will have higher GOR The generation potential of hydrocarbons strongly values (Jarvie et al. The2007). generation potential of hydrocarbonsdepends strongly on the depends original onquantity the original (original quantity total organic (original total organic The pristane/phytane (Pr/Ph) ratio in the Early carbon TOCo) and quality (original hydrogen index carbon TOCo) and quality (original hydrogen index HIo) of the organic matter in thermally immature source Silurian shales varies in the range from 1.24 to 2.42. HIo) of the organic matter in thermally immature This parameter, commonlyrocks (Peters indicating et al. 2005). the oxicityThe assessment source of rocks the hydrocarbon (Peters et al generation. 2005). The potential assessment of the of Early Silurian of the depositional environments,The generation however, potential is of also hydrocarbons the hydrocarbon strongly depends generation on the potential original of quantity the Early (original total organic maturity dependant shales(Peters in et the al .Western 2005). TheLithuania ratio of is basedSilurian mostly shales on inavailable the Western Rock–Eval Lithuania data is based(see Table mostly 1). Using the odd–to–even n–alkanescarbon – Carbon TOC oPreference) and quality Index (original on hydrogen available index Rock HI Evalo) of data the (seeorganic Table matter 1). Using in thermally the immature source (CPI), calculated forequations, the C –C proposed n–alkanes, by showsof G. E. the Claypool (Peters et al. 2006), the extent of the organic matter conversion to rocks23 (Peters32 et al. 2005). The assessmentequations, of proposedthe hydrocarbon by of G. generation E. Claypool potential (Peters of etthe Early Silurian values close to 1.0 forpetroleum all the samples (so–called analyzed. fractional Since conversion) al. 2006), was the determined. extent of the By organic making matter some assumptions,conversion the fractional CPI ratio values are maturityshales independant the Western and Lithuaniadecrease is tobased petroleum mostly (so–calledon available fractional Rock–Eval conversion) data (see Table was 1). Using the with increasing thermalconversion maturity (f), of originalthe organic TOC mattero, amount determined. of expelled hydrocarbonsBy making some (SIexpelled assumptions,) and expulsion the efficiency (ExEf) (Tissot, Welte 1984), valuesequations, about proposed 1.0 indicate by of highG. E. Claypool (Peters et al. 2006), the extent of the organic matter conversion to fractional conversion (f), original TOCo, amount maturity of studied organicwere derived matter. from the change in original hydrogen index (HIo) and original production index (PIo) to present petroleum (so–called fractional conversion)of expelled was hydrocarbons determined. By(SI makingexpelled) andsome expulsion assumptions, the fractional The analysis of theday fractionsvalues of ofthese the indexessaturated (HI hy -and efficiencyPI ; Peters et(ExEf) al. 2005). were derived from the change in drocarbons of high molecular weight and content ofp p conversion (f), original TOCo, amountoriginal of expelledhydrogen hydrocarbons index (HIo) and (SI originalexpelled) and production expulsion efficiency (ExEf) steranes and triterpanes forThe the extentshaley ofstrata the withorganic shal matter- index conversion (PIo) to present(fractional day conversion) values of these to the indexes liquid (HI andp gaseous were derived from the change in original hydrogen index (HIo) and original production index (PIo) to present low depth of occurrence would allow more accurate and PIp; Peters et al. 2005). assessment of the degreehydrocarbons of catagenesis. (f) could beHowever, assessed by adoptingThe extent the of equation: the organic matter conversion (frac- the organic matter of theday Early values Paleozoic of these indexessuccession (HI p andtional PIp conversion); Peters et al to. 2005). the liquid and gaseous hydrocar- HIp {1200 - [HIo / (1- PIo)]} comprises low content of theThe saturated extentf = 1 hydrocarbons−of the organic matterbons conversion(f) could be (fractional