New Data on the Gas Potential of the Pudino Arch

Yu. V. Naidanov

(Presented by Professor A. V. Aksarin)

The Pudino arch is one of the most interesting areas of Tomsk Oblast for the search for gas-condensate fields. After the discovery in 1967 of the Luginetskoye and Kazanskoye gas fields, its prospects increased considerably. The Luginetskoye gas-condensate–oil field is located in the northwestern part of the Pudino arch. The field is of interest in that, in addition to a gas pool with condensate, it contains a significant underlying oil pool. The Kazanskoye gas-condensate field is located in the southern part of the arch. A characteristic geological feature of the field is the subdivision of the productive beds Yu-1, Yu-II, Yu-III, which at Luginetskoye are combined into a single hydrodynamically connected series of beds Yu-I—III. After the discovery of these two fields, naturally, the Mirnaya and Ostaninskaya structures, located between the two indicated fields, became the most promising (Fig. 1).

The Mirnaya structure is located in the central part of the Pudino arch, has a sublatitudinal strike, and is contoured along seismic horizon II-a (the base of the Maryanovskaya suite of the Upper Jurassic) by the isohypse −2400 m. The dimensions of the structure along this isohypse are 16×5 km, and the amplitude is 60—80 m.

In the western and central parts of the structure, along the 2360 m isoline, two small domes are distinguished. The southwestern and southern flanks of the structure are steeper (1—3°), while the northeastern and eastern flanks are gentle (less than 1°; Fig. 1). Along reflecting horizon III (the lower Pokur suite), the Mirnoye uplift is an elevated zone extending almost latitudinally, with a very small amplitude—10—15 m.

Along horizon IV-v (the top of the Ipatovskaya suite), the Mirnoye uplift corresponds to a submerged zone. As of 1 January 1971, two exploratory wells, Nos. 410 and 411, had been completed with testing on the Mirnaya structure. In well 410, drilled on the western dome of the structure, bed Yu-I was penetrated in the interval 2469—2485 m (absolute elevations 2355—2371 m); when it was tested over its full thickness, 28 thousand m³/day of gas and 18 m³/day of condensate were obtained through a 12.5 mm choke. Such small flow rates are explained by deterioration of the reservoir properties in this part of the structure. Well No. 411 was drilled 6 km east of well 410, within the central dome of the structure. According to the results of the conducted electrical-

Fig. 1. Structural map of the Ostanino and Mirnaya structures along reference reflecting horizon II-a (base of the Maryanovo Formation of the Upper Jurassic): 1 — contour lines of reference reflecting horizon II-a; 2 — well number; 3 — Ostanino structure; 4 — Mirnaya structure

logging, bed Yu-1 in it was penetrated in the depth interval 2541–2581 m (absolute elevations −2426 to −2466 m), which is 71 m lower than in well 410. The bed practically contains no reservoir. Thus, from west to east (from well 420 of the Ostanino area to wells 410 and 411 of the Mirnaya area) there is a rather sharp deterioration in the storage properties of the main productive bed Yu-1, which was subsequently also confirmed by testing. In addition, well 411, according to the seismic basis, was placed under structural conditions equal to those of well 410; however, according to the drilling results it revealed a fairly significant difference from the seismic data: the top of bed Yu-1 in it proved to be sharply lowered by 60–70 m, whereas in well 410 this difference amounts to only a few meters. Such a discrepancy makes it possible to assume a displacement of the reflection boundaries in zones with poor reservoir properties, since in this case there is no sharp facies difference in the rocks necessary for obtaining reliable reflections.

The most interesting new data on the gas potential of the Pudino arch are associated with the Ostanino structure, located in the central part of the arch 1.5–2 km west of Mirnaya. Along the main reflecting horizon II-a, the Ostanino structure is outlined by the −2400 m seismic contour; its strike is submeridional, and its form is close to dome-shaped. The dimensions of the structure within the closing contour are 12×11 km, and the amplitudes are 75–85 m (Fig. 1). Along the overlying reflecting horizons (III and IV-v), the Ostanino structure is also traced rather clearly, although it gradually flattens (a decrease in the dip angles of the flanks and in amplitude). At the end of 1970, only one well, No. 420, had been drilled on the structure, located—

located in the southeastern part of the area. This well became the discovery well of the Ostanino gas field.

According to production geophysics data (BKZ), four promising beds have been identified in the section of the Tyumen and Vasyugan suites (Yu-IV, Yu-III, Yu-II, Yu-I). During testing of the lowermost bed Yu-IV in the interval 2578–2585 m, a small amount of oil, condensate, and formation water was obtained. Judging from the electrical logging, the bed has good reservoir properties, and the recovery of oil with water is associated with the presence of a small oil rim which, evidently, occurs in “patches” beneath the gas pool. We as yet know nothing about the hydrodynamic connection of these oil “patches.”

Testing of bed Yu-III in the interval 2560–2542 m confirmed the prospects of the Jurassic deposits; however, it has not yet been possible to establish the commercial value of the bed (8 thousand m³/day of gas, 0.5 m³/day of condensate, and 2.9 m³/day of formation water were obtained on a 4.5 mm choke). It is still premature to speak of the productivity of the bed as a whole on the basis of testing it in a single well. Evidently, we shall obtain an answer to this in subsequent wells. Bed Yu-II, occurring at the top of the Tyumen suite, was not tested in well No. 420. According to production-geophysical data, it has a resistivity of 15 ohm·m and is interpreted as productive (possibly gas-saturated). And, finally, the main productive bed Yu-I confirmed the geologists’ forecasts regarding the prospects of this area. After perforation of the interval 2431–2463 m, a commercial gas flow was obtained at a rate of 175 thousand m³/day through a 12.5-millimeter choke. The presence of condensate was noted in the gas stream.

On the basis of the results of preliminary exploration, it is still difficult to say whether there will be a separate gas pool on the Mirnaya structure, or whether here, perhaps, the periclinal termination of the more significant Ostanino gas-condensate field is emerging. In any case, the supposition of a single large field does not meet with any substantial objections. Moreover, the structural discrepancy between the seismic data and the drilling data in well No. 411 considerably reduces the dimensions of the Mirnaya structure, while the considerable condensate rate and the correspondingly small gas rate once again indicate that well No. 410 is probably located not far from the gas-pool contour.

At present geologists face a task of great practical importance: the search for zones with improved reservoir properties, since local structures situated in these zones may contain commercial accumulations of hydrocarbons. According to the results of the geological exploration work carried out, such zones are indicated first of all in the western and southwestern parts of the Pudino Arch.