Petrography and geochemical affinities of Spitsbergen Paleocene coals, Norway

Abstract

The petrography and geochemistry of the two main Paleocene coal seams mined at Spitsbergen have been investigated through detailed analyses of more than 350 samples across the coalfield. The results are used for seam identification during exploration, for production planning and for quality assurance. Geochemical compositions of duplicate samples were determined by Proton Induced X-ray Emission (PIXE) and by conventional techniques. Every dataset provides comparable qualitative results. Statistical evaluation of analytical results indicates that the distribution of three element-groups, (S + Fe), (Ca + Sr) and (Si + Al + Ti) describes nearly all variations within the dataset qualitatively. Electron microprobe investigations of fusinite identified some cells containing strontium and phosphorous, whereas other cells contain calcium, magnesium and iron. In contrast, mineral grains containing an association of phosphorous, calcium and strontium were also identified. The lower Svea seam, close to the Cretaceous/Tertiary boundary exhibits high lateral variability in thickness and maceral composition whereas the Longyear seam, 20 to 40 m above, has a more consistent lateral and vertical development. The Longyear seam contains about 80% by volume of the maceral group vitrinite which is dominated by detrovitrinite. In contrast, the Svea seam contains only 40% to 50% vitrinite and 35–50% inertinite. We interpret this to reflect that the early Paleogene topography gradually developed from laterally discontinuous mires of ombrogenous nature towards more widespread topogenous mires. Coarse grains and fragments of high reflecting fusinite with oxidation rims together with splinters of highly reflecting inertodetrinite are seen as expression of temporarily oxygen rich, arid, peat conditions and are correlated to the inorganic constituents of the coal. Random vitrinite reflectance of the Svea seam is 0.94% and the Longyear seam is 0.71%. This steep reflectance gradient is probably due to the lower seam having acted as an insulation layer for the heat flow from below. The vitrinite reflectance within the Longyear seam varies from 0.76% at the base to 0.62% at the top. This could be due to suppressed reflectance from impregnation by bitumen, which is supported by the occurrences of cleats filled with exsudatinite. Oil expulsions were observed on polished sections of coal and host rocks of both seams. GC–MS analyses proved that the oil is coal derived. As the total liptnite of the seam is < 3% this may require further investigations.