assessed(1); by conversion) adopting the to theequation: liquid and gaseous HIo {1200 -[HIp / (1- PIp)]} hydrocarbons (f) could be assessed by adopting the equation: here PIp – the present day production index (measured by means of Rock–Eval pyrolysis HIp {1200 - [HIo / (1- PIo)]} f = − (1); PIp=S1/(S1+S2), that indicate1 the amount of generated hydrocarbons); we assume that HIo – the original HIo {1200 -[HIp / (1- PIp)]} hydrogen index and PIo – the original production index for the most of immature source rocks are equal to here PIp – the present day production index (measured by means of Rock–Eval pyrolysis 500 and 0.02 respectively (Peters et al. 2005); 1200 – the maximal amount of hydrocarbons that could be here PIp – the present dayPIp production=S1/(S1+S2 ),index that (measuredindicate the amounthydrocarbons of genera thatted couldhydrocarbons); be formed we assuming, assume that that HI o – the original by means of Rock Evalformed pyrolysis assuming, PI =S that/(S organic+S ), that carbon organic comprises carbon 83.33% comprises of total 83.33% hydrocarbon of total hydrocarbon composition (Peters et al. hydrogen indexp and1 PI1 2– the original production index for the most of immature source rocks are equal to indicate the amount of generated hydrocarbons);o we composition (Peters et al. 2005). assume that HI – the2005). original hydrogen index and PI – o 500 and 0.02 respectively (Peterso et alThe. 2005); extent 1200 of fractional– the maximal conversion amount of of thehydrocarbons Early that could be the original production indexThe for extent the most of fractional of immature conversion of the Early Silurian shales has been calculated based on data from source rocks are equalformed to 500 assuming, and 0.02 respectivelythat organic carbonSilurian comprises shales has83.33% been ofcalculated total hydrocarbon based on data composition from (Peters et al. (Peters et al. 2005);well 1200 Ramu – thečiai maximal–1 (Table amount 1). of well Ramučiai–1 (Table 1). 2005). 136 {1200 - [500 / (1- 0.02)]} The extentf = 1 −of fractional conversion of the Early= 0Silurian.832 ; shales has been calculated based on data from 500 {1200 -[136 / (1- 0.4)]} well Ramučiai–1 (Table 1). Therefore, 83% of the hydrocarbon generation process in the area of the Western Lithuania adjacent to Therefore, 83% of the hydrocarbon generation136 {1200 to- [500 well /Ramučiai–1 (1- 0.02)]} has been completed. well Ramučiai–1 fhas = 1been− completed = 0.832 ; process in the area of the Western Lithuania adjacent500 {1200 -[136 / (1- 0.4)]} The original total organic carbon (TOCo) content in the source rocks before they get matured, could be 96 Therefore, 83% of the hydrocarbon generation process in the area of the Western Lithuania adjacent to calculated adopting the mass balance equation: well Ramučiai–1 has been completed 83.33× HIp× TOCp The originalTOCo = total organic carbon (TOCo) content in the source rocks (2) before they get matured, could be [HIo× (1- f)× (83.33- TOCp) + HIp× TOCp] calculated adopting the mass balance equation: Thus, the original content of TOC for the area adjacent to well Ramučiai–1 is calculated as: 83.33× HIp× TOCp TOC = (2) o × 83.33× ×136× 4.5 + × TOCo = [HIo (1- f) (83.33- TOCp) HIp TOCp]= 7 .06 wt.%; [500× (1- 0.832)× (83.33 - 4.5) +136× 4.5] Thus, the original content of TOC for the area adjacent to well Ramučiai–1 is calculated as: The estimated value implies that the mature Early Silurian shales in well Ramučiai–1 contain high 83.33×136× 4.5 TOCo = = 7.06 wt.%; amount of organic matter (4.5[500 wt%)× (1.- Moreover,0.832)× (83.33 before- the4.5) maturation +136× 4.5] sediments in this well contained excellent quantities of the organic matter– 7.06 wt%. The estimated value implies that the mature Early Silurian shales in well Ramučiai–1 contain high amount of organic matter (4.5 wt%). Moreover, before the maturation sediments in this well contained excellent quantities of the organic matter– 7.06 wt%. The generation potential of hydrocarbons strongly depends on the original quantity (original total organic

