География и геология/1. Регионоведение

Doctor of science professor Konyukhov A.I.

Moscow State Lomonosov University, Russia

EUROPE AND OIL-AND-GAS BEARING BASINS

AROUND IT

Most of recent oil- and gas-bearing basins are incorporated in the group of five belts of oil-and-gas accumulation. They are confined to continent/ocean transition zones, which existed in the Phanerozoic [Konyukhov, 2009]. Three of them (Tethyan, Laurasian and Paleo-Ural) were situated around Europe and include continental margins of Europe in the two modern oceans, Atlantic and Arctic, as well as in the Paleo-Ural Ocean, that had existed in the Paleozoic.

In the Tethyan (Median) or arid belt of oil-and-gas accumulation, the largest basins are represented by the Persian Gulf, the Oman, the Sirte and the Western Desert basins at the Arabian-African region. The belt also includes basins confined at one time to active margins of the Tethys Ocean e.g. the Aquitanian, Adriatic, Austrtian, Ciscarpathian, East Caucasian, South Caspian and other basins. In the Persian Gulf basin, an example of carbonate-evaporite platform is provided by very thick pile of limestones, dolomites, anhydrites and other salts, which were accumulated with short intervals during nearly 300 million years beginning from the Permian up to the terminal Miocene. The base of this platform includes the Late Permian-Early Triassic Dalan and Kangan limestones and dolomites, which are separated the Dashtac and Nar evaporates beds. All these four members are united into the Huff formation, in which the largest HC gas pools were discovered. Evaporites that serve as regional seals for oil-and-gas pools at several levels of the Persian Gulf Basin sequence ensured preservation of the major portion of HC produced by black shale and marls of the Sargelu, Kashdumi, Pabdeh and other formations with high oil source potential.

The main HC reserves are also confines to carbonate platforms in several other basins of the Tethyan belt. For example, we can identify the lower (Upper Precambrian-Cambrian) and the upper (Cretaceous-Paleogene) evaporate-carbonate complexes in Oman basin. The Cambrian Ara formation is a carbonate-evaporite succession as thick as one thousand meters. These sediments were deposited along with organic-rich clayey carbonates that are an important source of the HCs, which generated in the Ghaba and Fahud Salt depressions. The Middle Cretaceous Shuaiba and Natih carbonates account for most of the oil production in the Fahud Salt subbasin. The Upper Cretaceous- Eocene carbonates and evaporates are also wide spread in the Sirte basin. Reservoir rocks range in age from Cretaceous to Eocene with siliciclastic reservoirs more common to the Cretaceous and carbonate collectors dominating the Paleogene.

Carbonate rocks serve as natural reservoirs in many basins of South Europe: East Caucasus, Aquitaine, Adriatic and others. The Late Cretaceous – Eocene carbonates, broken by most fractures, serve natural reservoirs in the Terek-Sungean ridges in East Precaucasus and in south, Albanian part of the Adriatic Basin. The most part of liquid and gaseous hydrocarbons (HC) that form oil-and-gas fields in South Europe petroliferous basins were generated black carbonate shale of Late Cretaceous (Albian-Cenomanian-Turonian) and Eocene ages, minor – organic rich carbonates of Late Triassic- Early Jurassic ages.

The Laurasian or boreal belt comprises, in addition to basins of the Atlantic and Arctic margin of North America (Scotian, Jeanne d’Arc, Alaskian and Beaufort Sea), the Norwegian-Greenland, North Sea and Barents Sea basins in the northern sector of Europe. Most of them were formed in the course of complicated multistage rifting that involved the northern sector of Pangea after completion of the Hercynian folding. Tectonic movements leading in the initiation of first riftogenic structures took place in a suture zone that separated the Greenland and Norwegian continental blocks. The great thickness of Triassic rocks (> 5 km) which fill up isolated rift depressions indicates that the early rifting stage was most intense precisely in the whole Laurasian belt. The second Middle Cimmerian (Middle Jurassic) phase of tectonic movements was initially manifested in the North Sea region. Then, these processes occupied the Voring basins and Barents Sea basins. The third, Late Cimmerian phase of tectonic activation was most intense in the Viking Graben and some depressions of the Barents Sea. The Laramian phase of tectonic movements, which occurred in the terminal Cretaceous-initial Paleogene, provoked breakup of the continental crust and opening of the North Atlantic and after that the opening of Arctic Ocean. These movements mostly affected the Voring basin and the western part of the Barents Sea Basin, where the formation of submarine fans was accompanied by the accumulation of volcanogenic sediments.

