Dégradation et lessivage des hydrocarbures de la formation ordovicienne de Thumb Mountain encaissant le gîte Zn–Pb de Polaris (Territoires du Nord-Ouest, Canada)

摘要: Carbonate rocks of the Ordovician Thumb Mountain Formation, host to the Zn–Pb Polaris deposit, contain hydrocarbons that show types of alteration classically attributed to biodegradation and water washing. The hydrocarbons of the upper part of this formation and of the overlying Irene Bay Formation indicate alterations due to water washing only. The hydrocarbons of the impermeable shales of the overlying Cape Phillips Formation display indices of only higher thermal maturity than the underlying units. Contrary to classical concepts of hydrocarbon biodegradation, n-alkanes, cyclohexylalkanes, even isoprenoids and perhaps also steranes seem to have been degraded simultaneously and not successively in this sequence. This alteration process is mainly responsible for a log-linear decrease of the amounts of n-alkanes and cyclohexylalkanes with increasing depth. The severe and uniform alteration of aromatic hydrocarbons throughout the interval, which is opposite to the progressive alteration of associated n-alkanes, can be attributed solely to water washing. This conclusion necessitates a reconsideration of previous interpretations attributing the loss of short-side chain substituted polyaromatic compounds to microbial activity. Hopanes, tri-and tetra–cyclic terpanes as well as aromatic steroids and hopanoids seem to have been unaffected by the alteration phenomena. The increase in the degree of alteration of the hydrocarbons with increasing depth implies that the responsible migrating fluid circulated per ascensum.