Magnetic information may provide new ways to examine the Mupe Bay record of oil migration. Migrating fluids such as hydrocarbons have been shown to cause chemical conditions suitable for the alteration or formation of authigenic magnetite resulting in associated chemical remanent magnetization (CRM). Magnetic characterization reveals both the matrix and clasts contain multi-domain magnetite but abundant hematite only exists within the clasts. Hysteresis parameters show the matrix has more multi-domain and likely larger magnetic grains than the clasts.
Magnetic directions are different in the clasts and matrix supporting a two-phase oil-staining event. Moreover, paleomagnetic directions for the clasts after tilt correction (82.6¡ãN and 155.2¡ãE) are consistent with biodegradation processes in the Early Cretaceous. Consistent directions in separate clasts imply the biodegradation and magnetite formation took place following transportation and incorporation of the oil-cemented clasts into the conglomerate bed. Magnetic directions reveal that the Mupe Bay matrix has a remanent magnetization corresponding to today's magnetic field, confirming the matrix represents an active oil seep.
This study represents an unprecedented use of magnetic data to date the onset of oil migration in a basin. The classic two-stage oil-staining scenario, which constrains the onset in the Wessex Basin to the Early Cretaceous, is supported by our data.