YURYEVA Z.P. JSC “Arkhangelskgeoldobycha”, Russia

Lower Devonian deposits of the northern Timan-Pechora Province (TPP) are characterized by facial diversity and variable thickness and stratigraphic volume.

Zones of these deposits development are correlated with paleodepressions existing in pre-Middle Devonian time. Maximum thickness reaches up to 1500-2000 m. Stratigraphic volume and thickness declines on slopes of paleouplifts. In their tops the deposits are absent. Occurrence depth varies 1200 m to 4200 m [3]. Lower Devonian deposits are exposed in Subpolar Urals and Timan areas.

Cyclic structure is the main feature of Lower Devonian deposits. This facilitates correlation of multi-facial deposits based on well-logging data. Early Devonian deposits accumulated in marine shelf environment. Total regressive tendency of sedimentation in Early Devonian is reflected in properties of Lochkovian, Pragian, and Emsian deposits.

Lower Devonian deposits are represented by a single sedimentary cycle. Lochkovian regional stage is in its turn subdivided into two cycles corresponding to Ovinpamia and Sotchemkyrt formations of the local stratigraphic division. They are traced in sections located in different tectonic zones. Reliability of correlation is based on biostratigraphic and lithological criteria.

Reference sections, members, beds made it possible to compare multi-facial Lochkovian deposits of different thickness [4]. Reference beds and members are considered isochronous surfaces for the purposes of formations correlation. Using these beds and members it was established that on the paleouplift slopes thickness of the sediments decreased not only in the context of consedimentation and due to erosion of the upper part of the deposits but also due to absence of basal beds of Ovinparma formation. On the eastern slope of the Bolshezemel uplift Lower Devonian sediments wedging-out can be observed.

Ovinparma gray shaly-carbonate and carbonate rocks (Khatayakha formation) are widespread within the area of Varandey-Adzva structural zone (VAZ), in the eastern part of the recent Khoreyver Depression, in Kosyu-Rogov Depression and in the southern part of Kolva Ridge. Thickness of Khatayakha formation varies a few meters to 1100 m. In Denisov Depression Ovinparma formation is corresponds to Kumzha formation (120-600 m) composed of terrigenous gray and red rocks.

Shaly and sulphate-carbonate sediments of Sotchemkyrt formation correspond to Toravey formations (200-400 m). In the northwestern sections its analogue (Vaneyviss formation) is composed of shallow marine red sediments.

Terrigenous sulphate-carbonate sediments of Pragian and Emsian regional stages complete the Lower Devonian section, hi the northeastern Varandey-Adzva structural zone their thickness is 300-350 m. Emsian deposits are partly exposed in the northern Kolva Ridge. They are represented by light-gray and red terrigenous rocks.

However, the main feature of Lower Devonian deposits is their commercial oil content. About twenty oil fields have been discovered in the northern Timan-Pechora province where Lower Devonian deposits are commercially oil –bearing. In Varandey-Adzva structural zone, in the eastern Khoreyver Depression oil is contained in carbonate reservoirs.

In Kolva Ridge area oil is contained in terrigenous beds of the same age [2]. Granular oil-saturated Pragian reservoirs extend the stratigraphic interval of Lower Devonian oil-bearing deposits in the northeastern part of the province.

Relations between of oil pool location and facial changes are distinctly traced. In the northern VAZ regressive parts of two cycles comprising Ovinparma formation (members П and IV) are represented by limestone and secondary dolomite. Carbonate biogenic reservoirs contain oil pools. Transgressive parts (members I and III) are composed of shaly and shaly-carbonate rocks being fluid traps.

Five pay carbonate layers of member II of Ovinparma formation are the main targets of field development within the area of Medyn Ridge. Shaly rocks of Member III are fluid traps. Towards south, within North-Saremboy field, reservoirs located in the top of Member IV are also oil-bearing, hi Saremboy field and in fields of the Gamburtzev Ridge rocks of Member III loose their fluid-hap properties and represent pay interval of oil-bearing carbonate reservoirs together with rocks comprising Member IV.

In the zone of Lower Devonian deposit wedging-out on the eastern slope of the Bolshezemel paleouplift location of Ovinparma sheet oil pools depends upon facial properties of sediments. Pools of R.Trebs field are located in carbonate reservoirs of Members II and IV. In Kolva and Titov oil fields secondary dolomites of Members III and IV are productive.

Sedimentation environment in Early Devonian and impact of further geological events resulted in reservoir formation. Sedimentation within VAZ was especially favorable for natural reservoirs formation [1]. Complexity of carbonate reservoir structure depends upon leaching during pre-Middle Devonian and pre-Early Frasnian breaks in sedimentation. Doloinitization, fracturing, and leaching impacted capacity and filtration properties of rocks. Ovinparma rocks having organogenic structure and fractured rocks were impacted by leaching in the greatest extent.

Multi-layer structure is one of peculiar features of oil fields in Lower Devonian deposits. Oil pools of sheet and dome type were discovered in Ovinparmian carbonates of Toboy, Myadsey, North-Saremboy and some other fields.

Oil traps of different type were discovered in wedged-out Lower Devonian carbonates on the eastern slope of Bolshezemel paleouplift. Stratigraphic traps of sheet type were discovered in Titov. Kolva fields. Combined structural and stratigraphical traps were discovered in Trebs field and in the southern Sorokin Ridge. Stratigraphical traps result from angle overlapping of reservoirs by shaly Lower Frasnian rocks. Here, the most important and reliable condition of oil pool preservation is observed: a fluid trap directly overlaps a reservoir.

Massive fault-screened traps were discovered in fields of the Gamburtzev Ridge. In terrigenous Pragian sandstone of Toboy field a lithological oil pool of sheet type was discovered.

In course of Lower Devonian deposits of Timan-Pechora province studies the following special features of their structure were established:
facial diversity, variable thickness and stratigraphic volume:

regressive sedimentation in Early Devonian and cyclic structure of the sections:

commercial oil-bearing capacity of Lower Devonian deposits with oil pool locations impacted by facial change;

diversity of oil pool types result from different types of sheet and massive traps formation.

References
V.L.Duzin, E.P.Simonenkot V.V.Zaripova. Detection and appraisal of Lower Devonian carbonate reservoirs in Timan-Pechora province: A case study from West-Lekeyaga field. //Problems and the main lines of research work at JSC "Arkhangelskgeoldobycha", 29-45. M.:OAO"VNIIOENG", 2001.
T.V.Maidl, N.A.Malyshev, V.S.Chuprov. Features of structure and oil and gas bearing capacity of Lower Devonian deposits of Kolva megaridge. //Lithology and oil and gas bearing capacity of carbonate deposits (Proceedings of the 2nd All-Russian lithological meeting and of the 8n symposium on fossil corals and reefs), 150-151. Institute of Geology of KSC of the Urals branch of RAS, Syktyvkar, 2001.
A.V.Martynov. Structure of Lower Devonian natural reservoir in the northeastern Timan-Pechora petroliferous province. //280-282. Abstracts of the International symposium "Devonian Geology" , Syktyvkar, 2002.
Z.P.Yuryeva. Reference levels for correlation of Devonian subDomanik deposits (the northern part of Timan-Pechora province), 174-175. //Topical issues of geology of combustible fossils in the sedimentary basins of the European North of Russia: (Proceedings of Ail-Russian conference, Syktyvkar, 2000). Institute of Geology of KSC of the Urals branch of RAS. Syktyvkar, 2000.'

ARCTIC SHELF OIL AND GAS CONFERENCE 2004