Magnetic characterization of oil sands at Osmington Mills and Mupe Bay, Wessex Basin, UK

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 (R2=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.

Remagnetization and chemical alteration of sedimentary rocks

“…This suggests a pedoclimatic control. Emmerton et al (2012) found an inverse correlation between the magnetic susceptibility and the extracted organic matter content for samples from the Wessex Basin in SW England. These results indicate a complex relationship between existing magnetic minerals within the sandstones and the alteration of these magnetic minerals due to the multiplex biological activity and biodegradation of the oil.…”

Using magnetic techniques to calibrate hydrocarbon migration in petroleum systems modelling: A Case Study from the Lower Tertiary, UK Central North Sea

“…SEM imaging is particularly important for differentiating between-usually-detrital pyrrhotite and authigenic greigite, which have very different morphologies, but similar magnetic behaviour during magnetic hysteresis when a maximum field of only ∼1 T is available (Roberts et al 2014). As the abundance of magnetic minerals was less typically <<0.001 per cent, magnetic extraction was needed before imaging (Emmerton et al 2012;Emmerton et al 2013b). Samples were crushed to an even grain size (∼50 μm), and passed through a Frantz electromagnet magnetic separator three times following the protocol developed by Chang Figure 1.…”

“…The growth mechanisms for greigite and pyrite are similar, therefore we treat them as a single iron sulphide signal. There are several possible mechanisms for this greigite and pyrite formation: (1) Abubakar et al (2015) demonstrated that source rocks in the oil kitchen can precipitate nanometric greigite (<100 nm), (2) diagenetic changes caused by hydrocarbon presence, are known to lead to the replacement of magnetite, hematite or siderite by iron sulphides such as greigite and pyrite (Burton et al 1993;Emmerton et al 2012) and ( 3) anaerobic biodegradation by sulphate reducing bacteria generates reduced sulphur (HSand H 2 S) which reacts with dissolved Fe 2+ (from replacement or dissolution of magnetite or hematite) to form diagenetic greigite (Reynolds et al 1990).…”

Pyrolysis of Carboxylic Acids in the Presence of Iron Oxides: Implications for Life Detection on Missions to Mars

“…Solvent extraction was carried out using a standard liquid extraction protocol (e.g. Emmerton et al, 2012) as follows: 1-3 g of powdered sample was suspended in a 93:7 v/v DCM/MeOH solution and agitated in a sonic bath. The resulting suspension was then centrifuged and the supernatant solution pipetted off.…”