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Geochemical and Sedimentation History of Neogene Lacustrine Sediments from the Valjevo-Mionica Basin (Serbia)

Valjevo-Mionica Basin is one of the numerous lacustrine Neogene basins in Serbia. After Aleksinac Basin, according to the quality and amount of oil shale, it is one of the main deposits of this raw material in Serbia. The most important oil shale deposits in ValjevoMionica Basin are located in the central part of the basin (Bela stena series, Suseocka and Radobicka Bela Stena). The kerogen content in oil shales ranged from 8 16 %. The average oil yield of 6.3 % is of economical value.


Introduction
Valjevo-Mionica Basin is one of the numerous lacustrine Neogene basins in Serbia. After Aleksinac Basin, according to the quality and amount of oil shale, it is one of the main deposits of this raw material in Serbia. The most important oil shale deposits in Valjevo-Mionica Basin are located in the central part of the basin (Bela stena series, Sušeočka and Radobićka Bela Stena). The kerogen content in oil shales ranged from 8 -16 %. The average oil yield of 6.3 % is of economical value.
Total of 62 samples of Neogene lacustrine sedimentary rocks to the depth of 400 m were investigated in this study. The first objective of the study was to reconstruct geological history (evolution) of the sediments i.e. to determine the palaeoconditions in depositional environment during its formation. For this purpose numerous geochemical methods and approaches were used. The second objective of the study was to determine the origin, type, maturity and liquid hydrocarbon potential of organic matter (OM). Aimed at detailed estimation of the oil shale OM potential, and prediction of the conditions necessary to become active oil generating source rock, pyrolytic experiments were performed on the bitumen-free sample. Bearing in mind that some metal ions (e.g. Al(III)-ion in clay minerals) (Jovančićević et al., 1993;Peters et al., 2005) have catalytic influence on most of the maturation processes, and that Pt(IV)-and Ru(III)-ions are often components of catalysts in many laboratory investigations and industrial procedures (Hu et al., 1994;Kawaguchi et al., 2005), the pyrolytic experiments of bitumen-free rock were performed also in the presence of simple inorganic compounds, H 2 [PtCl 6 ] and RuCl 3 , to investigate if their presence changes the yield and hydrocarbon composition of liquid pyrolysates.

Geological characteristics of the investigated area
Valjevo-Mionica Basin is situated in the western part of Serbia, covering an area of 350 km 2 . (Fig. 1). The Valjevo-Mionica Basin consists of lacustrine and marine sediments (Jovanović et al., 1994). The current investigations were focused on the lacustrine sediments from the 2 drillhole Val-1 at depth interval of 0-400 m. Interval from 15 to 200 m depth is made of sediments of the Mionica series which covers an area of approximately 40 km 2 (Dolić, 1984). Lithological characteristics of the Mionica series based on cores from the drillhole Val-1 down to depth of 200 m reflect transitions of oil shale, relatively rare thin beds or lenses of sandy siltstone and laminated shale, marlstone (dolomitic, sandy and clayey as well as tuffaceous), tuff, lenses enriched with searlesite and analcite and limestone with chert concretions. Another sedimentary interval underlying oil shale series is from 200 to 400 m depth. These sediments are represented by marlstone (dolomitic, sandy and clayey as well as tuffaceous), lenses of carbonates, siltstone, tuff and pyrite (Šajnović et al., 2008a).

Mineral composition
Mineral composition of sediments is characterized by predomination of dolomite and calcite, which were found in all samples. Contents of quartz, illite and chlorite were changeable. All samples from depth interval 15 to 200 m, which contain oil shale, are characterized by the presence of analcite (Fig. 2). Analcite is mainly linked with marine or lacustrine sediments which are formed in conditions of increased salinity and alkalinity (Remy & Ferrel, 1989). Feldspars, smectite and amphiboles were indicated by X-ray analyses, but they should be confirmed by detailed studies. According to certain specificities of the mineral composition, two important depth intervals were defined in the drillhole Val-1. The first interval is from 15 to 75 m depth. It is characterized by presence of searlesite ( Fig.  2a), which is genetically linked to volcanogenic material. Another geochemically specific interval is at the depth of 360-400 m, and is characterised by interstratified clay minerals most probably of illite-smectite composition (lithium-bearing Mg-smectite) (Fig. 2b).

Geochemical parameters
Conditions which existed in the sedimentation environment, like water level, salinity, and climatic conditions, are reflected in the values of geochemical parameters (Ng & King, 2004).

