Jurassic-Lower Cretaceous Source Socks of the Yenisei-Khatanga Trough, Russia

Natalya Kim

Discovery of the unique reserves of the Vankor group of fields with oil pools in the northeastern part of West Siberia has led to the need to identify oil source strata in the Mesozoic section of this Arctic area and refine the hydrocarbon potential. Using liquid chromatography and gas chromatography-mass spectrometry, saturated fraction of chloroform extracts were investigated in 280 core samples from Jurassic-Lower Cretaceous mudstones of the Yenisei-Khatanga trough. The organic carbon content in the rocks was determined and Rock-Eval pyrolysis was made in 387 samples. Stable isotope compositions of carbon were studied in 120 samples of insoluble organic matter. In addition, vitrinite reflectance R0vt has been measured in the 139 rock samples by the coal-petrographic method. The total organic carbon (TOC) contents in Cretaceous rocks are lower than those of Jurassic rocks (average TOC=1.0 and 2.1 wt. %, respectively). The Vym, Malyshevka and Sig Formations are highly enriched in organic carbon (TOC>2.3 wt. %). For the stratigraphic analogues of the Bazhenovo Formation, Yanovstan and Golchikha Formations, the TOC values average 1.6 and 2.0 wt. %, respectively. The biomarker distribution in the examined extracts and the pyrolysis of the rocks made it possible to identify two groups: with terrigenous (type III) and aquatic (type II) types of organic matter (OM). The first group is characterized by the low values of hydrogen index HI and the predominance of C₂₉ hydrocarbon in steranes, and C₁₉, C₂₀ low-molecular homologs in tricyclanes. The aquatic OM in source-rock extracts, which were identified in the Malyshevka, Yanovstan, Golchikha Formations and in some Lower Cretaceous extracts (Lower Kheta, Shuratov Formations), exhibited a uniform distribution of sterane homologs and the prevalence of middle-weight molecular tricyclanes. The values of the Pr/Ph and S₃₅/S₃₄ homohopane ratios and the presence of diahopanes indicate the OM burial in suboxidative coastal marine environments. Thus, it has been established that in the Upper Jurassic Yanovstan and Golchikha Formations, and, to a lesser extent, in the Middle Jurassic Malyshevka Formation, the levels are widespread, where the OM in rocks is aquatic, and the organic carbon concentrations and the level of OM maturity in these rocks allow them to be regarded as an oil source. Lower Cretaceous extracts with aquatic OM are confined to thin regional deep-water members of clays. TOC in samples from these members account for 0.5-2.3 wt. % (averaging 1.6 wt. %). For the Lower Cretaceous rocks, analyzed by the coal-petrographic method, the OM maturity suggests that 78 % of all samples have reached the oil window. Therefore, Lower Cretaceous rocks of the Lower Kheta and Shuratov Formations could generate liquid hydrocarbons, but not in so significant amounts as the above-indicated Jurassic deposits. The other examined deposits contain OM related to higher land plants and are assigned to gas-generating ones.

AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013