Deposition of huge volumes of sandy material transported into rift basins of the North Atlantic and Arctic margins promoted the formation of thick sequences of natural reservoirs, which were filled with HCs generated by both humic and sapropelic organic matter. The major petroliferous complexes mainly comprise deposits of riverine deltas and coastal alluvial plains, more rare of the submarine slopes: turbidites and debrites. Collectors are represented by sandstones and siltstones, seals are by clayey rocks. The HC-generation potential was primarily governed by black shale of Middle-Late Triassic and Late Jurassic (Kimmeridgian and Tithonian) ages [Konyukhov, 2010].

The basins in both global belts composed of depressions exhibiting different stages of evolution. Between them there are structures, inheriting from the rift, continental margin, collisional and foredeep stages. The geological history of largest ones is as long as several hundred million years. Taken together, they units with more than 50% of oil and gas reserves discovered to date in our planet.

The Paleo-Ural Ocean was elongated in the N – S direction along the eastern margin of the East European plate and included Pechora Sea, Timano-Pechorian, Volga-Ural and Precaspean basins, which were formed by rifting during the Late Precambrian. Last time the geophysical researches and drilling activities were concentrated in south part of Volga-Ural basin, in region of the Pugachev swell and the Buzuluk depression. In the base of sedimentary cover here lay siliciclastics of Early and Middle Devonian age. The different sediment types appear to correlate with the presence or absence of deep ruptures. Due to short marine transgression successions in the Late Eifelian time had led to change siliciclastics by carbonate deposits/ There was time of fast growing biostroms and small reefs, which now contain many oil-and-gas pools. The Givetian was the time, when a great river delta was formed in the Buzuluk depression. Evidences of it are different bodies of sandstones, mostly buried bars that serve natural reservoirs for HC-accumulations.

An emergent phase was certainly taking place by the Middle Fransian epoch, in which may record an initial stage of the margin's collision in Paleo-Ural Ocean. After that there was long phase of tectonic subsidence. Starting in the South, this event is documented in the more northern zones by syn-sedimentary growing of reefs and evaporite deposition in the large region, including Buzuluk, Perelube and other intra-platform depressions. The great Karachaganak reef was growing most quickly [Konyukhov et al., 1998]. During the Carboniferous and Early Permian a thick carbonate platform was formed not only in Volga-Ural Basin, but also in Timano-Pechora and Precaspean basins. The most famous source rock on the Paleozoic margin of the Russian plate is the black carbonate shale and siliceous deposits of the Domanic formation of Late Devonian age.

The history of the Precaspean basin properly is not yet fully understood, but may have been influenced by both the Late Precambrian-Early Paleozoic opening of south part of Paleo-Ural Ocean and the development of rifting zones in the basin. The nature of the facies suggests that the extensive carbonate platform was formed. In the eastern margin of this basin an extent carbonate units of Carboniferous age are underlain by Middle-Late Devonian siliciclastics, mostly sandstones, siltstones and conglomerates, A little or no sea influence there is apparent. Limestones (packstone and grainstone) that contain shells of the foraminifera, oysters and bioclasts of reef predominate among carbonates. Secondary dolomites play less important role as collectors. Thick Late Permian salt succession (3-4 thousand meters thickness) covers the most part of the Precaspean Basin. Along with limestone and dolomite the role of collector are playing sandstones and siltstones, while source rocks mainly represented by black shale and carbonate—clayey-siliceous deposits of Carboniferous – Permian age that were deposited in the condition of submarine slope [Konyukhov et al., 2006].

References:

1. Konyukhov A.I. Continental Margins: Global Belts of Oil and Gas Accumulation //

Lithology and mineral resources. 2009. V. 44. No 6. P. 563-583.

DOI 10.1134/S0024490211060083

2. Konyukhov A.I. Continental Margins: Global Belts of Oil and Gas Accumulation.

Laurasian Belt // Lithology and mineral resources. 2010. V. 45. No 2.

P. 151-170. DOI 10.1134/S0024490210020045

3. Konyukhov A.I., Frolov S.V., Tran Ahn Hao. Devonian History and Paleogeo-

graphy of the Northern Margin of the Precaspean Basin // EAGE 60-th

conference and technical exhibition. Germany. Leipzig. 1998. P. 534.

4. Konyukhov A.I., Baymagambetov B.K., Kan A.N. Eastern Border Zone of the

Precaspean Depression: Sedimentary Complexes and Environments of

Middle Carboniferous Time // Lithology and mineral resources. 2008. No 6.

P. 592-610.