OIL, GAS RESOURCES ESTIMATED IN THE FORMER SOVIET UNION
Gregory F. Ulmishek
U.S. Geological Survey
Denver
Charles D. Masters
U.S. Geological Survey
Reston, Va.
This report provides an assessment of undiscovered oil and gas resources in countries of the former Soviet Union (Table 1).
The assessments were made by participants of the World Energy Resources Program of the U.S. Geological Survey using a modified Delphi (subjective) method and based on multiyear studies of the geology of FSU basins and exploration results.
Reserves and undiscovered resources are allocated by basin (Fig. 1) to the newly independent states (NIS).
Identified reserves have not been published, and the corresponding numbers in the table present a "best guess" based on reserves of the largest fields and/or basin production rates.
Identified reserves include approximately economically recoverable proved, probable, and possible reserves in an American sense and hence incorporate significantly more resources than commonly reported proved reserves. For proved reserves estimations, the reader might consider a reserves to production (R/P) ratio of 13 as applied to 1991 production (Table 1).
The reserve estimation is complicated by the "heterogeneity" of the Russian C1 and C2 reserves categories that are part of the identified reserves referred to herein. These categories include so-called "inactive reserves," which for various reasons are noncommercial by western standards.
Inclusion of these reserves plus the probable and possible reserves components of an identified reserve results in a high identified R/P ratio for many FSU basins relative to the commonly reported proved R/P ratio for U.S. basins.
The present assessment does not include poorly known frontier basins of the Arctic shelf east of the Kara Sea, basins of the Bering Sea, Sea of Okhotsk (except for the North Sakhalin basin), Japan Sea, Black Sea, the Moscow basin, and a number of relatively small nonproductive depressions located primarily in the Russian Far East and in Kazakhstan.
About 76% of the undiscovered oil and gas resources (mode) of the FSU are located in basins of Russia with gas constituting three quarters of the total resources.
Among other FSU states, very significant resources of oil and gas, some 11% of the total for FSU, are located in Kazakhstan (primarily in the North Caspian basin).
Turkmenistan possesses by far the largest share of the remaining undiscovered resources of gas. Moderate amounts of undiscovered oil and gas resources belong to Azerbaijan, Ukraine, and Uzbekistan.
Other NIS have only limited oil and gas resources: Belarus, Tajikistan, Kyrgyzstan, Lithuania, and Georgia, or are completely devoid of them: Armenia, Moldova, Latvia, and Estonia.
BASINS, PROVINCES
WEST SIBERIA
The West Siberian basin is the principal producer of petroleum in Russia and in the FSU and possesses the largest undiscovered resources of both oil and gas.
The main plain is in structural traps that include the Neocomian (Lower Cretaceous) deltaic section of the Middle Ob region. The play is maturely explored, and much of the remaining oil potential is expected to be found in stratigraphic traps in Neocomian rocks and in structural and stratigraphic traps in the pre-Neocomian (mainly Jurassic) section.
Most of the discovered gas reserves are found in Cenomanian (Upper Cretaceous) continental clastic reservoirs in huge structural traps of northern West Siberia. All the larger structures onshore have been drilled. The main gas play clearly extends into the southern Kara Sea region; three recent huge gas discoveries support the very high resource assessment numbers.
VOLGA-URAL
The Volga-Ural province has been maturely explored.
The main oil reserves are in Middle-Devonian and lower Frasnian clastic reservoirs in structural traps and in Upper Devonian reefs and drape structures that overlie them. The gas reserves are in Carboniferous to Lower Permian carbonate reservoirs in the southeast part of the province.
The undiscovered potential of the basin is limited. Remaining oil resources are expected to be in a large number of small fields and in the somewhat less explored southern and northwestern parts of the province.
TIMAN-PECHORA
Exploration has been successful during the last decade in the immaturely explored northern region of the Timan-Pechora basin.
The main potential plays are in carbonate reservoirs below the pre-Middle Devonian unconformity, in Upper Devonian reefs and drape structures overlying them, and in Lower Permian carbonate rocks (including reefs) below the pre-Late Permian unconformity.
BARENTS SEA
The principal potential of the Russian part of the Barents Sea shelf is connected with the large South Barents and North Barents depressions on the eastern side of the sea.
Our assessment for this province is based on the presumption that the major source rocks occur in the Triassic section of the depressions and that the Upper Jurassic source rocks are immature over the shelf. This suggests a strongly gas-prone character for the depressions because of both the significantly coaly nature of the source rocks and their very deep occurrence.
LENA-TUNGUSKA
Exploration has been concentrated in the southern half of the province, and two plays have been proved productive.
