U.S.S.R. OFFERS VARIETY OF EXPLORATION TARGETS

G. A. Gabrielyants
Minister of Geology of the U.S.S.R.
Moscow

For many decades the U.S.S.R. fuel-energy complex developed dynamically. During 1960-89 production of organic fuels--including oil, gas, coal, peat, oil shales, and firewood--tripled to 2.273 billion metric tons on a fuel-equivalent basis (see Table 1 and footnote).

General exploration for oil, gas, coal, and shales has ensured study of the geological structure of the Soviet Union and identified the major energy resource bases, which in turn have determined how the oil and gas industry developed and worked. Discovery of unique fields in the western Siberian, Pre-Caspian, and Amudarya provinces became events of worldwide significance and influenced the general progress of geological science, methodology, and practice.

In the 1980s the Soviet Union's fuel-energy complex and its resource base entered a new stage of development, characterized by aggravation of economical, technical, ecological, and organizational problems. It became more expensive to find and develop oil and gas reserves.

Nevertheless, oil and gas will remain primary sources of energy worldwide through 2010-2020, and the Soviet Union will remain a major component of worldwide supply. It has never been more critical that the country's petroleum resource be thoroughly assessed and developed.

SOVIET PROVINCES

In the U.S.S.R., oil and gas fields have been discovered in several oil and gas bearing provinces (Fig. 1).

For the last 25-30 years the prepared resource base has paved the way for a rapid rise in oil and gas production. 1 From 1961 to 1990, reserves of oil increased by 3.4 times and gas by 22.5 times. In the same period, oil production increased by 3.5 times and gas production by 15 times (Figs. 2, 3). Reserves and production of condensate also increased very rapidly.

At present, about 80% of the oil and 94% of gas discovered reserves are concentrated in major fields.

This high degree of concentration in major fields is evident in production as well. In 1985, 81.5% of the U.S.S.R.'s oil production was from major fields. This compares with 81% in 1980, 77.4% in 1975, 75.7% in 1970, 64.3% in 1965, and 58.3% in 1960. During this period, oil and gas made considerable inroads into an energy fuel mix previously dominated by coal (Table 1).

Despite the intensive exploitation of many oil and gas bearing regions of the country, the present rate of geologic and geophysical study is not uniform.

Of 206 million m of exploratory hole drilled in the Soviet Union through the beginning of 1990, 56% was drilled in the European part of the country, 15% in western Siberia, and the rest in the vast territory of Middle Asia, Kazakhstan, eastern Siberia, and the Far East (Table 2).

Nationwide, 31% of the oil resource and 25% of the natural gas resource have been converted into reserves. The highest degrees of conversion are characteristic of those pay rock/stratigraphic units which contain major fields (Neocomian and Cenomanian in western Siberia, Middle Devonian-Lower Permian and Upper Visean-Lower Permian in Timano-Pechora, Cretaceous in Amudarya, and Devonian and Carboniferous in Volga-Ural).

Worldwide experience indicates that at these rates of resource-to-reserve conversion the efficiency of exploration drops. In order to raise exploration efficiency it is important, first, to have a clear notion about general distribution characteristics of oil and gas reserves and, second, to scrutinize the most burning problems which appeared at the present rate of study of the country. Doing so will make it possible to outline a general strategy for the search for oil and gas and to define directions of exploration in different regions.

RESERVES DISTRIBUTION

About 99% of the Soviet Union's hydrocarbon reserves are associated with platformal areas. In folded zones, reserves are comparatively small.

Oil and gas have been discovered in all stratigraphic units (Fig. 4). On the young platforms (western Siberia, the southern part of the U.S.S.R.), most of the oil and gas reserves are confined to Cretaceous and Jurassic rocks.

On the East European and Siberian ancient platforms the stratigraphic range of oil and gas occurrence is broader. It embraces Upper Proterozoic, Paleozoic, and Mesozoic strata.

On the East European platform 94% of hydrocarbon reserve volumes are in Paleozoic rocks (Devonian to Lower Permian), while on the Siberian platform about half of the discovered reserves are in the Vendian-Lower Cambrian rocks and more than 35% of the reserves are confined to Mesozoic complexes developed on platform margins.

It is also noteworthy that in the U.S.S.R., as compared with other countries, ancient rock/stratigraphic units are characterized by rather high productivity.

Fig. 5 shows the pattern of distribution of discovered hydrocarbon reserves in folded areas.

Deep horizons within the U.S.S.R. are not sufficiently explored. Only 5% of all discovered oil and 12% of all gas reserves are at depths exceeding 3 km (Fig. 6).

The portion of hydrocarbon reserves in nonconventional traps is small: Stratigraphic and lithologic traps contain 17% of oil and 6.7% of gas reserves; reefs contain 2.5% of oil and 5% of gas discovered reserves.

