PROSPECTS FOR THE EXPLORATION OF LITHOLOGICAL OIL AND GAS ACCUMULATIONS IN THE SOUTHERN PART OF TOMSK OBLAST: THE EXAMPLE OF THE B-Kh RESERVOIR OF THE MYLDZHINSKOYE FIELD

V. G. CHERTENKOV, L. S. BABIKOVA

(Presented by Professor A. V. Aksarin)

The search for lithological accumulations of oil and gas, as a rule, requires the expenditure of a greater amount of effort and resources than the search for other types of accumulations. At the same time, it is generally known that such accumulations possess large reserves of oil and gas and are of enormous practical interest.

The presence in the south of Tomsk Oblast of already discovered lithological accumulations in the Myldzhinskoye gas-condensate field makes it possible to speak of finding the same type of oil and gas traps in this region.

The most interesting lithological accumulation is the accumulation of the B-Kh reservoir of the Myldzhinskoye field.

The Myldzhinskoye gas-condensate field is located within the southeastern part of the Sredne-Vasyugan complex arch. Administratively, it belongs to the Kargasok District of Tomsk Oblast. The field is multilayered; the Jurassic and Cretaceous deposits are commercially gas-bearing. Productive reservoirs Yu-I + II, B-XVI–XX, B-X, and B-VIII are distinguished in them.

The gas accumulation in reservoir Yu-I + II is massive; in reservoirs B-XVI–XX and B-X it is lithological; in reservoir B-VIII it is stratified.

Reservoir B-X is confined to the middle part of the section of the Kulomzino Formation of the Valanginian. The reservoir is facially variable and is traced only on the northeastern flank of the Myldzhinskaya structure. It has been penetrated by drilled wells at depths of 2142–2242 m. The reservoir is commercially gas-bearing; the gas contains condensate. The condensate factor according to a measurement in well No. 21 is equal to 95.3 cm³/m³.

Below are the test results for several wells.

The thickness of the reservoir varies from 6 to 34.8 m; the effective thickness ranges from 1.2 to 22 m.

The reservoir is composed of frequently interbedded sandstones, siltstones, and argillites.

The sandstones are gray, fine-grained, polymictic, of medium hardness, with rare inclusions of plant detritus, and with the odor of condensate.

The siltstones are gray, sandy, platy, micaceous, dense, of medium hardness, layered, with interbeds of gray, fine-grained sandstone.

The argillites are dark gray to black, micaceous, dense, hard, with inclusions of plant detritus and fauna.

Fig. 1. Structural map of the top of the B-Kh reservoir of the Myldzhinskoye field.
1 — drilled wells; numerator — well number, denominator — absolute elevation of the top of the B-Kh reservoir; 2 — structural contours of the top of the B-Kh reservoir; 3 — lithological boundary of the B-Kh reservoir; 4 — gas–water contact (conditional).

Fig. 2. Profile along line 1—1

Visible annotations in the figure:

• SW — NE
• Depth marks: −2040, −2060, −2080, −2100, −2120, −2140, −2160, −2180
• Wells: well 1, well 25, well 21, well 20, well 8
• Reservoir: B-Kh
• Notional GWC — 2145 m

The number of sandy interbeds constituting the effective thickness of the reservoir varies from 3 to 13. Owing to the frequent alternation of interbeds according to BKZ and RK logs, the position of the gas–water contact is not clearly distinguished. Taking into account the results of tests of wells Nos. 17 and 34, the GWC is provisionally drawn along the lower perforation hole in these wells, at the mark of −2145.

Table 1

On the structural map compiled for the top of reservoir B-Kh, it is evident that the gas-saturated part of the reservoir tends toward the steep northeastern flank of the structure, which gradually passes into the flank of a second-order negative structure—the Salat depression. The size of the accumulation is 20 × 5 km.

The improvement of the reservoir properties of the B-Kh reservoir and the increase in its effective thickness occur regularly in an easterly direction. According to deep exploratory drilling data, toward the crest of the Myldzhinskaya structure there is an almost complete replacement of sandstones by dense argillites, practically impermeable. The lithological screen passes along line A—A (Figs. 1, 2, 3), bounding the accumulation from the west.

Fig. 3. Profile along line II—II

Undoubtedly, the productivity of the B-Kh reservoir depends on its position on the structure and on the position of the structure itself on the swell. Pri-

confined to the steepest eastern flank of the Sredne-Vasyugan swell, the accumulation has as its feeding area the Verkhne-Salatskaya depression. There is no doubt that on the flanks of structures located under similar conditions there may be the same kinds of traps, possibly oil- and gas-saturated.

Such prospective areas must include, first of all, the marginal southeastern parts of the Sredne-Vasyugan swell, namely: the eastern and southeastern margins of the Yuzhno-Myldzhinskaya and Verkhne-Salatskaya structures. It is probable that similar conditions will also be observed on the western margin of the Pudinsky swell, at the Ostaninskaya structure.

REFERENCES

1. Gurari F. G., Nesterov I. I., Rostovtsev N. N., Rudkevich M. Ya. Basic regularities in the distribution of gas and oil accumulations. Proceedings of the Zap.-Sib. NIGNI, issue 1, Moscow, 1965.

2. Gurari F. G., Kazakov F. E., et al. Methodology and directions of geological prospecting and exploration for oil and gas in the West Siberian Lowland. Proceedings of the Zap.-Sib. NIGNI, issue 1, Moscow, 1965.

3. Rostovtsev N. N. On the methodology for determining the productivity of structures in oil- and gas-bearing regions. Geologiya nefti i gaza, No. 7, 1964.