Yenisey-Lena System of Downwarps, Northern Siberia

The Yenisey-Lena system of downwarps extends across the northern margin of the Siberian craton from the Yenisey River on the west to the delta of the Lena River on the east. Total area is 390,000 sq km. It consists of the Yenisey-Khatanga downwarp (900 by 300 km, area of 300,000 sq km) on the west, the Anabar-Lena downwarp on the east, and the Khatanga saddle between the two. On the west the Yenisey-Khatanga downwarp opens into the West Siberian basin, of which it is actually a part. The eastern boundary of the Anabar-Lena downwarp is drawn arbitrarily where the structure swings south into the Cis-Verkhoyansk downwarp. These downwarps are bounded on the north by the Paleozoic terrane of the Taymyr Peninsula and on the south by the Siberian carton. The region is well above the Arctic Circle. Average January temperature at Dudinka is -30 degrees C. Permafrost is continuous, reaching thickness of 600 m. No permafrost is present beneath the Yenisey River and is thin beneath large lakes and river valleys.

The initial stage of oil exploration was from 1936 to 1953, when wildcats were drilled to depths of 2.5 km in the Ust'-Port area, which is located where the Yenisey River empties into the Yenisey Gulf downstream from Dudinka. Gravity, magnetic, and seismic surveys during this time yielded the first information on the subsurface structure. Several anticlinal structures were recognized, and exploration drilling was begun in 1939 on one of them - the Malokhet.

Oil and gas shows were recorded during the period 1941-45 in sedimentary rocks of the Upper and Middle Jurassic, Triassic, and Upper Permian. It was found that the Malokhet high was cut by a series of faults, and the opinion was expressed that the Mesozoic could not contain commercial oil and gas. Attention at this stage turned to the Paleozoics, which occur at considerably greater depths. Work in the western part of the basin was discontinued in 1953. A stage of renewed exploration began at the end of the Fifties. Much of the western part of the basin was covered by seismic reflection surveys, and several local highs were delineated on the Jurassic-Cretaceous sedimentary complex.

The main stage of discoveries began in 1967, when there was a significant increase in the volume of seismic surveys and drilling. Messoyakha gas field was discovered in 1967, and a large multi-pay gas-condensate field was found in 1968 on the north flank of the Solenin high in sandstones of Valanginian-Hauterivian age. Several other discoveries were made during the following years. In the second half of the Eighties attention turned toward stratigraphic traps. Three sedimentary complexes fill these downwarps: Riphean (Precambrian), Middle Carboniferous-Permian, and Mesozoic. Good source beds are present throughout this section. The best source rocks are shales of the Middle Jurassic, Lower Permian, and Riphean. The most favorable targets for oil are the Upper Paleozoic rocks in the north part of the Khatanga saddle and possibly in the Yenisey-Khatanga downwarp. The best shot for gas condensate is the Neocomian-Jurassic in the latter (Petroleum Geology, vol. 30, no. 1, p. 48-55, 1996).

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