MULTIPLE OIL FAMILIES IN THE WEST SIBERIAN BASIN

Two major oil families are identified in the West Siberian basin. Twen ty-six of 32 analyzed oils occur in Jurassic and Cretaceous reservoirs and are derived from anoxic marine Upper Jurassic Bazhenov source roc k, based on geochemical comparison of oils and source rock extracts. T hese oils are widely distributed both north and immediately south of t he Ob River, and their biomarker ratios indicate a wide range of sourc e rock thermal maturity from early to middle oil window (Van-Egan, Rus skoye, Samotlor, Sovninsko-Sovyet, Olyenye, Ozynornoye, and Kogolym), to peak oil window (Srednekhulym, Yem-Yegov, Vostochno-Surgut, Khokhry akov, Fedorov, and Urengoi), to late oil window (Salym). Some of these oils have been mildly (e.g., Fedorov 75) to heavily (e.g., Russkoye) biodegraded in the reservoir. The Bazhenov-sourced oils show different compositions that support regional variations of organic facies in th e source rock. For example, the anoxic marine Bazhenov facies, which g enerated the oils in the Samotlor, Fedorov, and nearby fields, was par ticularly clay poor and sulfur rich. Six nonbiodegraded, highly mature oils show geochemical characteristics that suggest they were derived from clastic-rich lacustrine or nearshore marine source rocks dominate d by terrigenous higher plant input like those in the Lower to Middle Jurassic Tyumen Formation, although no correlation was observed betwee n the oils and a single rock sample (Yem-Yegov 15) from the formation. The six oils occur in the Tyumen (Taitym, Geologiche, and Cheremshan) and fractured basement/Paleozoic (Gerasimov, Yagyl Yakh, and Verchnek ombar) reservoirs in positions readily accessible to any oil migrating from the Tyumen source rock. For example, at the Gerasimov location, the Tyumen Formation lies unconformably on weathered basement-Paleozoi c reservoir rocks. Most of the probable Tyumen-sourced oils are from s outh of the Ob River, but the occurrence of Geologiche oil to the nort h suggests that related oils may be widespread in the basin.