The generationcarbon TOC potentialo) and ofquality hydrocarbons (original hydrogenstrongly depends index HI ono) theof the original organic quantity matter (originalin thermally total immature organic source

carbonrocks TOC o(Peters) and quality et al. 2005). (original The hydrogen assessment index of the HI ohydrocarbon) of the organic generation matter in potential thermally of theimmature Early Siluriansource rocks (Petersshales inet theal. 2005).Western The Lithuania assessment is based of the mostly hydrocarbon on available generation Rock–Eval potential data of(see the Table Early 1). Silurian Using the shales equations,in the Western proposed Lithuania by of isG. based E. Claypool mostly on(Peters available et al. Rock–Eval2006), the extent data (see of the Table organic 1). Using matter the conversion to equations,petroleum proposed (so–called by of G. fractional E. Claypool conversion) (Peters et was al. determined.2006), the extent By making of the someorganic assumptions, matter conversion the fractional to

petroleumconversion (so–called (f), fractionaloriginal TOC conversion)o, amount was of expelled determined. hydrocarbons By making (SI someexpelled assumptions,) and expulsion the efficiency fractional (ExEf)

conversionwere (f),derived original from TOC the ochange, amount in oforiginal expelled hydrogen hydrocarbons index (HI (SIo)expelled and )original and expulsion production efficiency index (PI (ExEf)o) to present

were derivedday values from of the these change indexes in original (HIp and hydrogen PIp; Peters index et al (HI. 2005).o) and original production index (PIo) to present

day values ofThe these extent indexes of the (HI organicp and PI matterp; Peters conversion et al. 2005). (fractional conversion) to the liquid and gaseous Thehydrocarbons extent of the(f) couldorganic be matter assessed conversion by adopting (fractional the equation: conversion) to the liquid and gaseous

hydrocarbons (f) could be assessedHIp {1200 by adopting - [HIo / the(1- equation:PIo)]} f = 1− (1); HIp {1200HIo {1200 - [HIo - /[HIp (1- PIo)]} / (1- PIp)]} f = 1− (1); here PIp –HIo the present {1200 day-[HIp production / (1- PIp)]} index (measured by means of Rock–Eval pyrolysis

herePIp=S PI1/(Sp –1 the+S2 present), that indicate day production the amount index of genera(measuredted hydrocarbons); by means of Rock–Eval we assume pyrolysis that HIo – the original

PIp=S1hydrogen/(S1+S2), thatindex indicate and PI theo – theamount original of genera productionted hydrocarbons); index for the most we assume of immature that HI sourceo – the rocks original are equal to

hydrogen500 index and 0.02 and respectivelyPIo – the original (Peters production et al. 2005); index 1200 for – the the most maximal of immature amount ofsource hydrocarbons rocks are thatequal could to be 500 andformed 0.02 respectively assuming, that (Peters organic et al carbon. 2005); comprises 1200 – the 83.33% maximal of total amount hydrocarbon of hydrocarbons composition that could(Peters be et al. formed2005). assuming, that organic carbon comprises 83.33% of total hydrocarbon composition (Peters et al. 2005). The extent of fractional conversion of the Early Silurian shales has been calculated based on data from Thewell extent Ramu čofiai fractional–1 (Table conversion 1). of the Early Silurian shales has been calculated based on data from

well Ramučiai–1 (Table 1). 136 {1200 - [500 / (1- 0.02)]} f = 1− = 0.832 ;

136 {1200500 {1200- [500 /- (1[136- 0.02)]} / (1- 0.4)]} f = 1− = 0.832 ; Therefore,500 83% {1200 of the -hydrocarbon[136 / (1- 0.4)]} generation process in the area of the Western Lithuania adjacent to Therefore,well Ramu 83%čiai– 1of has the been hydrocarbon completed generation process in the area of the Western Lithuania adjacent to The original total organic carbon (TOC ) content calculated adopting the mass balance equation: The original total organico carbon (TOC ) content in the source rocks before they get matured, could be in the sourcewell Ramu rocksč iaibefore–1 has they been get completed matured, could be o

