Silurian organic-rich sedimentary rocks – an example from Severnaya Zemlya of the Russian Arctic

The Silurian interval in the history of the Earth was characterized by widespread deposition of source rocks. They generate 9% of the world’s petroleum reserves (Klemme & Ulmishek 1991) and are known from many parts of the world in Europe, Africa, Asia, North and South America, and Australia. At present, 26 basins are characterized by the presence of Silurian organic-rich sediments, represented mainly by graptolite black shale and bituminous cephalopod (“Orthoceras”) limestone, preserving pelagic cephalopods, thin-shelled ostracods, bivalves, and conodonts. Of particular interest and detailed study so far is the early Silurian “hot” shale of the Middle East and North Africa (Lüning et al. 2000). However there is an apparent lack of data and uncertainties from the other areas of potential Silurian source rock distribution (e.g. some European countries, and the territory of the former Soviet Union).

In the Russian Arctic, black shale and bituminous limestone sequences, are known from Novaya Zemlya, Pai-Khoi, the north-western part of East Siberia, Taimyr, Severnaya Zemlya, the New Siberian Islands and Kolyma. In north-western parts of East Siberia, the basal Silurian black shale is believed to be one of the main Palaeozoic source rock (Zinchenko 1985). These rocks were used as a fuel, since they generally burn without any processing. Accumulation of black shale and bituminous cephalopod limestone in East Siberia took place on a passive continental shelf margin and deposition was initiated by an early Silurian transgression. Rapid sea-level rise led to the formation of an upwelling zone at the western margin (modern coordinates) of the basin, which caused nutrient-rich anoxic waters to emerge on the edge of the platform with a subsequent strong increase in the nektonic fauna.

We discuss the Silurian succession of Severnaya Zemlya, and our studies focus on the central part of October Revolution Island, where the early Silurian sequences contain the organic-rich black shale and bituminous limestone interval (Bogolepova et al. 2000). A number of studies have been undertaken to map the rocks, document the stratigraphy, establish and describe facies, reconstruct the palaeogeography, and identify the fossils. Using Rock-Eval Pyrolysis and other organic geochemistry methods (kerogen pyrolysis-gas chromatography, stable carbon isotopes, gas chromatography-mass spectrometry and solvent extract geochemistry) organic-rich black shale, black limestone and bitumen were analysed to obtain independent parameters on organic matter source, composition, thermal history and to investigate their possible oil-oil and oil-source correlation.

The occurrence of hydrocarbons in the Silurian of Severnaya Zemlya is clear evidence of an active petroleum system in the region. This information provides an important insight into the likely nature of successions and petroleum systems of the North Kara Shelf.