THE ROLE OF DISJUNCTIVE FAULTS IN THE DISTRIBUTION, FORMATION, AND DESTRUCTION OF GAS POOLS ON THE MALOKHETSKY SWELL

(NORTHEASTERN WEST SIBERIAN LOWLAND)

S. N. GULYAEV

(Presented by Professor A. V. Aksarin)

Disjunctive faults in the sedimentary cover of the West Siberian Plate have been noted by many researchers [7, 8, 9]; however, their role in the formation and destruction of gas pools has not yet been clarified.

In the northeast of the West Siberian Lowland, the zone most complicated by disjunctives is confined to the Malokhetsky swell, which lies within the Malokhetsko–Turukhansky deep fault [4] and is a contrasting anticlinal structure, extending for 160 km in a northeastern direction, with an amplitude of more than 1000 m. Here the disjunctives emphasize the contours of the swell and cut or split many local uplifts (Sukhodudinskoye, Malokhetskoye, Semenovskoye, Zimneye, Tampeyskoye). These faults have been identified by seismic exploration, and some of them have been confirmed by drilling. Thus, correlation between closely spaced deep wells 4-r and 5-r, 7-r and 13-r on the Malokhetsky local uplift makes it possible to determine not only the dip but also the type of disjunctive. The absence from the section of the lower part of the Valanginian clayey sequence in well 5-r and of individual Lower Cretaceous horizons in well 7-r indicates the normal-fault character of the disturbances in the crestal part of the Malokhetsky local uplift. The largest reverse-fault-type disturbance here was intersected by well 6-r. This reverse fault complicates the southeastern flanks of the Malokhetsky and Tochinsky uplifts and the northeastern flank of the Sukhodudinsky local uplifts, and then continues farther northeast beyond the limits of the swell. The amplitude of the reverse fault in the area of the Malokhetsky local uplift exceeds 900 m [9], on the southeastern flank of the Tochinsky uplift it is 500 m, and northwest of the Sukhodudinsky structure it is only 150 m, i.e., it dies out in the northeastern direction. It should be noted that the rupture fault also bounds the most uplifted part of the Malokhetsky swell (the Malokhetskoye, Funtusovskoye, and Tochinskoye uplifts) from the northwest. The Malokhetsky local uplift is split by a system of normal faults into four blocks; moreover, in the most downthrown of them Aptian deposits have been preserved, while in the others Neocomian sediments have been preserved. The amplitude of the normal faults here is 50–150 m; in other parts of the swell, along the base of the Jurassic deposits, it ranges from 50 to 200 m and usually dies out upward in the section. Some faults can be traced only as far as the Valanginian, others partly penetrate into the Neocomian complex, and still others die out only in the Albian deposits. Higher in the section, the disjunctives usually are not traceable. This fact confirms the previously expressed supposition that disjunctive tectonics mani-

also occurred here in Albian–Cenomanian time [4], when an intensification of tectonic activity was observed in the northeast of the West Siberian Plate. In Neogene time, a general uplift of the territory under consideration was observed here, along with rejuvenation of local structures, disjunctive faults, and fracture zones.

