Relevance. Geological prospecting carried out recently throughout the Lena-Tunguska oil-and-gas province makes studies of carbonates of the Osinskaya subformation particularly relevant, as they make it possible to establish the patterns of hydrocarbon trap formation. Aim. To present the results of the lithological and facial, biostratigraphic and petrophysical studies of the Osinskaya subformation in the southern and central part of the Nepa-Botuoba anteclise. Objects. Carbonate rocks of the middle Osinskaya subformation of the Usolskiy Formation of the Lower Cambrian Tommotian Stage, studied from well cores. Methods. GIS complex, detailed lithological and sedimentological description of the core, sequential stratigraphic analysis, petrographic analysis of 300 samples and examination of paleontological remains taken from the core; analysis. Results. Based on these studies, three sedimentation zones were identified, traced, and described for the first time: deep-water, marginal, and shallow-water. The deep-water zone is characterized by uncompensated sedimentation, the presence of single reefs, and confinement to depressions and troughs. It was flanked by an edge zone of carbonate platform associated with elevated thicknesses of the subformation and distribution of reefs over the area. It was followed by a shallow-water zone, represented by medium and reduced thicknesses of the Osinskaya subformation, its sediments are composed of carbonate grained and clayey rocks with small single reefs. The localized marginal zone of the extended carbonate platform agrees with the previously identified Chambinsko-Altybsko-Mirninskaya and Verkhnetokhomsko-Katsko-Pilyudinskaya zones of distribution of reef-like organogenic structures. On the basis of the studied wells, the structure of three formations according to the lithologic-facial profile was described. The paper introduces the authors’ variant of conducting the boundaries of the members. It was revealed that in the deep-water zone the prospects of searching for reservoir rocks are associated with single carbonate structures, in the marginal zone with area bioherms, and in the shallow-water zone with detrital deposits and shallow bioherms. The paleontological finds are shown to be confined to the selected sedimentation zones and beds. It was determined that algae diagnosed in the boudstones (autochthonous) were involved in the formation of the second and third strata, the edge zone of the carbonate platform. Archaeocyaths were diagnosed in detrital carbonate rocks of the second pack (allochthonous) in well no. 7 and autochthonous in boudstones of well no. 2X. Cribriciates were diagnosed in the boudstones of the third member (autochthonous). Archaeocyaths, cribriciates, namacalatusses, and calcareous algae studied from the cores of wells West Yaractinian no. 45, 361, Bolshetyrskaya no. 7, 3X, 2X, 4X, 5X, 6X were major edificators of Early Cambrian organogenic structures (biostromes, bioherms, bioherms massifs, reef formations).

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 biostrati-graphic 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.

