LITHOLOGY OF VENDIAN CARBONACEOUS PRODUCING HORIZONS IN THE CENTRAL PART OF THE NEPA-BOTUOBA ANTECLISE

I. V. Varaksina, E. M. Khabarov

The paper considers composition, structure, depositional environments and post-sedimentary alteration of carbonaceous deposits of the Erbogachen, Preobrazhenka, and Ust’-Kut producing horizons within the Vendian – Lower Cambrian petroleum play in the central part of the Nepa-Botuoba anteclise. The studied horizons are characterized by prevailing early diagenetic (practically synsedimentary) dolomites: stromatolithic (formed by columnar and laminithic stromatolites), granular, micritic-granular, micritic (micrite-siltite) and clayey. Granular (rarer micritic-granular) dolomites are divided into varieties (oolitic, lumpy, intraclastic etc.) depending on composition and size of grains. There are mudstones, often dolomitic and anhydritic ones. Siliciclastic sandstones and siltstones occur in the Vendian base. Carbonaceous and clayey rocks form clusters during various-scale shallowing cycles (mudstones and clayey dolomites to stromatolithic and granular dolomites). It has been shown that widespread occurrence of pores, caverns, and mycrostylolites can be found in the upper parts of sedimentary cycles, which were formed within ooliticpisolitic shallows or stromatolithic banks. Anhydrizing, halytizing, and rarer magnetizing have substantially reduced the void space, although these processes appeared variably. Intensive anhydrizing of the Erbogachan horizon’s rocks had practically destroyed reservoir in one of the studied sections.

Key words: Siberian Platform, Vendian, carbonaceous producing horizons. 

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