PIMENOV B.A., CHUPROV V.S. Institute of geology Komi SC UB RAS

Area of investigations includes northern parts of Khoreyver depression and Varandey-Adz'va structural zone.

The geological section, like the whole Pechora sedimentary basin (SB), is divided into two structural stages - Upper Proterozoic metamorphic basement and Paleozoic-Mesozoic sedimentary cover.

The sedimentary cover is characterized by extended and well correlated reflections with predominantly parallel and subparallel pattern. In the wave field of the sedimentary cover vertical and subvertical fault zones are determined with loss of correlation and vertical displacement of phase axes. On the whole the wave field is favorable enough for the isolation and seismic fades analysis of seismic sequences, and dynamic and cinematic characteristics of reflections well depict the local features of lythological boundaries.

The sedimentary cover section on the basis of seismic stratigraphic analysis is subdivided from the bottom upward into the following seismic sequences:

Ordovician-Lower Devonian terrigenous-carbonate sequence characterized by seismic wave velocities 4000-6000 m/s;

Middle Devonian-Lower Frasnian terrigenous sequence with wave velocities about 4000 m/s;

Upper Devonian-Lower Carboniferous and Carboniferous-Lower Permian carbonate sequences characterized by wave velocities 4000-5000 m/s;

Upper terrigenous sequence combining terrigenous deposits from Lower Permian to Mezozoic, with seismic wave velocities 2300-4500 m/s.

Each of distinguished seismic sequences corresponds with a particular historic stage of formation of the sedimentary cover of Pechora SB. The complex boundaries are reflectors controlling regional unconformity surfaces. These major section intervals consist in their turn of thinner strata and rock members differing by their acoustic features and causing the occurrence of constant inner reflectors.

Ordovician-Lower Devonian sequence is characterized by dynamic reflector V, related to the top of Ordovician deposits. Reflector IV is connected with the Silurian, predominantly carbonate deposits. The reflection is formed in contact zone of the Silurian and Lower to Upper Devonian terrigenous and carbonate-terrigenous deposits. Top of Silurian is erosion surface, and wave IV is interference-like and not stable. Above reflector IV Lower Devonian onlap are observed. Well and log data allow to mark in the wave field the reflections confined to different intervals - Llandoverian, Venlokian stages of the Lower Silurian and Ludlovian, Pridolian stages of the Upper Silurian on the basis of which cutting zones of these deposits are traced from the limbs of buried Bolshezemelsky arch toward its top. Inside the Silurian wave interval local anomalies of reef type are observed. Reflector IIID₁ is formed in the boundary interval of Lower to Middle Frasnian terrigenous with Lower Devonian-Silurian carbonate deposits. In the Lower Devonian development zone in the eastern and north-western paits of the territory reflector corresponds well with the top of Lower Devonian. In the east of Klioreyver depression in the time interval, corresponding to the Lower Devonian Lokhkovian stage, intermediate "a", "b", "c", "d" wave groups are marked. Lythologically group "a" corresponds to clay-carbonate Ovinpaima horizon, "b" – to carbonate Ovinparma horizon, "c" - clay-carbonate Sotchemkyrta horizon, "d" - anhydrite-dolomite Sotchemkyila member of Lokhkovian stage.

Middle Devonian-Lower Frasnian sequence on the wave field pattern is subdivided into Middle Devonian and Lower Frasnian intervals. With the top of Middle Devonian corresponds reflector III2. The Lower Frasnian terrigenous deposits top is characterized by the wave IIIdm confined to the contact with overlying, predominantly carbonate complex. The IIIdm reflector corresponds with Domanik formation base, it is formed in interbedding of argillaceous and carbonate rocks, it has wedge-like pattern, interferentional and unstable.

Upper Devonian-Lower Carboniferous sequence. Stratigraphic correlation of borehole sections within the sequence is often based only on log data, the most important is the tracing of argillaceous members in predominantly Upper Devonian carbonate strata. Due to their reduced seismic wave velocities against carbonate rocks these members contribute to the formation of intense reflectors. Stable thickness of argillaceous members, accumulated during relative sea level maximums, allow to reliably tracing the reflections laterally. The middle part of the complex is characterized by reflector IIIf3. The reflection is formed in the contact zone of Famenian limestones and dolomites with more argillaceous upper part of Upper Frasnian depositions. The wave is traced well enough but its pattern is getting complicated in the reef development zones. In the greater part of the Khoreyver depression characteristic reef anomalies are observed in the interval between waves Illdm and IIIf3. To the base of carbonate-argillaceous Middle Famenian Usf-Pechora horizon ("Usf -Pechora marker") reflector IIIfm2 corresponds. Between reflectors IIv and IIIfm2 in Famenian section the reef anomalies are observed too. Due to this observation the knowledge of correct structural plan of reflector nifm2 is getting the most important for clay-carbonate Usf-Pechora horizon appears to be a seal for oil fields in carbonate buildups of Central Khoreyver reef system.

Reflector IIv is related to Upper Devonian - Lower Carboniferous seismic sequence top. Its formation interval is studied in many wells. The wave is formed in member of thin interbedding of limestones and argillites in the bottom of Lower Carboniferous Visean-Toumaisian or Visean deposits and related to Early Visean unconformity surface. It determines interference and dynamic variability of IIv reflector.

Carboniferous-Lower Permian sequence. The top of the sequence is characterized by reflector la. The reflection is formed within interbedding of argillaceous and carbonate rocks in the boundary of the sequence and overlying Upper Permian terrigenous formations. Stratigraphic interval of wave formation - top of Asselian-Sakmarian deposits. Reflection la is related to pre-Artinskian wash-out and has all features characteristic of such waves. However, due to high intensity, the reflection is well determined in the wave field, hi the wave field of the complex reef anomalies are observed.

Upper terrigenous sequence, hi the lower part of Triassic section near unconformable boundary between Permian and Triassic terrigenous deposits the wave A is formed. The reflection is not stable, difficult to correlate. Below reflector A in the Upper Permian interval of the section erosion truncation are observed.

ARCTIC SHELF OIL AND GAS CONFERENCE 2004