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Geochemical and Sedimentation History of Neogene Lacustrine Sediments from the Valjevo-Mionica Basin (Serbia) 5 For this purpose numerous group and specific geochemical parameters (Šajnović et al., 2008a, 2008b, 2009, 2010) were determined based on detailed investigation of inorganic part of sediments and its organic matter (kerogen and bitumen) (Tables 1 and 2). The differences in mineral composition and geochemical characteristics of the sediments indicate that the conditions in the sedimentation area changed over the time. That allowed defining four different depth intervals (Table 1) LOI -loss of ignition; Corg -organic carbon content from elemental analysis; S1 -free hydrocarbons in mgHC/g rock; S2 -pyrolysate hydrocarbons in mgHC/g rock; HI -hydrogen index = S2x100/TOC in mgHC/gTOC; HC -hydrocarbons; TOC -total organic carbon; Tmax -temperature corresponding to S2 peak maximum; SD -standard deviation Table 1. Characteristical depth intervals and values of geochemical parameters

Depth interval 15-75 m
Relatively low values of the main inorganic geochemical parameters like SiO 2 , Al 2 O 3 , Fe 2 O 3 , TiO 2 and CaO in this interval indicate that the share of alumosilicate and carbonate fraction was low ( Table 1). Change of contents of K 2 O is similar to behaviour of SiO 2 and Al 2 O 3 , what indicates the connection between K 2 O and alumosilicates. This is confirmed by minerological analysis, that is presence of illite and rarely K-feldspar (Fig. 2a). Presence of potassium and terrigenic component is explained by the fact that potassium is mainly accumulated in clays by weathering and leaching processes as a result of syn-and postdepositional adsorption and ion exchange in salty or salted waters (Grim, 1968). Total iron (Fe 2 O 3 ) may be found in crystal lattice of clay minerals, especially illite and chlorite. The 6 other possible connection is with colloid oxides and hydroxides of manganese (MnO) and titanium (TiO 2 ), which are, apart from clays, important constituents of recent sediments. The mentioned oxides and hydroxides may be found alone or in form of film on clay or other minerals. Contents of Li in depth interval 15-75 m is relatively low, whereas Sr content is relatively high and in positive correlation with LOI, indicating that it is connected with carbonate fraction (Table 1) (Bray & Evans, 1961); Pr -pristane; Ph -phytane; Sq -squalane; i-25 -C25 regular isoprenoid; %C27, C28, C29 regular sterane relative contents calculated from the peak areas of C27-  Table 2. Characteristical depth intervals and values of specific organic geochemical parameters What makes this depth interval specific compared to the others is very high contents of Na 2 O in main elements, B and As in the microelements (Table 1). It is well known that in comparison to other environments, Neogene lacustrine sediments are enriched in B and As (Alonso, 1999;Yudovich & Ketris, 2005). The contents of boron and arsenic in lacustrine sediments depend on: active volcanism, closed basin, arid to semi-arid climate, tectonic activity, pH, salinity, redox potential, temperature, type of the surrounding minerals in the depositional environment (Helvaci & Alonso, 2000;Kazanci et al., 2006;Valero-Garcés et al., 1999). The highest levels of boron in detrial sedimentary rocks are usually associated with argillaceous facies and are related to the amount and type of the mineral presents (Aggarwal www.intechopen.com Geochemical and Sedimentation History of Neogene Lacustrine Sediments from the Valjevo-Mionica Basin (Serbia) 7 et al., 2000). Hydrated borate minerals accumulate as evaporate deposits in an arid, closed basin environment (Alonso, 1999;Floyd et al., 1998). Also, in arid areas, boron is likely to be co-precipitated with Mg and Ca hydroxides as coatings on the particles of the sediments, and it may also occur as Na-metaborate. Mineralogical analyses showed that dolomite and calcite were predominant in the investigated sediments and were found in all the examined samples (Fig. 2). Conditions of sedimentation, characterised by high salinity and pH and the presence of aluminosilicates and calcium and magnesium minerals, were suitable for boron accumulation. Therefore, sediments from this depth interval contained an order of magnitude higher amount of boron than sediments from other depth intervals (Table 1). Also, these sediments are characterised with increased contents of Na 2 O and As compared to the