Karelian shungite - an indication of 2.0-Ga-old metamorphosed oil-shale and generation of petroleum: geology, lithology and geochemistry

• Published in: Earth-Science Reviews
• Main Authors: Melezhik, V., Fallick, A., Filippov, M., Larsen, O.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 1999
• Subjects: Onega; karelia*; karelian
• Online Access: http://hdl.handle.net/21.11116/0000-0005-4CC2-B; http://hdl.handle.net/21.11116/0000-0005-4CC4-9

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 ...