Oil and Gas Potential of the Mesozoic in the Northeastern Part of the West Siberian Lowland

S. N. GULYAEV

(Presented by Prof. Dr. I. V. Lebedev)

In the last two or three years, the Northern Integrated Petroleum Exploration Expedition of the Krasnoyarsk Geological Administration has carried out a complex of geological and geophysical work in the northeastern part of the West Siberian Lowland, from the basin of the Turukhan River in the south to the basin of the Tanama River in the north. This work has made it possible, to a certain extent, to decipher the deep geological structure and to assess the prospects of its oil and gas potential.

It should be noted that this territory had previously been poorly studied. The area of the Ust-Yenisei port had been studied most fully, where drilling and geophysical work was carried out from 1936 to 1953; almost nothing was known about the Turukhan and Tanama regions. At present, the Turukhan region has been studied most fully: core profile drilling has been carried out, the Turukhan reference well and a number of deep wells on the Ermakovo uplift have been drilled, regional geophysical work has been conducted, as well as a small volume of areal seismic exploration.

In the Ust-Yenisei region, in 1960, a small amount of core drilling was carried out and seismic exploration work was begun, which by the present time has refined the understanding of the geological structure of the region.

In the Tanama region, a regional seismic profile was run from the settlement of Nosok, situated on the Yenisei River, northwestward to the Olen’i Roga tract, i.e., into the area of the Tanama uplift identified earlier.

The oldest of the Mesozoic sediments with which the prospects for oil and gas potential can be associated are the Lower Jurassic deposits, established as a result of deep drilling in the area of the Ust-Yenisei port. According to V. N. Saks (1), the formation of the Lower Jurassic sandy-clayey deposits in this area occurred on coastal-alluvial plains and under marine conditions.

Lower Jurassic deposits in the Turukhan region have not yet been precisely established.

In Middle Jurassic time, in the Turukhan region, apparently, continental conditions of sediment accumulation generally existed, as evidenced by the coarse bedding of sandstones and siltstones in the lower part of the section, the presence of coal interbeds in the rocks, the abundance of plant detritus, root remains, and imprints of flora. At the same time, the presence of foraminiferal fauna in the upper part of the Middle Jurassic deposits,

Sketch Map of the Northeastern Part of the West Siberian Lowland

[Map labels:]

• Tanama
• Bolshaya Kheta
• Yenisei
• Turukhan
• Karaul
• Ust-Port
• Dudinka
• Potapovo
• Igarka
• Turukhansk
• Vereshchagino

Legend

● — Uplifts:
1. Tanama
2. Malokheta
3. Ermakovo

✚ — Turukhansk reference well

very varied fine stratification, better sorting of the clastic material, the presence of pyrite and isolated grains of glauconite—all this indicates that by the end of Middle Jurassic time nearshore-marine conditions of sediment accumulation sometimes appeared. It should be noted that contemporaneous deposits of the area of the Ust-Yenisei port are more “marine” and were formed under marine and nearshore-lagoonal conditions. Taking into account that the sea penetrated into the West Siberian Lowland from the north, it may be assumed that in the area of the basin of the Tanama River the deposition of Middle Jurassic sediments took place under conditions very similar to those that prevailed in the area of Ust-Port.

In Late Jurassic time, in the northern part of the West Siberian Lowland, marine conditions of sediment accumulation prevailed everywhere, as is evidenced by finds of ammonite, belemnite, and foraminifer faunas.

In the Valanginian, marine conditions of sediment accumulation persisted both in the Turukhansk area and in more northern areas; moreover, in the Turukhansk area, at the beginning and middle of the Valanginian, sediment formation took place under shallower-water conditions than in the northern areas.

Toward the end of the Valanginian, lagoonal conditions of sediment accumulation were established everywhere; these continued to exist in the Hauterivian, but in the Barremian were replaced by nearshore-lagoonal facies.

The Aptian–Albian deposits in the Turukhansk area were formed under continental conditions and are characterized by an alternation of sandy and clayey rocks with interbeds of coal. The sandy rocks of this age are characterized by large-scale cross-bedding, caused by accumulations of plant detritus, as well as by the light-gray, whitish color of the rocks due to an admixture of kaolinite.

Farther north, toward the area of the Ust-Yenisei port, the continental facies of the Aptian–Albian are already replaced by lagoonal ones.