other samples (Table 1) and the presence of the mineral searlesite ( Fig. 2a), which is formed through the contact of sodium-rich alkaline saline waters with volcanic glass, which was the source of boron (Peng et al., 1998). This interval is characterised by the highest average values of all bulk organic geochemical parameters (Corg, S1, S2, HI), with the exception of maturity parameter, temperature of maximum generation, Tmax (Table 1). Samples from depths 15-75 m contained relatively high amount of organic matter (Corg). This is also holds for the content of soluble OM expressed as S1 and for the content of hydrocarbons formed by pyrolysis, expressed through S2 (Table 1.) Relatively high values of hydrogen index (Table 1) show that OM of the samples consists predominantly of Type I and/or I/II kerogen, with a good potential for liquid hydrocarbons generation. The average value of Tmax indicates low maturity degree of OM, which is expected since at these depths OM was not exposed to more significant thermal stress.
The n-alkane distribution is characterised by domination of n-C 17 and relatively low proportion of longer chain n-alkane homologues (Fig. 3a, Table 2). In the immature samples, n-C 17 origin is associated to cyanobacteria and/or algae. Reducing conditions in saline lacustrine environments are caused by the high salinity of water and linked density stratification impeding vertical mixing of strata water body. This results in extremely anoxic conditions in the depositional environment (Peters et al., 1996), documented by very low Pr/Ph ratio of 0.05 (Table 2, see *).
In relatively immature sediments, pristane and phytane are presumed to originate from phytol, being a side chain in chlorophyll a structure of phototrophic organisms. However, there are other sources of phytane, like membrane lipids from methanogenic or halophilic archaea (Anderson et al., 1977;Volkman & Maxwell, 1988). Squalane is present in all of these sediments in relatively high quantities ( Fig. 3a; Table 2). Squalane is presumed to originate from Halophihlic archaea (Grice et al., 1998), and is interpreted as indicator for hypersaline depositional environment. Very high quantities of phytane, C 25 (i-25) and C 30 (squalane) regular isoprenoids were found in a numerous saline lakes of non-marine origin in China (Wang & Fu, 1997). In this contest, the Sq/n-C 26 ratio is often calculated, averaging a value of 2, indicating environments with very high salt content. Sediments from depth interval 15-75 m are characterized with high phytane and squalane contents (Sq/n-C 26 > 1, and in some samples even over 3), whereas i-25 was not identified, or was present in small quantity (Fig.  3a). This result shows that in the current study area such extremely saline anoxic conditions did not suitable for precursors of i-25.  C 27 steranes in saturated lipid fraction of sediments in this depth interval accounts for over 40 %, and in some cases reaches even 50 %, whereas C 28 sterane content accounts for approximately 30 % in total distribution of C 27 -C 29 regular sterane homologues ( Fig. 3b; Table 2). Considering that distribution of regular steranes might serve even in classification of sediments or basins compared to the degree of salinity (Wang & Fu, 1997), the mentioned data, apart from the high contents of isoprenoids squalane and phytane, represent another confirmation of hypersaline conditions of depositional environment in depth interval 15-75 m. The distribution of 14(H)17(H)20(R) C 27 -C 29 regular steranes is often used in the evaluation of the OM type (Peters et al., 2005;Volkman, 2003). Based on high contents of C 27 and C 28 steranes, distributions of n-alkanes dominated by C 17 and high HI values (Tables 1  and 2) it might be concluded that the dominant source of OM during formation of sediments in this depth interval was from algal origin.
Concerning the distribution of terpane biomarkers, compounds with biological configuration and -moretanes are predominant in investigated samples representing immature microbial biomass (Fig. 3c). This agrees with the low level of thermal maturity. Gammacerane, which is most often considered as indicator of water column stratification and environments with high salinity (Sinninghe Damsté et al., 1995), is present in relatively small quantities (Fig. 3c). This confirms to the fact that extremely saline conditions are not suitable for its precursors e.g. protozoa Tetrahymena (Brassell et al., 1988;Šajnović et al., 2008b).

