A New Gas-Bearing Region in the North of the West Siberian Lowland

S. N. Gulyaev

(Presented by Professor A. V. Aksarin)

In the north of the West Siberian Lowland, where all hydrocarbon deposits are associated with local uplifts, three gas-bearing regions can currently be distinguished: western, central, and eastern. The western region is located in the Cis-Ural part of the lowland and is characterized by the presence of numerous single-reservoir fields with a productive horizon in the Upper Jurassic sandy basal beds, lying on formations of the Paleozoic basement, within the Severo-Sosvinsky arch, and by the multi-reservoir Novoportovskoye field with gas pools in Middle Jurassic and Valanginian sediments on the South Yamal swell.

In the central region, large and very large gas pools are confined to sandy beds occurring in the Cenomanian and in the lower part of the Turonian stages beneath a thick Turonian–Paleogene clayey caprock. The largest gas fields have been discovered here: Tazovskoye and Zapolyarnoye on the Tazovsky arch, and Gubkinskoye on the Purpeisky arch.

In the northeast of the lowland, gas shows at the ground surface have been known since 1934 in the area of the Malokhetsky swell, where they are represented by small releases of gas bubbles and powerful gas jets with a flow rate of more than 2000 m³/day [4]. In the 1950s, subcommercial accumulations of gas were discovered here in Valanginian and Jurassic sediments on the Malokhetsky local uplift, and in January 1966 an industrial inflow of gas was obtained in this region. Here, on the Nizhnekhetsky local uplift (Fig. 1), located 70 km northwest of the town of Dudinka, in the crest part of the Malokhetsky swell, a gas fountain with a flow rate of up to 400 thousand m³/day was obtained from well 1-R from a depth of 892–905 m. Somewhat later, gas pools were discovered on the Zimnee uplift.

The Malokhetsky swell, which controls almost all known oil-and-gas shows and gas accumulations in the northeast of the West Siberian Lowland, is confined to the zone of the Turukhan–Malokhetsky deep fault [1] and is a contrasting anticlinal structure with an amplitude of more than 1000 m, extending for 170 km in a northeastern direction, complicated in its arch by brachyanticlinal and dome-shaped uplifts, and in its northeastern part also by disjunctive disturbances. Beneath the Quaternary cover on the Malokhetsky swell, Cretaceous deposits everywhere occur, from the Valanginian in the most fav—

raised part up to the Cenomanian in the zones of subsidence. The dip angles on the flanks of the structure in the Cretaceous deposits reach 5–10°, and in the Jurassic deposits up to 10–15°. The local uplifts within the swell have various dimensions, from 4 × 4 to 10 × 16 km, and the amplitude of some of them is 200 m or more. At the base of the sedimentary cover here lie Lower Jurassic deposits, which, with angular and stratigraphic unconformity, overlap the tuffaceous-argillite formations of the Lower Triassic or the carbonates of the Cambrian–Silurian, already belonging to the pre-Jurassic basement.

Fig. 1. Diagram of the location of local uplifts in the southwestern part of the Malokhetsky swell: 1 — well that yielded a commercial inflow of gas; 2 — isohypses of the reflecting marker horizon in the Jurassic deposits; 3 — local uplifts: I — Nizhnekhetskoye, II — Semenovskoye, III — Zimneye, IV — Mayskoye, V — Tampeyskoye, VI — Gorchinskoye, VII — Messoyakhskoye.

The Nizhnekhetskoye local uplift, to which the gas field is confined, is a small dome-shaped structure, the dimensions of which at the base of the sedimentary cover are 4 × 4 km, and the amplitude about 100 m. Upsection, the structure gradually flattens out, and at the top of the productive horizon its amplitude is less than 50 m. These data indicate that a small gas field has been discovered here. The productive horizon at the Nizhnekhetskoye field, as shown by preliminary correlation of wells in this area and judging from the electric-log characteristics, is confined to a sandstone bed overlain by a 35-meter clayey Volgian member, which in turn lies at the base of the Upper Jurassic–Valanginian clayey sequence, a regional oil-and-gas seal over much of the West Siberian Plate.

The Upper Jurassic–Valanginian clayey sequence in different areas of the northeastern lowland has varying thickness (Fig. 2), differing lithological composition, and time-transgressive boundaries. Over most of the territory under consideration, this sequence includes the Kimmeridgian–Volgian stages and the lower substage of the Valanginian; moreover, near the Paleozoic framing south of the Arctic Circle it has been completely destroyed by erosion, and beneath the Quaternary cover here there crop out predominantly

Fig. 2. Isopach map of the Upper Jurassic–Valanginian clayey unit in the northeast of the West Siberian Lowland:
1 — isopachs; 2 — outcrops of Middle Jurassic deposits beneath the Quaternary cover; 3 — exploratory and reference wells (numerator — well number, denominator — thickness); 4 — boundary of the West Siberian Plate.

sandy rocks of Kelloway–Kimmeridgian and Middle Jurassic age (Fig. 2).

