THE KIEV-EGAN OIL FIELD

Yu. V. Naidanov, V. N. Rostovtsev

(Presented by Professor A. V. Aksarin)

In the northeastern part of the Kargasok District of Tomsk Oblast, a new oil field, the Kiev-Egan field, was discovered in 1970.

According to the map of the tectonics of the Mesozoic–Cenozoic cover of the southeastern West Siberian Plate, compiled in 1971 by a large group of staff members of SNIIGGiMS and the Tomsk Territorial Geological Administration under the editorship of F. G. Gurari, this field is located in the central part of the Kiev-Egan dome-shaped uplift (KP), identified in the northeastern part of the Ust-Tym depression.

The development of the Kiev-Egan structure was complex and discontinuous in character. This uplift underwent its most intense growth in Middle Jurassic time. The development of this uplift had a definite influence on the accumulation of hydrocarbons in the Kiev-Egan structure of the third order, within which an oil field was discovered in 1970.

The deep structure of the structure under consideration has been studied by seismic exploration along reflecting horizons 1-6 (the upper parts of the Tyumen Formation) and III (the lower parts of the Pokur Formation). Structural maps were not constructed for the other marker horizons.

Along horizon 1-6, the Kiev-Egan structure is confidently outlined by the 2650 m contour; its strike is northwestern, the dimensions of the structure within the indicated contour are 15×18 km, the amplitude is 100–130 m, and the dip of the flanks is usually small (up to 40–50′), but in a number of cases reaches 1.5–2° (Fig. 1).

In the northwest and southeast of the structure, two independent domes have been delineated by the —2550 m contour.

The structural scheme for reflecting horizon III is somewhat simplified; the dimensions of the structure increase to 18×20 km, and its amplitude decreases.

Analysis of the results of gravity prospecting and aeromagnetic surveying shows that the Kiev-Egan local uplift corresponds in plan to a gravity minimum and to a transition zone from positive to negative values of the magnetic field. As of January 1, 1973, three wells had been drilled on the structure: No. 350—the discovery well of the field, and Nos. 351 and 352. Drilling of wells Nos. 350 and 351 was stopped in deposits of the Tyumen Formation (Lower + Middle Jurassic). Well No. 352 reached a maximum depth of 3152 m and penetrated pre-Jurassic deposits.

Fig. 1. Structural map of the Kiev-Egan oil field.
1. Contours of the reflecting horizon 1-b of the Tyumen Formation. 2. Contours of the top of bed Yu-1 according to drilling. 3. Drilled wells. 4. Inferred oil-water contact.

The composite geological section of the Kiev-Egan field, taking into account core and geophysical material from three wells, is given in the table.

Table

The section penetrated by the wells is represented by sandy-clayey deposits of marine, coastal-marine, and continental origin, typical of the West Siberian Lowland.

The productive horizon at the field is horizon Yu-1 of the Vasyugan Formation of Upper Jurassic age.

In well No. 350 this horizon was penetrated in the interval 2622—2677 m and has a three-member structure. At the top of the horizon there is a permeable sandstone bed with a total thickness of 16 meters. In the upper part of this bed, in the interval 2622—2629 m, an interbed was identified with a resistivity of 12 ohm·m and SP of 55 mV; in the lower part of the bed, separated from the upper by a meter-thick clayey interlayer, in the interval 2631—2636.8 m, the resistivity is 7.5 ohm·m. The decrease in resistivity in the lower part of the bed may be explained either by an increase in its clay content or by a change in the character of saturation of this part of the bed.

The middle part of the horizon is represented by a 24-meter argillite-siltstone bed with thin sandstone interlayers. In practice this is an impermeable part of the horizon, dividing it into two permeable beds (upper and lower). The lower permeable bed lies in the interval 2662—2677 m; its total thickness is 15 meters. According to field-geophysical data, the resistivity of this bed varies from 3.7 to 3.1 ohm·m, SP — 57—52 mV.

