Steve Thompson
Intera Information Technologies Ltd.
Henley, U.K.
Vladimir Matveev
Yakutskgeofisika
Yakutsk, Sakha
The Republic Sakha (Yakutia) covers more than 3 million sq km across the eastern portion of the Siberian platform (Fig. 1).
The political and ethnic climate in the new Sakha Republic is stable. Its geographic location far from the troubled European and central Asian states of the former U.S.S.R. and relatively close to the markets of the Far East will be a benefit to long term investors.
More than 1,000 wells have been drilled in Sakha (Yakutia) since exploration began in 1937. About 500 of these wells are in the 30 discovered fields (Fig. 2).
More than 400 structures have been identified on existing seismic data, and many remain untested. The scope for constructive reassessment of existing structures and for discovery of additional traps in areas where seismic coverage is sparse is immense.
The announcement of a first licensing round for Yakutia serves to highlight the local government's determination to promote its indigenous oil and gas industry. The announcement and presentation made in Tokyo on Mar. 22 1994, provided a full review of Yakutia's oil and gas legislation and development plans.
Currently estimated recoverable reserves from discovered fields are as follows:
Oil - 15 billion bbl
Gas - 35 tcf.
Condensate - 185 million bbl.
PROSPECTIVE AREA
The total area regarded as prospective for hydrocarbon exploration in the Sakha Republic exceeds 1.64 million sq km, with trapped hydrocarbons recognized in reservoirs ranging from Late Precambrian to Cretaceous age. In excess of 195,000 km of seismic data have been shot.
Major discoveries of gas, condensate, and oil have been made in the republic but have not been greatly exploited due to remoteness from major population and industrial centers. All production is used for domestic consumption.
The discovery of Markovo oil and gas field in 1962 indicated the potential for Cambrian and Proterozoic discoveries across the Nepa-Botuobian region. Giant Srednebotuobi field, with estimated reserves of 6 tcf of gas, 150 million bbl of condensate, and 475 million bbl of oil, was discovered in 1970, and its discovery was quickly followed by several other major fields in that region.
Cambrian formations are frequently saturated by bitumens, and the Olenek tar sands in the Anabarian region are believed to contain 30-35 billion bbl of oil in sediments of Riphean to Jurassic age, representing half the bitumen resources of the entire C.I.S.
Nine major gas and condensate fields have been discovered in the Upper Paleozoic and Mesozoic sandstone reservoirs of the Vilyuian region. Srednevilyui field is a giant with estimated reserves of 7 tcf and 20 million bbl of condensate.
Although large regions of the republic have gone unexplored, regional seismic grids exist and well data from major structural features and deep stratigraphic tests are available.
Gas production is 53 bcf/year at present, all for local consumption. Oil fields are not commercially exploited with Yakutia importing 14.5 million bbl/year.
CURRENT ACTIVITY
Considerable interest in the future potential of the republic is developing.
A joint consortium combining the Russian and Yakutian governments and Korean Petroleum Development Corp. is undertaking a study to assess the feasibility of a natural gas pipeline from Yakutia to the Pacific coast. This pipeline could supply the huge and expanding markets of the Pacific Rim and Southeast Asia with natural gas through the next century.
Maxus Energy Corp., Dallas, and OMV AG, Vienna, Austria, have exploration ventures in Yakutia, and Intera Information Technologies, Henley-on-Thames, U.K., in partnership with Yakutskgeofisika and Sakhaneftegas, has prepared a geographic exploration database and regional data package for the republic.
REGIONAL TECTONIC SETTING
The structural basement of Yakutia is a complex system of crustal blocks dated as old as Early Archean, related to the stable Central Siberian platform (Fig. 3). This crops out in the central portions of the Anabar and Aldan regional highs, areas which have been the sites of continuous uplift since Late Precambrian.
Rifting and graben development commenced in the Late Proterozoic and continued through Mesozoic times. The basement is broken by a series of large regional north-south fracture systems. Movement along these systems formed a series of major Proterozoic-Early Paleozoic grabens trending north-south on either side of the Anabar high, in the north of Yakutia. These grabens are characterized by block faulting, thinning of continental crust, and thick Proterozoic and Paleozoic sedimentary infill.
The Tunguss basin, on the west and southwest borders of the Anabar high, developed fully in the Late Paleozoic and filled with a major series of Permian and Triassic flood basalts.
Across the northern, eastern, and southern flanks of the Siberian platform, Late Proterozoic to Early Paleozoic terrestrial and shallow-marine sediments were deposited in marginal basins.
Riphean, Vendian, and early Cambrian strata progressively overstepped basement to the north. An unconformity developed at the top of the Lower Cambrian sequence and subsequently thin Middle Cambrian and younger lagoonal-carbonate evaporate sequences were deposited.
In the south of the country, the dominant basement structure is the Nepa-Botuobian high (Fig. 4). Post basement thicknesses of sedimentary rocks range from 2 km or less at the crest to more than 8 km in adjacent basins.
Within the flanking basinal areas Early Proterozoic-Silurian sedimentary sections occur. Marginal basin development was closely associated with the evolution of fold systems adjacent to the platform. In the northeast and east the Late Carboniferous through Jurassic Verkhoyansk fold belt, a former geosyncline, was thrust towards the platform from the east. The Angara Lena trough forms the southern border of the Central Siberian platform.
A basinal area occupied the present areas south and east of the Nepa-Botuobian high through much of the Proterozoic and Early Paleozoic. This geosyncline was deformed during middle Proterozoic and Middle Devonian orogenies. This deformation is seen today as complex northwesterly directed overthrusting and folding in the Subpatomian trough area.
Two major northeast-southwest trending rifts commenced in the Devonian and controlled sedimentation in the Vilyui basin through the Mesozoic. In these grabens the basement is 10-12 km deep. Up to 8 km of Late Devonian-Lower Carboniferous carbonate and evaporitic sequences were deposited in the Kempendyaian depression, a southwesterly extension of the Vilyui basin.
The Devonian rifts appear to be related to several triple junctions on the margin of the craton at the eastern end of the Vilyui graben. The continent apparently overlay a hot mantle region at the time. This resulted in thinned continental crust.
Geophysical evidence suggests that under the eastern part of the Vilyui basin the crust is either considerably thinned or rifted to oceanic crust. The thinned northeast margin of the Vilyui aulacogen was subsequently overlain by a thick prism of clastic sediments from the Verkhoyansk orogenic belt.
Tectonic quiescence has predominated in western Yakutia since the Mesozoic. Thin and extensive relatively underformed Cretaceous and Jurassic strata mantle the older formations over much of the southwest.
Next: Exploration history and discussion of fields.
Copyright 1994 Oil & Gas Journal. All Rights Reserved.
