Hydrocarbons in Nigg Bay sediments

Abstract

SynopsisAs part of the Moray Firth environmental monitoring programme Britoil has commissioned a series of studies on the mudflats in Nigg Bay commencing in 1981. These cover an area of about five square miles and are situated in the Cromarty Firth adjacent to the oil terminal that handles crude oil from the Beatrice Field.Capillary gas liquid chromatography (GLC) analysis of the mudflat sediments in 1981 and 1982 showed a major autochthonous input of algal hydrocarbons as indicated by high n-C17 and C17 alkene concentrations. In addition there were low levels of petrogenic contamination, with concentrations of unresolved complex mixture (UCM), in general, being less than 10 ppm. These concentrations altered little between the two surveys during which time effluent discharge commenced from the oil terminal.In many of the GLC traces the dominant feature was an homologous series of n-alkanes in a smooth envelope from n-C22 to n-C34 with a maximum concentration at c. n-C25. This unusual distribution of n-alkanes does not appear to be associated with the UCM seen in many samples and may therefore not be of petrogenic origin. Possible sources of these components include bacteria and the erosion of local organic-rich shales, a selection of which have been sampled and analysed. Gas chromatography–mass spectrometry (GLC/MS) has been used in attempt to further clarify the origin of these n-alkanes.