Matthew R. Silverman
Gustavson Associates Inc.
Boulder, Colo.
The Soviets and their Japanese partners have expressed frustration with recent exploration and production efforts in the Sakhalin Island area.
Production levels have remained stagnant, and goals set in the late 1970s have not been met.
Current plans call for major increases in exploratory drilling, growth in capital investment in the petroleum sector, and development of at least two offshore fields.
To achieve these goals the Soviets are looking increasingly to joint venture participation by foreign firms.
The best future prospects almost certainly lie in post Eocene strata, but the production history and shallow burial of Neogene reservoirs suggest gas prone conditions almost everywhere except the North Sakhalin basin.
Modest potential exists in southern Sakhalin's Paleocene and perhaps the island's deep Cretaceous strata as well. The coastal shelf will be the key area to establish significant new reserves.
Prime targets offshore have been identified in several areas, but many await foreign technology and/or capital to exploit.
Exploration for subtle stratigraphic, hydrodynamic, and other nonanticlinal traps is still in its infancy, especially offshore.
Significant exploration potential exists on Sakhalin Island and in its offshore basins for those able to develop these resources.
SAKHALIN AREA
Sakhalin Island, U.S.S.R., is located on the western rim of the Pacific Ocean in the Sea of Okhotsk (Fig. 1).
As the Soviet Union's only secure source of petroleum in the Far East, Sakhalin's oil and gas production has a strategic importance greatly exceeding its market value.
The first commercial production was established in the early 1920s at Okha field (Fig. 2). Katangli field, also on the northeast coast, has produced since 1929.
Almost all of the island's production is still centered around these two fields. More than 60 fields have been discovered, with total cumulative production estimated at about 500 million bbl of oil and 1 tcf of gas.
A 1975 agreement between the Soviets and a Japanese consortium, Sodeco, was designed for the joint development of Sakhalin's offshore petroleum reserves. The joint venture was anticipated to boost Soviet hard currency earnings and offshore technology and experience.
Two offshore fields have been discovered, but exploration and development projections have not been met.
Estimated production from more than 30 fields is 17-18 million bbl/year. The crude is transported by pipeline under the Nevelskovo Strait to mainland refineries at Komsomolsk na Amure and Khabarovsk.
PRINCIPAL BASINS
Oil and gas shows have been noted along virtually the entire length of Sakhalin's eastern coast and in scattered locations elsewhere.
The hydrocarbon productive basins of the area are limited, however, to four narrow troughs of folded Cenozoic strata (Fig. 1).
NORTH SAKHALIN BASIN
The North Sakhalin basin is the island's most important oil and gas province.
The onshore portion of the basin is almost completely developed for oil and gas; however, large offshore structures have been mapped by seismic.
This basin, especially its offshore portion, is considered the most prospective area in Sakhalin. More than 30 zones are hydrocarbon productive.
Production occurs at fairly shallow depths; most reservoirs are above 6,560 ft.
Source rocks and seals are interbedded deltaic shales related to the paleo-Amur river delta. Marine shales in the Neogene are also a likely source.
Depositional environments for the reservoirs include offshore bars, beaches, bar-fingers, and other regressive transitional sandstone facies. A pattern of alternating sandstones and shales is characteristic.
Paleogeographic mapping suggests that remigration of previously trapped hydrocarbons occurred.
POGRANICH GRABEN
The Pogranich graben is located 62 miles south of the North Sakhalin basin.
Cenozoic sediments are as thick as 16,400 ft and dominated by shallow marine and transitional units.
The structure of the area is characterized by multidirectional block movements, including thrust-faulting (Fig. 3).
Okruzhnoye oil field was discovered in 1971, and commercial production has been established in five Miocene horizons.
Reservoirs are clayey-silty, occasionally tuffaceous sandstones, 33-656 ft thick; and low permeability, fractured siliceous deposits, similar to California's proliferously productive Monterey formation.
This is Sakhalin's second most prospective basin.
TERPENIYA BASIN
Drilling in the Poronayskaya depression (Fig. 3) of the basin established the presence of a complete Cenozoic section similar to that in the North Sakhalin basin and the block fault character of the structures.
Cretaceous source rocks are present, but Cretaceous reservoirs are not abundant. Principal objectives in this sparsely drilled area will probably remain Miocene and younger strata.
This basin has modest exploration potential.
ANIVA GULF BASIN
Aniva may have large gas reserves by Sakhalin standards, especially offshore.
Small quantities of low gravity oil have also been recovered.
