ON THE OIL AND GAS POTENTIAL OF UPPER CRETACEOUS SEDIMENTS IN THE NORTHEASTERN WEST SIBERIAN LOWLAND

S. N. Gulyaev

(Presented by Professor A. V. Aksarin)

Upper Cretaceous deposits in the north of the West Siberian Lowland contain the richest gas accumulations, which are confined to the sands and sandstones of the Cenomanian stage. Associated with them is the world’s largest gas field—the Urengoy field—and a number of other giant gas fields. The formation and distribution of the largest gas fields, along with the presence of reservoirs of high capacity and large local uplifts, are to a considerable extent also determined by the development here of a thick, 400–700 m Turonian–Paleogene clayey sequence, which is a high-quality seal.

In the northeast of the lowland, Paleogene sediments are almost everywhere absent; the entire section of Cretaceous deposits becomes considerably more sandy and is distinguished by a sharp predominance of sandy–silty rocks, which is especially characteristic of the southern part of the territory under consideration [1], and to a lesser degree of the northern part. In the Turukhan–Ermakovo area, in the Cretaceous deposits, conditions for oil generation were less favorable than for gas, but sufficiently thick and laterally persistent clayey members that could serve as reliable gas seals are absent. Here even the most clayey Turonian deposits are highly variable, both laterally and through time. Thus, whereas in the section of well 23 of the Turukhan profile a homogeneous clayey member 35 meters thick is recorded in the Turonian stage (Fig. 1), in the Turukhan reference well, located 70 kilometers west of it, a sand bed up to 7 meters thick appears in the middle part of this member, and its total thickness is 18 meters. Sanding and a decrease in the thickness of the Turonian clayey member are observed in the crest part of the Malokheta swell (Semenov and Zimnee uplifts) and in the marginal areas of the eastern part of the lowland. Moreover, here over a considerable area this member has been completely destroyed by erosion (Fig. 1).

Intensive upward tectonic movements in Neogene time led here to a deep erosional truncation of the Cretaceous deposits, the penetration into them of waters active in geochemical and microbiological respects, and enhanced dissipation of hydrocarbons. The great duration of the infiltration stage (the entire Neogene, partly the Anthropogene and, apparently, the Oligocene) intensified the processes of destruction and dissipation of hydrocarbons in the Upper Cretaceous sediments.

The factors noted, and the absence in the Mesozoic section of the central part of the Ust-Yenisei depression of clearly expressed local uplifts, reduce, in our opinion, the possible stratigraphic level of oil-and-gas potential even in areas with a sufficiently thick (more than 100 m) Upper Cretaceous clayey sequence, which may serve as a gas seal. And only on local uplifts lying along the path of jet migration is it possible to find gas pools beneath a less thick Upper Cretaceous clayey member and at a comparatively short distance from erosion windows [3]. Among such promising areas earlier was included the Messoyakha local uplift, where in 1967, at a depth of 815–880 m beneath the Turonian clay member 100 meters thick, a large pool with reserves of 44 billion m³ of gas was discovered in fine-grained Cenomanian sands and sandstones. The absolutely open gas flow rates from wells in the Messoyakha field range from 332 to 1142 thousand m³/day.

Fig. 1 — Diagram of isopachs of the Upper Cretaceous clayey sequence in the northeast of the West Siberian Lowland. 1 — isopachs; 2 — outcrops of pre-Quaternary sedimentary rocks; 3 — stratigraphic wells; 4 — exploration and reference wells (numerator — well number, denominator — thicknesses); 5 — Messoyakha gas field; 6 — boundary of the West Siberian Plate.

The discovery of the large Messoyakha field under less favorable geological conditions in comparison with other gas fields of the West Siberian Lowland, the presence in other oil-and-gas-bearing provinces of individual hydrocarbon fields in sequences with hydrogeological and geochemical conditions unfavorable for oil-and-gas potential, and confirmation by factual material of lateral and vertical jet migration even in fields not affected by disjunctive tectonics [4]—all this indicates the need, when substantiating the oil-and-gas prospects of marginal peripheral parts of depressions and troughs in general, and of adjacent areas of Krasnoyarsk Krai and Tomsk Oblast in particular, to use migration criteria.

REFERENCES

1. Gulyaev S. N. On coal-bearing deposits in the Mesozoic section of the northeastern part of the West Siberian Lowland. Scientific Notes of NIIGA, Regional Geology, issue 1, 1963.

2. Gulyaev S. N. Prospects for the oil-and-gas potential of the Mesozoic in the northeastern part of the West Siberian Lowland. Izv. TPI, vol. 127, issue 2, 1965.

3. Gulyaev S. N. On the gas potential of the Malokhetskii swell. Neft i Gaz, No. 4, 1967.

4. Dikenshtein G. Kh., Arzhevskii G. A., Stroganov V. P. The role of clayey seals in the formation of gas pools. Geologiya nefti i gaza, No. 3, 1965.