Mapping petroleum migration pathways using magnetics, geochemistry and seismic mapping. Case study: Wessex Basin, southern England, UK

This thesis presents, for the first time, direct evidence for the formation of magnetic minerals during hydrocarbon generation, it also reports on the nature and type of magnetic mineralogy formed during the generation of oil and gas. This thesis also reports experimental results that explain variations in magnetic mineralogy along potential crude oil migration paths in the Wessex Basin, southern England, UK. Potential oil-source rocks from the Wessex Basin, England, were pyrolysed (artificial maturation) at various temperatures under conditions similar to those for hydrocarbon-generation to reveal the formation of nano-metre sized magnetic minerals, mainly magnetite and pyrrhotite, with traces of greigite. The potential of these magnetic minerals to migrate along with the generated crude oil is also reported in this thesis. To assess the possibility of using magnetic characterisation to test for potential hydrocarbon migration, I examined the Wessex Basin, southern England, using 2D seismic data, geochemical analysis and mineral magnetic techniques. Potential hydrocarbon migration pathways from the hanging wall of the Purbeck fault into Sherwood Sandstone reservoir at Wytch Farm was identified via interpretation of 2D seismic data. Geochemical characterisation of oils extracted from core materials from the Wessex Basin revealed oils found at Stoborough and Waddock Cross are less mature than oils at Wytch Farm, which likely indicates spillage from the top of the reservoir at Wytch Farm due to regional tilting. In addition, the geochemical data from the core materials also show probable migration paths for oils found at Chickerell. Magnetic characterisation of reservoir core material (Bridport Sandstone and Inferior Oolite) revealed variations in magnetic signatures along potential migration routes, with variations also found with sampling depth. The magnetic signature was characterised using low-temperature magnetic remanence and hysteresis measurements, because much of the magnetic signature was found to be from nanoparticles that are thermally activated at room temperature (superparamagnetic). The study found magnetite predominates in the hydrocarbon proximal plume, pyrrhotite in the distal plume environment with mixed pyrrhotite and magnetite in shallow proximal plume environment. It is suggested that magnetic mineralogy has the potential to be used as a proxy for hydrocarbon migration identification in the future.Open Acces