Two north-south offshore seismic refraction profiles were run in 1990 off the northeastern coast of Sakhalin. From south to north they cross Lunskoye, Veni, Chayvo and Pil'tun-Astokh (Odoptu) fields, which are part of the North Sakhalin oil-gas basin.
Common depth point seismic surveys were carried out on nets of 1 x 1 and 2 x 2 km in the regions of the refraction surveys, and 18 exploration wells were drilled. This exploration targeted upper-middle Miocene in the northern part of the shelf and middle-lower Miocene in the southern part. This work resulted in discovery of three large oil, gas, and condensate fileds with total reserves of 5.5 BBOE.
Estimate of level of catagenesis on a basis of interval velocity of reflected waves is used widely in the Sea of Okhotsk region to predict hydrocarbon potential. Use of reflected wave velocity has its limits, however, and consequently refracted wave velocity was used here. Velocity models were used to estimate total thickness of the sedimentary section, volume of siliceous deposits (cherts), and volume of oil and gas windows.
Maximum thickness of the sedimentary fill is estimated at 10-11 km. Consolidated basement consists of highly lithified Mesozoic volcanic- sedimentary rocks. The stratigraphic subdivisions of the basin fill are quite variable in composition. Rocks of the same age may range from sub-continental to deep-water marine varieties. In its Cenozoic history the North Sakhalin basin has experienced two stages of tension and one of compression. Consequently, sediments of the same age have experienced different thermo-barometric conditions. At the base of the sedimentary pile are clayey-siliceous deposits of Late Paleogene to middle Miocene age. This unit with a density of 2.5 is characterized by de-densification due to post-depositional fracturing. These rocks are regarded as the main oil-source rocks and as reservoirs. Their thickness ranges from 3.6 km in the south to 2 km in the north.
Six regional velocity layers are recognized in the section above basement. The upper three layers have low velocity at up to 3.9 km/s, and the lower layers - as much as 5.1-5.4 km/s.
Active generation of hydrocarbons has been in the oil window with vitrinite reflectance from 0.5 to 0.85 o/o and in the thermal gas window at values greater than 0.85 o/o. Seismic velocities in these windows are 3.9-5.2 km/s and 5.2-6 km/s, respectively. Thickness of the oil window according to refraction data is 2 to 5.5 km, and depth to its top is 2.5 to 3 km. Stratigraphic range is from upper Miocene to Upper Paleogene. The thermal gas window is at depth of 5 km and more. Thickness is up to 2.5 km. Total thickness of the oil-gas source beds at 4 to 7 km suggests the possibility of a several-fold increase in hydrocarbon potential of the North Sakhalin Shelf.
Thickness of the main zone of oil-gas accumulation is estimated at 1.5-4 km and includes upper and lower Miocene rocks. (Taken from Konov and others, 1998; digested in Petroleum Geology, vol. 35, no. 1, p. 30-38, 2001, in preparation. One map, three cross sections, and one graph.)
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