Ссылка для цитирования: Токарев Д.А., Плюснин А.В., Терлеев А.А. Новые результаты комплексного литолого-фациального и биостратиграфического изучения осинского горизонта нижнего Кембрия юга Cибирской платформы (скважины Западно-Ярактинские 45, 361) // Известия Томского политехнического университета. Инжиниринг георесурсов. – 2022. – Т. 333. – № 4. – С. 157-167. Актуальность. Геологоразведочные работы, проводимые в последнее время по всей Лено-Тунгусской нефтегазоносной провинции, делают исследования карбонатов осинской подсвиты особенно актуальными, поскольку они позволяют установить закономерности образования ловушек углеводородов. Цель исследования – изучение осевой и склонной части крупного Ичединско-Большетирского барьерного рифа на юге Непско-Ботуобинской антеклизы. Объекты: карбонатные породы Ичединско-Большетирского барьерного рифа, средней (осинской) подсвиты усольской свиты томмотского яруса нижнего кембрия, вскрытые скважинами № 45 и 361 Западно-Ярактинской площади. Методы: комплекс ГИС, детальное литолого-седиментологическое описание 90 и 33 м керна соответственно, секвенс-стратиграфический анализ, петрографический анализ 120 образцов и изучение палеонтологических остатков, отобранных из керна скважин Западно-Ярактинской площади. Результаты. Были выделены семь литологических типов пород, закономерно сменяющие друг друга по разрезу. Накопление пород осинского пласта происходило в следующем диапазоне фациальных обстановок (по направлению от бассейна осадконакопления к континенту): верхняя сублитораль, литораль, супралитораль (себха). Образование карста происходило в субаэральных условиях. Проведенный биостратиграфический анализ подтверждает нижнекембрийский возраст. В скважине № 45 обнаружены остатки известковых водорослей Renalcis polymorphum, Renalcis levis, Botomaella zelenovii, Epiphyton zonatum, Epiphyton celsum, Epiphyton scapulum, Chabakovia ramosa, Chabakovia nodosa, а также мелкораковинная фауна Namacalathus sp. В скважине № 361 были обнаружены и определены известковые водоросли Renalcis polymorphum, Renalcis granosus, Renalcis sp., Renalcis levis, Renalcis seriata, Korilophyton sp., Chabakovia sp. и мелкораковинная фауна (SSF) рода Namacalathus sp. В разрезе подсвиты выделены три секвенса четвертого порядка. Границы секвенса фиксируют субаэральные перерывы, представленные в керне седиментационной брекчией, зонами карста. Наибольшее по амплитуде и продолжительности понижение уровня моря приурочено к границе второго и третьего секвенса. Известковые водоросли, изученные из керна скважин, являлись основными рифостроителями раннекембрийских органогенных построек. Для этих организмов характерны небольшие глубины литорали – сублиторали, что хорошо согласуется с результатами проведенного литолого-фациального исследования.

The section of the Osa Subformation of the Usolka Formation of the Lower Cambrian Tommotian stage penetrated by the Bolshetirskaya 7 well in the south of the Nepa-Botuoba anteclise has been studied. As a result of lithological-facies investigations, seven lithological types of rocks were identified, they are regularly replaced by each other along the section. Facies environments of the reef complex were dеtected: bar bank, rear part of the reef, organogenic bioherm bildup, reef rear bars, reef buildup, core of reef buildup. In the section of the subformation, three fourth-order sequences are identified. The sequence boundaries are fixed by subaerial hiatuses represented in the core by sedimentation breccias, karst zones. The largest lowering of sea level, in terms of amplitude and duration, is confined to the border of the second and third sequences. The conducted biostratigraphic analysis confirms the Lower Cambrian age. Remains of calcareous algae Renalcis polymorphum, R. gelatinosum, R. granosum, Botomaella zelenovi were found. Small shell fauna of the genus of Namacalathus Grotzinger, Watt ers et Knoll. also was found. It has global distribution in the Lower Vendian (Ediacaran) deposits in Namibia, Brazil, Canada, Oman, Spain, China, Russia, etc.

Complex researches of the pyrite Early Cambrian Kyzyl-Tashtyg deposit (East Tuva) have allowed to reconstruct paleohydrothermal systems, to find out physical and chemical conditions of hydrothermal processes, and also to establish features of existence of ancient hydrothermal biota in the operating zone of the solutions, participating at all stages of formation of ore-bearing structures, since influence of postmagmatic fluids and finishing low temperature hydrotherms. The representative data on biota has been received at studying of the objects, which were in a zone of influence of rather low temperature hydrothermal systems. In one cases findings of microorganisms are dated for basalt complexes, containing ore body, and in other cases the found fauna is connected with ferruginous-siliceous sediments. It has been found out that microorganisms in the tonsil cavities of basalts evolved at the temperatures nearby 110-140°С under the influence of hydrothermal solutions. Much lower temperatures (to 100 °С) existed at development of micro-fossils in the quartz-hematite hydrothermal constructions of Kyzyl-Tashtyg deposit.

