Multiple Oil Families in the West Siberian Basin

Abstract

Two major oil families are identified in the West Siberian basin. Twenty-six of 32 analyzed oils occur in Jurassic and Cretaceous reservoirs and are derived from anoxic marine Upper Jurassic Bazhenov source rock, based on geochemical comparison of oils and source rock extracts. These oils are widely distributed both north and immediately south of the Ob River, and their biomarker ratios indicate a wide range of source rock thermal maturity from early to middle oil window (Van-Egan, Russkoye, Samotlor, Sovninsko-Sovyet, Olyenye, Ozynornoye, and Kogolym), to peak oil window (Srednekhulym, Yem-Yegov, Vostochno-Surgut, Khokhryakov, 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 n the reservoir. The Bazhenov-sourced oils show different compositions that support regional variations of organic facies in the source rock. For example, the anoxic marine Bazhenov facies, which generated the oils in the Samotlor, Fedorov, and nearby fields, was particularly 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 dominated by terrigenous higher plant input like those in the Lower to Middle Jurassic Tyumen Formation, although no correlation was observed between 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 Verchnekombar) 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-Paleozoic reservoir rocks. Most of the probable Tyumen-sourced oils ar from south of the Ob River, but the occurrence of Geologiche oil to the north suggests that related oils may be widespread in the basin.