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Abstract:
The ca. 2.0-Ga-old, 600-m-thick upper Zaonezhskaya Formation near Lake Onega, NW Russia, contains unusually high concentrations of C-org (up to 98%), averaging around 25%. The formation contains an estimated 25 X 10(10) tonnes of organic carbon accumulated within an area of 9000 km(2). Organic material is represented by shungite, which forms a black, dense, amorphous or nanocrystalline mass consisting of C with traces of N, O, S, and H. Autochthonous shungite occurs as disseminated organic material (0.1-50% C-org) which, when mixed with migrated bitumen (now pyrobitumen), appears as coal-like seams and lenses of semilustrous and semimat layer-shungite rocks (oil shales, 50-75% C-org). The migrated bitumen (originally petroleum), represented by the lustrous vein- and layer-shungite, conformably fills interbedding spaces or cross-cutting joints and usually contains 80-98% C-org. The shungite-bearing rocks of the upper Zaonezhskaya Formation represent one of the most richest accumulations of organic material reported from the Palaeoproterozoic, and one of the geologically earliest stages of petroleum generation. The sediments of the Zaonezhskaya Formation were initially deposited in brackish water in a non-euxinic, lagoonal environment. The high C/S ratio (8-1000) with a zero intercept on the C-S cross-plot indicates that deposition occurred in sulphur-poor water. Intensive synchronous volcanism may have contributed to both the enhanced delivery of nutrients and elevated sedimentation rate, and eventually to the high degree of preservation of organic material. The integrated data suggest that the organic material has a biogenic origin, most likely algal or bacterial. The organic material suffered complex catagenetic and metamorphic alteration which is reflected in: (1) the four-modal distribution of C,, content (with maxima at 5, 30, 65 and 95%); (2) highly variable delta(13)C(org) (-45 parts per thousand to -17 parts per thousand); (3) bimodal distribution of delta(13)C(org) (with maxima at -28 and -39 parts per thousand); and (4) low H/C ratios (0.005-0.2). Abundant diagenetic carbonates associated with shungite rocks (delta(13)C(carb) = -5 to -26 parts per thousand) and the presence of pyrite (delta(34)S - 22 to +31 parts per thousand), reflects substantial loss of organic matter via bacterial reduction of sulphate during diagenesis. The shungite rocks are characterised by a further substantial loss (> 50%) of biologically produced organic material in the course of thermal maturation and by a depletion in C-12 (> 10 parts per thousand). The isotopic composition of carbonate concretions does not indicate the involvement of fermentative diagenesis. Conservative estimates give delta(13)C(org) of -34 parts per thousand as the best value of the initial biomass. Lustrous vein- and layer-shungite containing more than 80% C-org are considered to be allochthonous, migrated bitumen (originally petroleum). The semilustrous and semimat layer shungite rocks containing 55-75% C-org represent oil shales with both migrated bitumen (originally petroleum) and autochthonous kerogen residues. The oil source rocks were apparently hosted in the Zaonezhskaya Formation. The generated oil has migrated both vertically and laterally with the highest concentration in cupola structures. The locality at Shunga represents the most significant volume of trapped petroleum from the study area. (C) 1999 Elsevier Science B.V. All rights reserved.
