Intera Information Technologies Ltd.
Henley, U.K.
Vladimir Matveev
Yakutskgeofisika
Yakutsk, Sakha
Hydrocarbon exploration in Yakutia began in 1935. Extensive geological mapping and shallow drilling took place with oil recovered from wells drilled through the Cambrian sequence in the Nepa-Botuobian region.
In 1943 small amounts of oil--50-100 b/d--were recovered from Permian reservoirs in the Nordvik area on the far northwest Arctic coast, and by 1948 four small oil fields had been discovered.
Through the 1950s and 1960s extensive regional seismic acquisition took place and many stratigraphic wells were drilled. Exploration of the Subverkhoyanian region and the Vilyui basin was undertaken, and gas-condensate reservoirs were discovered at Ust-Vilyui in 1957.
Through 1975 exploration was focused on the Mesozoic and Permian sandstone plays of those regions with considerable success, thereafter the focus shifted to the Cambrian and Precambrian of the Nepa-Botuobian.
Discovered fields can be conveniently divided into those with reservoirs of Cambrian or Proterozoic age occurring in the Nepa-Botuobian and Subpatomian regions and those with reservoirs of Permian to Cretaceous age occurring in the Vilyuian and Subverkoyanian regions (Fig. 2).
CAMBRIAN, PROTEROZOIC HYDROCARBONS
Nepa-Botuobian and Subpatomian regions. The largest fields occur in the Nepa-Botuobian region, being Srednebotuobi, Tass-Yuryakian, and Verkhne-Vilyuchan gas fields, where exploration and evaluation is completed. Reserves of these three fields are estimated at a combined 14.1 tcf.
Additionally, preliminary studies indicate that the Chayandin gas field, regarded as a single unit with the Ozernaya and Nijnekhamakin fields, is the biggest gas field in Yakutia. Studies are under way to fully evaluate these fields.
Talakan oil and gas field also has significant reserves. Intensive studies and preparatory work have been carried out in order to commence test and commercial production of the oil bearing part of the field, which contains 27% of Yakutia's oil reserves. Srednebotuobi field, with 24.4% of Yakutia's oil reserves, is another significant discovery in this region.
Production has been concentrated in Cambrian fractured carbonates and in Precambrian fluvio-deltaic sandstone reservoirs. Structures are huge, gentle anticlines developed over subtle basement highs often covering thousands of square kilometers. Other trap types include stratigraphic pinchout and those related to salt tectonics and thick Cambrian halites.
Traps are faulted anticlines (e.g. Srednebotuobi field, Fig. 5) with a strong stratigraphic element, with higher reservoir quality sands wedging out updip across the high, in the basal Vendian clastics. Seals for sandstone reservoirs are interbedded shales and siltstones, for carbonate reservoirs shales, argillaceous dolomites, or interbedded salts. Fields are gas-condensate, some with an oil ring.
In the Nepa-Botuobian region, the stratigraphy of which is reviewed in Fig. 6, a total of 13 producing horizons exist, five of which are carbonates. All the known fields have multiple reservoirs, from two to four, and 14 fields are oil and gas bearing, five being gas bearing. Facies changes and unconformities in the geological section complicate field geometry.
Stratigraphic trapping plays a significant role in the southern and central parts of the region. In particular Chayandinian, Ozernoe, and Nijnekhamakin fields are controlled by a single large stratigraphic trap covering an area of 3,300 sq km.
Fields in the region display abnormally low formation pressures and temperatures, around 8-100 C., pressures being dose to hydrostatic. Gas fields display oil margins, and gases have commercial concentrations of helium, ethane, propane, and butane; hence it is necessary to use complex and nontraditional approaches when considering commercial development of fields in this region.
Subsalt reservoirs, fluviatile, deltaic, and marine bar sands are pure quartz or arkosic sandstones 20-30 m thick. Riphean reservoirs have porosities up to 15% and permeabilities around 100 md. Vendian reservoirs display porosities up to 25% and permeabilities recorded up to 2 darcies. The Botuobi horizon up to 30 m thick, a fine to medium grained arkosic sandstone, is the main Vendian reservoir.
In the Vendian-Cambrian limestone and salt section, the limestones contain reefal buildups. Reservoirs are limestones and fractured vuggy dolomites, with non fracture porosity in the region of 5-10%. Permeabilities are erratic up to 300 md or more. Productive thicknesses for carbonates in the Nepa-Botuobian average 30-46 m.
Cambrian carbonate reservoirs are described from Talakan, Vostochno-Talakan, Iktekh, Srednebotuobi, Verkhnevilyuchan, and Vilyui-Djerba fields, the main horizons being the Osin and Yuryakh.
The section is neither thick enough nor buried deeply enough for generation of gas, but source beds, interbedded shallow marine, and lacustrine shales with total organic carbon values up to 10% have reached the oil window. Migration of gas was lateral from the more deeply buried Proterozoic and Lower Paleozoic sediments of the Subpatomian trough to the east and south, probably during the Cainozoic.
