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Case history and structure features of northern part of Varandey-Adzivinskaya structural zone Печать E-mail


FSUE «Arktikmomeftegazrazvedka»

The Varandey-Adzvinskaya structural zone was distinguished in a separate structural element by previously conducted studies (V.A. Dedeev, V.G. Getsen - 1982, A.V. Zhuravleva - 1979. etc.) and was referred to category of aulacogen which "combines" different-complicated fault-angle structures (essentially, horsts and grabens) such as the Sorokin rampart Moreyuskaya depression, etc.). In the Northern water part of the structural zone, one traces regularities of its structure and development as aulacogen.

On the base of paleotectonik constructions with regard to paleogeographic reconstructions, one traced development stages in the North-Eastern part of the Varandey-Adzvinskaya aulacogen in the area of the Severo-Saremboiskaya. Myadseiskaya, Toboiskaya. Medynskaya, Perevoznaya, Medynskaya-more 1 local structures, that were referred to the Medyn-Saromboiskava anticlinal zone, and also the Medynskaya-more 2, Varandei structures and with exit to the Khoreiversky trough in the area of the Varknavsky structure. In the work one made use of data from prospecting and exploratory wells and also, in a single case, involved thickness of deposits by seismic data within the offshore extension of the Moreiyuskaya depression, hi the work one analyzed deposit thickness of Devonian-Carboniferous. Permian. Trias, and Jura, tapped by wells, and drew schematic paleotectonic profiles across the trend of the Varandey-Adzvinskaya aulacogen and by the trend of the Medyn-Saremboiskaya anticlinal zone.

The main stages of structural plans reconstruction occurred on the boundaries of Middle-Lower Devonian and Trias-Jura. The stages ended with a significant rise and erosion of previously accumulated deposits.

Before the beginning of Frasnian time, the Varandey-Adzvinskaya structural zone was forming as aulacogen and succeeding the basement structure. Lower Devonian within aulacogen is presented by deposits of Lokhkovian, Praguian Emsian stages and in part, by deposits of Middle Devonian. The uncovered deposit thicknesses of Lower and Middle Devonian reach 1000-1300 m without considering eroded deposits in Prefransnian time. Outside of aulacogen - the Varknavskaya structure, one detected deposits of Lokhkovian stage with thickness of no more than 200 m and in the Medynskava - more 2 the Lokhkovian + Praguian thicknesses were 450 m. In that time the Varknavskaya and Medynskaya-more 2 areas were separated from aulacogen by ruptures with amplitude of 800 and 1000.

In Late Devonian time, development of the Northern part of aulacogen and near-edge zones occurred synchronously, the fractures limiting it, were not active that reflected on the deposits thickness - 300-400 m of Frasnian and 400-550 m of Famenian.

Fracture limiting the North-Eastern part of aulacogen, became active in Toumasian time and, judging by rate of deposition, was active prior to Pretriassic washout. The Southwestern fracture did not appear itself or was weakly active.

The main inversion and final formation of the Varandey-Adzvinskaya structural zone occulted at the boundary of Trias-Jura. That time in the water area of the structural zone there appeared the Northern end of the Moreyuskaya depression, the Sorokin and Medyn ramparts.

At the same time, there occurred isolation on the paths of lateral migration of fluids from the Northern-East part of the Varandey-Adzvinskaya structural zone to the South-Western one. On the boundary of Trias-Jura there occurred reconstruction of the migration paths and on the lateral of the Medyn-Saremboiskaya anticlinal zone.

For Prefrasnian deposits, the paleoslopes and present slope coincide; the Northwestern part is always was and is below the South-Eastern one.
In the end of Permian the uplifted zone of the Carboniferous carbonates roof was in the center of the Medyn-Saremboiskaya anticlinal zone, i. e. within the Myadseiskaya, Toboiskaya, Medynskaya, Perevoznaya local structures, and the lowered zone was on the territory of the Medynskaya-more 1 and Severo-Saremboiskava structures. On the boundary of Jura-Trias, the zone uplifted part subsided and the Medynskaya-more 1 and Severo-Saremboiskaya structures turned out higher than the central zone and we observe it in the present structure of the zone as well. Hence, if prior to Pretriassic washout, the area of the Myadseiskaya, Toboiskaya, Medynskaya, Perevoznaya structures served as a place for fluid aggregation, so in the beginning of Jura and up to now the area served as a place for discharging and fluids in Carboniferous carbonates migrate towards the Medynskaya-more 1 and Severo-Saremboiskaya structures, hi the area of the Severo-Saremboiskaya structure there are Jurassic sandstones above Carboniferous carbonates, i.e. there are no caps and fluids migrate farther, hi the area of the Medynskaya-more 1, Carboniferous limestones are overlapped by Lower Triassic clays and therefore, all the section of Visean and Serpukhovian stages is oil-saturated. Visean and Serpukhovian carbonates of the Medynskaya-more 1 structure contact through low-amplitude fracture with Carboniferous and Permian limestones of the Medynskaya-more 2 that are also reservoirs and, therefore, by the Medvnskaya - more 1 and Medvnskava - more 2 which relate to different structural and tectonic elements, we have a common deposit.

Deposit in Ovingparmian dolomites on the Medynskaya-more 2 structure contacts through the fracture with Praguian mudstones and clays of the Medynskaya-more 1 and is separate.

From the above, it follows:

- The Medynskaya-more 1 and Medynskaya-more 2 structures are in different structural and tectonic zones. Oil deposits in Ovingparmian horizon of Lower Devonian are different on those structures.

- Oil deposits in Permo-Carboniferous on the Medynskaya-more 1 and Medynskaya more 2 structures are common.

- The Medynskaya-more 1 structure can be the North-Western pericline of the Medyn-Saremboiskaya anticlinal zone. Oil deposit in Ovingparmian horizon of Lower Devonian detected on the Myadseiskaya, Toboiskaya, Medynskaya, Perevoznaya structures, may be common with deposit on the Medynskaya-more structure.



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