The Upper Cretaceous deposits, with the exception of the Danian stage, are represented by marine and nearshore-marine, predominantly sandy-silty facies.

The Danian deposits in all areas are characterized by continental facies and are represented in the Turukhansk area by light-gray sands with plant detritus and isolated inclusions of amber, and in the Ust-Yenisei area by variegated sands and siltstones.

As is evident from the foregoing, almost the entire Mesozoic section is characterized by nearshore-marine and nearshore-continental sediments—zones of transition from marine facies to freshwater coal-bearing facies, with which the main centers of oil and gas formation are usually associated.

In the northeastern part of the lowland, from the Turukhansk area in the south to the Tanama area in the north, favorable structural conditions exist for the accumulation of oil and gas.

As shown by the drilling of shallow boreholes on the profile along the Yenisei River from the village of Vereshchagino in the south to the settlement of Potapovo in the north, in the very marginal part of the West Siberian Lowland, on the eroded surface of the Siny-Paleozoic basement, which in its lithological characteristics is similar to contemporaneous deposits exposed within the western margin of the Siberian Platform, there occur Mesozoic complex sediments of various ages.

Thus, in the Turukhansk–Vereshchagino sector, the basement is apparently overlain by Middle Jurassic sandy-clayey coal-bearing deposits, which are traced farther north beyond the Ermakovo field station, where they are replaced by Upper Jurassic deposits. This is confirmed by borehole No. 6 of the Igarka profile, located 60 km south of the town of Igarka, in which

the occurrence on the basement of Upper Jurassic deposits has been established. Farther north, closer to the town of Igarka, the Mesozoic sediments seem to recede westward; here, beneath the Quaternary cover, wells have penetrated the basement.

In the Dudinka sector of the Yenisei profile, two wells located in the area of Potapovo station established the occurrence, on the Paleozoic basement, of Cretaceous sediments. It should be noted that of all 13 wells drilled along the profile of the Yenisei River, nine penetrated the basement; this indicates the shallow occurrence of the basement in the very near-border part of the West Siberian Lowland. Exceptions, as seismic data show, are the sectors Yakuty–Angutikha, near Yermakovo station and north of Luzino settlement, where the basement subsides to a considerable depth.

West of the Yenisei River and north of Ust-Port, there is a general subsidence of the pre-Jurassic basement and, correspondingly, of the Mesozoic sediments to great depth. The nature of the subsidence of the pre-Jurassic basement in a westerly direction has been determined only in the Turukhansk region, in the interfluve of the Yenisei—Turukhan—Taz rivers; moreover, as established by seismic exploration, the subsidence of the basement surface and, accordingly, of the Mesozoic deposits has a gentle step-like character.

Thus, from the area of the Yermakovo uplift, located 12 km northwest of Yermakovo settlement, westward over a distance of 16 km the basement subsides by 900–950 m, i.e., the angle of inclination of the basement surface to the west is 3°–3.5°.

Farther west, toward the Taz River, against the background of the general monoclinal subsidence of the basement surface and, correspondingly, of the Mesozoic deposits, two structural terraces are distinguished, separated from one another by a gentle flexure-like bend, the amplitude of this bend over a distance of 35 km being 1000 m. If the dip angle of the conventional reflecting horizon, and correspondingly of the Middle Jurassic rocks, in the sector of the flexure-like bend is about 2°, then in the sectors of the structural terraces it does not exceed 35′. The behavior of the contour lines of the basement surface indicates the presence in it of a projection of northeastern strike.

It should be noted that, in plan, the first structural terrace corresponds to a certain degree to a gravity minimum, with which some authors associate the presence within the basement body of a depression filled with Paleozoic deposits.

Thus, in a number of sectors confined to the first structural terrace, extended reflecting benches were recorded at depths of about 5000 m, which apparently are located within a Paleozoic complex of sediments.

Analysis of all geological and geophysical data for the region suggests that in plan each structural terrace corresponds to different types of blocks of the pre-Jurassic basement.

Thus, the first structural terrace apparently corresponds to a depression in the pre-Jurassic basement, filled with Paleozoic weakly dislocated deposits of platform type; and the second structural terrace, located west of Lake Sovetskoye to the Taz River, corresponds to more ancient and considerably dislocated rocks of Sineian–Proterozoic age. It should be noted that the thickness of the Meso-Cenozoic in the area of the second structural terrace reaches 4500–5000 m, i.e., the basement in this sector is submerged to great depth.