Depth interval 75-200 m
Contents of SiO 2 , Al 2 O 3 , Fe 2 O 3 and TiO 2 are higher, comparing to sediments from 15 to 75 m, whereas the contents of MgO, Sr and LOI are lower. Contents of Al 2 O 3 and TiO 2 are the measure of clastic share of material (terrigenic origin), or erosion activity. In general, it may be said that in depth interval 75-200 m, due to increased erosion activity, alumosilicate contents grows, and carbonate content falls. The greatest and most dramatic change was noticed in the reduction of the boron content (Table 1), what is mineralogically followed by absence of searlesite. In these sediments, lower contents of Na 2 O and As were observed, although these changes are not that prominent as in content of boron (Table 1).
Sediments from this depth interval are characterised with lower values of all bulk organic geochemical parameters than in previous interval, especially those connected with the quantity of organic matter (Table 1). Lower HI values indicate that OM is composed of mixed terrestrial-algal precursor biomass (kerogen types II and mixture I/II; Table 1). The maturation degree of organic matter of the sediments is low.
n-Alkane distribution of the saturated fraction is characterized by relatively high proportions of n-C 17 , and n-C 27 , n-C 29 , n-C 31 long-chain odd n-alkanes (Fig. 4a). Decreasing of the n-C 17 /n-C 27 ratio indicates higher contribution of terrestrial precursor biomass (Table 2). Low value Pr/Ph ratio, in some cases of 0.07 (Table 2) suggests extremely anoxic conditions in the depositional environment (Peters et al., 1996). In addition, sediments from 75 to 200 m are characterized with low contents of i-25 and squalane ( Fig. 4a; Table 2).
Sterane distribution with domination of C 27 and C 29 in similar proportions confirms mixed terrestrial algal precursor biomass, consistent with HI value and n-alkane distribution ( Fig.  4b; Table 1). In distribution of terpane biomarkers, compounds with biological configuration and -moretanes are predominant, whereas gammacerane is present in relatively low quantity (Fig. 4c).  (Table 1). Obtained results indicate significant contribution of clastic material.
Sediments of this depth interval contain the least quantity of the organic matter in the whole vertical profile ( Table 1). As values of both HI and parameter S2 are the lowest compared to the other intervals, it is obvious that the OM of these sediments is of the lowest quality, composed mainly from kerogen type III and II/III with a low potential for production of liquid hydrocarbons. These bulk data provide further indication that the terrestrial OM significantly contributed to samples from depth interval 200-360 m. This is confirmed by biomarker distributions, which are characterized by domination of longer chain odd n-alkane homologues (C 27 , C 29 and C 31 ) and pronounced proportion of C 29 regular sterane (Fig. 5a,b; Table 2). Moreover, the samples contain low content of squalane, whereas i-25 is absent (Fig. 5a; Table 2). All the mentioned changes in composition and quality of OM of sediments in depth interval 200-360 m are caused by expressed erosion activity which resulted in high contribution of clastic material. Relatively high values of gammacerane index (Table 2) could be explained by water stratification, which was most probably result of the temperature changes (Sinninghe Damsté et al., 1995).

Depth interval 360-400 m
The last drilled interval is characterised by high share of dolomite and calcite, but also with presence of already mentioned lithium clay minerals. The most important geochemical link in these sediments is related to the most likely presence of interstratified clay mineral type illite-saponite (lithium-bearing Mg-smectite). This is indicated by high concentrations of MgO, K 2 O and Li and their mutual geochemical correlation (Table 1), as well as X-ray analysis (Fig. 2b).
In this depth interval the quantity of the OM is higher in comparison with previous depth interval, as well as the content of boron. However, this increase is not as pronounced as in depth interval 15-75 m. Value of HI indicates that the OM is composed of different types of kerogen, and that it is on relatively low degree of maturation ( Table 1).
The saturated lipid fraction of these samples is characterized by relatively high proportions of n-C 17 , phytane and pristane (Fig. 6a). The maximum in the short-chain length range (n-C 17 ) of the n-alkanes is higher than in the long-chain range, resulting in a n-C 17 /n-C 27 ratio higher than 1 ( Table 2). Odd homologues predominate among longer chain n-alkanes, and for peak assignments, see legends, Figs. 3, 4 and 7 Fig. 6. GC-MS ion fragmentogram of n-alkanes and isoprenoids, m/z 71 (a), steranes, m/z 217 (b) and terpanes, m/z 191 (c) representative for sediments from depth interval 360-400 m the maximum is at n-C 29 or n-C 31 (Fig. 6a). In sample 62 the relative proportion of pristane is highest among all investigated samples, causing the highest Pr/Ph and Pr/n-C 17 ratios in the whole sample set. The isoprenoid alkane with 25 carbon atoms is present in relatively high quantity (Fig. 6a; Table 2). This indicates that the conditions with high pH values, i.e. alkaline environment are suitable for precursors of C 25 isoprenoid. Literature data show that the most frequent precursors of this isoprenoid alkane are Archaea haloalkaliphiles, for which, apart from the alkaline environment, suitable is the environment with increased salinity (de Rosa et al., 1986). However, in case of sediments of depth from 360 to 400 m, there is no indication of the increased salinity during their formation.
In some samples, C 27 -steranes accounted for approximately 40 % (Table 2), and this observation was corroborated by high contents of n-C 17 . The maturity of the organic matter of these samples being low, their high concentrations indicated a significant proportion of planktonic and cyanobacteric precursor organisms, which might have been favoured by increased alkalinity. Distributions of terpanes of the investigated samples are shown in figure 6c. The presence of thermodynamically less stable homologues with (moretane) and configurations confirms that the OM of the investigated sediments has a low level of maturity. Sediment samples are additionally characterized by the presence of gammacerane. However, the values of gammacerane index for samples from this depth interval are low (Table 2). This data lead to the assumption that high alkalinity conditions were not very favourable for survival of gammacerane precursors e.g. protozoa Tetrahymena (Šajnović et al., 2008b;ten Haven et al., 1988).

