PROCEEDINGS
OF THE TOMSK ORDER OF THE OCTOBER REVOLUTION
AND ORDER OF THE RED BANNER OF LABOR
S. M. KIROV POLYTECHNIC INSTITUTE

Volume 236　　　　　　　　　　　　　　　　　　　　　　　　　　　　　1976

ON THE NONHORIZONTALITY OF THE OIL–WATER CONTACT OF HORIZON A-1 (VARTOV FORMATION) OF THE SOVETSKOYE OIL FIELD

F. Ya. Borkun

(Presented by Professor D. S. Mikov)

The Sovetskoye oil field is located in the central part of the Sosninsky swell of the Nizhne-Vartovsk arch and is associated with the Sosninsky, Sovetsky, Medvedevsky, and North-Western local uplifts. The field is multilayered. The largest natural reservoir of this field is horizon A-1, occurring in the top of the Vartov Formation and combining all the above-named local uplifts into a single large oil pool with an area of about 1000 square kilometers.

During exploration of this horizon it was established that, in the course of testing beds lying hypsometrically above the initially accepted position of the oil–water contact (OWC)—1650–1652 m—it was possible to obtain an inflow of oil with formation water. The amount of water predominated in this case. Sometimes only inflows of formation water with an oil film were obtained. It should be noted that the initial position of the OWC was adopted from test data for wells located in the crestal parts of the structures. Field-geophysical data were used in subsequent exploratory work to determine the nature of the behavior of the “oil–water” interface.

To clarify the reasons for the appearance of formation water from those beds which, proceeding from their high hypsometric position, should have yielded clean oil, the suspicion was checked that formation water might appear from overlying and underlying aquiferous collectors as a result of poor isolation of the productive part of the horizon. It turned out that, along with confirmation of these suspicions by work with radioactive isotopes (well 41), there exists the real fact of nonhorizontality of the “oil–water” interface.

This fact is confirmed by careful interpretation of field-geophysical material, by testing beds in the open borehole with formation testers, and also by control verification of the filtration paths of formation fluids using radioactive isotopes. Since for each well there was the necessary set of field-geophysical information, while data from the formation tester, core data, and especially data from work with radioactive isotopes were available for a limited number of wells, the main task of determining the OWC in each well and for the field as a whole was assigned to field geophysics. In doing so, all geological data were taken into account which, to one degree or another, could help in solving the problem posed.

Lithologically, the productive horizon A-1 has a complex structure and consists of an alternation of sandstones, siltstones, and clays. The ratio of the individual lithological rock varieties in the wells is highly variable. The percentage content of sandstones in the total thickness of the bed varies from 40 to 80%, siltstones from 10–25%, and distinguishable clay beds from 7–25%. The number of permeable interbeds varies from 3 to 9. The beds assigned to effective intervals are characterized by increased clay content, reaching 35–38%.

Sandstones are fine-grained, less often coarse- to medium-grained, most often with the presence of clayey material in a dispersed state and in the form of lenses, interbeds, and nests. The average median size of sandstone grains over the area varies from 0.09 mm to 0.24 mm; sizes of 0.1 mm are encountered most often. In the southeastern and eastern parts of the field, sandstone samples with a median grain size of 0.21–0.24 mm have been encountered. The weight content of clayey material, the particle size of which is less than 0.01 mm, regularly increases from the base to the top of the horizon within the range from 5 to 20%.

Siltstones play a significant role in the total volume of rocks composing the horizon and are confined mainly to its upper part. In the granulometric composition, grains of 0.05–0.1 mm predominate (40–70%); the content of fine-silt grains, less than 0.05 mm in size, is 10–20%, rarely 25%. For the siltstones of the horizon, as for the sandstones, a gradual increase in clayey material from the base to the top is characteristic, up to their complete replacement by argillite-like clays at the top of the horizon (the Koshai member). The most frequently occurring value of weight clay content in the siltstones is 25–40%. Only by taking into account the fact that not all clayey material is in a dispersed state can the siltstones be regarded as oil reservoirs.