Thecalculated original adopting total organic the mass carbon balance (TOC equation:o) content in the source rocks before they get matured, could be calculated adopting the mass balance equation:83.33 × HIp× TOCp TOC = (2) o × × + × [HIo (183.33- f) ×(83.33HIp×-TOCpTOCp) HIp TOCp] TOCo = (2) Thus, the original[HIo content× (1 of- f)TOC× (83.33 for the- TOCp)area adjacent + HIp to× wellTOCp] Ramu čiai–1 is calculated as: Thus, the original content of TOC for the area adjacent to well Ramučiai–1 is calculated as: 83.33×136× 4.5 Thus, the original contentTOC of = TOC for the area adjacent to well Ramučiai–1 is calculated= 7.06 wt.%; as: o × × + × [500 (183.33- 0.832)×136(83.33× 4.5 - 4.5) 136 4.5] TOCo = = 7.06 wt.%; The estimated[500 value× (1 implies- 0.832) that× (83.33 the mature- 4.5) Early +136 Silurian× 4.5] shales in well Ramučiai–1 contain high Theamount estimated of organic value matter implies (4.5 that wt%) the. matureMoreover, Early before Silurian the maturationshales in well sediments Ramuč iai–1in this contain well contained high The estimated value implies that the mature the maturation sediments in this well contained excel- Early Silurianamountexcellent shales of organic in quantitieswell matter Ramučiai–1 (4.5of the wt%) organic contain. Moreover, matter– high before7.06lent quantitieswt%. the maturation of the organicsediments matter– in this 7.06 well wt%. contained amount of organic matter (4.5 wt%). Moreover, before excellent quantitiesThe amount of the of organic the expelled matter– hydrocarbons 7.06 wt%. (SIexpelled) and expulsion efficiency (ExEf) could be The amount of the expelledThe amount hydrocarbons of the expelled (SI )hydrocarbons and expulsion (SIexpelled efficiency) and expulsion (ExEf) couldefficiency be calculated (ExEf) could be The amount of the expelled expelledhydrocarbons (SIexpelled) and expulsion efficiency (ExEf) could be Thecalculated amount accordingly: of the expelled hydrocarbons (SIaccordingly:expelled) and expulsion efficiency (ExEf) could be calculated accordingly: calculatedcalculated accordingly: accordingly: 1000(TOCo - TOCp) SI = 1− (3) expelled 1000(TOCo1000(TOCo - TOCp) - TOCp) SIexpelled1000(TOCo = 83.331− - TOCp- TOCp) (3) ; SIexpelled− = 1− (3) SIexpelled = 1 83.3383.33 - TOCp - TOCp (3) 83.33(1 --TOCpf)[PIp/(1 - PIp)] ExEf = 1− ×100 (4) (1- f)[PIp/(1(1- f)[PIp/(1- PIp)]- PIp)] ExEf(1-−f)[PIp/(1 = 1f −+ [PIo/(1- PIp)]- PIo)] × ×100 (4) ; ExEf =ExEf 1− = 1 f + [PIo/(1×100- PIo)] (4)100 (4) + f + [PIo/(1- PIo)] here TOCp is the presentf [PIo/(1day content- PIo)] of the total organic carbon. here TOCp is the present day content of the total organic carbon. here TOC ishere the TOC presentp is the day present content day of content the total of the organictotal organic carbon. carbon. herep TOCp is theThus, present the amountday content of hydrocarbons, of the total organic expelled carbon. in the Early Silurian shaley succession in Western Thus, the amount ofThus, hydrocarbons,Thus, the amount the amount expelled of hydrocarbons, of hydrocarbons,in the Earlyexpelled expelledSilurian in the shaley in Early the Earlysuccession Silurian Silurian shaley in Western shaley succession succession Lithuania in Western in Western LithuaniaThus, the equals amount to: of hydrocarbons, expelledequals in the to: Early Silurian shaley succession in Western Lithuania equals to: LithuaniaLithuania equals equals to: to: 1000 × (7.06 - 4.5) SIexpelled = = 32.47 mg HC/g rock; 10001000× (7.06× (7.06 - 4.5) - 4.5) SIexpelled1000 = × (7.0683.33 -- 4.5)4.5 = mg HC/g rock; SIexpelled = = 32.4732 mg.47 HC/g rock; SIexpelled = 83.33 - 4.5 = 32 . 