CLAYTON, JERRY, U.S. Geological Survey, Denver, CO

ABSTRACT: Oil and Gas Basins in the Former Soviet Union

The Pripyat basin is a Late Devonian rift characterized by a typical fault-block structure. Two synrift salt formations separate the Devonian stratigraphic succession into the subsalt, intersalt, and postsalt sections. Oil is produced from carbonate reservoirs of the subsalt and intersalt sections. Traps are controlled by crests of tilted fault blocks.

We analyzed 276 shale and carbonate-rock samples and 21 oils to determine oil-source bed relationships in the basin. Maturities of the oils are from very immature, heavy (9 degrees API), to very mature, light (42 degrees API). All fields are in a narrow band on the north side of the basin, and only shows of immature, heavy oil have been obtained from the rest of the basin. Three genetic oil types are identified. Oil type A has high pristane/phytane ratios (>1.0), high amounts of C29 18a (H) trisnorneohopane, and (delta)13C of hydrocarbons in the range of -31 to -27%. Oil types B and C contain very high amounts of gammacerane, which suggests that the oils were derived from carbonate-evaporite source facies. Type B oils are isotopically similar to type A, whereas type C oils are isot pically light (about -33%). Organic carbon content is as much as 5%, and kerogen types range from I to IV. Our data indicate that rocks within the intersalt carbonate formation are the source of the type B oils of low maturity. Thermally mature rocks that might be the source for the mature oils have not been found. Such rocks may occur in depressions adjacent to tilted fault blocks.

Higher levels of thermal maturity on the north part of the basin in the vicinity of the most mature oils may be related to higher heat flow during and soon after rifting or to a suspected recently formed magmatic body in the crust below the northern zone. Present-day high temperatures in parts of the northern zone may support the latter alternative.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.