Hopanes in bitumen extracts of Mesozoic deposits of the Yenisei-Khatanga trough, Russia

Kim N.¹ and Rodchenko A.²

¹A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia; ² Novosibirsk State University, Novosibirsk, Russia

[e-mail]

Discovery of the unique reserves of the Vankor oil field in the northeastern part of West Siberia has led to the need to identify oil source rocks in the Mesozoic section of this area and refine the hydrocarbon potential. For this purpose, using gas chromatography-mass spectrometry, saturated biomarkers were investigated in 68 core samples from Jurassic and Cretaceous mudstones of the Yenisei-Khatanga trough (wells Nanadyanskaya-310, Payutskaya-1, Pelyatkinskaya-15, Turkovskaya-2 and Ushakovskaya-1). Analysis of saturated fraction of bitumen chloroform extracts was carried out on chromatograph Hewlett Packard 5890 - mass spectrometer 5972A MSD with the computer system (ChemStation) of recording and processing of information HP GI034.

In this paper, special focus has been placed on the composition and distribution of biomarker hydrocarbons, such as hopanes. Biomarkers are complex molecular fossils, derived from biochemicals, particular lipids, in once-living organisms. They occur in sediments, rocks, and crude oils and show little or no change in structure from their parent organic molecules in living organisms. Hopanes are pentacyclic triterpanes commonly containing 27-35 carbon atoms in a naphthenic structure composed of four six-member rings and one five-member ring. Hopanes originate from precursors in bacterial membranes. Hopanoids in these membranes, such as bacteriohopanetetrol, originate by cyclization of squalene precursors [1, 2].

In the studied samples the peak of distribution of hopane hydrocarbons usually is marked at С₃₀ hopane. All the extract rocks have high concentrations of C₃₀ diahopane (17a(H)-15a-methyl-27-norhopane). Diahopane C₃₀ originates from bacterial input to sediments containing clays deposited under oxic or suboxic conditions [1, 3]. Diahopane reaches significant concentrations (diahopane-C₃₀/hopane-C₃₀=0.53-2.82) in the bitumen extracts from the Ushakovskaya (Vym Formation), Payutskaya (Leontievskoe Formation) and Nanadyanskaya (Malyshevka Formation) areas. The concentration of С₃₅ homohopane is lower than that of С₃₄ homohopane (C₃₅/C₃₄ ratios <1), which is indicative of oxidizing environments in diagenesis during accumulation of sediments.

There have also been identified rare occurring rearranged hopanes (Figure): hZ hopane (C (14α)-homo-26-nor-17α-hopane) [4,5], and hY hopane of unknown structure [6].

Trendel et al. [4] discovered an unusual series of C₂₇-C₃₅ pentacyclic triterpanoids in severely biodegraded oil extracted from sandstone (Congo), which was identified as C(14a)-homo-26-nor-17a-hopanes. In the studied bitumen extracts of Mesozoic deposits of the Yenisei-Khatanga trough only the C₃₀ compound, which is called “hZ”, elutes slightly

Fig. Mass chromatograms at m/z 191 and m/z 177 indicate that 25-norhopanes occur in the extracts of Jurassic and Cretaceous mudstones of the Yenisei-Khatanga trough.

earlier than ab-30-norhopane in m/z 191 ion chromatogram. The origin and utility of these compounds are unknown. Trendel et al. [4] considered it unlikely that they are biosynthesized by bacteria. Rather, they may have formed by the oxidation of the C-26 methyl to an alcohol, either in the reservoir or during early diagenesis, followed by a concerted rearrangement and migration of the C8(14) bond.

On the m/z 191 mass fragmentograms, C₃₀ component of novel series of early eluting rearranged hopanes appears between Ts and Tm. This hopanes of unknown structure occurs in the terrigenous oil found in China [6, 7]. The precursors of these compounds and genetic mechanisms remain to be determined. Zhu et al. [6] suggest that the abundant rearranged hopanes is evidence for the presence of effective highly mature and clay-rich source rocks formed under a suboxic depositional setting.

Both of these rearranged hopanes C₃₀ (hY and hZ) also are found in oils of Like Baikal [8].

The 28,30-bisnorhopanes C₂₈ are identified in studied bitumen extracts using the m/z 355 mass chromatogram. Nytoft et al. [9] detected a complete series of 28-norhopanes (C₂₆ and C₂₈-C₃₄) in oils and rock extracts from west Greenland and the North Sea. They believe that 28,30-bisnorhopane originates from different biological precursors than other 28-norhopane homologs, but no biological precursor for 28,30-bisnorhopanes has yet been found, and their origin is still unclear.

In this study 25-norhopane series (also called 10-desmethylhopanes or degraded hopanes) have been first determined in the extracts of Jurassic and Cretaceous mudstones of the Yenisei-Khatanga trough (see Figure). We propose that the 25-norhopanes in these mudstones are indigenous. The 25-norhopane series can be assigned best by using m/z 177 chromatograms [1]. The carbon number distribution of 25-norhopanes series consists of the carbon number suite from C₂₇ to C₃₀. 25-Norhopanes is usually used as the reliable indicators of oil biodegradation. The presence of 25-norhopanes in mudstones has been report several times [10-12]. However, their origin is still controversial. There are three hypotheses about the possible origin for these compounds: 1. microbial demethylation of hopanes to 25-norhopanes through the removal of the methyl group at C-10 in the hopane ring structure; 2. a relative enrichment in preexisting source-derived 25-norhopanes; 3. directly from microbes (but this is unlikely as no 25-norhopanes have been found in microorganisms) [1, 12]. Cao J. et al. [12] note that right conditions responsible for the formation of 25-norhopanes in Qaidam Basin (China) are associated with anaerobic and restricted situations. They suggest that in such a nutrient limited environment the microbial consortia scavenge the existing hopanoids, contributing biomass in order to release nutrients to perform 25-norhopane-forming reactions [12].

The distributions 25-norhopanes in studied mudstones are only represented by the C₂₉ 25-norhopane, with a few other members of series. In our cases, 25-norhopane are associated with organic matter of terragene type (input of lipids of higher land plants), formed under the suboxic conditions in diagenesis. We found co-existence of the 28,30-bisnorhopane and the 25-norhopane in studied extract rocks.

Thus, this study provides additional evidence for the existence of 25-norhopanes in rocks. The identified rare occurring rearranged hopanes C₃₀ hY and hZ may be used for biomarker correlations between source rocks and crude oils.

The study was supported by grant of the President of Russian Federation for the young scientists (МК-4893.2012.5).

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