ABSTRACT: Diverse Super-Giant Petroleum Deposits in the West Siberian Oil-Gas Basin

James W. Clarke, James A. Peterson

The West Siberian sedimentary basin is a relatively uncomplicated basin where deposition has continued with little significant interruption during the almost 200 m.y. since the beginning of the Pliensbachian Age in the Early Jurassic. The petroleum deposits, however, are diverse, as exemplified by the Salym, Samotlor, and Urengoy fields.

During the Volgian Age, in the latest Jurassic, bituminous siliceous clays were deposited during a tectonic calm in an anoxic basin. These clays, now the cherty shale of the Bazhenov Formation, were enriched in sapropelic organic matter and became source rock, reservoir rock, and seal for the oil of Salym field. The oil occurs along fractures generated by the hydraulic effect of the generating oil. No significant water or gas accompanies the oil.

Following the tectonic calm, progradational deltaic deposits began to fill the sedimentary basin during the Early Cretaceous. Deltaic sands deposited on a mid-basin high were winnowed by currents that carried the finer material farther west. These sands became reservoirs whose source beds were either the distal pelitic facies to the west or the underlying bituminous Bazhenov Formation. The oil pools of Samotlor field, which accounts for about 5% of the world production, are in these progradational sandstones.

Deposition of sandy facies continued with little interruption through the Cenomanian, after which clay was deposited during the Turonian in an anoxic environment similar to that of the Bazhenov. Urengoy field, probably the largest gas deposit in the world, occurs in the sandstones beneath the Turonian clays. The gas is almost pure methane and was sourced probably by both the humic organic matter in the rocks beneath and adjacent to the pool and by organic matter in the overlying Turonian bituminous clays. The deposit is sealed by the Turonian clay and perhaps also by permafrost.

The source beds of the West Siberian basin are presently at those depths favorable for petroleum generation.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990