INTERREGIONAL CORRELATION OF OIL- AND GAS-BEARING STRATA BY POSITION IN SEDIMENTATION CYCLES

S. N. Gulyaev

(Presented by Professor A. V. Aksarin)

For the Mesozoic deposits of the Siberian and West Siberian platforms, the confinement of regionally oil- and gas-bearing strata to the lower parts of transgressive and regressive complexes has long been noted (Gurari et al., 1967). However, this is also characteristic of other platforms, which makes it possible to correlate productive strata according to their position in sedimentation cycles.

The following terrigenous regionally oil- and gas-bearing strata are associated with the lower part of the transgressive complex: the Eifelian–Lower Frasnian in the Volga–Ural region, the Lower Permian, Middle Jurassic–Callovian, and Middle Jurassic–Oxfordian, respectively, on the West European, Turan, and West Siberian platforms, where they contain numerous oil and gas fields, including such major ones as Romashkino, Slochteren, Leman Bank, and Uzen. In the upper part of the transgressive complex, clayey and carbonate deposits predominate, and only in Western Europe are there halogen deposits (Zechstein), which form regionally persistent seals that screen the largest oil and gas accumulations.

The gas pools of the giant Hassi R’Mel and Rhourde Nouss fields in Algeria are likewise located in the lower part of the transgressive Triassic complex, unconformably overlying deposits of the Lower and Middle Paleozoic. Here the productive sandstones are separated by clay packets, which are zonal seals. The regional seal for the gas accumulations is a 500-meter sulfate–halogen Triassic sequence, which belongs to the upper part of the transgressive complex.

However, the richest in oil and gas is the Albian, or Albian–Cenomanian, terrigenous basal sequence of the Upper Cretaceous maximum transgression.

Albian sands and sandstones are regionally oil- and gas-bearing on the Scythian Platform, where they are overlain by Upper Albian clays and Upper Cretaceous carbonates.

The Turonian clayey unit, which is a zonal seal on the Bukhara stage of the Turan Platform, screens the largest gas pool in the underlying Cenomanian sands and sandstones at the Gazli field.

The unique and largest gas pools of the West Siberian Plate in sands and siltstones of the lower part of the Albian–Upper Cretaceous transgressive complex extend beneath a thick (400–600 m) Turonian–Senonian ...

with a clayey seal forming its upper part. It should be noted that the second transgression here most likely began in Albian time, since in the Turukhan River basin the deposits of the Upper Albian–Cenomanian Makovskaya Formation transgressively overlap Jurassic and Cretaceous sediments of different ages.

On the Arabian Platform, in Kuwait and Saudi Arabia, unique oil accumulations in Albian–Cenomanian sands and sandstones at the giant fields of Greater Burgan, Raudhatain, Wafra, and Safaniya-Khafji are associated with the same transgressive sediments, which lie with a regional unconformity on Aptian Shuʿaiba limestones (Kalinko, 1969). The productive interval, represented here in its lower part by coarse- and medium-grained, and in its upper part by fine-grained sands and sandstones with interbeds of clays and siltstones, is reliably sealed by clays and clayey limestones of the Ahmadi and Magwa formations of Cenomanian–Turonian age.

On the North African plate in Libya, the largest oil pool of the Sarir field occurs in Albian–Cenomanian sandstones lying directly on the basement. Above them lie variegated clayey shales with interbeds of anhydrites and sandstones, which upward are replaced by marine clays and limestones of the Upper Cretaceous (Varentsov et al., 1969).

The facts cited from different regions indicate a clear relationship between the largest fields and their position in the lower part of the transgressive Albian–Cenomanian complex, the upper part of which, composed of clays and sometimes also carbonates, often serves as a regional seal for numerous oil and gas pools. Thus, interregional correlation makes it possible to explain the at first glance surprising confinement of the richest oil and gas accumulations to Albian–Cenomanian terrigenous sediments on different platforms.