As was already stated earlier, the monoclinal subsidence of Mesozoic deposits westward in the regional plan is complicated by structural terraces and flexure-like bends.

Against the general background of monoclinal subsidence, a significant number of local anticlinal flexures have been established, in which the western limbs are somewhat steeper than the eastern ones; this is probably connected with the greater subsidence of the basement in the western direction. The dip angles of the limbs of these structures along the lower horizons reach 3°–4°, and the amplitude of the uplifts ranges from 30 to 100 m.

These anticlinal flexures are in part, possibly, uplifts of the brachyanticlinal type. At present, in various parts of the Turukhansk district five uplifts have been identified, whose trends are varied—meridional, northeastern, sublatitudinal, and northwestern.

It should be noted that, simultaneously with the subsidence of the Mesozoic deposits in a westerly direction, an increase in their thickness is observed, as well as the appearance of new sedimentary complexes both in the upper part of the section (Upper Cretaceous) and in the lower part (possibly Lower Jurassic). A particularly significant increase in the thickness of the sediments from east to west has been established in the Upper and Middle Jurassic (approximately by a factor of 2–2.5). Near the base of the Mesozoic deposits in some parts of the Turukhansk district, a discordance is observed between individual reflecting surfaces obtained by seismic exploration, with the lower surfaces having a greater angle of dip than the overlying ones.

Such behavior of the reflecting surfaces may indicate the possibility of wedging-out of beds. The wedging-out of Lower Jurassic deposits and of the lower horizons of the Middle Jurassic sediments as one approaches the marginal part of the lowland undoubtedly exists and has a regional character. This is evidenced not only by data obtained from seismic exploration, but also by the results of deep drilling and by KMV seismic-sounding data.

All this gives grounds to assume that in the Turukhansk district, along with structural traps, there are also stratigraphic-type traps associated with the wedging-out of individual stratigraphic horizons as one approaches the marginal part of the lowland.

One may also suppose that there is a slight wedging-out of the lower Mesozoic horizons on the western limbs of local structures in the direction toward the crests of the uplifts. True, this assumption still requires verification in the course of petroleum exploration work.

The territory of the northeastern part of the lowland between the areas of Ust-Yenisei Port and Turukhansk has been illuminated only slightly by investigations, and we shall not dwell on it.

The latest KMV seismic-exploration work along the Yenisei River confirms the presence of the Dolgan syncline north of the village of Sitkovo up to the Malokheta anticline, which had been established earlier in the area of Ust-Yenisei Port. In this area the Malokheta anticline is the largest and most thoroughly studied structure, extending for 150 km in a northeastern direction; in the crest of the anticline five dome-shaped uplifts are observed. Beneath Quaternary deposits on the Malokheta anticline, Cretaceous sediments lie everywhere—from Valanginian, in the most elevated part, to Turonian–Coniacian in the zones of subsidence. The dip angles on the limbs of the structure reach 5°–10° in the Cretaceous deposits and 10°–20° in the Jurassic. The anticline as a whole has a box-like form and is broken by numerous faults.

North of the Malokheta anticline, the base of the Mesozoic subsides to a depth of 4000–5000 m, and the Tanama district falls within the central part of the Ust-Yenisei depression. As seismic-exploration work along the route from the Nosok settlement, located on the left bank of the Yenisei River, to the Tanama uplift—that is, to the northwest—has shown, the thickness of the sedimentary cover in this area is no less than 4000–4500 m. From

from the Nosok settlement in the northwestern direction, a general rise of the Mesozoic deposits is observed, against the background of which two large anticlinal flexures, 20–30 km in length, with amplitudes of about 50 m, have been noted. The presence of these flexures may be linked with second-order structures, which, apparently, are typical of the central part of the Ust-Yenisei depression.

In addition to the large flexures, in the area from Nosok station to the Olenii Roga locality there are a number of small flexures, with which third-order structures may possibly be associated.

