MALYSHEV N.A., OBMETKO V.V., BORODULIN A.A., BARINOVA E.M., IKHSANOV B.I.
Rosneft Oil Company, Russia

This article was developed based on the generalization of a substantial volume of geological and geophysical material, reinterpretation of over 17,000 km of seismic lines, acquired by MAGE, SMNG and BGR in 1989-2005.

For the geological structure analysis and hydrocarbon potential forecast, the authors tracked 6 reflecting horizons (LS1-6) through time sections, related to the main depositional breaks in the region; their stratification was executed based on the geological history reconstructions accounting for the sea level variations, reflected on the eustatic curve, and depositional breaks determined from outcrops and wells on coastal margins of the basin, as well as thickness analysis, also paleogeographic reconstructions were performed in more details.

There are different conceptions regarding stratification of the Laptev Sea sedimentary cover. And if for the eastern part of the Laptev Sea shelf, the majority of explorers have close enough opinions, then for the western part the ideas are significantly different. Basically, all conceptions on the western (largest) part the Laptev Sea shelf can be combined into two groups:

- The Laptev Sea basement is formed by the Verkhoyansk formations, and the sedimentary cover is composed by the Lower Cretaceous (?) - Cenozoic formations;

- The western part of the Laptev Sea is located on the East-Siberian platform extension, the basement is formed by the Proterozoic formations, and the sedimentary cover begins with the Upper Riphean.

This variant of stratification is not satisfactory by the following reasons:

- Stratification of the basement in western part of the Laptev Sea shelf as the Verkhoyansk complex is not proved enough, because below the base of the Omoloisky-Ust-Lensky rift system there is a seismic complex up to 8.5 km thick, interpreted by the authors. This complex on the wave field is characterized by rather extended regular reflectings, not typical for the acoustic basement, and has an areal extend. Its depocenter is significantly shifted westward of the Laptev Sea rift system.

- Comparison of this seismic complex and the Late Riphean-Lower Cretaceous formations is not possible from the thickness analysis, as their thickness in the southern margins of the Laptev Sea is not less than 11 km and increase towards the water area.

In this connection, the authors propose the following alternative of reflecting horizons stratification. The top of acoustic basement in the western part of the Laptev-sea basin is accepted at the level of the most brightly indicated Pre-Permian unconformity, described in the outcrops and wells on the southern sea margins. The schematic paleogeologic map for the beginning of the Permian time shows that the Proterozoic up to Carboniferous formation are exposed to erosion. On the land the large swell-like zone with maximum erosion amplitudes up to Permian formations was determined [2]. On the Laptev Sea shelf we tracked its extension (line BGR9703) as a big consedimentation structure.

Indirect confirmation of the Pre-Permian age of this unconformity is the abrupt change of interval velocities on its boundary from 4000-4200 m/s, typical for the clastic rocks, up to 4800-5600 m/s, more characteristic for carbonate and clastic-carbonate rocks.

Accounting for the magnitude of the Pre--Cenomanian uplift, the underlying Riphean-Carboniferous mainly carbonate formations, which are up to 3 km thick on paleolifts, based on thickness and time section analysis, were buried to the lower depths, and at present forms a part of the basement.
Thus, from the wave field character and thickness analysis, the western part of the Laptev-sea shelf is occupied by the Permian - Lower Cretace ous pericratonic complex. Based on the geological history reconstruction, the region was formed in the environment of continental-marginal basin, located within the northern passive margin of the East-Siberian platform.

On the Novosibirsk Islands, in the Verkhoyansk folded system and, apparently, in the eastern part of the Laptev Sea (beyond the Lazarev break), the Pre-Aptian formations are significantly lithified, cleavaged, and included into the basement. The formation of this complex is referred to the final stage of the Verkhoyansk folding and closing of the Southern Anuisky paleoocean with the subsequent development of the Belkovsko-Sviatonosovsky graben troughs. The age of folding is dated as the beginning Early Cretaceous [1]. Thus, the age of the sedimentary cover in the east part of the Laptev-sea basin is proposed as Aptian-Cenozoic.

