Biomarker Hydrocarbons in Autochthonous and Migrated Bitumens in the Precambrian of Ust’-Mayskaya-366 Well, Siberian Platform

Abstract

The distribution of biomarker hydrocarbons in samples from Ust’-Mayskaya-366 well in the Aldan–Maya depression, Siberian platform, was studied in detail using modern techniques to elucidate biogeochemical features of Precambrian source organic matter (OM) and naphthides generated by it. To correlate OM and assess the effect of migration on biomarker parameters, allochthonous and mixed bitumens (AllB and MB, respectively), as well as open-pore bitumens (OPB) extracted from the uncrushed rocks of the Neryuen, Ignikan, Kandyk, Ust-Kirbin (Riphean), and Sardana (Vendian) formations were studied in comparison with the autochthonous bitumen (AutB) of the source-rock horizon of the Neryuen Formation. Saturated cyclanes and aromatic-fraction compounds from this well were analyzed for the first time. The Neryuen mudstones are enriched in organic matter (TOC > 1%), accumulated in clays [the steranes have βα/(αα + ββ) = 0.5], survived diagenesis in reducing environments without excess H₂S (the homohopanes have C₃₅/C₃₄ = 0.4), and were thermally transformed at MC₂–\({\text{MC}}_{3}^{1}\) (T_max = 452–465°C). The maximum in tricyclanes C_19–31 in almost all of the studied samples falls onto C₂₃, as in the Neryuen AutB. Even homologues of 3,7-dimethylalkanes found in the Neryuen AutB and Ust-Kirbin MB, as well as those detected in trace amounts in the Neryuen and Ignikan MBs and in the OPBs of the Sardana Formation, were apparently disseminated during migration. All bitumens were found out to contain 12- and 13-monomethylalkanes, which are most abundant in the Kandyk OPBs. These compounds can likely be accumulated during migration. The migrated bitumens differ from the AutB in containing a higher proportion of steranes C₂₉ (up to C₂₉/nC₂₇ > 2) and a higher nC₂₇/nC₁₇ ratio (up to >1). Relationships with the bitumen coefficient was found for Pr/Ph, nC₂₇/nC₁₇, ΣnC_i/ΣizoC_i in acyclic hydrocarbons, C₂₉/C₂₇ in steranes, C₃₅/C₃₄ in homohopanes, hopane/tricyclane ratios; these parameters are likely susceptible to effects of migration. The most probable source of the Riphean bitumens was mainly the carbonaceous rocks of the Neryuen horizon, and the variations in characteristics were caused by catagenesis and migration processes of accumulation or dissemination of low-molecular-weight mobile compounds. The OPBs of the Sardana Formation differ from the Riphean ones. The upper two bitumens are immature parautochthonous, and the others are probably a mixture of Riphean and Vendian bitumens, which were influenced by solutions from the Sardana base-metal stratiform deposits (which is located at approximately 35 km southeast occurs of the area) and the Perevalnoye deposit (at about 20 km), whose mineralization is of Vendian age.