The noted mobility of the Malokhetsky swell in Mesozoic–Cenozoic time explains the failures in prospecting for oil and gas fields over most of it. Intensive tectonic movements in the crest part of the Malokhetsky swell (the Tochinsky, Funtusovsky, Semenovsky, and Tampeisky uplifts), which led here to deep erosional truncation of Mesozoic deposits and to the formation in the latter of numerous faults and fracture zones, contributed to the destruction of hydrocarbon accumulations. At present this is evidenced by numerous occurrences of combustible gas at the surface within the most disjunctively disturbed Malokhetsky local uplift and near the faults bounding it (Fig. 3), although the latter have been identified by seismic prospecting only recently. Attention is drawn by the similarity in the composition of the gas from surface gas shows and from wells in the Mesozoic complex [11]. The confinement of oil shows and noncommercial gas accumulations only to the Malokhetsky local uplift, and their absence on the Tochinsky structure, which is hypsometrically higher but lies outside the zone broken by faults, permits the conclusion that the migration of oil and gas and the formation of noncommercial gas pools took place here after disjunctive tectonics, i.e., after Cenomanian time. Migration along disjunctive faults and fractures also partly occurred from Paleozoic formations, as indicated by films and drops of oil in the tuffogenic-argillite complex of the Lower Triassic in wells 6-r, 7-r, and 13-r on the Malokhetsky local uplift. Traces of vertical migration of hydrocarbons here are also oil shows, predominantly in the sandy Barremian sequence, where oil-saturated sands and sandstones are often not controlled by clayey beds and interlayers. The absence of gas pools on the Semenovsky local uplift, which is hypsometrically higher than the Zimnee gas field, together with the absence here of an Upper Jurassic seal [5], is also explained by the presence of a fault penetrating down to and including the Albian deposits. On this uplift only insignificant gas inflows were obtained, although in the section of the Jurassic complex here there are sandstone beds with direct indications of oil and gas potential.

However, on the Malokhetsky swell, alongside conducting disjunctive faults, there are also sealing ones. In the Zimnee gas field, whose total reserves do not exceed 8 billion m³, a fault penetrating down to the Valanginian is a seal for the Bathonian gas pool. The presence of a fault here is confirmed by the absence of gas in the Bathonian sandstones of wells 4-r and 5-r, which are hypsometrically higher than well 1-r, which penetrated the gas pool in the northern block (Figs. 1, 2). The possibility is not excluded that the overlying Valanginian gas pool formed through migration of gas along the fault from the Bathonian deposits of the southern downthrown block. Thus, A. I. Levorsen notes that oil and gas pools in traps formed by normal faults are almost always located in the upthrown wing of the fault. In his opinion, this is due to the fact that oil and gas escape upward along the dip around the edges of the fault [6].

The geological material considered above and the data on the distribution of oil and gas pools in other regions [1, 2, 3, 10] make it possible to draw the following conclusions:

1. Faults are often, during certain intervals of time, pathways for the vertical migration of gas. The most conductive are normal faults at shallow (down to 1000 m) depths, which is confirmed by the absence of significant accumulations of gas in sediments above shallowly occurring salt domes in the Pre-Caspian, Pre-Mexican, North German, and other basins of the USSR and foreign countries [1, 3].

Fig. 1. Structural map of the Zimnee uplift along the top of the Bathonian productive bed (compiled by S. N. Gulyaev).

1 — wells (numerator—the well number; denominator—the absolute elevation of the top of the Bathonian productive bed); 2 — contour lines along the top of the Middle Jurassic deposits; 3 — normal fault; 4 — outer boundary of gas saturation of the Bathonian pool

Fig. 2. Gas pools of the Bathonian and Valanginian deposits of the Zimnee field in section along line I—I (compiled by S. N. Gulyaev).

1 — gas-bearing sandstones; 2 — water-bearing sandstones; 3 — clays; 4 — siltstones; 5 — normal fault

Fig. 3. Schematic arrangement of surface gas shows on the Malokhetsky uplift.

1 — deep wells; 2 — contour lines along the base of the Jurassic deposits; 3 — disjunctives; 4 — zones of surface gas shows

1. Where there are numerous faults that cut the entire sedimentary complex up to the surface or to the Quaternary cover, large gas pools can almost never be encountered, and this should be taken into account when prospecting for gas fields. Exceptions to this may be areas (the Shebelinka field) with halogen rocks, whose plasticity sharply reduces the conductive role of disjunctives.

2. Disjunctives confined to the lower part of the sedimentary cover and

not affecting the overlying gas-bearing intervals, are a favorable factor for increasing the stratigraphic range of gas potential and for the formation of multi-reservoir gas fields.

REFERENCES

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