Proceedings of the Tomsk Order of the October Revolution and Order of the Red Banner of Labor Polytechnic Institute named after S. M. Kirov

Vol. 237　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1975

GEOLOGICAL STRUCTURE AND OIL POTENTIAL OF THE MALORECHENSKOYE OIL FIELD

N. M. Babikov

(Presented by Professor A. V. Aksarin)

The Malorechenskoye oil field is confined to the uplift of the same name, located within the Lar-Egan swell of the Nizhnevartovsk arch. The structure was identified by areal seismic surveys (MOV) in 1962–1963. Along reflecting horizon XII-a (the base of the Maryanovka Formation of the Upper Jurassic), the uplift is a brachyanticlinal fold of submeridional strike, measuring 5 × 11 km, complicated by three small domes.

In 1965, well No. 117 was drilled in the crestal part of the central dome and yielded a commercial oil fountain.

The geological section of the Malorechenskoye field is composed of Paleozoic, Jurassic, Cretaceous, and Paleogene rocks. The Paleozoic deposits are represented by light-gray limestones, strong and weakly micaceous. The Jurassic deposits, which unconformably overlap the Paleozoic, have a total thickness of up to 240 m and are represented by an alternation of argillites, siltstones, and sandstones. The Cretaceous deposits conformably overlie the Jurassic and are also composed of argillites, siltstones, sandstones, sands, and clays. The Paleogene deposits are composed of clays, sandstones, and sands.

A total of 7 wells have been drilled in the Malorechenskoye oil field (Nos. 117, 118, 119, 120, 121, 122, 123). The oil pool is confined to bed Yu-1, occurring in the upper part of the Vasyugan Formation of the Valanginian. The productive bed occurs at depths of about 2438–2497 m; the total thickness of the bed varies within 32–52 m. The effective thickness of bed Yu-1 ranges from 17.2 to 30.4 m, and the oil-saturated thickness from 0.9 to 17.2 m.

Lithologically, productive bed Yu-1 is composed of sandstones, argillites, and siltstones.

The sandstones are light gray, medium-grained, micaceous, with fine horizontal lamination due to very thin interbeds of gray argillite with inclusions of plant detritus.

The argillites are gray, dense, weakly micaceous, with isolated inclusions of pyrite, and with horizontal thin lamination due to thin interbeds and lenses of light-gray sandstone.

The siltstones are light gray, dense, strong, micaceous, with inclusions of plant detritus, and with wavy horizontal thin lamination due to thin interbeds of light-gray sandy material.

Fig. 1. Structural map of the Malorechenskoye oil field:

1 — contour lines for reflecting horizon II-a (base of the Maryanovka Formation of the Valanginian);

2 — contour lines of the top of productive horizon Yu-1 (Vasyugan Formation of the Valanginian);

3 — deep drilled wells.

The reservoir properties of the sandstones are quite good. The average value of open porosity ranges from 18.4 to 20.8%, and permeability from 50.5 to 794.1 millidarcies.

According to field-geophysical studies, productive bed Yu-1 is characterized by rock resistivity values of up to 4 ohm.

As a result of well testing, oil flows of up to 146 m³/day were obtained through an 8 mm choke.

According to core data, field-geophysical studies, and well testing, the oil-water contact (OWC) is drawn at an absolute elevation of −2418 m. However, the drilling results for well No. 123, placed in the northern part of the field, changed the understanding of the geological structure of the oil pool. This well penetrated the top of bed Yu-1 at an absolute elevation of −2427 m, that is, 9 m hypsometrically below the previously accepted OWC outline. From the upper part of bed Yu-1, oil-saturated sandstones were recovered in the intervals 2478.4–2486.4 m and 2490.4–2491.4 m. During testing of this bed in the interval 2488–2480 m (absolute elevations −2437–2429 m), an oil flow was obtained at a rate of 21 m³/day through a 5 mm choke.

Thus, as a result of testing well No. 123 it was established that in the northern part of the Malorechenskoye field there is a separate oil pool lying 19 m hypsometrically below the main pool. There are grounds to suppose that well No. 123 penetrated the oil pool near its OWC, and consequently it extends somewhat north of this well (see Fig. 1).

The oil of the Malorechenskoye field has a specific gravity of 0.84–0.87 g/cm³. The initial boiling temperature varies over a wide range from 45° to 118°C. Light fractions boiling off up to 200°C amount to 14.7–37.5%. The oil is low-sulfur (0.35–0.60%); its kinematic viscosity under surface conditions at 20°C is 5.5–18.9 centistokes. In terms of hydrocarbon composition the oil is methane-naphthene-aromatic. Methane hydrocarbons predominate (30.4–38.2%); naphthenic hydrocarbons are somewhat fewer (25.7–37.5%), and aromatic hydrocarbons amount to 21.1–24.3%.

The features of the geological structure and oil-and-gas potential of the Malorechenskoye field must be taken into account in prospecting for similar pools in other oil-and-gas-bearing districts of Tomsk Oblast, in order to achieve the greatest efficiency of geological exploration work.

References

1. F. G. Gurari. On the search for oil and gas in the Mesozoic of the West Siberian Lowland. Proceedings of SNIIGGiMS, issue 17, 1961.

2. E. E. Danenberg. The Sosninskoye oil field. Vestnik ZSGU, No. 3, 1963.