Geochemical characteristics of oils and source rocks in the Yamal peninsula, West Siberia, Russia

Eight oil and condensate samples and four samples of possible source rocks from the Yamal peninsula, northernmost West Siberia, have been characterized geochemically in order to determine source rocks and their maturity level. The high content of C29 steranes (m/z = 217) in extracts combined with geological data indicate nearshore marine and continental depositional environments for Cretaceous argillites. Jurassic source rocks, with a predominance of C27 steranes and Pr/Ph ratios of 2.4–2.6, have been deposited in the deeper marine basin and contain oil-prone type II kerogen. A number of parameters 20S/20S + 20R, ββ/ββ + αα steranes (m/z = 217), 22S/22S + 22R, hopane/hopane + moretane (m/z = 191), ratios of tri- and monoaromatic steroids (m/z = 231, m/z = 253), and methylphenanthrene ratios show an increase in maturity with depth, from immature Cretaceous source rocks to moderately mature Jurassic sources (Bazhenov and Tyumen Formations). Oil-oil and oil-source rock correlations based on genetic parameters and maturity indicators have demonstrated that fluids found in Jurassic and Cretaceous reservoirs, including heavy biodegraded oil from the Cenomanian, belong to a single family and could be derived jointly from the Bazhenov (J3) and Tyumen (J1–2) Formations. The origin of naphthenic condensate found at shallow depth is still controversial. Although earlier work has suggested that biodegradation has been a dominant factor in the formation of naphthenic condensates, their formation from Cretaceous source rocks with type III kerogen could also be possible.