Paleozoic-sourced petroleum systems of the Western Siberian Basin - What is the evidence?

Mesozoic petroleum systems in the West Siberian Basin have been studied extensively and are relatively well understood. The two main source systems are the Upper Jurassic marine Bazhenov suite (J3) and the Lower-Middle Jurassic, more terrigenously influenced Togur/Tyumen suites (J1-J2). Most of the oils known to be generated from these sources are produced from Mesozoic reservoirs. However, there is also production from Paleozoic strata. The petroleum systems related to these latter reservoirs are much less well understood, in particular with respect to source, and form the basis of this study. The oils studied here are from Paleozoic and Jurassic reservoirs located in the western part of the basin and the southeastern area. Distinct sources were recognized on the basis of the general molecular composition of the whole oil, and saturate and aromatic fractions, in combination with detailed biomarker evaluation and isotopic composition. Comparison with other oils from the West Siberian Basin showed that the two main contributions were from the two well known Jurassic sources. However, two samples from the southeast part show evidence of a Pre-Jurassic source, which could relate to the Proterozoic carbonate petroleum system similar to that sourcing oils of East Siberia.