Carbonate reservoirs in the Soviet Union contain only 8% of the oil reserves and 17% of gas reserves (30% of all discovered pools are in carbonates). This is one-half to one-third the share of reserves in carbonates evident in other countries. Carbonates nevertheless are widely spread over the U.S.S.R., particularly in the Pre-Caspian, Timano-Pechora, Volga-Ural, Amudarya, and Lena-Tungussk provinces.

In most of the U.S.S.R., potential oil and gas resources have been explored to depths of 3.5-4 km.

INDUSTRY CHALLENGES

Among the challenges facing the Soviet petroleum industry are discovery of more major fields, exploration of deep horizons, exploration funding, and utilization of produced hydrocarbons.

Before 1960, fields considered major or unique represented 11.5% of the total. The share peaked in 1960 at 15.3% and declined to 14.7% in the 1970s and 10% in the 1980s. Correspondingly, the average new field today holds one-fourth the reserves of the average field discovered 20 years ago.

This trend is normal as the degree of conversion of resources into reserves rises. It is not unique to the Soviet Union.

As in other countries, discovery of major fields in the U.S.S.R. occurred in the initial stages of exploration, when only 10-15% of the potential resources had been converted into reserves.

It was during that stage of the Soviet Union's exploration that most of the country's unique fields were discovered, including Samotlor, Urengoi, and others in western Siberia and Tengiz as well as Astrakhan and Karachaganak in the subsalt strata of the Pre-Caspian province.

Nevertheless, possibilities of discovering new large fields in the U.S.S.R. are not exhausted. Exploration will be increasingly difficult, however, because it will have to concentrate on poorly studied strata, unconventional traps, and marginal areas.

Some major and unique fields are underdeveloped, leaving the prospect open for large reserves additions years after discovery. Significant reserves additions recently have been made in fields such as Urengoi and Medvezhye, for example. In Shebelinka field in the Ukraine Republic, 100 billion cu m was added to natural gas reserves after 33 years of development and production and recovery of 98% of the originally estimated reserves volume.

During the 1980s, despite intense production and owing to supplementary exploration, average reserves of producing Soviet oil fields decreased by only 7%, while average reserves of producing gas fields increased by 80%.

DEEP EXPLORATION

Exploration of deeper zones, which brings large prospective depth intervals of sedimentary rocks into the process of geologic study, is the second important and widespread characteristic of the long history of exploration in the U.S.S.R. The country has many zones of deep subsidence which are natural objectives of the search for oil and gas.

Deep horizons have been studied intensively since the 1960s, in the U.S.S.R. and elsewhere. Results have been an increase in the share of reserves in horizons below 3 km.

Between 1965 and 1985, the proportion of reserves at depths of 3-4 km increased 1.6 times for oil and 1.7 times for gas. In the same period, the portion of reserves in the 4-5 km depth increment increased 3.6 times for oil and 2.5 times for gas. 2

The amount of deep exploration in the U.S.S.R. exceeds that of the U.S.

During 1960-88, the average depth of exploratory wells in the U.S.S.R. increased to 2,829 m from 1,928 m. The 901 m increase contrasts with 268 m in the U.S. during the same period--to 1,778 m from 1,510 M.

The most striking comparison involves the number of superdeep wells in relation to total drilling in both countries.

During 1986-89, 957 boreholes were drilled in the U.S.S.R. to depths exceeding 4,000 m. This represents 12.2% of total wells drilled and includes 262 wells (3.3%) deeper than 5,000 m.

In the U.S. during the same period, 1,205 boreholes were drilled to depths of more than 12,500 ft (3,81 0 m), including 404 exceeding 15,000 ft (4,572 m). Those totals amount to 6.2% and 2.1% of total wells drilled in the U.S. during the period, respectively--about half the share sizes of the corresponding deep well categories in the U.S.S.R.

Deep drilling in the U.S.S.R. has resulted in the discovery of 450 hydrocarbon pools at depths of 4,000-6,330 m. They include major pools in Karachaganak and Tengiz fields in the Pre-Caspian depression.

However, these discoveries are considered to be the exceptions because the bulk of the deep pools contain medium, small, and very small reserves.

In recent years, 13% of all wells drilled in the Soviet Union have been deeper than 4,000 m. They have accounted for 13.4% of gross oil reserves added and 8.7% of gas reserves added. 3

This comparatively low efficiency of deep drilling can be explained when analyzing a pattern of reserves distribution at different hypsometric levels of sedimentary basins.

In basins with high rates of study (Volga-Ural, Pre-Caucasus, Southern Caspian) most reserves occur at 1,000-3,000 m. Below this depth, amounts of reserves and numbers of large accumulations decrease. This observation holds for basins as a whole and for individual productive formations.

For example, average geological reserves in the Lower Permian-Carboniferous complex of the Dnepro-Donetsk trough total 61.9 million metric tons in a depth interval of 1,000-2,000 m, 24.4 million tons at 2,000-3,000 m, 26.4 million tons at 3,000-4,000 m, and 3 million tons at 4,0005,000 m.

