Giant Gas Fields of Northern West Siberia

Abstract

The 66 fields discovered since the 1960s in the northern West Siberian basin contain at least 22 trillion m3 (777 tcf) of proved gas, almost one-third of the world's reserves. Half of these fields are giants (> 85 billion m3 or 3,000 bcf of reserves). These include the largest and second-largest gas fields in the world--Urengoy (8.099 trillion m3 or 286 tcf of gas) and Yamburg (4.81 trillion m3 or 170 tcf of gas)--as well as most of the other ten largest gas fields in the world. The West Siberian basin occupies a 3.4-million km2 (1.31-million mi2) arctic lowland immediately east of the Ural Mountains, extending north under the Kara Sea. It is a composite basin, with Mesozoic-Cenozoic basin fill on top of a Paleozoic basin that overlies a crystalline Archean-Proterozoic framework. The productive zones in the northern basin are principally in the Neocomian section (at an average depth of 2,800 m or 9,200 ft) and the Cenomanian section (at an average depth of 1,100 m or 3,600 ft). The former contains reservoirs with gas, condensate, and oil; the latter contains two-thirds of the region's gas. Gas in Cenomanian reservoirs is almost pure methane. Hydrocarbons in Neocomian reservoirs were generated by thermal maturation of sapropelic organic matter contained principally in the Tithonian Bazhenov shale. Methane in the Cenomanian section appears to be a combination of thermogenic gas from the Bazhenov Suite (or deeper) and biogenic gas generated in the Cenomanian section itself, although workers disagree over how much gas came from each source. Continental glaciation during the Pleistocene may have been important in concentrating the methane in Cenomanian reservoirs. Regional petroleum geology of the northern basin is reviewed, with detailed descriptions of the largest fields: Urengoy, Yamburg, Bovanenko, Zapolyarnoye, and Medvezh'ye. Analyses of the size/frequency distributions and discovery rates of Neocomian and Cenomanian pools indicate that future discoveries will probably be much smaller than in the past, with new giants most likely to be found in remote areas, which are presently untested for transportation and technical reasons. Into the 1990s, the most severe constraint on production from the region will be imposed by available pipeline capacity.