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Distribution of Pore and Mixed Pore-Fractured Reservoir Rocks in the Neocomian Deposits of Deep-Water Fans, Western Siberia

Michail D. Belonin and Vladimir V. Shimansky
VNIGRI, St. Petersburg, Russia

The Achimian highly prospective oil-gas complex occurs in the base of Neocomian clinoform deposits of Western Siberia. The Achimian reservoirs are represented by aleurosandy deposits formed by turbidite and grain flows.

The internal structure of the submarine fans is established. The proximal part of the turbidite system forms mainly by grain, very dense flows at a slope base, in the mouths of feeding channels. The pore type of reservoir rocks predominates in this zone. The middle part of the turbidite complex is formed by turbidite flows; it forms lobes and different embankments. This turbidite part is characterized by the A. Bouma succession. The distal part of the turbidite complex is an intermediate zone of filling between the lobes. The middle and distal parts of the turbidite system, composed of fine-grained sandstones and siltstones, are characterized by the presence of mixed pore-fractured reservoir rocks.

In the western part of the Neocomian basin which is at a considerable distance from a sediment source, a share of coarse-grained material decreases. The submarine fans are composed of thin-alternating siltstones, sandstones, and mudstones. Therefore, the clinoform complex deposits in the axial part of the Neocomian sedimentary basin are not considered to be prospective. The zones of developing mixed pore-fractured reservoirs formed as a result of irregular deformations of lithologically non-uniform rocks (thin-alternating silty sandstones, siltstones, and clays) in katagenetic compacting are established. A number of new hydrocarbon fields are discovered in these zones.