ON THE ROLE OF A DEEP FAULT IN POSSIBLE OIL AND GAS ACCUMULATION IN THE NORTHEAST OF THE WEST SIBERIAN LOWLAND

S. N. Gulyaev

(Presented by the scientific seminar of the Department of Combustible Minerals)

For petroleum geology, faults in the basement of plates are of great interest, since they often control the position and determine the formation of local uplifts in the sedimentary cover. The geophysical indicators of deep faults here are usually zones of increased gravity gradients (zones of crowding of isoanomalies), and sometimes also extended linear belts or “chains” of positive magnetic anomalies.

Many investigators have expressed views on the presence of regional faults in the basement of the northeastern part of the West Siberian Plate [1, 3, 4].

D. V. Levin and S. M. Kryukov [4] note, at the southern margin of the Ust-Yenisei depression, the Malokhetsk belt of positive magnetic anomalies, which can be traced for hundreds of kilometers and is produced, in their opinion, by basic and ultrabasic rocks intruded along fault zones.

V. A. Dedeev, V. D. Nalivkin, T. N. Simonenko, V. N. Sokolov, and N. V. Shablinskaya [3], in the tectonic scheme proposed by them, show a deep fault coinciding in its position with the Malokhetsk belt of positive magnetic anomalies.

V. S. Golubkov [1] distinguishes the Malokhetsk–Turukhansk deep fault, which is displaced somewhat from the Malokhetsk fault zone noted by D. V. Levin and S. M. Kryukov.

Considering and comparing the elements of geophysical fields characteristic of deep faults in the northeastern part of the West Siberian Lowland (Fig. 1), we establish that the Malokhetsk belt of positive magnetic anomalies extends southward from the Arctic Circle, while zones of increased gravity gradients either directly adjoin this belt or are observed in its separate segments.

It should also be noted that on a map (compiled by the Taimyr Geophysical Expedition), with the effect of gravity of the sedimentary sequence of the Meso-Cenozoic cover removed, along the presumed deep fault one can trace not separate, mutually disconnected segments, but a single, continuous zone of increased gravity gradients, which in its position also agrees with the Malokhetsk belt of magnetic anomalies.

Thus, the geophysical data cited confirm the assumptions regarding the presence in the northeast of the West Siberian Lowland of—

...of the Malokheta–Turukhan deep fault, and the material obtained as a result of drilling and seismic prospecting makes it possible to be still more convinced of this.

Because the territory under consideration has been poorly studied, it is not yet possible everywhere to trace how the Malokheta–Turukhan deep fault is reflected in the Mesozoic sedimentary cover, although in some of its segments (in the basin of the Turukhan River, in the lower reaches of the Bolshaya and Malaya Kheta rivers, and in the upper reaches of the Pyasina River) it has already been established that the zone of the deep fault is associated with (Fig. 1)

Fig. 1. Schematic comparison of elements of geophysical fields characteristic of deep faults in the northeast of the West Siberian Lowland.
1 — areas with positive magnetic anomalies;
2 — zones of increased gravity;
3 — positive Mesozoic structures: I — the Iral’e uplift, II — the East-Sovetskoe uplift, III — the Malokheta swell, IV — the Rassokha swell;
4 — zone of the Malokheta–Turukhan deep fault;
5 — boundary of the West Siberian Lowland.

the East-Sovetskoe and Iral’e local uplifts, and the Malokheta and Rassokha swells.

Of these structures, the best studied is the Malokheta swell, which is a long (more than 150 km), clearly expressed anticlinal structure, complicated by local dome-shaped uplifts and numerous ...

disjunctives. Disjunctive disturbances with displacement amplitudes from 50 to 830 meters here break the pre-Jurassic basement, and sometimes the Mesozoic cover as well, into blocks. The presence here of numerous faults, often cutting Cretaceous deposits, the considerable amplitudes of these disturbances, and the relatively great contrast of the Malokhetsky swell distinguish it from many positive structures of the second order of the West Siberian Plate and indicate increased mobility of individual segments of the deep fault in Meso-Cenozoic time.

