Aberrations in the coalification patterns of the offshore coalfields of Northumberland and Durham, United Kingdom

摘要

The offshore extensions of the Northumberland and Durham Westphalian coalfields are formed of continuous and linked sequences of sediments which lie in contrasting structural regimes, respectively 'Basin' and 'Block'. The coalfields have suffered different geothermal histories. Offshore Northumberland ('Basin') sediments endured low heating rates and low maximum temperatures to give seams which, even at the base of the succession, hardly reached vitrinite reflectances of Roil = 0.60%, apart from raised values due to igneous activity. The offshore Durham ('Block') succession was exposed to higher temperatures and heating rates due to the proximity of the Lower Devonian Weardale granite underlying the 'Block', which was re-activated during Carboniferous times. These geothermal conditions not only led to higher levels of reflectance and steeper maturation gradients offshore in the 'Block' succession than offshore in coals of the 'Basin', but also produced contrasting relationships between correlations of vitrinite oil reflectance and volatile-matter yield of coals for the two structural zones―and probably also for other property relationships as well. Reflectance profiles have been modified due to three circumstances: first, considerable variation locally in the raising of reflectance levels caused by igneous activity; second, lowering of vitrinite reflectance values due to arenaceous sediments being deposited immediately on top of vitrinite precursors within the coal-forming organic matter; and third, possible interference from liptinite-rich accumulations within coal seams or near to them in the succession, which exuded aliphatic materials relatively early in coalification which were then absorbed by vitrinites or, more likely, deposition of the vitrinite precursors in a marineear marine and calcium-rich environment, so reducing reflectance values on coalification. Similar contrasting property relationships to those described here may have been produced by Lower Devonian granitoid bodies which have been identified on the North Sea High, if they also were re-activated during the Carboniferous.