Geochemistry of Selected Oils and Rocks from the Central Portion of the West Siberian Basin, Russia

Abstract

Six analyzed oils, produced from Middle Jurassic to Upper Cretaceous strata in the Middle Ob region of the West Siberian basin, show biomarker and stable carbon isotope compositions indicating an origin from the Upper Jurassic Bazhenov Formation. The chemical compositions of these oils are representative of more than 85% of the reserves in West Siberia (Kontorovich et al., 1975). Bazhenov-sourced oil in Cenomanian strata in the Van-Egan field underwent biodegradation in the reservoir, resulting in a low API gravity, an altered homohopane distribution, and the appearance of 25-norhopanes without alteration of the steranes. High API gravity oil from the Salym field has surpassed the peak of the oil window, consistent with abnormally high temperatures and pressures in the Bazhenov source rock from which it is produced. The remaining oils are very similar, including samples from Valanginian and Bathonian-Callovian intervals in a sequence of stacked reservoirs in the Fedorov field. Bazhenov rock samples from the study area contain abundant oil-prone, marine organic matter preserved under anoxic conditions. Organic matter in a Bazhenov core from the Pokachev 58 well was compared with the oils because it is thermally mature and shows total organic carbon (TOC = 13.8 wt.%) and hydrogen index values (HI = 489 mg HC/g TOC) representative of the average for the formation (10.7 wt.% for 840 samples; 420 mg HC/g TOC for 75 samples). Other rocks in the Middle Ob region are far less likely to have generated these oils than the Bazhenov Formation. No prospective source rocks older than Middle Jurassic were available. Geochemical analyses indicate a lack of thermally mature Hauterivian or younger source rocks in the study area. Core samples from the Lower Cretaceous Frolov Formation either are thermally immature (Hauterivian) or show little oil-generative potential (Berriasian-Valanginian). The Upper Jurassic Vasyugan Formation shows lower oil-generative potential than the Bazhenov Formation. The average TOC for Vasyugan rocks is 3.20 wt.% (240 samples). Average atomic H/C ratios for Vasyugan and Bazhenov kerogens are 0.90 (10 samples) and 1.13 (25 samples), respectively. The Vasyugan Formation cannot be excluded as a source rock because insufficient sample was available for biomarker analysis. Core from the Lower to Middle Jurassic Tyumen Formation in the YemYegov 15 well was compared with the oils because it is thermally mature and shows TOC and HI values (2.78 wt.% and 137 mg HC/g TOC, respectively) indicating slightly more favorable oil-generative characteristics than the average for the formation (2.75 wt.% for 720 samples; 95 mg HC/g TOC for 25 samples). The core contains terrigenous, gas-prone organic matter that shows no relationship with the analyzed oils.