Only the upper and middle parts of horizon Yu-1 have been characterized by core material. From the interval 2622.5—2631.7 m, 1.71 m of gray coarse-grained sandstone, weakly oil-saturated—with an odor of oil—was recovered. The interval 2631.7—2651.4 m is represented in the core by light-gray fine-grained sandstones, black shaly coals, and dark-gray argillites. The open porosity of sandstones taken for analysis from the interval 2622.5—2628.7 m ranges from 17 to 14%, and permeability parallel to the core axis from 1.67 to 2.26 millidarcys.

The total effective thickness of horizon Yu-1 is 23.2 m.

During the joint testing of the lower part of the Bazhenov member and the upper part of horizon Yu-1 in the interval 2618—2626 m, on a 3 mm choke, a pulsating flowing of oil was observed, with an average daily rate of 8.4 m³. It was not possible to work out the regimes on a large-diameter diaphragm, since the well watered out. To determine the interval of influx

formation water from entering the well. Considering that the lower perforation holes are 5 meters higher than the last signs of oil saturation in the core, the influx of formation water may be attributed to insufficient quality of the well cementing. According to cementometry data obtained by the GGK method, only the interval 2607–2633 m is cemented satisfactorily; in the interval 2655–2633 m the cementing is one-sided. It is quite probable that, during perforation of the testing interval, the cement sheath in the interval 2633–2626 m could have been disturbed, and formation water from underlying aquiferous horizons flowed in through the disturbed zones.

In well No. 351, drilled in the crest part of the northwestern dome, horizon Yu-1 lies hypsometrically 11 meters higher than in well No. 350. The structure of horizon Yu-1 is the same as in well No. 350, that is, three-membered. However, the permeable upper and lower interbeds are reduced owing to their replacement by clayey varieties. The effective thickness of the horizon is up to 11.6 m. In view of the heterogeneity and the shielding of the upper part of the horizon, it is impossible to draw a conclusion on reservoir saturation from the results of field geophysics. The lower part of the horizon and the deposits of the Tyumen Suite are not characterized by electric logging because of an accident. In core material the horizon is characterized incompletely and is represented by gray sandstones, mainly dense and clayey, with interbeds of dark-gray argillites, siltstones, and coals. During drilling, when testing the upper part of the horizon in the interval 2608.6–2620.7 m with a formation tester, over 32 minutes, at a drawdown on the formation of 98.4 atm, 0.1 m³ of clay mud filtrate with an oil film was obtained.

According to the core in well No. 351, signs of oil saturation were noted in horizon Yu-1 in the interval 2890–2898 m; however, for technical reasons it was not possible to test it.

In well No. 352, horizon Yu-1 was penetrated in the interval 2650–2722 m, which is hypsometrically 38 meters lower than in well No. 350. In the section of this well, deterioration of the reservoir properties of horizon Yu-1 and a reduction of effective thickness to 10.4 meters are noted.

Permeable intervals are marked by elevated values of apparent resistivity, which is associated either with poor reservoir properties or with the nature of saturation. According to the core, horizon Yu-1 is represented by interbedding of argillites, sandstones, and siltstones. Deposits of the Tyumen Suite and the Paleozoic are practically impermeable. Owing to cement not rising behind the production string, horizon Yu-1 was not tested.

During well construction at the Kiev-Egan field, deposits of the Paleozoic, as well as of the Kulomzino, Tarskaya, Kuznetsovo, and Ipatovo suites, were tested both during drilling and in the casing.

In these deposits no influx of oil or gas was established. In conclusion, the following points may be stated:

1. With the discovery of the Kiev-Egan field, the territory of lands with proven commercial oil productivity has shifted considerably eastward in Tomsk Oblast.

2. In the section of the field, deposits of the Vasyugan and Tyumen suites are promising.

3. The principal productive horizon Yu-1 has a complex structure; the regularities of its development have not yet been identified.

4. Further prospecting and exploration work must be carried out

work in this region for the purpose of a detailed study of its geological structure, tectonics, and oil-and-gas potential, with the aim of establishing the regularities governing the distribution and prospecting of oil fields in the depressions.

REFERENCES

I. I. Nesterov, F. K. Salmanov, et al. Oil and Gas Fields of Western Siberia. Moscow, “Nedra,” 1971.