The presence of reservoirs and impermeable seals has been established for the Turukhansk and Ust-Yenisei regions, where deep drilling has been or is currently being carried out. Both in the Turukhansk region and in the Ust-Port area, in the upper and middle parts of the Upper Jurassic deposits (Volgian, partly Kimmeridgian), a rather thick clayey sequence is distinguished, which in the Turukhansk reference well reaches 150 m, and in well No. 10 on the Malokhetskaya anticline—400–500 m.

It should be taken into account that accumulation of this clayey sequence took place under typically marine conditions, which permits one to assume the regional character of its distribution throughout the entire northeastern part of the West Siberian Lowland; the Turonian clayey unit is also of interest, since it too may possibly have a regional distribution.

Apparently, individual clay beds in the Valanginian, Hauterivian, and Apt–Albian may also serve as good seals for fluids.

Significant inflows of formation water obtained during testing of deep wells on the Malokhetskaya area indicate the presence of well-permeable rocks in the Jurassic and Lower Cretaceous section.

The reservoir properties of the rocks of the Turukhansk region can so far be judged mainly from the results of determining porosity and permeability on core from the Yermakov deep wells and the Turukhansk reference well. The open porosity of Jurassic sandstones from these wells reaches 30%, and permeability 1137 millidarcies; moreover, upward in the section the permeability of the sandstones increases. It should be noted that the permeability of sandstones in the Jurassic and Lower Cretaceous deposits varies widely, from poorly permeable to very well permeable, but permeable sandstones are observed throughout the entire Mesozoic section.

The thickness of sandstone beds reaches 20–30 m.

Oil and gas shows in the Ust-Port area have been mentioned more than once by various authors. It should only be noted that the Valanginian deposits are especially rich in oil and gas shows; to a lesser degree, the rocks of the Hauterivian, Barremian, and Middle and Lower Jurassic are so. Of all 14 deep wells drilled on the Malokhetskaya area, only five (4-R, 7-R, 8-R, 9-R, 13-R) were tested for inflow of oil and gas. As a result of well testing, inflows of water saturated with gas (7-R, 9-R) and with an oil film (wells 9-R, 4-R, 13-R) were obtained. In well No. 13-R, from Valanginian sandstones in the interval 594.7–597.7 m, a gas inflow of up to 12,000 m³/day was obtained; and of the other 9 horizons tested in this well, 8 yielded various gas inflows, often with water. The gases in the Ust-Yenisei port area are hydrocarbon in composition, with methane content up to 98%, nitrogen up to 6%, and heavy hydrocarbons up to 5.5%. The groundwaters of the Ust-Yenisei region, of the sodium bicarbonate and chloride types, with mineralization of 5–12 g/l, are characterized by a significant chlorine content, are poor in sulfates, and contain microcomponents:

iodine, naphthenic acids, as well as dissolved methane and heavy hydrocarbons.

For the Turukhansk region, clear signs of oil and gas occurrences have not yet been established, with the exception of insignificant traces of light bitumens in the core of the Turukhansk parametric well in the interval 2633–2658 m in Middle Jurassic deposits.

However, as the first results of testing the Ermakovo wells show, a high gas saturation of formation waters is observed. Thus, according to the results of testing Middle Jurassic deposits (interval 1382–1362) in Ermakovo well 1-R, the gas factor was 1.58; moreover, the gas is of hydrocarbon composition: methane averages 96%, nitrogen up to 4%. The water obtained from this horizon is mineralized (15.6 g/l), of the calcium chloride type, and contains the microcomponents iodine, 2 mg/l, and bromine, 66 mg/l. Thus, the salt and gas composition of the groundwaters indicates stagnant conditions, which is of great importance in assessing prospects for oil and gas potential, since a stagnant water regime is a necessary condition for the formation and preservation of oil and gas fields; in addition, one should note the somewhat elevated value of methane elasticity in the formation water, which is 63 atm. All this makes it possible to assess positively the prospects of the Middle Jurassic deposits of the Turukhansk region with respect to oil and gas potential.

Thus, structural, facies, and gas-hydrochemical data make it possible to highly assess the oil and gas prospects of the entire northeastern part of the West Siberian Lowland.

REFERENCES

1. V. N. Saks, Z. Z. Ronkina. Jurassic and Cretaceous deposits of the Ust-Yenisei depression. Proceedings of NIIGA, vol. 90, Gostoptekhizdat, Moscow, 1957.