Reconstruction of geological history based on geochemical and mineralogical parameters
The relatively low degree of OM maturity in all investigated samples (diagenetic phase), provides an opportunity to relate values of organic geochemical parameters with OM origin and palaeoconditions in sedimentation environment. Interpretation of those parameters, combined with mineralogical data and content of macro-and microelements allows reconstruction of the geological history of sediments in the drillhole Val-1. Obtained results showed that the conditions, and consequently sources of OM in the sedimentary environment changed significantly, based on which different geochemical intervals (zones) were defined. In certain periods sediments were deposited under very specific conditions. Depth interval 360-400 m. Sediments from this interval were formed in alkaline conditions, with a variable bicarbonate to carbonate ratio. They are characterized by high content of magnesium, potassium and lithium, and also by presence of clay minerals of probably saponite, hectorite or interstratified illite-smectite types ( Fig. 2b; Table 1), which needs further research. Results of elemental analysis and Rock-Eval pyrolysis indicate a moderately amount of immature OM (average organic carbon content, Corg, is 1.07 %; Table 1). Organic matter consists of kerogen types II and II/III. Biomarker distribution is characterized by domination of short chain over long chain n-alkanes, significant amount of phytane and regular isoprenoid C 25 (i-C 25 ), as well as by domination of C 27 -homologue in the distribution of C 27 -C 29 regular steranes (Fig. 6a,b; Tables 1 and 2). These results indicate significant contribution of algal biomass to OM in sediments (Peters et al., 2005). Therefore, it may be supposed that alkaline conditions are suitable for algae, and for some specific organisms such as Archaea haloalkaliphile, which is the main precursor of i-C 25 (de Rosa et al., 1986).

Depth interval 200-360 m.
With time, environment is changed from calm to turbulent. Contents of magnesium, lithium and potassium in sediments decreased, whereas the contents of clastic material, SiO 2 , Al 2 O 3 , Fe 2 O 3 and TiO 2 are strongly increased, being the highest in the whole drillhole (Table 1). Mentioned changes resulted in decrease of OM content and quality (S1, S2 and HI; Table 1). Distribution of biomarkers, which is characterized by domination of odd homologues of long chain n-alkanes and high content of C 29 sterane indicated significant contribution of terrestrial biomass to OM in sediments (Fig.  5a,b). Sedimentary OM from this interval consists predominantly of Type III kerogen, with a low generative liquid hydrocarbons potential.
Depth interval 200-75 m. Significant changes in the sedimentary environment occurred when the formation of sediments from the depth of 200 m, which belongs to Mionica series with oil shale, started. Those changes were primarily related to the increase in salinity, reflecting also in the increase of sodium content. The share of clastic sediments decreased, whereas the carbonate one increased (Table 1). Such conditions in sedimentary environment, followed by arid or semi-arid climate, allowed formation of analcite and better preservation of the OM, whose content increased in this depth interval (average, Corg, 2.42 %; Table 1). Biomarker distribution, which is characterized by high contents of n-alkanes (n-C 17 , n-C 27 , n-C 29 , n-C 31 ) and phytane, uniform distribution of C 27 -C 29 regular steranes and also by low abundance of squalane, i-25 and pristane, indicates that sedimentary OM originates from mixed terrestrial/algal biomass, deposited under slightly anoxic conditions (Fig. 4a,b). It contains kerogen types II and I/II.