Southern Sakhalin's first discovery was made in 1971 with the opening of the Eastern Lugov gas field from shallow Miocene sandstones.
The presence of a gas province was confirmed with the discovery of the Southern Lugov gas field 3 years later.
Results of the first offshore drilling in the basin near the apparently small Aniva field are unavailable.
Structures include gentle folds, moderately deformed blocks, and thrust faults (Fig. 3). Marine and deltaic deposits in the Miocene section provide adequate source rocks, reservoirs, and seals in this poorly explored basin.
TATAR TROUGH
Despite the presence of oil and gas shows in several wells, no production has been established in this large basin separating Sakhalin from the Soviet mainland.
Tertiary sediments as thick as 16,400-22,960 ft and several large, elongate anticlines have been mapped.
Potential for commercial hydrocarbons in this area is considered poor due to intensely deformed structures, inadequate seals, inferior reservoir development, and insignificant fields in nearby geologically similar western Hokkaido.
STRUCTURE
The principal structure of North Sakhalin is the East Sakhalin deep fault, a late Cretaceous feature which experienced early Miocene to post-Pliocene vertical and dextral slip.
It is regarded as the boundary between the island's fold belt and the North Sakhalin basin. Faulting has controlled the vertical migration of oil, and 90% of North Sakhalin's reserves are adjacent to three major faults.
Search for other fault related zones of accumulation is a key aspect of exploration in Sakhalin.
Most hydrocarbon traps in the study area are faulted, compressional anticlines, which have undergone later tensional and vertical modifications. Growth faults and stratigraphic pinchouts, often on structure, are critical elements in many traps.
Several traps are modified by hydrodynamic influences. An impressive array of trap styles is present among Sakhalin's fields.
STRATIGRAPHY
Cretaceous and older strata have typically been considered economic basement on Sakhalin.
Paleogene strata include thick clastics, but no significant petroleum reservoirs. Upper Tertiary deposits (Fig. 4) are 16,400-19,680 ft thick.
Folded and faulted Neogene sediments are dominated by three lithofacies: marine, continental, and volcanic. Neogene strata are characterized by a westward shift from marine to transitional and terrigenous deposits, toward the continental mountains that were the principal provenance.
Thickness of the Miocene sediments ranges from a wedge edge in North Sakhalin to 12,792 ft to the south.
Middle Miocene strata (including the prolific Dagi Suite) are highly petroliferous and include a basal marine unit, continental deposits in the middle, and marine sediments at the top. This section is as thick as 6,232 ft.
The Upper Miocene sediments include marine claystones, sandstones, and siliceous units. Maximum thickness is about 12,464 ft.
Continental conglomerates, cross-bedded sandstones, and coals comprise the Pliocene section, which is approximately 8,200 ft thick. Hydrocarbon production of secondary importance has been established from the Pliocene.
RESERVOIR ROCKS
At least 36 separate reservoir rocks produce oil and/or gas, and as many as 16 units are productive in individual fields.
Production has predominantly been from middle Miocene through early Pliocene fluvio-deltaic and nearshore marine sandstones. The source of the continental sandstones is the paleo Amur River delta, now located on the Soviet mainland, west of Sakhalin.
Separation of the island from the mainland is post Pliocene.
The principal onshore exploration objectives have traditionally been the late Miocene and Pliocene Okobykai and Nutov formations and the middle Miocene Dagi formation.
The Dagi consists of 1,640-8,200 ft m of sandstones and shales that range from predominantly continental on the west to deep-water siliceous to the east and northeast.
Shales comprise 60-80% of the section, but sandstones beds are up to 98 ft thick, Stacked pays within the Dagi are common.
Reservoirs have porosity of approximately 20% and permeability in the range of tens to a few hundred millidarcies. Minor production is obtained from sandy shales.
SEALS
Shales within the Okobykai, Nutov, and Dagi provide vertical and lateral seals for sandstone reservoirs.
Sections dominated by sandstone have proved to be nonproductive due to the absence of seals.
Thick Okobykai shales also act as seals for Dagi sandstones. Fault seals are common.
SOURCE ROCKS
By 11 million years ago, the lower Neogene section had entered the principal zone of hydrocarbon generation, at depths in north Sakhalin of 8,200-13,120 ft.
The area's hydrocarbon source rocks are almost entirely Neogene shales, often interbedded with sandstone reservoirs. Source rocks are both oil-prone and gas-prone in the North Sakhalin basin and Pogranich graben, with the possible exception of the gassy Sakhalin Gulf. Associated-gas volumes vary widely.
