Jurassic Bazhenov Unit in the Salym Oil Field, West Siberia--an Excellent Source Rock and Fractured Shale Reservoir

PETZOUKHA, Y., A. ROVENSKAYA, M. ZONN, and A. KONONKOV, Institute of Geology and Exploration of Fossil Fuels, Moscow, U.S.S.R.

The Upper Jurassic Bazhenov Unit covers approximately 1 million km of West Siberia and is an excellent source rock. This Unit has proved to be a prolific shale reservoir in 14 local highs with the Salym oil field the major discovery. More than 300 wells have been drilled to date in this area, covering approximately 5800 sq km.

The Bazhenov formation is composed of calcerous, organic-rich mudstone and siliceous shale lithofacies. Average thickness of the Unit in the Salym is 30 m. A significant heterogeneity is observed in the shale reservoir properties, both locally and regionally. Producing capacities of individual wells range from a few barrels daily to a maximum of 1000 bbl/day, and occasionally reaching 10,000 bbl/day. On average the porosity is 6.1%. The rock matrix is virtually impermeable and oil flow occurs via various types of fractures. All production is water free. A sharp difference is observed in the distribution of formation pressures and temperatures through the Bazhenov Unit in the various parts of the Salym field. The initial formation pressure ranges from normal hydrostatic to abnormally h gh, reaching 4060 and 6525 psi, respectively, at 2800 m depth. The temperature ranges from 180 to 280 degrees at depths from 2750 to 2950 m. The oil density varies between 30 and 46 degrees API, and the sulfur content ranges from 0.08 to 0.48%. The range of CO(2) content in the associated gas is between 0.2 and 11.5%.

Material of marine origin was the precursor organic matter of the kerogen for the Bazhenov Unit. The organic carbon content is high, ranging from 5 to 7% and occasionally reaching 15-20%. The oil generating potential of such source rocks is very high. These are type II kerogens. The Bazhenov source rocks generate in situ high quality paraffin-base oils.

Meanwhile, oil-oil and oil-source rock correlation revealed controversial discrepancies in the distribution of various hydrocarbon compounds that are hard to explain using the conventional approach to source rock maturation and hydrocarbon generation. In this particular case, it is apparent that the governing maturation factor has probably been related to the same process that controlled Bazhenov rock deformation and development of fractured shale reservoir, as well as the origin and irregular excess pressure and temperature distribution.

Proper understanding of the oil pool formation mechanism in the isolated body of the Bazhenov type formations will certainly help to develop a successful exploration program for fractured shale production.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)