Depth interval 75-15 m. Salinity continues to increase in the sedimentation environment.
Sediments from this interval were formed under the conditions of high salinity. The presence of clay minerals and calcium and magnesium minerals allows the accumulation of boron. Sediments from this interval have the highest content of boron, sodium and arsenic in the whole drillhole Val-1 and are characterized by the presence of searlesite ( Fig. 2a; Table  1). Searlesite was probably formed by reaction of saline waters rich in sodium with thin beds o f v o l c a n i c t u f f . S u c h c a l m e n v i r o n m e n t w i th high salt concentration and intensive evaporation, allowed increased bioproductivity of algal biomass and preservation of the deposited OM, due to pronounced stratification of water column. All that resulted in high content of the OM in sediments from this interval (average Corg is 3.32 %; Table 1), which consists of kerogen types I and I/II with good generative liquid hydrocarbons potential. Biomarker distribution is characterized by predominance of short chain over long chain n-alkanes, significant amount of phytane and squalane, as well as by domination of C 27 -homologue in the distribution of C 27 -C 29 regular steranes (Fig. 3a,b).

Pyrolysis and catalyzed pyrolysis in the investigation of an oil shale potential
The generative potential of an oil shale from the Valjevo-Mionica Basin was investigated using conventional pyrolysis and pyrolysis in the presence of Pt(IV)-and Ru(III)-ions . Pyrolysis was performed on bitumen-free oil shale sample from the most interesting depth interval (15-75 m).

Characteristics of organic matter in the oil shale sample
Group organic geochemical parameters obtained by elemental analysis and Rock-Eval pyrolysis indicate the oil shale has significant generative potential with a total organic carbon content (Corg) of 3.40 %, a Hydrogen Index (HI) of 600 mgHC/ gTOC, and a Tmax of 428 °C. These values are consistent with the presence of immature to marginally mature, oil-prone organic matter composed primarily of kerogens type I or I/II. Soxhlet extraction of the shale with an azeotrope mixture of dichloromethan and methanol yielded 5054 ppm of bitumen. The relatively high bitumen content in an immature sample may be explained by the presence of a significant amount of polar fraction (94.83 %), which is not readily expelled from the kerogen or did not incorporate into the kerogen matrix during late diagenesis Šajnović et al., 2010).
The n-alkane distribution in bitumen extracted from the oil shale is characterised by pronounced n-C 17 domination, typical for organic matter of predominantly algal origin (Fig.  3a). CPI value for full amplitude of n-alkane range of 3.38 indicates low maturity, which has also been confirmed on the basis of the values of group organic geochemical parameters (Tmax and group bitumen composition). Pr/Ph ratio in bitumen extracted from the initial shale is 0.29, which indicates reducing conditions during deposition of the organic matter that contributed to its preservation.
Sterane distribution of saturated fraction of the extracted shale bitumen is characterised by the predominance of homologues with unstable (R)-and (R)-configurations (Fig.  3b), which again confirms a low maturity. Among them, C 27 -and C 28 -steranes are in higher proportions, which is in agreement with predominantly algal origin of the organic matter. Steranes with (R)-, (S)-configuration and typical geoisomers, -and -diasteranes were not identified, and only C 29 sterane with (S)-configuration is present in low amount (value of C 29 (S)/C 29 (S+R) ratio = 0.20).
Distribution of terpane biomarkers in bitumen isolated from the initial shale is characterised with domination of thermodynamically less stable -and -isomers, the most abundant being C 30 -moretane (C 30 M/C 30 H ratio > 5; Fig. 3c). Terpanes typical for extracts of more mature source rocks and crude oils, such as, Tm, Ts, C 29 Ts and series of 22(S)-homohopane isomers, were not identified with exception of C 31 (S), which as minor component that coelutes with 2-gammacerene. C 29 and C 30 -Hopanes are present in small quantity, as well as thermodynamically less stable epimer of C 31 -homohopane with biological 22(R)configuration (Fig. 3c).
Components of aromatic fraction, methyl-dimethyl-and trimethylnaphthalenes, as well as methyl-and dimethylphenanthrenes typical for more mature source rock bitumens and oils, were not identified or are present in traces in the shale extract, with exception of phenanthrene. The observation is consistent with the low maturity of the organic matter since the main quantity of alkylaromatics is generated during catagenesis.