47 mg HC/g rock; Assuming that oil and shale 83.33 densities 83.33- 4.5 - 4.5equals respectively to 850 mg/cm3 and 2.4 g/cm3, the average thickness 3 3 Assuming that oil and shale densities equals respectively to 850 mg/cm3 and 2.4 3g/cm , the average thickness AssumingAssumingof the that Early oil that and Silurian oil shale and blackdensitiesshale shaledensities equals strata equals respectively is 150 respectively m (Fig. to 8503) andto mg/cm 850 the mg/cmar3 andea of 2.4 oneand g/cm square2.43 ,g/cm the kilometer, average, the average thickness the estimated thickness Assuming that oil and shale densities equals re- equals to 13.75 × 106 kg. This mass of hydrocarbons of the Early3 Silurian black3 shale strata is 150 m (Fig. 3) and the area of one square kilometer, the estimated spectivelyof the to of850Earlyvolume the mg/cm EarlySilurian of expelled Silurian and black 2.4 hydrocarbonsblackshaleg/cm stratashale, the averagestratais (SI)150 is mequals 150 (Fig.is mroughly to 3)(Fig. 13.75 and 3) equivalentthe ×and ar 10 eathe6 kg.of ar toone eaThis 389of square massone × square 10kilometer,of 5hydrocarbons cubic kilometer, meters the estimated theisof roughly estimated 6 thickness of the Early Silurian black shale strata is methane (the calculations6 presented below). The graph volume of expelled 5hydrocarbons (SI) equals to 13.756× × 10 kg. This mass of hydrocarbons is roughly 150 m (Fig.volume 3)volumeequivalent ofand expelled the of areaexpelled to hydrocarbons389 of one× hydrocarbons 10squarecubic (SI) kilometer, meters equals (SI) equals ofto methane13.75of toJ. W.13.75× (the Schmoker10 calculationskg. 10 This kg. (1994) mass This presented was ofmass hydrocarbons used of below). hydrocarbons for conversation The is roughly graph is roughly of J. W. 5 the estimated volumeequivalent of expelled to 389 hydrocarbons ×5 10 cubic (SI)meters ofof massmethane hydrocarbons (the calculations to methane. presented below). The graph of J. W. equivalentequivalentSchmoker to 389 to (1994)× 389 10 5 × wascubic 10 used cubicmeters for meters conversationof methane of methane (the of masscalculations (the hydrocarbons calculations presented presented to methane. below). below). The graph The graphof J. W. of J. W. Schmoker (1994) was used for conversation of mass hydrocarbons to methane. SchmokerSchmoker (1994)32.47 (1994) was× 2400 used was for used conversation for conversation of mass of hydrocarbonsmass hydrocarbons to methane. to methane. SI = × 106 × 150 = 13.75× 106 kg HC = 389× 105 m3 of methane. 32.47 × 2400 6 6 5 3 32.47SI32× 2400=.47 850× 2400 6× 10 × 150 = 13.756× 10 kg HC = 3895× 103 m of methane. SI = × 6 ×× 10 × 150 = ×13.756× 10 kg HC =× 3895 × 310 m of methane. SI = 850 85010 150 = 13.75 10 kg HC = 389 10 m of methane. Based on parametersBased850 on estimatedparameters above, estimated the expulsion above, the efficiencyexpulsion efficiency(ExEf) is (ExEf)calculated: is calculated: Based on parameters estimated above, the expulsion efficiency (ExEf) is calculated: BasedBased on parameters on parameters estimated estimated above,(1- 0.832)above, the ex ×thepulsion[0.4 ex pulsion/ (1efficiency- 0.4)] efficiency (ExEf) (ExEf) is calculated: is calculated: ExEf = 1− (1- 0.832)×[0.4 / (1- 0.4)]×100 = 86.9% ExEf(1- 0.832) =(1 10.832-−0.832)×[0.4 + [0.02 ×/ [0.4(1 -/0.4)] (1/ (1- 0.02)]- 0.4)] ×100 = 86.9% ExEf− = 1− × ×=100 = 86.9% ExEf = 1 0.8320.832 + [0.02 + [0.02 / (1- 0.02)]/ (1100- 0.02)]86.9% Thus, the numbers presented0.832 + [0.02above /clearly (1- 0.02)] indicate the very high expulsion efficiency of the Early Silurian blackThus, shales the in numbers Western presented Lithuania. above Obviously, clearly these indicate are onlythe ve tentativery high expulsionvalues, as efficiencyonly the measurements of the Early Silurian from Thus, Thus,the numbers the numbers presented presented above above clearly clearly indicate indicate the ve thery high very expulsion high expulsion efficiency efficiency of the ofEarly the EarlySilurian Silurian the area,black described shales in byWestern very high Lithuania. maturity, Obviously, were used these for calculations.are only tentative As some values, of the as onlyinput the parameters measurements for the from Thus,black the numbersblackshales shales in presented Western in Western Lithuania.above Lithuania. clearly Obviously, indicate Obviously, thesethe arecalculationsthese only are tentative only have tentative notvalues, been values, as determined only as the only measurements analytically, the measurements from from calculationsthe area, have described not been by verydetermined high maturity, analytically, were adoptingused for calculations.some derivative As somevalues of instead, the input the parameters accuracy of for the the verythe high area, expulsionthe describedarea, described