Lying to the southeast of the Nepa-Botuobian high, the Subpatomian region consists of two north-northeast trending troughs, the Subpatom and the Berezovsk, separated by the similarly trending Djeyuktinian crest.
The stratigraphic section which is similar to that for the Nepa-Botuobian region, thickens southeastwards to more than 7,000 m.
Seismic coverage reflects exploration having been concentrated along the southern flanks of the Nepa-Botuobian high and within the Berezovsk trough. Relatively small (5 by 10 km) anticlinal folds exist along northeast-plunging thrust faults that cut across the trough. Eleven of these features have already been drilled, with two discoveries. Forty-five additional closures have been identified in mapping completed by Yakutskgeofisika.
Bysakhtakh field is productive from a tight anticlinal fold measuring about 40 km by 3-5 km. Closure is about 450 m. Productive intervals are Basal Vendian sand at Riphean unconformity surface (Bysakhtakh horizon) at a depth of 2,500 m, and Upper Vendian-Lower Cambrian dolomites (Yuryakh horizon) at depths ranging from 1,500-2,000 m.
The field was discovered in 1986, and reserves are estimated at 1.4 tcf of gas and 9 million bbl of condensate.
Since the trough is known to have generated much of the oil and gas now trapped across the Nepa-Botuobian structure, it is believed that many structures in the Subpatomian region represent ii low risk exploration target.
Pinchouts of basal Riphean and Vendian sands, either northwestwards against the Nepa-Botuobian structure or eastwards where they abut the Aldan block, may also provide a series of extensive exploration targets.
PERMIAN AND MESOZOIC HYDROCARBONS
Vilyuian and Subverkhoyanian regions. The two main fields in the region are Srednevilyui gas field with proved reserves of 7 tcf and Srednetyrung (Fig. 7) with proved reserves of 6.1 tcf. The combined reserves represent 65% of the gas reserves of the region.
Only gas and condensate fields occur in this region. Some wells on anticlinal traps (Mastakhskaya, Srednevilyui, Srednetvung fields) have given commercial oil flows of 5-10 bbl of oil, indicating the presence of small oil margins.
Reservoirs are Permian, Triassic, and Jurassic sandstones. Reservoir parameters are directly related to depth of burial, dropping from greater than 32% porosity with up to 1 darcy permeability to less than 10% porosity and less than 10 md permeability at depths greater than 5 km. Facies changes to finer grained more distal, dirtier sands occur in an easterly direction towards the Verkhoyansk fold belt, also adversely affecting reservoir properties.
Recorded reservoir pressures are abnormally high in the Permian, only slightly above hydrostatic in the Triassic, and described as underpressured in the Cretaceous. Reservoir temperatures are generally low, from 60-900 C. between 3,000-4,000 in.
Interbedded clays and shales form the main seals to all the discovered fields. Impermeable shales are discontinuous laterally, hence their effectiveness as seals is impaired, thus affecting oilfield distribution.
The Vilyuian region is a roughly triangular downwarp between the Anabarian regional high to the northwest and the Aldan regional high to the south. Total sedimentary section, Permian up to Cretaceous, is frequently in excess of 6,500 in (Fig. 8), reaching up to 14,000 m in the basin center.
In the central part of the region is the Khapchagay arch, which divides the basin into two depressions. The arch is about 300 km long and 30-50 km wide. Closure on the top of the Permo-Triassic sediments is 800 in. Faulting of these structures is rare, and the flanks of the folds dip gently at 1-30.
The Permian is an alternating succession up to 1,600 in thick of sands, silts, and shales of deltaic and shallow marine origin. Sandstones are frequently massive pure quartz lithologies and shales dark and bituminous grading to coals, and three main productive sands occur.
Triassic sediments (1,200-1,600 in) are also alternating sandstones, siltstones, and shales. Massive fluviatile and deltaic sands dominate the 300 in thick Middle Triassic interval, and conglomerate beds occur in the sequence in the Upper Triassic.
The Lower Triassic reservoir interval is up to 600 in thick and contains from one (Badaran) to five (Mastakh) productive sandstones, carrying the main reserves of large Srednevilyui field, currently supplying gas to the Vilyui-Yakutsk gas pipeline.
Compared to Permian reservoirs, the Lower Triassic displays enhanced reservoir characteristics with porosities up to 24% and high permeabilities. Formation pressures are hydrostatic, with producing intervals at depths of 2,350-2,950 in. Reservoirs are sheet sands in anticlinal structures frequently with a strong element of stratigraphic trapping. Fold geometry is such that many structures are offset at depth '
The Jurassic succession exceeds 1,000 in, the lower beds being a repetitive series of dark massive shales and Siltstones, with developments of deeper water grits and greywackes.
The influence of the proto-Verkhoyansk began to be felt in the Upper Jurassic with influx of easterly derived coarse, ill sorted, molasse-type sediments. This influence continued into the Cretaceous with thick spreads of coarse molasse sediments thinning to the west.
