GEOLOGICAL STRUCTURE AND OIL POTENTIAL OF THE STREZHEVOY UPLIFT

E. E. DANENBERG

(Presented by Professor A. V. Aksarin)

The geological results obtained in recent years have confirmed the high assessment previously given by geologists to the oil prospects of the territory of the Lar-Yegan oil region, located in the northern part of Tomsk Oblast. By 1968, within its limits the Sovetskoye, Malorechenskoye, and Alenkinskoye oil fields had been discovered; at the first of these, commercial oil production began in 1966.

In 1968, on the Strezhevoy area, situated in close proximity to the Sovetskoye field, an oil gusher was obtained from Upper Jurassic deposits.

The geological structure of the Strezhevoy field involves deposits of the Jurassic, Cretaceous, and Paleogene systems, resting unconformably on Paleozoic formations. The total thickness of the Meso-Cenozoic deposits is about 2900 m.

Within the Jurassic deposits, the Tyumen (Lower–Middle Jurassic), Vasyugan (Callovian–Oxfordian), and Maryanovskaya (Kimmeridgian–Volgian) formations are distinguished. The Tyumen Formation is represented by a frequent alternation of argillites, sandstones, and siltstones accumulated under continental conditions. At the top of the formation lies bed Ю-II, which has regional development throughout the entire Lar-Yegan region. The Vasyugan Formation is characterized by marine formations, which in the lower part are represented by argillites and in the upper part by sandstones. Bed Ю-I is confined to the latter and is the main productive horizon of Tomsk Oblast. The Maryanovskaya Formation is composed of strongly bituminous argillites of marine genesis, which serve as the seal for productive bed Ю-I.

Within the Cretaceous deposits, the Kulomzino and Tara (Valanginian), Vartovskaya (Upper Valanginian–Hauterivian–Barremian), Pokur (Aptian–Albian–Cenomanian), Kuznetsovskaya (Turonian), Ipatovskaya (Coniacian–Santonian), Slavgorodskaya (Campanian), and Gankinskaya (Maastrichtian–Danian) formations are distinguished.

The Kulomzino Formation is represented predominantly by clayey rocks with rare sandstone beds. The most persistent are beds Б-XVI–XX, Б-X, and Б-VIII; the last of these is the principal productive horizon of the Sovetskoye field. The Tara Formation is composed of sandstones with subordinate interbeds of argillites; in the upper part of the formation lies bed Б-II, containing an oil pool at the Sovetskoye field. The Vartovskaya Formation is represented by clays, silt—

clays and sandstones. In the section of the formation several beds of group “A” are distinguished; these are productive at the Sovetskoye field. In the upper part of the formation lies bed A-1, overlain by argillites of the Koshai member.

The Pokur Formation is composed of sandstones, sands, and siltstones accumulated under continental and nearshore-marine conditions.

The Kuznetsov, Ipatov, Slavgorod, and Gankin formations are represented by marine clayey, siliceous, and clayey-limestone varieties of rocks.

Paleogene deposits are subdivided into the Talitsa (Paleocene), Lyulinvor (Eocene), Chegan (Eocene—Lower Oligocene) formations and

Fig. 1. Structural map of the eastern part of the Sosnino arch (after A. A. Beglyarov, 1965) and logging diagram of well II of the Strezhevoy uplift:
A — structural map of the eastern part of the Sosnino arch: I — Medvedev uplift, II — Strezhevoy uplift, III — Zvezdnoye uplift; 1 — reliable position of the contour lines of reflecting horizon IIa; 2 — contour lines drawn at 20 m intervals; 3 — uncertain position of contour lines; 4 — deep drilled well; B — logging diagram of well II of the Strezhevoy uplift in the interval 2500–2605 m.

the Nekrasov (Middle—Upper Oligocene) series. The Talitsa, Lyulinvor, and Chegan formations are represented by marine clayey varieties of rocks with rare interbeds of sands and sandstones. The Nekrasov series is composed of sands, alternations of sands and clays, and siltstones of lacustrine-alluvial genesis.

Lacustrine-bog Quaternary formations cover the Paleogene deposits as a continuous blanket.

The Strezhevoy field is confined to the eastern part of the Sosnino arch, situated on the eastern flank of the Nizhne-Vartovsk arch. The field is an anticlinal fold of latitudinal strike, complicated by four domes. The structure, with respect to reflecting horizon “B” (Maryanov Formation), is outlined by the −2560 m contour and has dimensions of 14 × 3 km with an amplitude of 65 m. With respect to the higher-lying reflecting horizon A (Pokur Formation), the structure is outlined by the −1580 m contour and has dimensions of 6.5 × 1.5 km. The minimum depth marks for horizon A reach −1560 m and coincide with the most uplifted parts of the Strezhevoy structure on horizon “B”.

In the general structural plan of the Sosninsky swell, the Strezhevoy structure, in the Jurassic and Cretaceous horizons, is located hypsometrically lower than the Sovetskoye field.

The identified oil pool is confined to productive bed Yu-I. Lithologically, the bed is represented by sandstones with subordinate interbeds of siltstones and argillites. The total thickness of the bed is 40.4 m, and the effective thickness is 26 m. According to field-geophysical studies, the bed is characterized by favorable reservoir properties. The structure of the pool has not been studied, but by analogy with other fields of the Lar-Egansky oil district it may be assumed that it will have a stratified character.

During testing of bed Yu-I in well No. 11, from the interval 2559–2539 m, a flowing oil production rate of 96 m³/day was obtained through an 8-mm choke, with bottom-hole pressure of 200 atm and a gas factor of 58 m³/m³. The reservoir pressure is 254.3 atm, and the productivity coefficient is 1.8 m³/day·atm.

One well has been drilled in the field, penetrating the section of the sedimentary complex only down to the upper part of the deposits of the Tyumen Formation. On the basis of comparison with the nearby Medvedevskoye field, it may be expected that within the flank parts of the Strezhevoy structure development of horizon “M” is possible; lithologically, this horizon is favorable for the formation of lithologically screened oil pools.

The prospects of the Strezhevoy field are also associated with the deposits of the Vartovskaya Formation, in whose section permeable beds of group “A” are noted; these are characterized by lithological inconsistency along strike. Oil pools in these beds will belong to the lithological type and will be characterized by small dimensions.

The main productive horizons B-VIII and A-1 of the Sovetskoye field are well traced in the section of the Strezhevoy field and are characterized by analogous favorable reservoir properties. However, in the section of the Strezhevoy field they lack oil saturation, which is explained by their low hypsometric position in comparison with the Sovetskoye field.

The discovery of the Strezhevoy field makes it possible to increase substantially the recoverable oil reserves for the group of fields of the Sosninsky swell, which in turn ensures growth of oil production in Tomsk Oblast not only in the current five-year plan but also in subsequent years.

REFERENCES

1. F. G. Gurari. Geology and prospects of oil-and-gas potential of the Ob-Irtysh interfluve. Trudy SNIIGGiMS, issue 3, L., 1959.

2. Yu. K. Mironov. Prospects of oil potential of the regions of the middle course of the Ob River, West Siberian Lowland. Sovetskaya geologiya, No. 2, 1959.