The Terpeniya Gulf and the Aniva basin are apparently gas-prone areas. Gas prone source rocks may also be dominant in deeper strata offshore Sakhalin's northeast coast.
Many Soviet geologists consider marine shales beneath the Sea of Okhotsk to be the principal source for petroleum found on Sakhalin. Others favor the influence of deltaic shales related to the paleo-Amur River.
Probably, both marine and deltaic source rocks are mature, organic-rich and have generated significant volumes of hydrocarbons. Vertical oil migration played a controlling role in the development of the largest fields.
FIELD EXAMPLES
Discovered in the early 1920s, Okha field (Fig. 5) is an asymmetric, north-south trending anticline producing from upper Miocene sandstones.
Dips are 50-600 on the steep, faulted east flank and 20 25 on the west. Estimated ultimate recovery is less than 200 million bbl of oil.
Production is from 13 sandstone reservoirs in the Miocene Okobykai suite. The field has a water-drive mechanism, several fault seals, and no significant associated gas.
Discovered onshore in 1953, Odoptu field was extended offshore by Sodeco. The offshore portion, Odoptu More, consists of multiple structures.
The initial offshore production came from a directional well drilled onshore, and the first true offshore drilling was by jack-up in 1977.
Thirteen oil and gas/condensate zones in the Dagi and Okobykai Suites produce, and deepest pay is less than 8,200 ft. Some wells have flowed as much as 3,000 b/d of oil. Ultimate recovery may exceed 70 million bbl.
Chayvo-More field was discovered by Sodeco in 1979, approximately 50 miles south of Odoptu-More. It has eight oil and gas/condensate pay zones, some as deep as 9,840 ft.
Field development has been completed, with reserves estimated by the Soviets at 240 million bbl of oil, 65 million bbl of condensate and 4.2 tcf of gas.
HYDROCARBON PRODUCTION
Although gas production below 13,120 ft has recently been established, about 90% of Sakhalin's reserves have been found at depths less than 6,560 ft.
More than 80% of the oil reserves and 76% of the total hydrocarbons are located in 17 fields.
Cumulative production estimates vary widely. Oil production has been estimated at 400-600 million bbl. Gas production totals from 500 1,200 bcf are quoted in the literature.
Soviet reluctance to publish production statistics is still a problem.
RESOURCE ESTIMATES
Current, official Soviet reserve estimates are also typically secret.
One Soviet estimate for Sakhalin's coast shelf was 22 36.5 billion bbl of oil and 11 tcf of gas.
Tiratsoo predicted ultimate recoverable reserves from the entire Soviet Far East would be less than 1 billion bbl of oil and 100 tcf of gas.
Meyerhoff estimated total recoverable reserves in the Soviet Far East at 2 billion bbl of oil and 100 tcf of gas, most of which to be recovered from the North Sakhalin basin and Pogranich graben. Meyerhoff's estimates seem most credible, although much of the large gas resource will come from unconventional sources and completely unexplored and deepwater offshore areas.
Gustavson Associates research suggests estimated ultimate reserves of 2 billion bbl of oil and 20 tcf of gas. The offshore portion of the North Sakhalin basin will probably yield most of these resources.
CURRENT ACTIVITIES
Offshore development in recent years has focused on the delineation of Odoptu and Chayvo fields.
The Soviets have recently proposed further drilling by Sodeco. This would result in three to four platforms to be installed at Odoptu and two platforms at Chayvo; each platform containing approximately 60 wells.
Drilling is limited by weather and by the Soviet fleet; plans to add additional offshore rigs have been announced.
With the 1987 completion of the 373 mile gas pipeline from Okha to Komsomolskna-Amure, gas production is expected to reach about 100 bcf/year. It is widely anticipated that this will lift restrictions on several gas fields and spur gas development, especially in Southern Sakhalin.
Recent reports indicate that the Soviets have agreed to supply Sakhalin gas to South Korea through a pipeline across North Korea. A subsea line to Japan is also being discussed.
BIBLIOGRAPHY
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Central Intelligence Agency, Sakhalin Oil and Gas Project, in U.S.S.R. Energy Atlas, 1985, Washington, 79 p.
Golobov, Yu.N., Cyclicity of Cenozoic oil-and-gas deposits of Northern Sakhalin: Soviet Geology and Geophysics, Vol. 23, No. 7, 1982, pp. 25-30.
Meyerhoff, A.A., The oil and gas potential of the Soviet Far East: Scientific Press Ltd., 1981, 176 p.
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Japanese have new oil target-promising Sakhalin Island: OGJ, June 10, 1974, pp. 94-96.
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