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1 2006

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Hydrocarbon composition of bitumens of Upper Jurassic deposits of Kashpir oil shale-bearing basin (Russia)

Burdelnaya N., Boushnev D., Lyurov S.

Volumina Jurassica

2006

Vol. 4, no. 1

257-258

Kashpir oil shale deposit, located in the Volga region, is left one of two, previously developed, most important (after Baltic kukersites in Leningrad region and Estonia) oil shales resources in Russia. The organic carbon content (Corg) ranges >40%. Besides these oil shales are characterized by increased content of organic sulphur which has a high concentration due to specific sedimentation environment of organic matter (OM) in sediments (Riboulleau et al. 2000). The samples under study were taken from a section near the city of Syzran, in the vicinity of old shale mine 5. In the section of shale rocks the interbedding of oil shales, argillaceous oil shales, kerogen clays and carbonate grey, dark grey and brown clays is observed. All the rocks are saturated by fine-disperse OM of different concentration. Altogether 4 oil shales members are observed with the thickness from 0.08 to 0.3 m. All the rocks are littered by shell detritus. In argillaceous rocks there are many deformed shells and bivalve prints, also ammonites and belemnites are found. In OM-rich rocks macrofauna is found considerably rarer. Among ammonites the representatives of Zaraiskites, Dorsoplanites and numerous bivalves Buchia are determined. The visible thickness of the layer is 2.8 m. Corg content changes from 0.85 to 37.34% in the studied section. According to gas chromatography data the distribution of normal and isoprenoid alkanes changes upward the section. The OM content is sharply dominated by acyclic isoprenoid hydrocarbons. Pristan/phytan ratio <1 in all the samples which testifies to reducing environment predominance during sedimentation both with low and high Corg. Odd carbon number (C15-C19) of n-alkanes is <1 (except sample KS-1/7), and mid-chain (C21-C25) C high (C27-C31) n-alkanes ranges considerably >1, and for medium range CPI in oil shales are the highest (except sample KS-1/7). In OM of clays the hump in the range of high-molecular-weight hydrocarbons, characteristic for steranes, is absent. In the samples KS-1/2, KS-1/4, KS-1/10 normal hydrocarbons C16, C18 dominate, in other fractions the content of these compounds is not so expressed against the background of other n-alkanes. This fact could be connected with the change of redox environment during sedimentation, but Pr/Ph ratio in these samples insignificantly differs from these values of other samples. It can be suggested that isoprenoid components of initial OM were exposed to the least reworking by bacteria during OM formation of the stated above deposits. Practically in all the samples, except KS-1/7 and KS-1/10, low n-alkanes C13-C19 dominate. The increased concentrations of C15-C17 n-alkanes resulted from the participation of phytoplankton in the OM formation of the studied sediments. C27, C29, C31 normal hydrocarbons peaks, which are prevalent in high plant waxes, expressed to a lesser extent relative to the given compounds identified in the OM from oil shales of the Sysola shale-bearing region (Boushnev & Lyurov 2002), which resulted from the low input of terrigenous component to the composition of OM. A certain medium variant of n-alkane distribution presented by higher algae - characteristic C21, C23, C25 homologues predominance is fixed for all the samples. Desulphurization of oil shale bitumen polar fractions, done on Raney nickel, resulted in the equal content of hydrocarbon components in all three studied fractions. In desulphurization products the n-alkanes (C14-C31) are present. The distribution of normal and isoprenoid alkanes is slightly different from the hydrocarbon content of desulphurization products of bitumen polar fractions from the Sysola shale-bearing region which was obtained earlier. If the desulphurization products of Sysola bitumens are dominated by phytane on the background of n- and iso-alkanes, in the desulphurization products of Kashpir bitumens just minor change of isoprenoid/n-alkane ratio at the transition from free to desulphurized fractions is stated. Concerning the Sysola deposits a sharp isoprenoid predominance is observed relating to n-alkanes (3-4 times), basically due to increasing concentration of phytane. Considerable changes are observed in low n-alkanes where in the process of desulphurization even n-alkanes C16 and C18 concentration sharply increases. The content of the given compounds also slightly prevails over odd low structures in the content of free hydrocarbon fraction which testifies to the active contribution of initial biostructures C16, C18 to the content of Kashpir deposit OM. Thus, hydrocarbon content of bitumens of deposits with increased Corg is sharply different from the content of normal and isoprenoid hydrocarbons of organic carbon-poor deposits by great concentrations of isoprenoid hydrocarbons, by isolating two n-alkanes predominance areas, by the increased contribution of low hydrocarbons (C14-C18), supported by the content of desulphurization products of bitumen polar fractions.