Oil- and gas-bearing strata located in the lower part of a regressive complex of sediments are correlated with one another not only on platforms, but also in marginal troughs; in the latter they are represented by carbonates and are overlain by anhydrites and rock salt. Beneath regional sulfate–halogen seals formed in the regressive part of the sedimentation cycle lie numerous oil and gas pools in the Mesopotamian and Cis-Ural marginal troughs. The salt-bearing sequence of the Lower Fars in Iran screens the largest oil fields in the underlying Asmari limestones of Oligocene–Lower Miocene age. The sulfate–halogen sequence of the Kungurian controls the distribution of gas and oil fields in Lower Permian carbonates in the Cis-Ural marginal trough and in adjacent areas of the Russian Platform. The oil- and gas-bearing Permo-Carboniferous carbonates, lying beneath thick (100–1200 m) sulfate–halogen sequences in the lower part of the regressive complex, are correlated with one another on the North American (the Panhandle-Hugoton field) and Russian (the Orenburg field) platforms and in the Cis-Ural marginal trough, where the Vuktyl gas-condensate field is the largest. The presence of a laterally persistent permeable interval in the Permo-Carboniferous at the Vuktyl and Orenburg fields is explained to some extent by the position of the former in the marginal trough, and of the latter next to it, where increased tectonic mobility caused fracturing of the carbonates. However, the enormous (200 × 56 km) lithologic Hugoton pool in Lower Permian dolomites and limestones, confined to the gentle (1–1.5°) monocline of the western flank of the Dodge City basin (Bakirov, 1959), shows that tectonic factors were not the only ones decisive in the formation of unique reservoirs in Upper Paleozoic carbonates. Evidently, from that time onward for

formation of extensive thicknesses of porous and permeable carbonates in arid zones, favorable conditions arose which, in our opinion, are determined by a considerable decrease in carbon dioxide in the atmosphere.

In the eastern marginal part of the Arabian Platform, the giant oil fields Ghawar, Abqaiq, Manifa, Khurais, Lulu, Umm Shaif, and Murban in Saudi Arabia, Kuwait, and Abu Dhabi are associated with Kimmeridgian and Neocomian carbonates lying above and below the sulfate-halogen sequence (the lower part of the regressive complex).

Thus, correlation of the regionally oil-and-gas-bearing carbonates that contain the largest and giant accumulations of hydrocarbons shows that all of them formed in arid zones at the beginning of a marine regression.

Within the Turanian and West Siberian platforms, analogous in position within the sedimentation cycle are the Neocomian–Tithonian sequence in the first case and the Neocomian or Valanginian sequence in the second. Numerous oil and gas fields are confined to these sequences, which are composed (with the exception of the southeastern part of the Turanian Plate) predominantly of terrigenous sediments; among them are such major fields as Shatlyk, Samotlor, and Mamontovo.

The interregional correlation carried out for oil-and-gas-bearing strata by their position in sedimentation cycles shows that the confinement of the principal accumulations of hydrocarbons to definite stratigraphic subdivisions (Lower Permian, Middle Jurassic–Callovian, Neocomian, Albian–Cenomanian) is not accidental and is determined by the position of the sediments below and above them that correspond to the maximum of transgressions. This regularity is due to the optimal ratio here of oil-and-gas source deposits, reservoirs, and seals. Taking this into account, one may confidently assume the presence of large accumulations of oil and gas in the southern and central areas of the Caspian Depression above deeply submerged salt domes, in Middle Jurassic–Callovian sediments located here, as on the Turanian and West Siberian platforms, in the lower part of the transgressive complex.

A clear correlation between the principal oil-and-gas-bearing carbonate strata and arid zones indicates the great role of paleogeographic criteria in evaluating the prospects for oil-and-gas potential.

REFERENCES

1. A. A. Bakirov. Oil-and-Gas-Bearing Regions of North and South America. Gostoptekhizdat, 1959.

2. M. I. Varentsov, G. E. Ryabukhin, E. R. Alieva. New data on the oil-and-gas potential of Libya. Geology of Oil and Gas, 1969, No. 6.

3. F. G. Gurari, A. E. Kontorovich, K. I. Mikulenko, Yu. K. Mironov, G. B. Ostryi, G. M. Taruts. Some regularities in the distribution and conditions of formation of oil and gas pools in the Mesozoic deposits of the West Siberian Lowland and the Siberian Platform. Proceedings of the West Siberian Research Institute of Geology, Geophysics, and Mineral Resources, issue 3, 1967.

4. M. K. Kalinko. Oil-and-Gas Potential of the World’s Offshore Areas. Nedra, 1969.