In the composition of the pre-Jurassic basement, represented on the Malokhetsky swell by a tuffaceous-argillite sequence of the Lower Triassic and by Cambro-Silurian carbonates, the presence of basic intrusions and effusions attracts attention. The presence here, in the Silurian deposits, of effusives and tuffs, which are entirely absent in the Silurian of the northwestern part of the Siberian Platform, once again confirms that the Malokhetsky swell, with which the elements of the geophysical fields considered above coincide in plan, is confined to a deep fault that manifested its activity not only in Hercynian but also in Caledonian time.

There are as yet no data on the existence of a deep fault in the Lower Paleozoic and in the Precambrian. But if one takes into account that many investigators directly or indirectly indicate that approximately along the belt of the Malokhetsko-Turukhansk fault in the Late Precambrian there passed a peculiar boundary between different structural-facies zones [3], then it may be assumed that it was initiated in Proterozoic time.

All the data considered above show that the Malokhetsko-Turukhansk deep fault, extending for hundreds of kilometers and having a zone 20–30 km wide, developed over a long period and in stages over the course of several eras.

For assessing the prospects for oil and gas potential in the northeast of the West Siberian Lowland, the history of development of the deep fault is of greatest interest, especially the periods of intensified tectonic activity in its zone in Meso-Cenozoic time. In the Meso-Cenozoic, in the northeast of the West Siberian Lowland, there were apparently three periods of intensified tectonic activity and growth of local structures. Significant movements of individual basement blocks in the zone of the Malokhetsky swell occurred in Upper Jurassic, Late Cretaceous [6], and, probably, in Albian–Cenomanian time.

Intensive movements in the zone of the deep fault in the Upper Jurassic are indicated by a large (amplitude 830 m) pre-Cretaceous thrust fault on the southeastern limb of the Malokhetsky swell, and by the sharp reduction in thickness, and sometimes complete disappearance from the section due to erosion, of the Upper Jurassic sequence in its crest part.

Tectonic movements in Late Cretaceous time led to a deep erosional truncation of Mesozoic deposits in the near-margin part of the West Siberian Lowland, and in the crest part of the Malokhetsky swell caused the erosion of Cretaceous deposits with a thickness of more than 1000 meters [6].

The intensification of tectonic activity and the growth of local structures at the end of the Albian—beginning of Cenomanian time are indicated by the overlap of the Dolgan Formation (upper Albian–Cenomanian) onto the Upper Jurassic on the Farkovo uplift, the sharp reduction due to erosion of Aptian–Albian deposits in the crest part of the Ermakovo uplift [2], and, probably, the fact that disjunctive tectonics on the Malokhetsky local uplift manifested itself after deposition of the Aptian–Albian.

Periods of intensive growth of local structures in the northeast of the West Siberian Lowland in Meso-Cenozoic time are undoubtedly connected with the intensification of tectonic movements and the general uplift of the western part of the Siberian Platform. This is evidenced by the increase in the contrast of positive structures of the zone of the Malokheta–Turukhan fault in the direction of the Siberian Platform.

V. N. Saks [7] notes the increased uplift and erosion of the Siberian Platform in Late Jurassic and Albian–Cenomanian time, while V. D. Nakaryakov [5] points to the connection between intensified growth of positive structures during these periods and in Late Tertiary time in the northeast of the West Siberian Lowland with tectonic movements within the Siberian Platform.

Thus, the activation of tectonic movements in Meso-Cenozoic time in the northeast of the West Siberian Lowland reflects the intensification of oscillatory movements within the Siberian Platform; however, the degree of its intensity in particular parts of the lowland is determined not only by their distance from the Siberian Platform, but also by their association with an active or passive tectonic zone. Thus, tectonic movements were manifested most strongly in the zone of the Malokheta–Turukhan deep fault and in those of its sectors (the Malokheta and, apparently, the Rassokha arches) that are situated closer to the northwestern framing of the Siberian Platform.