efficiency by very by ofhigh very the maturity, highEarly maturity,Silurian were usedwereadopting for used calculations. somefor calculations. derivative As some values As ofsome instead,the ofinput the the parametersinput accuracy parameters for the for the black shales inthe Western totalcalculations estimatedLithuania. have volumes Obviously, not been should these determined be are considered of analytically, the total assuming estimated adopting quite volumes highsome errors. derivative should Though, be values considered it isinstead, well known the accuracy that of calculationscalculations have not have been not determined been determined analytically, analytically, adopting adopting some derivativesome derivative values values instead, instead, the accuracy the accuracy of of only tentative onlyvalues, thevery astotal smallonly estimated theamount measurements volumes of hydrocarbons should from be generated assumingconsidered (usually quite assuming high only errors. quite few percent)highThough, errors. may it is Though, havewell knownbeen it is preserved well known and that the totalthe estimated total estimated volumes volumes should should be considered be considered assuming assuming quite highquite errors. high errors. Though, Though, it is well it is known well known that that the area, describedbe latteronly by onvery very extracted. highsmall maturity, amount Only in ofwere very hydrocarbons used rare cases,that generated onlyup to very 10% (usuallysmall of the amount total only estimated offew hydrocarbons percent) volumes may generated haveof hydrocarbons been preserved and for calculations.only veryonly As small very some smallamount of theamount of input hydrocarbons of parameters hydrocarbons generated for generated(usually (usually (usuallyonly only few few only percent) percent) few percent) may may have have may been beenhave preserved preserved been preserved and and mightbe be latter produced. on extracted. Furthermore, Only in for very the rare more cases, accurate up to assessment 10% of the of total the estimatedshale gas generationvolumes of potential, hydrocarbons a be latterbe onlatter extracted. on extracted. Only inOnly very in rare very cases, rare cases,up to 10%up to of 10% the oftotal the estimated total estimated volumes volumes of hydrocarbons of hydrocarbons widemight range be of produced.the additional Furthermore, parameters, for such the more as accurate assessment of the shale gas generation potential, a might mightbe produced. be produced. Furthermore, Furthermore, for the for more the accuratemore accurate assessment assessment of the ofshale the gasshale generation gas generation potential, potential, a a wide range of the additional parameters, such as wide rangewide rangeof the ofadditional the additional parameters, parameters, such as such as 97 and be latter on extracted. Only in very rare cases, up the Cambrian and Ordovician oil bearing complexes to 10% of the total estimated volumes of hydrocarbons also show a good hydrocarbon generation potential, might be produced. Furthermore, for the more accurate the Early Silurian shale source rocks could also adsorb assessment of the shale gas generation potential, a wide hydrocarbons diffused from Cambrian and Ordovician range of the additional parameters, such as minera- source rocks. logical composition and the reservoir properties of the Early Silurian shales, gas storage capacity, formation Acknowledgements pressure and temperature etc. are required. Whereas the Early Silurian succession overlies The authors thank the reviewers Dr. Anatoly I. Cambrian and Ordovician oil bearing complexes in the Baltic Sedimentary Basin, the Silurian shaley Khoubldikov (Kaliningrad, Russia) and Dr. Niels H. sediments could both adsorb and absorb hydrocarbons, Schovsbo (Copenhagen, Denmark) for constructive generated also from Cambrian and Ordovician source comments that helped to improve the manuscript. Also rocks. we thank the Lithuanian Geological Survey under the Ministry of Environment and Geological Museum of CONCLUSIONS Lithuania for their assistance in providing the access to samples and archive data. The Early Silurian organic rich graptolite shales in Western Lithuania, described by high content (total organic carbon References > 1%) and the maturity (vitrinite like macerals reflectance Buchardt B., Lewan M.D., 1990. Reflectance of vitrinite– index R ~ 0.9–1.0%) of the organic matter, could be o like macerals as a thermal maturity index for Cambrian– considered in terms of shale gas prospective „black Ordovician Alum Shale, southern Scandinavia. American shales”. The depth of the occurrence of the base of the Association of Petroleum Geologists Bulletin 74 (4), Early Silurian increases to the southwest, ranging from 394-406. 