A minimum of nine commercial and 13 non-commercial discoveries has been made since 1962 in the Lena-Tunguska petroleum province of Eastern Siberia. However, the petroleum potential of this province has scarcely been tapped because the prospective area is greater than 1,737,000 sq km. Discovered reserves are in Proterozoic marine terrigenous clastic reservoirs and Early Cambrian fractured carbonates interbedded with evaporites. The Cambrian reserves are small and are associated with salt swells and salt pillows, whereas the Proterozoic discoveries are large and are in both stratigraphic and structural traps. Most of the Proterozoic accumulations were found as a result of drilling through overlying Lower Cambrian structural traps. Generally, the discovered hydrocarbons are as and gas condensate, although some oil has been found.

The original discovery field for the basin, Markovo field found in 1962, has proved and probable reserves of 622 Bcf of gas, 16 million bbl of condensate, and 10 million bbl of oil. The second largest field to date is the giant Sredne-Botuobin gas-condensate field, found in 1970. Sredne-Botuobin is a structural trap in both Proterozoic and Lower Cambrian strata. Proved plus probable reserves are 5.95 Tcf of gas and 149 million bbl of condensate. In both fields, the major reserves are in the Proterozoic. The largest field, Verkhnevilyuy, was found in 1975, and has 10.5 Tcf of proved plus probable reserves of gas and about 260 million bbl of condensate.

The Yaraktin oil field was discovered during 1971 in Proterozoic strata. Proved, probable, and potential reserves are about 210 million bbl. The field is a very large stratigraphic trap. The reserve estimate is subject to upward and/or downward revision as stepout and infill wells are drilled.

The potential of the area is very great. Several hundred, perhaps several thousand, oil and gas fields remain to be found. The province appears to be gas prone, but the discovery of the Yaraktin oil field indicates that some areas will have oil production. In terms of ultimate recovery, this region has the potential for producing 100 billion bbl of oil and 200 Tcf of gas, together with condensate. The existence of major to giant fields in Proterozoic strata--containing hydrocarbsons known from paleontological and geological data to have originated in Proterozoic beds--emphasizes the fact that nowhere should the Precambrian be regarded as economic basement.

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