Geochemistry of water-soluble gases in the oil and gas bearing sediments of the zone of junction between the Yenisei-Khatanga and the West Siberian basins (the Arctic regions of Siberia)

The results of the studies of geochemical and zoning features of water-dissolved gases in the Mesozoic sediments of the junction between the Yenisei-Khatanga and the West Siberian basins are reported. The stage of industrial oil and gas content is more than 3 km thick and involves the depth range from 750 to 4000 m. Waters occurring in the region contain methane, their total gas saturation is 0.3 to 4.6 dm³ /dm³ , and CH₄ content in 88.9 to 95.4 vol. % on average. An increase in the concentrations of carbon dioxide and methane homologues is observed with an increase in the depth, which is accompanied by a regular decrease in the concentrations of methane and nitrogen. In general, waterdissolved gases from the Neocomian and Jurassic reservoirs have similar compositions (С₁ > N₂ > C₂ > C₃ > n-C₄ > i-C₄ > CO₂ > i-C₅ > n-C₅ ). The water-dissolved gases of the AptianAlbian-Cenomanian sediments are distinguished by lower concentrations of methane homologues (С₁ > N₂ > C₂ > CO₂ > C₃ > i-C₄ > n-C₄ > i-C₅ > n-C₅ ), with the total content of heavy hydrocarbons (HHC) equal to 1.44 vol. %. Vertical zoning of the composition of water-dissolved gases is determined by the distribution of hydrocarbon pools in the Jurassic-Cretaceous section. A regular increase in the average values of the factor of enrichment with hydrocarbons (HC) (from 37 to 154) was established, along with a decrease in the ratio of CH₄ /ƩHHC (from 130 to 7), C₂ H₆ /C₃ H₈ (from 41 to 2) and i-C₄ H₁₀/nC₄ H₁₀ (from 2.6 to 0.6) from the Aptian-Albian-Cenomanian reservoirs to the Jurassic ones. The zone with anomalous He concentrations within the range of 0.4–0.9 vol. % was detected in the Neocomian and Middle Jurassic sediments. This zone is localized at the north-western slope of the Messoyakh inclined ridge (the Anomalnaya, Turkovskaya, Pelyatkinskaya and Sredne-Yarovskaya areas). The nature of this anomaly needs further studies.

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