Prospects for the Oil-and-Gas Potential of the Okunev Dome-Shaped Uplift of the Alexandrov Arch

E. E. Danenberg, N. M. Babikov

(Presented by Professor A. V. Aksarin)

The available materials on the oil-and-gas potential of the Mesozoic deposits of the Alexandrov Arch show that, in the rocks of its section from the Lower Jurassic through the Upper Cretaceous inclusive, numerous indications of oil and gas are encountered. This testifies to the broad regional oil-and-gas potential of the entire rock complex and makes it possible to rate highly its prospects in terms of oil and gas. At the same time, the degree of prospectivity of individual districts of the region depends on a number of factors, such as the time of formation of local structures, the character of seals, reservoir properties, and a whole series of other indicators.

One of the most promising areas is the Okunev dome-shaped uplift, located in the eastern part of the Alexandrov megaswell, in the zone of its junction with the Ust-Tym Depression.

Its high prospectivity is based on the well-known concept that the areas of hydrocarbon generation are negative first-order structures—in this case, the Ust-Tym Depression—and, consequently, the Okunev dome-shaped uplift represents an area of oil-and-gas accumulation directly adjoining the generation zone and occupying a higher hypsometric position in comparison with it.

Jurassic, Cretaceous, and Paleogene deposits, lying unconformably on the Paleozoic basement, take part in the structure of the Okunev dome-shaped uplift. The total thickness of the sedimentary complex is about 2700 m.

This dome-shaped uplift is a large positive second-order structure, clearly traceable in Jurassic deposits. Along reflecting horizon II a (the base of the Maryanov Formation of the Valanginian), it is contoured by the −2350 m contour line and has the form of an irregular oval. Within it, the Krugloozernoe, Mygtynskoe, East-Mygtynskoe, and Vartovskoe local uplifts have been identified.

The most promising of these local uplifts is Vartovskoe, where in 1968 an inflow of oil was obtained from Jurassic deposits.

The Vartovskoe uplift, located in the southeastern part of the Okunev dome-shaped uplift, is confidently traced in Jurassic, Cretaceous, and Paleogene deposits.

Along reflecting horizon II a (the Maryanov Formation of the Upper Jurassic), the Vartovskoe uplift is contoured by the −2300 m contour line and has the form.

an elongated oval of north-northeasterly trend. The dimensions of the uplift are \(23 \times 7\) km, and its amplitude is 70 m. Along reflecting horizon III (the Pokur Formation of the Aptian–Albian–Cenomanian), the uplift is contoured by the \(-1360\) m isohypse and retains its shape and trend. Its dimensions are \(46 \times 20\) km, with an amplitude of up to 95 m. The uplift is also traced along the overlying reflecting horizons IV b (the Kuznetsov Formation of the Turonian) and V a (the Talitskaya Formation of the Paleocene), where it is contoured, respectively, by the \(-560\) m and \(-240\) m isohypses. Along reflecting horizon IV b, the uplift retains a meridional trend and has dimensions of \(41 \times 21\) km, with an amplitude of 60 m.

Comparison of the structural plans shows that the morphological features of the uplift generally do not undergo change; only a smoothing of relief forms from older to younger horizons is observed, while the crest of the uplift shifts in a southeasterly direction. At the same time, a regularity is recorded in the increase in the dimensions of the uplift from

Fig. 1. Tectonic sketch map of the Alexandrov arch (after F. G. Gurari, 1967).
I — Poludennogor Ridge; II — Krivolutsky Ridge; III — Okunev dome-shaped uplift;

Third-order structures:
1 — Okhteuryevskaya, 2 — Severnaya, 3 — Taezhnaya, 4 — Traigorodskaya, 5 — Vostochno-Mygtynskaya, 6 — Alexandrovskaya, 7 — Chebachya, 8 — Krugloozernaya, 9 — Mygtynskaya, 10 — Baikalskaya, 11 — Nazinskaya, 12 — Vartovskaya, 13 — Poludennaya, 14 — Yuzhno-Kazinskaya, 15 — Ambarskaya; negative structural elements: A — Ust-Vakh trough; B — Negotkinskaya depression; a — Sosninsky Ridge.

from the older to the younger horizons, with preservation of significant amplitudes. Such a long period of structural growth is a favorable factor for the formation of oil and gas pools in the section of the Vartov uplift, from the Jurassic deposits through the Paleogene inclusive.

During testing in the course of drilling in the first well drilled on the Vartov uplift, well No. 330, a formation tester obtained an inflow of oil from beds in the lower part of the Tyumen Formation of the Jurassic. The productive horizon occurs in the interval 2554–2576 m and, according to production-geophysical studies, is characterized by favorable reservoir properties. The structure of the pool has not been studied, but it may be assumed that it will be developed within the limits of the entire crest of the uplift. In addition, in the deposits of the Tyumen Formation, production-geophysical studies have recorded several more permeable horizons, indicating an improvement in the reservoir properties of the Tyumen Formation as a whole.

A characteristic feature of both the Vartov uplift and the nearby Mygytinsk uplift is that, in comparison with the central and western parts of the Alexandrov arch, their sections show a significant improvement in the reservoir properties of bed Yu-1 of the Vasyugan Formation of the Upper Jurassic. Whereas within almost the entire Alexandrov arch this bed, which has regional oil saturation, has very low reservoir properties, within the Vartov and Mygytinsk uplifts, according to production-geophysical studies, it has a favorable reservoir characterization. Bed Yu-1 is overlain by a thick sequence of clayey formations of the Maryanovskaya (Upper Jurassic) and Kulomzinskaya (Valanginian) formations. The presence of a thick clayey seal and of a zone of favorable reservoirs is a favorable factor for the formation, in bed Yu-1, of commercial oil pools in uplifted parts of structures.

The oil-bearing prospects of the Vartov area are also connected with the deposits of the Kiyalin Formation (Hauterivian–Barremian), in the section of which a whole series of permeable beds of group “A” is recorded. These beds are characterized by lithological discontinuity along the strike; consequently, the oil accumulations associated with them will have the character of small lithological pools.

The preservation of the structural forms of the Vartov uplift in Upper Cretaceous deposits, with their considerable dimensions and amplitudes, and the presence of regional clayey seals of the Turonian and Campanian, are favorable factors for a possible commercial accumulation of oil and gas in well-developed reservoirs of the sandy varieties of Cenomanian and Santonian rocks.

The oil-bearing prospect of the Mygytinsk, East Mygytinsk, and Krugloozersk uplifts is connected only with bed Yu-1 and the deposits of the Tyumen Formation, since these uplifts are traced only in Jurassic deposits.

REFERENCES

1. Geology and Oil-and-Gas Potential of the West Siberian Lowland—a New Oil Base of the USSR. Novosibirsk, Publishing House of the Siberian Branch of the USSR Academy of Sciences, 1963.

2. N. N. Zapivalov, K. A. Shpilman, “On the prospects for oil-and-gas potential of Tomsk, Novosibirsk, and Omsk oblasts,” Geologiya nefti i gaza, No. 7, 1963.