The Upper Permian, with dark carbonaceous shales and coals, is regarded as the main source horizon. TOC values range from 1-5%. Generation of hydrocarbons was most intense in the deeper central and eastern sections of the basin. The oil window is loosely reported as around 3,500-4,000 in.
One of the biggest gas-condensate fields in Yakutia is Srednetyung field (Fig. 7), situated on the Loglor crest. This field has three Lower Triassic and 11 Upper Permian reservoirs. The total gas column in this field is in excess of 900 in. This field, 150 km from the Vilyui-Yakutsk gas pipeline, has been prepared for commercial development and is shut-in.
Exploration potential exists to search for traps across deeper uplifts and flanking to the now heavily explored Loglorian and Khapchagaian structures. In addition to structural plan,s developed across basin center highs a zone of stratigraphic, pinchout traps can be postulated both along the northwestern and southern flanks of the Vilyui basin. Seismic pinchouts have been mapped, and several early stratigraphic wells had strong oil and gas shows in Permian and Mesozoic sands.
The Subverkhoyanian region coincides with a foreland downwarp between the Siberian platform to the west and the Verkhoyansk fold belt to the east.
The stratigraphic section 7,000-10,000 m thick consists of Permian, Triassic, Jurassic, and Cretaceous rocks, with Carboniferous clastics exposed at the surface in the thrust belt. A thick Paleozoic section is present in the eastern extension of the region.
The Verkhoyansk thrust belt is up to 300 km wide with a series of westward directed low angle thrusts developed. Fold belt deformation commenced in the Jurassic, but the main features, the system of marginal depressions and over-thrusts, were created at the Upper Cretaceous-Paleogene boundary.
Mesozoic, and particularly Permian, source rocks are gas prone and overmature due to depth of burial. Reservoir targets are Permian, Triassic, and Jurassic sands, sealed by intraformational clays that are productive in the Vilyui basin to the west.
Ust-Vilyui gas-condensate field, the first discovered in Yakutia in 1957, was productive (70 bcf of gas) from Triassic and Jurassic sandstone reservoirs but is now depleted.
Associated with Ust-Vilyui was a similar discovery at Sobo-Khain, a small structure 5 by 4 km with 100 m of structural closure. Gas was trapped in two small pools in the Upper Jurassic. Reservoir quality was poorer than Ust-Vilyui, minor production being recorded at about 3.5 bcfd.
Exploration activity has been intense in this region as witnessed by the heavy seismic grids and extensive exploration drilling on both thrust and subthrust structures. Strong gas condensate shows have been found in wells on several of these structures.
CONCLUSIONS
The republic presents difficult operating conditions associated with permafrost and remoteness, but good investment opportunities exist on the basis of attractive hydrocarbons and minimal political risk.
More than 1.6 million sq km of the country is considered to be prospective. To date a number of major discoveries of gas, condensate, and oil have been made, but only small quantities of hydrocarbons have been produced for local consumption. Current production is focused on the Nepa-Botuobian and Vilyuian regions.
Of the 30 oil, gas, and gas-condensate fields discovered, all have been found in the traps in the sedimentary section of the stable east Siberian platform west of the continental overthrust zone of the Verkhoyansk fold belt.
Beyond the relatively well explored Nepa-Botuobian and Vilyuian regions a review of available geological and geophysical data reveals extensive potential for exploration success in traps ranging from Proterozoic to Cretaceous in age.
Although large regions of the republic have gone unexplored, regional seismic grids and well data exist across the whole territory.
Accessibility to high quality digital exploration data for all of Yakutia has been enhanced by recent joint venture projects undertaken in Yakutia.
The announcement of a licensing round indicates the government's commitment to expanding exploration and production in Yakutia which could well become a major producer supplying Far Eastern and world energy markets through the next century.
BIBLIOGRAPHY
Antisiferov, A.A., et a]., Nepa-Botuobinsk regional high--a new favorable region for oil and gas production in the east of the U.S.S.R., Petroleum Geology, Vol. 23, No. 11-12,1986, pp. 309-319.
Clarke, J.W., Petroleum Geology of East Siberia, U.S. Geological Survey Open File Report 85-367, 1986.
Gustavson Associates Inc., Petroleum Geology and Exploration Potential in the Former Soviet Republic, 1992.
Lubchuk, M.A., Lithology and oil-gas prospects of Jurassic sediments of Yenisey-Khatanga downwarp, Petroleum Geology, Vol. 24, No. 11-12, 1990, p. 309327.
Meyerhoff, A.A., The oil and gas potential of the Soviet Far East, Scientific Press Ltd., 1981.
Zonesham, Lev. P., et a]., Geology of the U.S.S.R., A Plate-Tectonic Synthesis, in Page, B.M., ed., American Geophysical Union Geodynamic Services, Vol. 21, 1990.
Eastern Siberia slow to yield reserves, OGJ, Vol. 85, No. 10, 1987, pp. 49-50.
Safronov, A.F., and Gausman, A.A., Prediction of capacity prospectus of deep sandstones of the Lena-Vilyuy oil-gas region, Petroleum Geology, Vol. 24, No. 11-12, 1990, pp. 338-342.
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