Abstract This study combines magnetic experimentation and geochemical analysis on oil sands from Osmington Mills and Mupe Bay, Wessex Basin, UK to investigate the possibility of a relationship between hydrocarbons and magnetic mineralogy. Removal of hydrocarbons by chemical extraction was conducted to allow comparison of (1) oil sands and (2) cleaned sands. Detailed magnetic analysis including low-temperature and high-temperature experimentation revealed that all but one sample was dominated by siderite, identified by the Néel transition at 37–38 K as well as containing large grains of multidomain magnetite (Verwey transition 110 K) and hematite (Morin transition 250 K). Scanning electron microscopy and energy dispersive x-ray analysis confirmed the presence of iron oxides, in particular framboids 500 nm–45 µm in diameter, probably magnetite. Hysteresis parameters showed distinct grouping of oil sands compared to their clean counterparts and a negative linear regression in log space was observed ( R 2 =0.7) between the percentage of extractable organic matter and magnetic susceptibility. These results suggest a relationship exists between magnetic minerals and the alteration of oil due to biodegradation, which is not yet fully understood. Possible mechanisms are suggested to be due to anaerobic bacteria or the transportation of the oil as it migrates through the host rock.

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