The increased mobility of the Malokheta–Turukhan fault, genetically connected with the depth of its inception, explains the failures in the search for oil and gas fields within the Malokheta arch. Intensive tectonic movements in the crestal part of the Malokheta arch (the Malokheta, Funtusov, and Tochino local uplifts), which here led to a deep erosional truncation (down to the Hauterivian–Barremian and Valanginian) of the Mesozoic deposits and to the formation in the latter of numerous disjunctive disturbances and zones of fracturing, contributed to the destruction of hydrocarbon accumulations. This is now evidenced by numerous shows of combustible gas at the surface within the Malokheta arch, where the principal field of gas shows is confined to the Malokheta local uplift, the most disturbed by disjunctions. Also noteworthy is the fact that the most numerous and maximum (up to 12,000 m³/day) gas inflows from the Jurassic and Valanginian in the Malokheta local uplift were obtained in well 13-R, located on the most downthrown block (where even the Aptian–Albian has been preserved), which, owing to these circumstances, was under more favorable conditions for the formation and preservation here of gas accumulations in Cenozoic and, possibly, Late Cretaceous time.

However, the increased mobility of the zone of the Malokheta–Turukhan fault in Meso-Cenozoic time played not only a negative but also a positive role in the processes of possible formation here of oil and gas pools. First, it contributed to the formation in this zone of local uplifts—structural traps for oil and gas. Second, the considerable tectonic activity of this zone, having produced strong disruption of the Paleozoic rocks (where they participate in the structure of the basement) and periodically renewed macro- and microfracturing in them, facilitated the migration of oil and gas from them into the Mesozoic deposits.

The oil-and-gas potential of the Paleozoic and the migration of hydrocarbons from it into the overlying formations are indicated by numerous oil shows in the tuffaceous-argillaceous sequence of the Lower Triassic on the Malo-

Khet Val and in the tuff-lava complex of the Lower Triassic in the Norilsk region.

Thus, the zone of the Malokhet–Turukhan deep fault is of great interest for oil and gas exploration in those of its sectors where tectonic activity in Meso-Cenozoic time, contributing to the formation of anticlinal structures in the Mesozoic cover and to the migration of fluids from Paleozoic and Mesozoic deposits, did not reach the level at which hydrocarbon accumulations are destroyed. These promising sectors apparently include the southwestern continuation of the Malokhet Val (possibly the Semenov uplift), the Rassokhin Val, and the positive structures presumed to lie south of the Malokhet Val, the probability of finding which in the zone of the deep fault and in its immediate vicinity is comparatively high.

REFERENCES

1. V. S. Golubkov. On the cyclicity of the geological development of the northwestern Siberian Platform. Tr. NIIGA, vol. 133, 1963.

2. S. N. Gulyaev. On coal-bearing deposits in the Mesozoic section in the northeastern part of the West Siberian Lowland. Uchenye zapiski NIIGA, regional geology, issue 1, 1963.

3. V. A. Dedeev, V. D. Nalivkin, T. N. Simonenko, V. N. Sokolov, N. V. Shablinskaya. The structure of the pre–Middle Jurassic basement of the West Siberian Lowland in light of new data. Sovetskaya geologiya, No. 7, 1962.

4. D. V. Levin, S. M. Kryukov. A scheme of tectonic zoning of the basement of the Arctic part of the West Siberian Lowland according to aeromagnetic data. Tr. NIIGA, vol. 96, 1959.

5. V. D. Nakaryakov. Tectonics of the Mesozoic deposits of the basin of the Turukhan River. Tr. SNIIGGiMS, issue 14, 1961.

6. V. N. Saks, Z. Z. Ronkina. Jurassic and Cretaceous deposits of the Ust-Yenisei depression. Tr. NIIGA, vol. 90, 1957.

7. V. N. Saks, Z. Z. Ronkina. On possible unconformities of the Siberian Platform in the Mesozoic. Informats. byull. NIIGA, issue 7, 1958.