Characteristics of liquid pyrolysates
Group organic geochemical parameters. Heated at 400 °C for 4 h, the sample generated a total liquid pyrolysate and hydrocarbons of 1700 ppm and 692 ppm, respectively (Table 3). The yields are consistent for source rock with good potential and support the assumption derived from an analysis of the immature oil shale. The presence of Pt(IV)-and Ru(III)-ions significantly increases the yields of liquid pyrolysate and hydrocarbons, with a bit more pronounced effect of Ru(III)-ion (Table 3).

Sample
Yield of liquid pyrolysates ** (ppm) Yields of hydrocarbons (HC) ** (ppm)  S400  1700  692  SPt400  2700  1266  SRu400  2700  1376  Sample  p o  p  S400 4.5 4.9 SPt400 4.5 5 SRu400 4.3 5.7 ** relative to the bitumen-free sample; po, initial pressure; p, pressure at the end of pyrolysis; S400pyrolysate of bitumen-free oil shale at 400 °C; SPt400 -pyrolysate of bitumen-free oil shale at 400 °C + H2[PtCl6]; SRu400 -pyrolysate of bitumen-free oil shale at 400 °C + RuCl3 Table 3. Values of group organic geochemical parameters in liquid pyrolysates The catalytic effect of the used metal ions is based on them acting as Lewis acids and their high affinity for forming complexes with organic matter, both with the functional groups, such as carboxylic, hydroxyl, and amino groups, and the aromatic systems in the so-called sandwich compounds (Filipović & Lipanović, 1995;Hagen, 2006;Sheldon et al., 2007). Apart from the liquid pyrolysate, the pyrolytic experiments also produced gaseous products that may be generated by direct degradation of kerogen or as secondary products of the degradation of liquid hydrocarbons. Gaseous products were not analyzed. However, their presence is proved by measuring pressure in the autoclave at the end of pyrolysis in relation to the initial pressure, which typically was ~ 4.5 atm (Table 3). As in case of liquid products of pyrolysis, the increase of pressure/yield of gas products was somewhat more pronounced for Ru(III)-ion compared to Pt(IV)-ion. The more pronounced influence of Ru(III)-ion may be explained by the fact that the Ru(III)-ion forms exclusively octahedral complexes, while Pt(IV)-ion forms both octahedral and square planar complexes, because of which the Ru(III)-ion has greater capacity for forming complexes with organic substance (Filipović & Lipanović, 1995;Hagen, 2006;Sheldon et al., 2007).