Precambrian (Vendian) to lowermost Cambrian clastic and carbonate rocks on the Nepa-Botuoba arch on the southeast, and Precambrian (Riphean) carbonate rocks beneath the pre-Vendian unconformity on the Baykit arch on the southwest. Source rocks although not geochemically identified, occur in the Riphean section. The potential of both plays is confined to the area covered by the Lower Cambrian salt seal.
The potential of the superposed middle Paleozoic to Triassic age rocks of the Tunguska basin is low. The main negative factor is the abundance of dolerite sills and dikes that could adversely affect preservation of petroleum.
ANABAR-KHATANGA
The basin is almost completely unexplored.
A few noncommercial oil discoveries were made in the 1950s in Permian continental clastics in structures related to Devonian(?) salt domes. The reservoir rocks are poor.
VILYUY
The Vilyuy basin containing late Paleozoic to Mesozoic age rocks is superposed on the margin of the early Paleozoic Siberian platform.
After several gas discoveries in the 1960s, additional exploration has been unsuccessful. The source rocks are Permian coaly clastics, resulting in a strongly gas-prone character for the basin.
NORTH SAKHALIN
Almost all discovered oil and gas in the North Sakhalin basin are in structural traps in Miocene to Pliocene deltaic sediments of the paleo-Amur river. The offshore area north of Sakhalin Island is gas prone.
The main potential lies in the offshore area east of the island, where several significant discoveries were made in recent years. The potential area is rather small, however, because the deltaic section thins eastward and probably passes into prodeltaic shales.
M. CASPIAN-AZOV-KUBAN
The so-called North Caucasus-Mangyshlak province includes the Azov-Kuban basin on the west and the Middle Caspian basin on the east. The onshore areas of the province are maturely explored; gas dominates in hydrocarbon reserves of the Azov-Kuban basin, whereas most of the Middle Caspian basin is oil-prone.
The principal part of undiscovered resources is located offshore in the unexplored central Caspian Sea of the Middle Caspian basin where Jurassic and Lower Cretaceous clastic rocks in structural traps are the prime exploration targets.
NORTH CASPIAN
During the last 15 years, three supergiant oil and gas condensate fields and a number of smaller but significant fields were discovered in rocks beneath the thick Lower Permian salt of the North Caspian (Peri-Caspian) basin.
The basin is still in an immature stage of exploration because of great depths to potential reservoirs, high reservoir pressures, and high sulfur content in the gas.
The potential of the basin remains high for subsalt Paleozoic carbonate rocks and associated reefs and clastic fans on the margins. Carboniferous and Lower Permian clastic fans are also widespread along the eastern and southern margins of the basin.
A shallow, Mesozoic, suprasalt salt-dome play has been explored for many years and still possesses a significant petroleum potential.
BALTIC
The onshore part of the Baltic basin has been substantially explored. A single play in Middle Cambrian sandstones contains all the reserves in the basin.
The offshore extension of the Middle Cambrian play likely will contain the bulk of undiscovered resources.
PRIPYAT
The Pripyat basin is a Devonian rift basin filled with salt and carbonate rocks and overlain by Carboniferous and younger sediments.
The basin is oil-prone. Production is from Upper Devonian subsalt (Frasnian) and intersalt (Lower Famennian) carbonate rocks in structural traps at crests of tilted fault blocks.
DNIEPER-DONETS
The Dnieper-Donets basin of the Ukraine is the southeastward continuation of the Devonian rift trend of the Pripyat basin. The Dnieper-Donets basin is oil productive in its northwestern part; southeastward, gas becomes dominant because of over-maturity of source rocks.
The entire Carboniferous through Lower Permian section is productive, but major gas reserves are concentrated in Devonian salt dome-related traps below the Lower Permian salt seal.
The basin is substantially explored to depths of 4-4.5 km. The remaining potential is mostly gas in stratigraphic and deeply buried structural traps (more than 4.5 km) in Lower Carboniferous clastics.
CARPATHIAN
The Ukrainian part of the Carpathian basin occupies a Tertiary thrusted foredeep of the Carpathian foldbelt and the adjacent foreland. Strongly folded thrust plates of the foredeep are dominantly oil-productive, whereas gas fields are mainly controlled by gentle uplifts on the foreland. Upper thrust plates are significantly explored to depths of 4-4.5 km.
CHU-SARYSU
Several hydrocarbon fields have been discovered in Carboniferous and Devonian rocks. Suspected source rocks in the Devonian to Carboniferous (Tournaisian) section are strongly overmature, and all discoveries have been gas.
SOUTH TURGAY
Exploration that began in the early 1980s revealed flat-lying tertiary and Cretaceous rocks underlain by a Lower-Middle Jurassic rift system. Several oil and gas fields have been discovered, but most reserves are concentrated in Upper Jurassic and Cretaceous (Neocomian) elastic rocks in a single large field, Kumkol.
