BEST SAKHALIN HYDROCARBON POTENTIAL SEEN LYING OFFSHORE

The hydrocarbon potential of the Sea of Okhotsk along the northeast coast of the U.S.S.R.'s Sakhalin Island dwarfs oil and gas reserves in the onshore portion of the North Sakhalin basin. The Soviets drilled their first strike there in the early 1920s. Most puzzling aspect of Soviet activity in this area is why it has taken so long to develop offshore reserves even with foreign financial and technical assistance. The minimum of five offshore fields discovered to date on Sakhalin

March 18, 1991

5 min read

The hydrocarbon potential of the Sea of Okhotsk along the northeast coast of the U.S.S.R.'s Sakhalin Island dwarfs oil and gas reserves in the onshore portion of the North Sakhalin basin.

The Soviets drilled their first strike there in the early 1920s.

Most puzzling aspect of Soviet activity in this area is why it has taken so long to develop offshore reserves even with foreign financial and technical assistance. The minimum of five offshore fields discovered to date on Sakhalin Island's northeast shelf are on average many times the size of the island's onshore fields.

Soviet reports indicate that besides the offshore fields found to date, there are at least a dozen promising structures on Sakhalin Island's northeast shelf. The structures span an area from near the main oil center of Okha southward about 160 miles to the region of Lunskaya Bay south of the town of Katangli.

All of today's offshore fields are near shore in water less than 1 00 m (328 ft) deep, Soviet maps show.

The Soviet magazine Geologiya Nefti i Gaza (Geology of Oil and Gas) says the largest hydrocarbon reservoirs in the offshore portion of the Northeast Sakhalin basin "are recognized within the intervals of the section characterized by the presence of superhydrostatic reservoir pressures and uncompacted clays with abnormally high pore pressures and hydrochemical inversion."

Timely development of Sakhalin's offshore reserves could have prevented the critical fuel shortage that struck the Soviet Far East during the 1980s and still persists.

WHAT'S BEEN FOUND

The Soviets' first offshore Sakhalin Island field, Odoptu-More, was found in 1977 in shallow water near Okha. A second offshore field, Chaivo-More, was found farther south in 1979.

Yet, as of 1990, the Soviets had not reported any commercial oil or gas production off Sakhalin Island. Directional wells drilled near the coast have, however, tapped the offshore sector of the onshore Odoptu field for many years (see map, OGJ, Nov. 19, 1990, p. 74).

The Soviets found their first onshore Sakhalin oil field, Okhinskoye, in 1923 in a Miocene formation. More than 60 onshore oil and gas fields tapping the Miocene and Pliocene have been found.

Even so, Sakhalin crude and condensate production has remained in the 40,000-60,000 b/d range since the 1960s. Gas flow increased during the 1980s following completion of a pipeline across Tatarsky Strait to the Siberian mainland but is still far below potential.

Japan and the U.S.S.R. agreed on offshore Sakhalin exploration early in 1975. Geophysical surveys began during the spring of that year.

Odoptu-More's discovery well flowed 7,000 b/d of oil in the summer of 1977. Miocene pay is from multiple zones in three domes.

Productive intervals range from 3,707 ft to just below 6,000 ft. Gross Miocene pay zone thicknesses are 20-207 ft, initial reservoir pressures were 1,836-2,879 psi, and porosity is 18-24%.

"Explored" (proved, probable, and some possible) Odoptu-More reserves have been estimated at 200 million bbl.

The Japanese-built Okha jack up rig found Chaivo-More oil field in 1979. The discovery well reportedly flowed 2,050 b/d of liquid hydrocarbons and 77 MMcfd of gas.

Chaivo-More's multiple pay zones are Miocene-Pliocene. One estimate places the field's reserves at 630 million bbl of crude, nearly 5 tcf of gas, and 142 million bbl of condensate.

Productive intervals in Chaivo-More range from 3,855 to 12,424 ft. Gross pay thicknesses are 49361 ft, initial reservoir pressures were 2,923-4,480 psi, and porosity 12-20%.

Piltun-Astokhskoye field, south of Odoptu-More, is believed to hold about 400 million bbl of explored oil reserves. The big Piltunskaya syncline is said to have Neogene sediments more than 8,000 m (26,246 ft) thick.

The Moscow newspaper Pravda last year estimated Lunskoye field, on the southern end of Sakhalin Island's offshore play, has more than 42 tcf of gas in place.

Some western observers believe recoverable reserves are considerably lower.

But Lunskoye alone could go far toward meeting the Soviet Far East's desperate energy shortage. Sales of Lunskoye gas to Japan and South Korea could provide badly needed hard currency for poverty stricken Sakhalin.

LATEST DISCOVERY

The Soviets' latest reported strike in the Sea of Okhotsk, made in late 1989, is Arkutunskoye field. The discovery well, drilled at an undisclosed site, flowed 1,336 b/d of oil, 403 b/d of condensate, and nearly 9.6 MMcfd of gas through a 16 mm choke.

By comparison, an onshore Sakhalin discovery tested about the same time near the Sea of Okhotsk coast yielded only 219 b/d of oil.

Estimates that the Sea of Okhotsk has ultimate oil reserves of 28-36 billion bbl were made as early as the mid-1970s. More recent evaluations place oil reserves at about 1 0 billion bbl just in the Sakhalin sector of the Sea of Okhotsk.

Pravda's estimate of almost 17 trillion cu m (600 tcf) of "already explored" gas reserves in offshore areas of the Soviet Far East, mainly in the Sea of Okhotsk, has not been confirmed by other Soviet sources.