Specific organic geochemical parameters based on biomarkers.
All pyrolysates have similar n-alkane distributions in which n-alkanes C 17 -C 23 are predominant (Fig. 7a), typical of organic matter of algal origin. CPI values for all the pyrolysates are close to 1, typical of a mature oil distribution (Table 4). Unlike bitumen in the immature oil shale, squalane is absent in the pyrolysates (Figs. 3a and 7a).
Values of Pr/Ph ratio are higher than in the initial bitumen, which may be explained by the fact that degradation of kerogen during laboratory simulations results in uniform formation of both pristane and phytane (Tables 2 and 4; Stojanović et al., 2009Stojanović et al., , 2010. Compared to the pyrolysate of sample alone, the pyrolysates obtained in presence of metal ions have greater relative contents of pristane and phytane, which is reflected through the increase in Pr/n-C 17 and Ph/n-C 18 ratios and have higher values of Pr/Ph ratios (Table 4). Ru(III)-ion exhibits a somewhat more pronounced catalytic effect on both n-alkanes and isoprenoids, which also is observed at interpretation of group organic geochemical parameters (Tables 3 and 4).
Sterane distributions in pyrolysates obtained at 400 C are typical for oils, which confirms once again good potential of the investigated sediment and shows that catagenesis has been successfully simulated by pyrolysis. Apart from the regular (R)-steranes, C 27 -C 29 isomers with thermodynamically more stable (S)-, (R)-, and (S)-configurations, as well as, typical geoisomers, -and -diasteranes were present (Fig. 7b). All the three pyrolysates contain greater quantity of thermodynamically more stable C 29 and C 30 hopanes, compared to corresponding -moretanes (C 29 M/C 29 H and C 30 M/C 30 H below 1; Table 4), whereas unstable -hopanes and unsaturated hopenes were not identified. On the basis of mass spectra of corresponding peaks, the presence of Tm, Ts, C 29 Ts and 22(R and S)epimers C 31 -C 33 homohopanes was determined in all pyrolysis (Fig. 7c). Values of the most used sterane maturation parameters based on the ratios of C 29 sterane isomers, C 29 (R)/C 29 ( (R)+ (R)) and C 29 (S)/C 29 (S+R) in pyrolysates are lower than quilibrium values. On the other hand, values for C 31 (S)/C 31 (S+R)-homohopanes show that in isomerisation 22(R) → 22(S) the equilibria has been achieved in all pyrolysates (Table 4). Based on these results, it may be assumed that during pyrolysis at 400 C the investigated oil shale reached the value of vitrinite reflectance equivalence between 0.60 and 0.80 % (Peters et al., 2005). All pyrolysates obtained in presence of metal ions are characterised with higher values of C 29 M/C 29 H, C 30 M/C 30 H ratios, and lower values of C 29 (S)/ (S+R), Ts/(Ts+Tm), C 29 Ts/C 29 H ratios compared to pyrolysate of pure oil shale, especially in case of Ru(III)-ion (Table 4). The only exception among the sterane and terpane maturation parameters is the C 29 (R)/C 29 ( (R)+ (R)) ratio (Table 4). The reported observations lead to the assumption that metal ions, especially in case of Ru(III)ion have greater impact on kerogen degradation, which directly reflects on the increase in the quantity of hydrocarbons, than on isomerisation reactions: moretanes → hopanes, hopanes → neohopanes and C 29 (R) → C 29 (S). This conclusion is not surprising since kerogen contains functional groups for which the used metal ions show much stronger affinity, compared to saturated hydrocarbons.  (Peters et al., 2005)  Values of the naphthalene maturation parameters in pyrolysates suggest that the metal ions have a catalytic effect on isomerisations of methyl groups that lead to the generation of more thermodynamically stable naphthalene isomers. Again, the Ru(III)-ion exhibits a somewhat more pronounced effect compared to Pt(IV)-ion (Table 5). Maturation parameters based on isomerisation of methyl phenanthrene groups from -to -positions, as well as on the reactions of methylation of phenanthrene ring are higher in pyrolysates obtained in the presence of Pt(IV)-and Ru(III)-ions, than in the pyrolysate of pure oil shale. Thus, Pt(IV)-and Ru(III)-ions have catalytic effect on both the processes (isomerisation → and methylation) at 400 °C. Ru(III)-ion showed more pronounced effect on the reactions of isomerisation of methylphenanthrenes (parameters MPI 1 and MPI 3), and Pt(IV)-ion on the methylation processes, especially in case of methylphenanthrenes to dimethylphenanthrenes transformation (parameters PAI 1, PAI 2 and DMR) ( Table 5).

Conclusions
The differences in geochemical and mineralogical characteristics of the sediments indicate that the palaeoconditions in the sedimentation area changed over the time, which allow defining four different depth intervals in the drillhole Val-1. In certain periods sedimentation occured under very specific conditions. Sediments from 15-75 m depth interval were formed under the high salinity conditions, whereas sediments from depths 360-400 m were deposited in the conditions of the high alkalinity. Sediments from 15-75 m depths are characterized by the presence of searlesite and high amount of immature algal OM with good generative liquid hydrocarbon potential, deposited under reducing environment. From the organic-geochemical point of view, depth interval 200-400 m is less interesting due to the lower OM content with low liquid hydrocarbons generation potential. Interval at depths from 360 to 400 m is significant, since sediments may contain lithiumbearing clay minerals.
Pyrolytic experiments showed that oil shale from depth interval 15 to 75 m in a catagenetic stage may be a source of liquid hydrocarbons. Pt(IV)-and Ru(III)-ions, demonstrated significant positive effects on the yields of total liquid pyrolysate and corresponding hydrocarbons. The used metal ions had much greater influence on maturity changes on planar systems (naphthalene and phenanthrene rings) than on isomerisations in the molecules of polycyclic alkanes. Values of terpane and sterane and phenanthrene maturation parameters indicate that through pyrolysis at 400 °C the investigated sample reaches the value of vitrinite reflectance equivalent of approximately 0.70 %.
It was estimated that the investigated oil shale should be found at depth of 2300-2900 m in order to become active source rock. The calculated minimum temperature necessary for catagenetic hydrocarbon generation is between 103 and 107 °C.

Acknowledgment
Investigations within this study were done in cooperation with the company Rio Tinto Exploration from Serbia. The study was partly financed by the Ministry of Science and Technological Development of the Republic of Serbia (Projects number 146008 and 176006).