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Over 50 oil and gas fields with total reserves of about 6 × 109 BOE have been discovered in the Timan-Pechora basin, one of the most important Russian frontiers. Almost all the sequence is productive, although major reserves are confined to two stratigraphic intervals beneath regional seals. Principal source rocks are the so-called Domanik facies, 25 to 150 m (82 to 492 ft) thick, represented by rapidly alternating black shales, chert, marls, and siliceous and organic limestones. Exclusively sapropelic organic matter averages 5 to 7% and reaches 20% or more. Soluble bitumen is very abundant (1 to 2 wt. %) and contains all the components characteristic of crudes: from light oils to heavy tars and typical high-molecular asphaltenes. These characteristics exi t even on the basin's periphery where Domanik facies are only marginally mature. In other areas, Domanik facies are mature; they are probably overmature in the deepest troughs. Outside the area covered by Domanik facies, pools and even significant shows are absent.
Deposition of this prominent facies began during the end of early Frasnian time in a wide stagnant sea that covered the eastern edge of the Russian platform. Beginning in the late Frasnian, shallow-water carbonate sedimentation resumed along the basin's edges and on uplifted blocks. Condensed Domanik deposits continued to form in the gradually deepening sea on the east side of the basin. Barrier reefs and clastic terraces that prograded basinward formed along the northern and western boundaries. The deep-water trough was finally filled by thick clastics at the end of early Tournaisian time.
The unusual composition of Domanik facies and their exceptional enrichment by sapropelic organic matter result in their peculiarities as petroleum source rocks. Lithology of the rocks, particularly the abundance of huge carbonate concretions fully or partly replaced by silica, suggests a long delay in lithification and the relative importance of the late diagenetic stage of oil generation. This explains the presence of immature oils in underlying Devonian clastics and their absence elsewhere in the sequence. On the other hand, Domanik facies, owing to significant silicification that trapped giant amounts of bitumen in the rocks, became a "natural repository" of oil during geologic history. This oil migrated because of fracturing, especially during stages of tectonic activity. Spatial istribution of oil types and deposits of solid bitumen in traps having different ages of formation, clearly shows predominance of pulse-like vertical migration. Migration of oil from the Domanik continued during late stages of geologic history along with block uplifting, cooling of the sedimentary cover, and absence of sedimentation. Thus, methods of applied geochemistry that invoke models of heating should not be applied to Domanik-type rocks and more geologic data are required to assess their role as oil sources.
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