Combined geophysical acquisition methods heighten hydrocarbon prospectivity of the Sea of Okhotsk

Erling M. Frantzen and Kjell E. Trommestad, TGS-NOPEC Geophysical Company, explain how a modern marine 2D seismic data being acquired in a multi-client survey in the Sea of Okhotsk is being combined with gravity and magnetic data to challenge previous ideas about the limited petroleum prospectivity offshore Far East Russia. Numerous onshore discoveries beginning with the revelation of the Okha field on Sakhalin Island in 1923 have indicated that active petroleum systems underlie a large part of the Sea of Okhotsk, a mostly unexplored epicontinental sea covering a 900 km x 1200-km area in the Russian Far East. When the Tockinsky 1 well encountered a significant oil show at a depth of 800 m on the Kamchatka peninsula in 1932, it touched off more than three decades of exploration in the region and by 1971, 35 onshore oil, gas, and condensate discoveries had been reported. Offshore exploration activity in the Sea of Okhotsk was initiated in the early 1970s and for the most part has been limited to areas offshore Sakhalin Island. Significant oil and gas discoveries offshore Sakhalin, including Odoptu in 1977, Chaivo in 1979, Lunskoye in 1984, Piltun-Astokhskoye in 1986, and Arkutun-Dagi in 1989, established offshore Sakhalin as a world-class hydrocarbon province with reserves exceeding 5.5 billion barrels of oil and 35 trillionft3 of gas. Total reserves in place in the area were estimated to be more than 90 billion barrels of oil equivalent in a narrow corridor east of the Sakhalin Island, where all of the above discoveries are located. Several international exploration companies joined the hunt in the late 1980s and early 1990s, including BP, ChevronTexaco, ExxonMobil, Rosneft, Shell, and others. A joint venture of BP and Rosneft announced in October 2004 that the Pela Lache 1 wildcat well had encountered significant volumes of oil and gas in several high-quality sandstone reservoirs on the 6000 km2 Kaigansky-Vasukansky exploration licence block about 40 km northeast of Sakhalin, marking the first successful drilling activity outside the established fairway offshore Sakhalin in what is considered the modern era of oil and gas exploration. BP and Rosneft announced in October 2005 that a second exploratory well, Udachnaya 1, had encountered hydrocarbons in three zones at a surface location about 15 km west of the Pela Lache 1 wildcat.