The Ust'-Lena (Mouth of the Lena River) downwarp is an elongate structure of the Laptev platform. It extends southward from the continental slope on the north into East Siberia on the south between 125 and 135 degrees east longitude. The area of present interest for petroleum is between 71 and 73 degrees north latitude. The position of the Ust'-Lena downwarp had been determined by gravity survey before seismic surveys were made. The first predictions of petroleum potential were made in 1976. Refraction surveys were made in 1985-88. Two profiles were run across the trend of this feature and one along its length. Each was 200-240 km long. Water depths are generally 15-20 m. Refracting Horizons II, III, and IV (downward in the section) are traced almost everywhere. Their interface velocities are 4.2-4.7, 5.2-5.7, and 6.3-6.6 km/sec, respectively. Seismic velocity in the section above Horizon II is in the 2.8-3.2 km/sec range. Structural concordance of these horizons is notable. On one of the transverse profiles depth to Horizon II subsides regionally from 0-1 km on the east to 4.5-5 km on the west, and Horizon III from 2 km on the east to 8-9 km on the west. On a profile that runs up the middle of the downwarp all three refracting horizons dip evenly to the north. Structural relief on these horizons decreases upward in the section. A large number of faults cut the three refracting horizons, resulting in mosaic structure. Displacement on these faults is as much as 1.5-2 km on Horizon IV. Greatest density of faults is in the axial area of the downwarp, which is characterized also by greatest density of earthquakes. The upper part of the section is correlated with the Paleogene deposits of the numerous grabens of the Northern Verkhoyansk region. The stratigraphy of the lower part of the section is uncertain because none of the seismic profiles extend into areas where these rocks are at the surface. Based in velocity characteristics these underlying rocks are divided into two parts. The upper part is correlated with Middle Paleozoic to Lower Mesozoic clastics, and the lower part with clastic-carbonate rocks of probable Late Proterozoic to Middle Paleozoic age. Horizon IV is correlated with the top of the Precambrian crystalline basement. Thickness of the platform complex of the Ust'-Lena downwarp is 3-5 km, and width of this feature is 150-160 km in the area of the Lena Delta. The asymmetry of the downwarp and its very irregular western border are characteristic of rifts that have formed at the junction of substantially different lithospheric plates. All factors necessary for formation and accumulation of oil and gas are present in the Ust'-Lena downwarp. Good siltstone and sandstone reservoirs and thick clay seals suggest the potential of this structural feature, which is similar to the Suez, Yemen, and other basins. The area is also in the zone of Arctic shipping channels and is close to Tiksi with its large port facilities and airport. (Taken from Avetisov and others, 1994; digested in Petroleum Geology, vol 29, no. 5/6, 1995; two maps and three cross sections.) Copyright 2000 James Clarke. You are encouraged to print out this News Letter and the forward it to others. Earlier News Letters are available at our website: http://geocities.com/internetgeology/