Structural characteristic and tectonic evolution analysis of West Siberian basin northern areas in the Mesozoic and the Cenozoic (by results of regional seismic profiles 27 and 32 interpretation)

Surikova E.

A.A. Trofimuk Institute of Petroleum Geology and Geophysics SB RAS, Novosibirsk, Russia

Surikovaekaterina@gmail.com

West Siberia is the largest gas-bearing province of the world. There are such unique gas giants, as Urengoi, Yamburg, Polar, Medvezhii, Bovanenkovo, Harasavei and other fields in this region. The main gas resources of this region are concentrated in apt-alb-senomanian unit and are controlled by high-peak anticlinal structures. The region is also rich with oil resources. Large oil pools are discovered on Southern Russian, Bovanenkovo, Urengoi, Vankor and other fields.

In spite of the fact that many hydrocarbon fields of the North of West Siberia are opened nearly half a century back questions of their formation still remain debatable. It concerns both genesis of hydrocarbons, and formation conditions of huge traps. This paper is about structural and tectonic aspects of this problem including creation of seismogeological model, studying of tectonic evolution of the region and establishment of large anticlinal structures – traps formation time in northern areas of the West Siberian petroleum province.

The basis of work was sublatitudinal No 27, 32 and submeridional No 109, 105 regional time sections of common depth point method and data of deep drilling of 70 wells located near lines of seismic sections. The pro section file No. 27 intersects Medvezhii, Urengoi, Russian and other fields. The section extent is 760 km. The section No. 32 located in 160 km to the North from a section No. 27, passes from Yamburg area in the West to the Yenisei folded belt in the east, crossing Yamburg, Yurkharovskii, Pyakyakhinskii and Vankor fields. The section extent is 440 km.

The interpretation of seismic data including stratification and correlation of the reflecting horizons, creation of depth seismogeological sections and paleosections was realized using W-Seis software developed in IPGG.

Seismic sequence characterization

In the north of the West Siberian petroleum province the Mesozoic and Cenozoic sequences consist of Triassic, Jurassic, Cretaceous and the Cenozoic deposits. There are four basic reflectors in Mesozoic - Cenozoic sequence which clearly correlate on cross-sections.

The reflector A is a bottom of Jurassic - a top of the Triassic.

A number of the steady reflecting horizons are detected below the reflector A in sections No 27 and 32. It allows concluding that there is a platform Paleozoic and the lower Triassic deposits of considerable thickness in this region.

The reflector B in most parts of West Siberia is associated with the Bazhenov formation and its analogs. It is the most reliable seismic marker in the territory of West Siberia. The horizon possesses high power level and dynamic expressiveness. f

The reflector M forms on the Koshai shale member lying at the top part of the Alymskaya formation. It is the least reliable seismic marker in Mesozoic part of sequence. The horizon is well traceable, strong, excellent-quality reflector, clearly defined on sections.

The reflector G is formed on the mudstone member of the Kuznetsovo formation. The horizon is steady, possesses high power level and, along with the B horizon, is the most reliable seismic marker in a Mesozoic part of sequence.

Features of all seismic regional markers are that all of them are represented by thickness-persistent transgressive shale members formed in tectonic stable conditions and spread over vast areas [1-4].

Tectonic evolution

In recent work method of thickness is used to reconstruct history of tectonic development.

Nature of distribution of thickness of seismogeological complexes served in the work as a basis for studying history of tectonic development of the research area and restoration of stages of formation of key positive structures. The technique of paleotektonic reconstruction based on the analysis of thicknesses was offered for the first time by N.S.Shatsky [5] and then developed by V. V. Belousov, R.G. Garetsky, V. B. Neumann, K.A.Mashkovich, N. N. Forsh, V.E.Hain, A.L.Yanshina and etc. [6-8].

The regional section No. 27 crosses a number of contrast positive tectonic structures such as the Yarudeiskii megauplift, both the Medvezhii and the Central Urengoi mesobar and the Chasel` inclined megabar. The Medvezhii mesobar includes the Medvezhii and Nydinsk fields.The Central Urengoi mesobar includes the Urengoi field. The Chasel` inclined megabar includes Russian field in the northern part and South-Russian in the southern part.

The analysis of wave characteristics shows that all these tectonic structures are “rootless”, i.e. there are no basement highs under sedimentary cover of these structures.

The analysis of paleosections on line No 27 allows to draw a conclusion that for the Triassic - Jurassic, the Neokomian, and the Apt-alb-senomanian development stages, i.e., at least throughout 150 million years, the research area monotonously plunged in east direction and any of contrast positive structures existing nowadays had not relative growth tendency.

Formation of these traps and resulting formation of unique fields is associated only with postsenomanian stage of development.

The regional section No. 32 passes on more plunged zone and its considerable part is located within Bolshekhetsky syneclise. The section crosses Yamburg and Vankor fields in the west and east correspondingly.

The analysis of wave characteristics allows to conclude that unlike objects considered before, Yamburg and Vankor fields are included in the range of structures created over basement highs («root structures»), being characterized by chaotic pattern on seismic section.

The analysis of modern time cross-section and a series of paleosections shows that the Yamburg raising had a relative growth tendency throughout long history from the Paleozoic till the Cenozoic and in each stage its amplitude increased in structural plans of more ancient horizons. It is obvious that formation of a huge senomanian pool was influenced by exclusively postsenomanian stage of development which predetermined raising formation in a relief of the Kuznetsov formation and sand layers of the PK group. However, the impulse for these vertical tectonic movements, apparently, was put in the Paleozoic.

In spite of the fact that the Vankor field is also a basement high, the history of its formation essentially differs.

The analysis of a modern time cross-section and a series of paleosections on line No. 32 shows that this block of the basement had not growth tendency since the late Triassic and up to the end Senomanian. At this time monocline existed here, which gradually flattened until the sediments deposited. Formation of this tectonic structure in a relief of all Mesozoic horizons, including in structural plans of Lower Kheta and Yakovlev reservoirs with which the main oil pools are connected, occurred only during postsenomanian time.

Conclusions

1. The analysis of wave characteristics allows assuming that rocks of the top part of the Paleozoic and the bottom Triassic are consisting of sedimentary platform deposits in the research area.

2. Throughout all Mesozoic and Cenozoic history the research area plunged in the northern direction that affected in increase of thickness of both the sedimentary cover and separate seismogeological sequences.

3. In the research territory large positive structures had different ways to develope:

• Medvezhii, Urengoi, Russian, South-Russian fields are associated with «rootless» structures, there are no basement highs under sedimentary cover of these structures. These raisings didn't develop throughout the Triassic, Jurassic and the most part of Cretaceous and were created only as a result of postsenomanian tectonic movements.

• The Yamburg field is associated with the structure created over a basement high which had a tendency to relative growth throughout all of the Mesozoic and Cenozoic.

• The Vankor field is associated with the basement high which had not a growth tendency throughout the most part of the Mesozoic. Formation of this raising in structural plans of all Mesozoic markers is also connected only with postsenomanian tectonic processes.

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