An analysis of all materials on the lithologic–facies characteristics of the Upper Jurassic and Lower Cretaceous deposits in the northeastern part of the lowland, and the constructions carried out (Fig. 2), show that, as the basement subsides—especially from east to west—there is a regular increase in the Upper Jurassic–Valanginian clayey sequence. This occurs both through facies replacement of the sandy rocks of the Middle and Lower Valanginian, Kimmeridgian, and, in some areas, Oxfordian and Kelloway stages by clayey rocks, and through an increase in the thickness of individual stratigraphic units. At the same time, with the increase in thickness of this sequence, its clay content also increases. The noted regularity is manifested most clearly for the northern, and to a lesser degree for the southern, part of the territory described.

Certain features in the change of composition and thickness of the Upper Jurassic–Valanginian clayey sequence are observed in the crestal part of the Malokhetsky arch. Here the reduction of its thickness to 80–100 m occurs because of complete or partial erosion (Malokhetskoye, Semenovskoye, Funtusovskoye uplifts) or sandification (Tochinskoye uplift) of the Upper Jurassic deposits; moreover, on the Semenovskoye uplift (Fig. 1), the Volgian member, the most clayey and serving as the immediate gas seal for the gas pool on the Nizhnekhetsky uplift, has been destroyed by erosion. Such a change on the Semenovskoye uplift undoubtedly reduces the gas-bearing prospects of this structure. For the Middle and Lower Jurassic of this area, a rhythmic alternation of rather persistent clayey and sandy members is characteristic, with the thickness of both reaching 30–100 m. Thus, the clayey members situated in the lower part of the Toarcian and Aalenian can serve as gas seals, and the sandstones underlying them as reservoirs. The noted geological features of the Middle and Lower Jurassic section, together with the presence in the upper part of the Jurassic complex of a rather thick Upper Jurassic–Valanginian clayey sequence, create here favorable conditions for the formation of multilayer gas fields. Only such geological conditions [2] made possible the formation of the largest gas field in the Soviet Union, the Gazli gas field.

To the southwest of the Semenovskaya structure, on the Zimneye uplift (Fig. 1), which is considerable in size (16 × 8 km), a 30-meter clayey Kelloway member appears in the section, sealing the gas pool. The productive bed here is represented by fine-grained sandstones with individual interbeds of siltstones. During testing of this bed in well 1-R from the interval 1848–1870 m, a gas flow was obtained at a rate of 187 thousand m³/day through a 25 mm choke. An insignificant gas inflow of up to 10.8 thousand m³/day was also obtained here from the Lower Jurassic deposits at a depth of 2925–2944 m. It should be noted that the total thickness of the Upper Jurassic–Valanginian clayey sequence, at the base of which the Kelloway terrigenous seal occurs, reaches 156 m on the Zimneye uplift.

Taking into account the general regular increase of the Upper Jurassic–Valanginian clayey sequence (Fig. 2) and the increase in the content of clayey material in it in a western direction, it may be assumed that west and southwest of the Zimneye structure, on the Messoyakhskoye, Gorchinskoye, and other local uplifts (Fig. 1) situated along the path of jet migration of hydrocarbons into the most elevated axial part of the arch, this clayey sequence will serve as a gas seal, and gas accumulations will be found in the underlying deposits and in reservoir beds within it.

Thus, significant gas shows at the surface within the Malokhetsky swell, indicating an apparently continuing jet-like migration of gas from the interior regions along the axial part of the swell; the presence of semi-commercial accumulations of gas in the Jurassic–Cretaceous complex on the Malokhetsky uplift; the discovery of gas fields in the Nizhnekhetskaya and Zimnyaya areas; and favorable lithologic-facies, paleotectonic, and structural conditions [1] for the formation of relatively large multilayer gas fields in the southwestern part of the Malokhetsky swell and to the north of it—all make it possible to assert that in the northeast of the West Siberian Lowland a new gas-bearing region has been discovered, which in the near future will be able fully to supply natural gas to the Igarka–Norilsk industrial complex, which is in acute need of energy raw materials.

REFERENCES

1. S. N. Gulyaev. On the deep fault in the northeast of the West Siberian Lowland in connection with the geological structure and the problem of its oil-and-gas potential. Neft’ i gaz, No. 11, 1965.

2. G. Kh. Dikenshtein, G. A. Arzhevsky, V. P. Stroganov. The role of clay seals in the formation of gas pools. Geologiya nefti i gaza, No. 3, 1965.

3. V. N. Saks, Z. Z. Ronkina. Jurassic and Cretaceous deposits of the Yenisei depression. Tr. NIIGA, vol. 90, 1957.

4. V. N. Sokolov. Geology and prospects for oil-and-gas potential of the Arctic part of the West Siberian Lowland. Tr. NIIGA, vol. 100, 1960.