Accounting for previous paleogeographic constructions, seismic and drilling data, and biostratigraphic studies the depositional environment was updated for the Permian-Lower Cretaceous pericratonic complex. Due to absence of the Permian-Lower Cretaceous formations in the northern part of the Laptev Sea, the provenance area in Permian-Early Cretaceous time (Paleoarktida) was possibly located in the northern part. The main sea transgression was, apparently, occurred from the southeast - from the Southern Anuisky paleoocean. The Permian formations for the most part of Laptev-sea shelf were described as coastal and shallow-marine clastic formations. The Triassic time is distinguished by transgression and in the most part of the basin Triassic formations should include shallow and deep-water clastic-carbonate rocks with prevailed depression facies. The basin shallowing with several transgression stages started from the Jurassic time, which was ended in the Late Cretaceous by the basin-wide continental deposition.

The overlying formations, from their thickness behavior, were divided into two main complexes - synrift (LS2-4) and postrift (LS5-bottom).
Reflection horizon LS2, corresponding to the base of synrift complex, was reffered by authors to the Pre-Cenomanian break and prerift uplifting of the area. The Late Cretaceous age of the Laptev-sea rift system is proved by a basic magmatism on the Novosibirsk Islands and in the Momsky rift [1, 3].
Slope clinoforms were defined from the overlying reflecting horizon on time sections that pointed to the Euroasian basin opening. From the confident dating of magnetic anomalies, opening of the Euroasian basin began 58 million years ago [1]. Accordingly, the underlying unconformity cannot be younger that age. Therefore, reflecting horizon LS3 was compared with the regional Pre-Lower Paleocene break, determined from outcrops and wells through the entire Laptev Sea margins.

The uppermost reflecting horizon LS6 inside the postrift complex represents the youngest and clearest unconformity and can be confidently compared to the last Eo-Pleistocene regression.

Respectively, reflecting horizons between the Pre-Late Palaeocene and Eo-Pleistocene unconformities, by analogy to the regional breaks determined everywhere on the Laptev Sea margins, were stratified as the Lower Oligocene (LS4, within the synrift complex) and Pre-Middle Miocene (LS5, the postrift complex base) unconformities.

For the synrift formations and the base part of the postrift complex, analysis of seismic facies was performed accounting for the provenance area location, determined in the Laptev Sea margins and typical wave field characteristics [4]. The subsequent dynamic analysis showed rather good convergence with the results of seismic facies analysis. The maps of maximum peaks helped in updating boundaries of various seismofacies in the areas of their unreliable correlation.

The Late Cretaceous expresses the beginning of continental rifting, and continental facies were widely spread within this interval. An oceanic basin started its development in the Middle Paleocene time. In the corresponding interval and above it, mainly continental, deltaic, shallow-marine, slope and deep-water sediment were indentified in time sections.

The presented schemes show, that from the Late Paleocene and till present, there is a gradual marine transgression. The river Lena had the biggest impact on deposition of deltaic-wave-cut formations. In the Late Paleocene - Early Miocene, the river deposited the most of clastic material in the Ust-Lensky rift trough and, respectively, had the north-northwest orientation. Since the Middle Miocene there is an essential reduction of deltaic facies and the delta turn to a northeast direction, which was possibly related to downwarping decrease in the Ust-Lensk trough and active downwarping in the Omoloisky rift trough.

Thus, the presented study allows more accurate definition of the Laptev-sea shelf stratigraphic range in its western part and updating of the existing conceptions on the Laptev shelf sedimentary cover depositional environment.

References
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Mikulenko K.I., Sitnikov V.S., Timirshin K.V., Bulgakov M.D.. Evolution of structure and conditions of sedimentary basins’s of Yakutia oil and gas formation. Yakutsk, YaNC the Siberian Branch of the Russian Academy of Science, 1995. 180 p.
Under the editorship of Parfyonov L.M., Kuzmin M.I. Tectonics, geodynamics and metal formation territories Republics Sakhas (Yakutia). М., MAIK "Science/interperiodika", 2001. 571 p.
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Oil and gas of Arctic shelf 2008