In terrigenous Devonian rocks of the Volga-Ural region the largest reserves of an average accumulation are also confined to an interval of 1,000-2,000 m, equaling 33.9 million tons. At depths of 4,000-5,000 m they total 8.7 million tons.

In the Southern Caspian basin the largest reserves are confined to the uppermost interval--0-1,000 m--where they total 16.3 million tons, while at a depth interval of 5,000-6,000 m reserves average 8.4 million tons.

Similar patterns exist in major fields. Of 506 Soviet and non-Soviet fields, maximum hydrocarbon reserves are concentrated at depth intervals of 0-500 m (36.5%) and 500-1,000 m (26.7%). The reserves share decreases to 17% at 1-3 km, 2% at 3-4 km, and 0.7% at 4 km and greater. 4

CHANGING IDEAS

Nevertheless, hydrocarbon prospectivity of formations at great depth is well documented. 5 - 8 And ideas about the ratio of oil to gas at increasing depths are changing.

In the late 1970s, less than one fourth of all oil pools were found at depths of 3.6-4.2 km, and at depths of 6 km they were totally absent. Now reserves at depths of 4-8 km are divided as follows: oil pools 35%, gas pools 41%, and gas-condensate pools 24%. 9

In the U.S.S.R., the best prospects for discovery of significant reserves at great depth are in the Pre-Caspian depression, where the main zone of hydrocarbon accumulation is shifted to deep horizons due to the presence of a thick salt sealing stratum.

As oil demand and fuel prices rise, deep horizons will play an increasing role in Soviet production. By 2000 a mean depth of exploratory wells in the U.S.S.R. may reach about 3.5 km.

Along with deep horizons, condensate and natural gas liquids will become a more important exploration target in the Soviet Union as markets develop.

Western Siberia and the Pre-Caspian depression have particularly high concentrations of condensate reserves.

Liquids content of natural gas ranges from 5-10 g/cu m to 1,700-2,000 g/cu m. Concentrations in the high end of the range have been established in the Ukraine Republic, the northern Caucasus, and some other areas.

Commercial production of gas liquids is considered possible in western Siberia, the Pre-Caspian depression, Middle Asia, Volga-Ural, and some other regions.

FUTURE EXPLORATION

Exploration strategy in the U.S.S.R. in 1991-2010 will differ strongly from that of the preceding period. Changes will involve direction, tasks, and methods.

The search for oil and gas will be more oriented than before to unconventional traps, new formations, and increasing depths. Exploration for oil pools with complex geology and gas pools in rocks with poor reservoir properties, in reefs, and in low amplitude traps will become vital. Considerable attention will be paid to autochthons and allochthons.

In the Western Siberian province one exploration focus will be Lower Cretaceous and Triassic prospects in the northern areas of the Tyumen region. Also due attention are accumulation zones unassociated with anticlinal folds in Jurassic and Lower Cretaceous strata in the central and southern parts of the province. Oil and gas potential of Paleozoic rocks throughout the territory also will be studied.

Critical to further major discoveries in the Pre-Caspian province will be subsalt deposits as deep as 5-5.6 km in marginal and internal zones with conditions favorable to the occurrence of oil and gas.

Timano-Pechora province exploration will center on assessment of Ordovician-Carboniferous and Permian strata within the limits of Pre-Ural depressions. The effort also will trace zones where productive horizons pinch out as well as zones of reef occurrence in Permian and Devonian complexes in areas already well studied.

In the Eastern Siberian provinces, exploration's main task is investigation of the Paleozoic and Vendian-Lower Cambrian rocks of the Tungussk syncline and Mesozoic and Paleozoic rocks in the Yenisei-Khatanga, Lena-Anabar, and Pre-Verkhoyan foredeeps. As a whole, the amount of study of the Siberian platform remains low in comparison with other parts of the country.

Large discoveries in the Amudarya province depend mainly on the study of some poorly investigated zones of the Upper Jurassic subsalt carbonate deposits, which are regionally gas prone, especially in the Murgab depression. Lower and Middle Jurassic rocks throughout the region need further exploration. Also promising is the Pre-Kopetdag foredeep, involving Neocomian-Upper Jurassic rocks deeper than 4 km.

Exploration has covered no more than 10% of the U.S.S.R.'s continental shelf, focusing on the southern Caspian, Black, Azov, and Baltic seas and parts of the Barents, Kara, and Japan seas. Most other offshore areas have been poorly studied and are considered prospective.

CONCLUSION

The Soviet Union's petroleum resource base--despite negative tendencies that are the natural results of its mature state--can meet the present and long term demands placed upon it by the country's economy.

Possibilities of further growth are linked with exploration of promising new areas; elaboration of laws governing oil and gas fields; creation of highly efficient geophysical and geochemical methods; manufacture of technical facilities for drilling and well logging and testing; development of technology to enhance oil and condensate recovery; and complex utilization of hydrocarbons,

Great importance will be attached to international collaboration, exchange of advanced ideas between scientists and specialists of various countries, and use of worldwide advances in scientific investigation and geological exploration.

REFERENCES

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