OIL POTENTIAL OF THE CONTACT ZONE BETWEEN THE SEDIMENTARY COVER AND THE PALEOZOIC BASEMENT OF TOMSK OBLAST

N. M. Babykov, E. E. Danenberg, A. L. Naumov,
V. N. Rostovtsev, Yu. V. Naydanov

(Presented by Professor A. V. Anasarin)

One of the problems in assessing the prospects for oil-and-gas potential in Tomsk Oblast is the question of the oil-and-gas potential of the Paleozoic and, in particular, of the zone of its contact with the deposits of the sedimentary cover. The contact zone in the territory under consideration is heterogeneous along strike and is represented mainly by three varieties: the basal horizon of the Tyumen Formation, and permeable and impermeable basement rocks of diverse material composition. At present, numerous oil-and-gas shows have been established in the permeable varieties of rocks in this part of the section.

Thus, within Tomsk Oblast, an oil pool in the Medvedevskaya area of the Sovetskoye oil field is associated with the basal horizon. Here, in well No. 6, from the interval 2734–2690 m, an inflow of oil with a rate of 88 m³/day was obtained through an 8-mm choke. Lithologically, the productive bed is represented by fractured, weathered calcareous rocks.

In the Chebachya area, in well No. 217, from the surface of the weathered basement, represented by fractured granites, an oil inflow of 17 m³/day was obtained through an 8-mm choke. A characteristic feature of the structure of this area is the absence, in the section of its crest, of deposits of the Vasyugan and Tyumen formations. The rock of the Bazhenov Formation lies directly on the erosional surface of the basement.

A number of other areas in Tomsk Oblast are also known where indications of oil-and-gas potential of the contact zones have been noted—Aleksandrovskaya, Nazinskaya, Cheremshanskaya, Myldzhinskaya, Severo-Vasyuganskaya, and others.

In connection with the above, the study of the reservoir properties of the contact zone is of practical interest, and consequently so are the material composition and structure of the weathering crust of the basement, as well as the distribution of the basal horizon.

In the territory under consideration, Paleozoic formations are represented by the most diverse types of rocks. Among sedimentary rocks, argillaceous shales and limestones are widespread. It is probable that a continuous field of distribution of argillaceous shales covers the entire southern part of the Pyl-Karaminsky megaswell. Farther west these rocks are less widespread, but here too areas with their predominant development are distinguished. In all likelihood, it is difficult to expect in these areas the presence of fractured zones on the basement surface and, therefore, ...

the development of a thick basal horizon with good reservoir properties.

Limestones, which are widely distributed on the Sredne-Vasyugan, Pudino, and Parabel megaswells and partly on the Lar-Egan swell, deserve considerable attention as fractured reservoirs. The intensity of fracturing in the upper part of the limestones is indicated by catastrophic losses of drilling mud at the Ust-Silga, Yuzhno-Myldzhino, Kilchino, and Chinzhar areas.

Of special interest in the search for oil and gas pools are Paleozoic igneous rocks. They are composed of effusive and intrusive varieties metamorphosed to varying degrees.

Of the effusive complex, basic rocks (diabases) and acidic rocks (quartz porphyries) are widely distributed. In areas of their development, volcanic tuffs are often encountered; from the standpoint of reservoir properties, these have a negative characteristic.

Of the intrusive complex, granitoids are the most widespread. The zone of their development encompasses the Aleksandrov arch, the southern part of the Tym depression, and partly the Parabel megaswell. In isolated cases they have also been penetrated beyond the limits of the indicated regions—for example, in the Koltogor trough at the Pionerskaya area.

Intrusive rocks participate in the formation of two types of reservoirs. The first is the granulometric reservoir of the basal horizon of the Tyumen Formation, for which the intrusive rocks supplied coarse-clastic material. The second type formed during weathering of the basement surface, creating zones of fractured reservoirs. A reservoir of the first type was encountered at the Yuzhno-Nazino area, where testing of the basal horizon in well No. 6 yielded an inflow of highly mineralized formation water (67 g/l) at a rate of 27.6 m³/day at a dynamic level of 750 m. A reservoir of the second type has been penetrated at the Chebachye field.

On the basis of the data set forth above, which indicate the presence of reservoirs in the contact zone, it may be assumed that if the reservoirs contain formation water, then under certain conditions they may represent potential traps for liquid and, still more, gaseous hydrocarbons.

The question of the origin of oil in the basal horizon and in the basement remains debatable. Some investigators (A. E. Kontorovich et al.) believe that the oil entered from the cover deposits of the Tyumen Formation; broad vertical migration along faults from magmatic sources is also admitted (N. A. Kudryavtsev).

On the basis of physicochemical characteristics, A. E. Kontorovich (1964) assigns the oils of the basal horizons and the weathered surface of the basement to an independent group: oils of the basal horizons. A distinctive feature of the physicochemical properties of these oils, as compared with oils of the Upper Jurassic horizons, is their very low content of naphthenic hydrocarbons and even their complete absence in the higher fractions, low specific gravity (0.7–0.8), low sulfur content (up to 0.3%), and high paraffin content (up to 20%).

At the same time, the similarity of these oils to oils obtained from the lower part of the deposits of the Tyumen Formation at the Vartovskaya and Kolpashevskaya areas is noted, i.e., a possible unity of their genesis. Proceeding from this, it may be assumed that the deposits of the Tyumen Formation in the depression zones served as the source of hydrocarbon generation for the contact zone. The products of this generation, as a result of lateral migration, filled the hypsometrically higher areas.

favorable reservoirs of the basal horizon and the weathered surface of the basement. This assumption is also based on the fact that all oil-and-gas shows established at present are confined to local structures situated directly both within positive structures of the first and second orders and on their flanks.

Thus, the basal horizon of the Tyumen Formation and the fractured zone of the Paleozoic basement, in light of the data on their oil-and-gas content, should constitute, in the territory of Tomsk Oblast, an independent target for the search for commercial accumulations of oil and gas.

REFERENCES

1. A. E. Kontorovich, O. F. Stasova, A. S. Fominchev. Oil of the basal horizons of the sedimentary cover of the West Siberian Lowland. Proceedings of SNIIGGiMS, issue 32, 1964.

2. A. E. Kontorovich, E. L. Verman et al. Geology of Jurassic and Lower Cretaceous deposits of the West Siberian Lowland. Proceedings of SNIIGGiMS, issue 36, 1971.

3. N. A. Kudryavtsev. Deep faults and oil fields. Leningrad, Gostoptekhizdat, 1963.