The lower Ust'-Kundat Formation has been studied in detail in a section along the Kiya River (Kuznetsk Alatau, Altai-Sayan folded area). The section was deposited during the Tommotian stage and is a stratotype for the Lower Cambrian Ust'-Kundat sequence comprising several members of the Ust'-Kundat Formation. Member 2 contains newly discovered and previously known species of archaeocyathids near its top found stratigraphically lower than the oldest known archaeocyathan assemblage of the Nochoroicyathus mariinskii biozone. The found archaeocyaths cannot be dated precisely and may have either Tommotian or Atdabanian ages. Small shelly fossils from the same section cover a large stratigraphic range as well. The findings call for updating the age of the upper Ust'-Kundat sequence and the Tommotian upper boundary in the regional stratigraphy of the Altai-Sayan folded area. (c) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

Fossiliferous Upper Vendian strata are discovered in the Upper Proterozoic to Lower Paleozoic Fore-Yenisei sedimentary basin under a thick Mesozoic-Cenozoic cover in southeastern West Siberia. Two depositional systems are recognized based on sedimentological features: (1) wave-and current-agitated shoreface-forereef-biohermal reef system (Vostok-3 Borehole section) and (2) tidal flat-evaporite basin (Averinskaya-150 Borehole section). The forereef facies yielded fossilized tubular calcareous skeletons of reef-building metazoans Cloudina riemkeae, Cloudina hartmannae and Cloudina carinata, phosphatized Namacalathus-like fossils, and a diversity of tubular phosphatized and agglutinated tubular fossils. The fossil assemblage can be interpreted as the evidence of ecological complexity of the reef system. Paleontological characteristics suggest correlation of the Vendian strata with the lowermost Purella antiqua Assemblage Zone and the boundary interval with the underlying Anabarites trisulcatus Assemblage Zone of the Siberian Platform. Therefore, at least in the late Proterozoic, the Fore-Yenisei sedimentary basin was part of a larger pericratonic depositional system on the western margin of the Siberian Craton. (C) 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.

In this paper we discuss the mineralization features of Vendian-Early Paleozoic algae, preserved in several varieties: silicified, organic-walled, calcified, and sometimes partially phosphatized. Amongst these, the calcified taxa of the Chlorophyta and Rhodophyta divisions were dominant. Calcareous algae were subjected to mineralization twice: the primary structure arose during their lifetime, while the second carbonate cover formed as a result of calcium deposition on the dead thalli by bacterial-cyanobacterial associations acting as catalysts. In cases when algal fossils are found in localities near ore deposits, their porous carbonate envelope can change color depending on the prevalent chemical elements in the solution.

Study of Early Cambrian sedimentary-volcanogenic complexes hosting the Kyzyl–Tashtyg pyrite deposit in Eastern Tuva has revealed cyanobacteria and algae that existed in zones of ore-forming hydrothermal systems similar in characteristics to present-day “black smokers” at ocean bottoms. Along with archaeocyaths and Cyanophyta from the host sedimentary rocks and microfossils from basalt amygdules, various cyanobacteria, monocyatheans, and sponge spicules have been found in ferrosiliceous deposits and metasedimentary rocks. Scanning electron microscopic, mineralogical, and thermobarogeochemical studies helped to reconstruct their hydrothermal environment.

In recent years on the territory of the South-Eastern part of West-Siberian geosyneclise the distribution of Vendian deposits was determined at significantly greater territory than it was considered before. In this regard, the study of the conditions of Vendian formations accumulation becomes relevant because of possible association of oil and gas deposits to them. The aim of the research is to study the peculiarities of lithological composition of deposits based on the study of the faunal remnants and fair determination of Vendian deposits existence, and to identify the features of distribution of certain lithological type of rocks by the area of their distribution using the method of paleogeographic reconstructions. As a result, and based on the earlier analysis of the tectonic structure of the South-Eastern part of West-Siberian geosyneclise the authors have determined the widespread propagation of sedimentary Dolomites and limestones of the Vendian period of pure carbonate without admixture of terrigenous material composition. They are traced not only within Vezdekhodny and Tiysky structural-facial areas, as it was known before, but on the territory of Nurolsky and obviously Kolpashevsky structural-facial areas as well. It allows making conclusion on the existence of carbonate platform on the vast territory with the center near the Vezdekhodnaya ground. Its deposits become potentially hydrocarbon bearing at wide development of secondary transformation. The corresponding complex of works could promote the discover of new oil and gas fields in the Precambrian carbonate rocks.