NORTH USTYURT
The basin occupies a microcontinent in the Hercynian accreted terrane. Thick Triassic and Jurassic through Tertiary elastic sediments overlie a carbonate platform of the microcontinent. Most of the discovered reserves are heavy oil in Jurassic and Cretaceous (Neocomian) rocks at shallow depths on the extreme west side of the basin (Buzachi peninsula).
SOUTH CASPIAN
Most of the discovered oil reserves of the basin are in a thick elastic rock section of middle Pliocene age.
The largest and most productive fields are located in a narrow structural zone extending across the Caspian Sea along the northern boundary of the basin from Azerbaijan (Apsheron peninsula) to western Turkmenistan (Peri-Balkhan region). This zone contains good reservoir sandstones that were deposited in the paleo-Volga River delta. Southward, the quality of reservoir rocks deteriorates significantly. Oil dominates in the discovered reserves, but gas accumulations are more common seaward.
Offshore exploration has been largely limited to very shallow water depths on the west; the extensive Turkmenian shelf on the east is essentially undrilled. Recent large-field discoveries along the onshore part of the shelf suggest that a high resource potential exists for the adjacent area of the Caspian Sea.
AMU-DARYA
The central area of the basin (Murgab depression) is filled with a thick sequence of Lower to Middle Jurassic clastics, Upper Jurassic carbonates and salt, and Cretaceous through Tertiary primarily elastic rocks.
The basin is significantly gas prone due to the abundance of coaly organic matter in the Lower to Middle Jurassic-section and to the deep occurrence and overmaturation of Upper Jurassic marine source rocks. The gas producing Cretaceous section is well explored, and the main potential is related to subsalt Jurassic rocks where high-sulfur gas is expected.
SOUTH TAJIK
The South Tajik basin (an eastward continuation of the Amu-Darya basin) was deformed in the late Tertiary by the Pamir protrusion. The basin is moderately explored to depths of approximately 3 km.
The undiscovered resources of the basin are judged to be limited, owing to intensive faulting leading to poor preservation conditions for petroleum.
FERGANA
The intermontane Fergana basin is a deep Neogene molasse depression overlying Paleogene and Mesozoic platform rocks of a microcontinent in the Alpine foldbelt. The principal portion of hydrocarbon reserves is in Paleogene elastic and carbonate reservoirs.
The marginal zones of the basin are thoroughly explored, but drilling in the deep, inner areas of the basin began only recently, and a large oil field was discovered (with a recent catastrophic blowout) in Paleogene and lowermost Neogene rocks.
The resource potential of the central part of the basin appears to be high; however, drilling depths are great, exceeding 6 km and even 7 km over large areas.
ACKNOWLEDGMENTS
The text of this report derives largely from the Russian literature, which is not referenced herein. The reader is referred, however, to the several reports listed below on FSU petroleum basins by James Clarke, James Peterson, and Gregory Ulmishek of the USGS World Energy Resources Program.
SELECTED BIBLIOGRAPHY
Clarke, J.W., Petroleum geology of East Siberia, USGS Open-File Report 83-367, 1983, 167 p.
Clarke, Petroleum geology of the sedimentary basins of the Northeastern U.S.S.R., USGS Open-File Report 88-264, 1988, 54 p.
Masters, C.D., Root, O.H., and Attanasi, E.D., World resources of crude oil and natural gas, in proceedings of 13th World Petroleum Congress, Vol. 2, 1991, pp. 51-64.
Peterson, J.A., and Clarke, J.W., Petroleum geology and resources of the Volga Ural Province, USGS Circular 885, 1983, 27 p.
Peterson, I.A., and Clarke, Geology and hydrocarbon habitat of the West Siberian basin, AAPG Studies in Geology, No. 32, 1991, 96 p., 75 figures.
Ulmishek, Gregory, and Harrison, Wyman, Petroleum geology and resource assessment of the Middle Caspian basin, U.S.S.R., with special emphasis on the Uzen field, Argonne National Laborator Report/ES-116, 1981, 145 p.
Ulmishek, Gregory, Petroleum geology and resource assessment of the Timan-Pechora basin, U.S.S.R., and the adjacent Barents-Northern Kara shelf, Argonne National Laboratory Report/EES-TM-199, 1982, 197 p.
Ulmishek, Gregory, Geology and petroleum resource, of the Barents-Northern Kara Shelf in light of new geologic data, Argonne National Laboratory Report/ES-148, 1985, 88 p.
Ulmishek, G.F., and Masters, C.D., Estimated petroleum resources in the Former Soviet Union, USGS Open-File Report 93-316, 1993, 18 p.
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