N.A. Krylov

 

Substantiation of transitional sequences in platform column

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DOI 10.31087/0016-7894-2019-5-45-59

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Principle of three basic structural levels identificati on in the vertical secti on of the platform Earth’s crust is substantiated; these levels are: basement, transitional sequence, and platform (plate) cover. Considerati on of both stratigraphic elements well known in the open regions and rock associati ons below the plate cover studied in recent years using deep‑hole drilling and seismics make a basis for identi fi cati on of parti cular geological features of the transitional sequence. Transitional sequences of young and ancient platforms are considered separately. Transitional sequence of ancient platforms includes protoplatform covers (covers of ancient consolidation massifs) and ancient aulacogens. Transitional sequence of young platforms includes massif covers, Middle‑Upper Palaeozoic Caledonian depressions, and graben‑like troughs (taphrogens). Transitional sequences are scattered between basement and sedimentary cover, they are composed of formati ons pertaining to orogenic and platform classes, include volcanic rocks and rare granitoid intrusions; main types of dislocation is block faulting. Duration of transitional stage of development exceeds the actual platform (plate) stage in both ancient and young platforms. Transitional sequence as a whole cannot be related to the structural level of platforms that is promising for oil and gas. High but variable degree of catagenesis and even metamorphism deprives the terrigenous rocks of the required reservoir properties (porosity and permeability); and the originally clayey rocks lose the properties of good impermeables. To this should be added a high degree of metamorphism of organic matter in the rocks and complexity of their architecture. At the same time, there are hydrocarbon fields already discovered in the rocks of the transitional sequence — in Western Siberia, North‑Eastern Caucasus, Central Kazakhstan (Qazaqstan), and other regions. Reserves of the discovered accumulati ons are not so large. Analysis of these discoveries suggests that probability of new accumulations discovery should be associated with three classes of transitional sequences, namely: aulacogens of ancient platforms, taphrogens and intermontane troughs of young platforms. Productivity of carbonate formations is most probable within these structures; the expected types of traps are combination (structural and strati graphic).

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Key words: transitional sequences; protoplatform; graben-type trough; protoplatform cover; block faulting; fault; intermontane trough; aulacogens; taphrogen; formation series.

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For citation: Krylov N.A. Substantiation of transitional sequences in platform column. Geologiya nefti i gaza. 2019;(5):45–59. DOI: 10.31087/0016‑7894‑2019‑5‑45‑59.

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N.A. Krylov   Scopus

Gazprom VNIIGAZ, Moscow oblast, Russia;

d_krylov@vniigaz.gazprom.ru;