NEW RESULTS OF A COMPREHENSIVE LITHOLOGICAL-FACIES AND BIOSTRATIGRAPHIC STUDY OF THE OSINSKY HORIZON OF THE LOWER CAMBRIAN IN THE SOUTH OF THE SIBERIAN PLATFORM (WELLS WEST YARAKTA 45, 361)

Abstract

Relevance. Geological prospecting work carried out recently throughout the Lena-Tunguska oil and gas province makes the study of the Osinskaya subformation carbonates especially relevant, since they allow establishing the patterns of hydrocarbon traps formation. The goal of the research is to study the axial and inclined part of the large Ichedinsko-Bolshetirsky barrier reef in the south of the Nepa-Botuobinsky anteclise. Objects: carbonate rocks of the Ichedinsko-Bolshetirskiy barrier reef, middle (Osinskaya) subformation of the Usolskaya suite of the Tommot stage of the Lower Cambrian, penetrated by wells no. 45 and 361 of the West Yarakta area. Methods: GIS complex, detailed lithological-sedimentological description of 90 and 33 m of core, respectively, sequence-stratigraphic analysis, petrographic analysis of 120 samples and the study of fossils sampled from West Yarakta area core. Results. The authors have identified seven lithological types of rocks, consistently alternating each other along the section. The accumulation of rocks of the Osinskaya stratum occurred in the following range of facial environments (in the direction from the sedimentation basin to the continent): upper sublittoral, littoral, supralittoral (sebkha). Karst was formed under subaerial conditions. The conducted biostratigraphic analysis confirms the Lower Cambrian age. In the well no. 45 the remains of calcareous algae: Renalcis polymorphum, Renalcis levis, Botomaella zelenovii, Epiphyton zonatum, Epiphyton celsum, Epiphyton scapulum, Chabakovia ramosa, Chabakovia nodosa, as well as small shell fauna Namacalathus sp., were discovered. In the well no. 361 calcareous algae Renalcis polymorphum, Renalcis granosus, Renalcis sp., Renalcis levis, Renalcis seriata, Korilophyton sp, Chabakovia sp., were found and SSF Namacalathus sp. was identified. In the section of the subformation, three fourth-order sequences were identified. The sequencing boundaries are fixed by subaerial hiatuses, represented in the core by sedimentation breccia, karst zones. The largest, in terms of amplitude and duration, lowering of sea level is confined to the border of the second and third sequences. Calcareous algae studied from borehole cores were the main reef builders of the Early Cambrian organogenic structures. These organisms are characterized by shallow depths of the littoral – sublittoral, which is in accordance with the lithological-facies study results.

Keywords

• cambrian
• osinskaya subformation
• eastern siberia
• archaeocyaths
• cyanobacteria
• organogenic structures
• sequence-stratigraphic analysis