Integrated Petroleum Systems Analysis of Western Siberia Basin: Why So Oily in the South, Why So Gassy in the North?

Alexei V. Milkov
BP, Moscow, Russia

Western Siberia basin in Russia is the largest petroleum basin in the world (area 2.2 million km2, Ulmichek, 2003), where ~355 billion barrels of oil equivalent (boe) is known (~83 billion boe is produced and ~272 billion boe is in remaining reserves) and ~183 billion boe is estimated to be undiscovered (Ahlbrandt et al., 2005). Approximately 60% of discovered petroleum is gas, which occurs mostly in the northern part of the basin. Such regional distribution of oil and gas accumulations – mostly oil in the south, mostly gas in the north – has been often noted (e.g., Peterson and Clarke, 1991; Ulmichek, 2003, numerous Russian publications). However, surprisingly, no clear comprehensive explanations of the observed fluid distribution in the basin have been presented in publications available to the author.

A basin-wide petroleum systems study integrating both geochemical investigations of rocks (>7000 samples) and fluids (>1700 oil samples, >850 gas samples) and pseudo-3D (various maps describing structures, Gross Depositional Environments, temperature, heat flow, uplift etc. plus multiple calibrated 1D models) basin modeling was performed to understand factors that led to the observed fluid distribution in Western Siberia. In the southern part of the basin, Bazhenov (Upper Jurassic – Lower Cretaceous) suite is a rich oil-prone source rock, which expelled the majority of petroleum accumulated in Jurassic and Neocomian reservoirs. Other Jurassic sources (Mid-Jurassic Tyumen and Radom suites, Lower-Jurassic Togur suite) have a relatively low generative potential and are more gas-prone. However, none of the source rock intervals in the southern Western Siberia have ever entered gas window. These factors led to the occurrence of mostly oil fields in the south. Gas and gas-condensate accumulations are rare in the south. They can be explained by contributions of secondary microbial gas (e.g., in Severnoe field in Tomsk area, Goncharov et al., 2005 and likely in the Cenomanian reservoirs of Samotlor and other Mid-Ob fields) and mature thermogenic gas from locally important deep Paleozoic (Mid-Devonian and Mid-Carboniferous) source rocks (Luginezkoe, Myildzhinskoe and other fields in Tomsk area, Goncharov et al., 2005).

In the northern part of the basin, Bazhenov suite and its stratigraphic equivalents (e.g., Maryanov suite) are relatively poor and gas-prone source rocks, which have been buried into the gas window. While Mid-Jurassic and Lower-Jurassic source rocks are more oil-prone than in the south, they entered gas window 50-70 Ma ago and spent most of their generative potential. Triassic coals absent in the south are widely present in the north and could be important gas generators. Biodegradation of petroleum in Neocomian-Cenomanian reservoirs is common in the north and likely resulted in generation of significant volumes of secondary microbial gases (predominantly methane) accumulated in giant Cenomanian pools (Goncharov et al., 1983; Milkov, 2009). All these factors together led to the occurrence of mostly gas fields in the north. Other, more speculative and perhaps minor factors such as primary microbial and/or early thermogenic gas generation from Albian-Cenomanian coals (Nemchenko et al., 1999; Fjellanger et al., 2008) and exsolution of gas from water during Neogene uplift (Cramer et al., 1999) also could have contributed to the dominance of gas accumulations in the northern part of Western Siberia.

References

Ahlbrandt, T.S., R.R. Charpentier, T.R. Klett, J.W. Schmoker, C.J. Schenk, and G.F. Ulmishek (2005): Global resource estimates from total petroleum systems, AAPG Memoir 86, Tulsa, Oklahoma, USA

Cramer, B., H.S. Poelchau, P. Gerling, N.V. Lopatin, R. Littke (1999): Methane released from groundwater: the source of natural gas accumulations in northern West Siberia, Marine and Petroleum Geology, Vol. 16, pp. 225-244.

Goncharov, I.V., D.I. Krashin, K.A. Shpilman (1983): On the origin of oils and gases in the North of Tyumen area, Geologiya Nefti I Gasa, no. 3, pp. 34-38 (in Russian).

Goncharov, I.V., V.G. Korobochkina, N.V. Oblasov, and V.V. Samoilenko (2005): Nature of Hydrocarbon Gases in the Southeast of Western Siberia, Geochemistry International, v. 43, no.8, pp. 810-815.

Fjellanger, E., A.E. Kontorovich, S.A. Barboza, L.M. Burshtein, M.J. Hardy, and V.R. Livshits (2008): 3D basin simulation and gas generation of the northern West Siberia basin, AAPG International Conference & Exhibition, October 26-29, 2008, Cape Town, South Africa

Milkov, A.V. (2009): Secondary microbial origin of gas in giant Cenomanian pools of Western Siberia, AAPG Annual Convention & Exhibition, Abstracts Volume, submitted.

Nemchenko, N.N., A.S. Rovenskaya, M. Shoell (1999): Origin of natural gases in giant gas pools of the North of Western Siberia, Geologiya Nefti I Gasa, no. 1-2 (in Russian).

Peterson, J.A., and Clarke, J.W., (1991): Geology and hydrocarbon habitat of the West Siberian Basin: AAPG Studies in Geology, no. 32, 93 p.

Ulmichek, G.F. (2003): Petroleum geology and resources of the West Siberian basin, Russia, U.S. Geological Survey Bulletin 2201-G.

AAPG Search and Discovery Article #90091©2009 AAPG Hedberg Research Conference, May 3-7, 2009 - Napa, California, U.S.A.