The Bazhenov-Neocomian Petroleum System of West Siberia

John D. Grace and David B. Walsh

The Bazhenov-Neocomian petroleum system contains approximately 85% of the discovered oil In the West Siberian basin. The source is the Upper Jurassic Bazhenov Formation, a 30 meter thick highly bituminous shale covering 2.3 million square kilometers. It has a 10% average total organic carbon content. Approximately 80% is Type II kerogen.

Maturation modeling yields time series maps of oil and gas generation. Approximately 40% of Bazhenov organic matter converted to hydrocarbons, with peak generation occurring in the Paleogene Period. Hydrocarbon expulsion from the Bazhenov was high, around 80%. Secondary migration efficiency was low, between 5% to 10% on average across the basin, although higher in the structured Middle Ob region.

The reservoirs of the system are vertically stacked sandstones and occasionally siltstones in the Neocomian Megion and Vartov formations (and their equivalents). Reservoir sizes, continuities and quality are highly variable and sealed by locally extensive interbedded shales.

There are three principal plays in the Bazhenov-Neocomian system, defined by trapping mechanism: the "on-structure" play, which contains most discovered fields and the "off-structure," which consists of combination and stratigraphic traps. Finally, there is the Bazhenov shale play, in which the fractured Bazhenov shale forms a self-sourced reservoir.

Mass balance calculations indicate the range of undiscovered hydrocarbons in the system. Mapping of generation and migration on palinspastically restored surfaces at the time of peak generation indicates the areas and magnitude of undiscovered hydrocarbons in the system.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California