GEOCHEMISTRY OF OILS FROM THE NORTHERN TIMAN-PECHORA BASIN, RUSSIA

A geochemical study of 36 oils from the northern Timan-Pechora Basin i ndicates four oil types. Two types of sulfur-rich oils (range 1.4-2.5% sulfur) can be identified on the basis of both gross and molecular pr operties. These oils are found primarily in the Toravey, South Toravey and Roman Trebs fields. They are also present in the offshore Prirazl om field, thereby extending the geographic range of sulfur-rich oils i nto the Pechora Sea. Another oil type exhibits a distinctive odd predo minance of n-alkanes in the range n-C-11 through n-C-19 and a diminish ed abundance of n-alkanes greater than C-19. These oils, which occur p rimarily in the A. Titov and Labogan fields, show striking similaritie s to Ordovician-sourced oils from other Paleozoic basins throughout th e world. Their biomarker and isotopic compositions, for example, bear a strong resemblance to oils generated from the Ordovician Viola Limes tone of the Anadarko Basin. The fourth oil type occurs in the northwes tern portion of the basin in the Khylchuyu field. Some mixing between the various types is evident. Light oils and condensates of the Pescha no-ozyor, Shtokman, Rusanov and Vasilkovo fields cannot be readily ass igned to any of these four types. The Upper Devonian Domanik Formation is generally thought to be the major oil source in the Timan-Pechora Basin. However, geochemical characteristics of Timan-Pechora oils anal yzed do not closely resemble those of other Upper Devonian-source oils , such as those generated from the Duvernay Formation of Western Canad a. These differences might reflect variations in the organic matter pr eserved in the two basins during the late Devonian or may indicate tha t the Upper Devonian is not an important hydrocarbon source in the nor thern Timan-Pechora region. The results suggest that the Timan-Pechora oils analyzed are generated from Paleozoic carbonate or calcareous sh ale source rocks, at least one of which appears to be of Ordovician ag e.