As is known, in clayey reservoirs difficulties arise in determining the nature of bed saturation from production geophysics data because of the significant influence of surface conductivity on the measured physical parameters.

This influence is aggravated by the presence in the reservoirs of the horizon not only of dispersed but also of layered clay content, and it proves so significant that the apparent resistivities of water-bearing and oil-bearing beds turn out to be practically equal or differ by amounts within the limits of the accuracy of their measurement. Accordingly, it became necessary to find a set of geophysical parameters that would make it possible to resolve the question of the position of the WOC by preliminary subdivision of the horizon into oil-saturated and water-saturated beds. Of course, in this case the position of the WOC, which here and hereafter will be understood as the boundary above which the production of oil is guaranteed, is determined by a surface passing along the base of the lower oil-saturated bed.

After numerous calculations, the method of interval-by-interval determination of the nature of saturation of the beds composing horizon A-1 was chosen for determining the position of the WOC. The nature of saturation was determined by two methods, which checked and supplemented each other. The first method consisted in interval-by-interval determination of the nature of saturation of the horizon from data on specific electrical resistivity \((\rho_{\mathrm{p}})\) and the relative anomaly of spontaneous polarization \((\alpha_{\mathrm{ps}})\), taking into account the degree of clay content of the reservoir being processed [1].

The second method for determining the nature of saturation of clayey reservoirs was based on the results of applying the new recognition program “Statistics.” This program used all available geological and geophysical information on the recognizable—

in the reservoir under study and had high recognition efficiency in control reservoirs (100%).

After determining the nature of saturation of each reservoir subjected to processing, in 70 exploratory and 40 production wells data were obtained on the location of the WOC over the entire area, with a root-mean-square error in depth determination of ±1.95 m. The absolute depth marks of the WOC, in quantitative terms, were distributed as follows:

A WOC depth of 1640–1642 m was found in two wells; 1642–1644 m—in 9 wells; 1644–1646 m—in 9 wells; 1646–1648 m—in 13 wells; 1648–1650 m—in 11 wells; 1650–1652 m—in 9 wells; and in the remaining 57 wells, 40 of which were production wells, the WOC occurs at a depth of 1652–1654 m.

From the figures given it follows that the WOC surface is not horizontal, and may have a maximum range of fluctuation of 14 meters. The most probable position of the WOC, determined by statistical calculation methods, varies within 1649 m, and not 1650–1652 m, as was assumed at the first stage of exploration work.

Analysis of the areal distribution of the WOC revealed a regular decrease in the depth of occurrence of the WOC toward the crestal parts of positive structures. Thus, of 53 wells having low depths of WOC occurrence (1652–1654 m), 52 wells are located in the uplifted parts of structures with an absolute mark of occurrence of the top of the horizon within 1605–1615 m, and only well No. 69 had a lower top mark—1624 m, but it too is in the near-crestal part of the structure. With increasing distance from the crest of the structures toward the flanks, a regular rise of the WOC is observed up to the highest marks—1640 m in well No. 51, which is located on the northwestern flank of the field. The features of the areal distribution of the WOC are confirmed by testing of flank wells on the southeastern flank of the field, where wells No. 45 and No. 8, at corresponding depths of occurrence of the top (1644 and 1649 m), yielded formation water during testing even without an oil film.

The investigations carried out established:

1. The WOC of horizon A-1 of the Sovetskoye field is not horizontal, but has a complex character determined by the structural-tectonic configuration of the horizon.

2. The maximum difference in depths of WOC occurrence in the crestal and flank parts of the field is 14 meters.

3. The WOC of the horizon is raised toward the flanks of the field and lowered in the crest of the structures.

4. The most probable depth of WOC occurrence, when the entire field area is drilled out, will be 1649 m.

REFERENCES

1. F. Ya. Borkun, Yu. L. Brylkin, E. I. Leontiev, V. I. Romenko. Determination of the nature of saturation of clayey reservoirs in the central part of the West Siberian Lowland from production-geophysical data. Proceedings of SNIIGGiMS, issue 93. Novosibirsk, 1970.