200 m in the east to nearly 2,050 m in the westernmost Čyzienė J., Šliaupa S., Baliukevičius A., Lazauskienė J., part of Lithuania. Two major systems of WSW–ENE 2006. A set of the structural maps at a scale of 1:200 and SSW–NNE trending faults dissect the Silurian 000. Geological Survey of Lithuania, Annual Report, succession. The thickness of the Early Silurian strata 15-18. within the shale gas prospective area varies in a range of Espitalie J., Deroo G., Marquis F., 1985. La pyrolyse Rock– 40 meters, the thinnest section of ~ 120 m recorded in the Eval et ses applications, deuxieme partie. Revue de central part, whereas the thicknesses increases (up to 160 l’Institut Francais du Petrole 40, 755-784. m) towards the north and southwest or carries out rather Horsfield B., Schulz H. M., GASH Team, 2008. GASH: A local character in some wells. shale gas initiative for Europe. EGU General Assem- The Early Silurian strata are an organic–rich, contain- bly. Geophysical Research Abstracts 10, EGU2008– ing kerogen type II, oil–prone shales of marine origin. A–01508. Correlation of biomarker parameters, Rock Eval results Jarvie D. M., Lundell L. L., 2001. Amount, type and kinet- and vitrinite like macerals reflectance data allowed to de- ics of thermal transformation of organic matter in the termine the regional trend in the distribution of the thermal Miocene Monterey Formation. In Isaacs C. M., Rul- maturity of the organic matter, implying that the thermal lkotter J. (eds.): The Monterey Formation: from rocks maturity gradually increases to southwest and reaches the to molecules, New York Columbia University Press, „early oil” and „oil peak phase” in the south–westernmost 268-295. Jarvie D. M., Hill R. J., Ruble T. E., Pollastro R. M., 2006. part of Lithuania. A max reflectance value of 1.94% Ro, recorded in the sample from Ramučiai–1, shows the late Unconventional shale–gas systems: the Mississippian Barnet shale of north–central Texas as one model for gas generation phase. Such a sharp, but locally restricted, thermogenic shale gas assessment. American Association increase in the maturity of the organic matter could be of Petroleum Geologists Bulletin 91(4), 475-499. explained in terms of the temporal variations in the heat Kadūnienė E., 2001. Organic matter in oil source rocks. flow related to magmatic intrusion or the thermal events In Zdanavičiūtė O., Sakalauskas K. (eds.): Petroleum of the different origin. geology of Lithuania and Southeastern Baltic, Institute The Early Silurian shales in Western Lithuania might of Geology, Vilnius, 96-119. have had generate a large volumes of methane gas Kepežinskas K., Rastenienė V., Suveizdis P., 1996. The within the limits of „black shale” productive area, due West Lithuanian Geothermal anomaly. Vilnius, 68 pp. to a number of favourable conditions, such as excellent [In Lithuanian]. original organic richness of sediments, sufficient thermal Lapinskas P., 1996. Palaeogeographical peculiarities maturity and generation potential. The expulsion effi- of the Silurian in the Southern Peri–Baltic basin. In ciency, computed for the Early Silurian strata in Western Zdanavičiūtė O., Sakalauskas K. (eds.): The oil bearing Lithuania, is very high. The estimated volume of expelled complexes of Lithuania, Vilnius, 27-35. [In Lithua- hydrocarbons might have been as high as 13.75 × 106 kg nian]. or 389 × 105 cubic meters of methane (in the section of Lapinskas P., 2000. Structure and petroliferosity of the Silu- one sq. km and ~ 150 m thickness of shale strata). As rian in Lithuania. Vilnius, 203 pp. [In Lithuanian].

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