The enhancement of the rock magnetic signature at oil-water contacts (OWCs) provides
an alternative tool for identifying the base of present-day and paleo-oil columns, to assess
yield and reservoir fill history when traditional methods are inconclusive. This thesis uses the
Wessex Basin petroleum system, UK, as a data-rich laboratory to identify the characteristics
and universality of OWC magnetic enhancement and constrain if it can be used in conjunction
with petroleum systems modelling to establish the extent of Cenozoic hydrocarbon remigration.
Increased saturation magnetisation and magnetic susceptibility have been identified at the OWC in siliclastic and carbonate reservoirs at Wytch Farm. Potential magnetic paleocontacts (PCs) have also been observed above the OWC, indicating the formation of multiple stable OWCs. Magnetic measurements reveal an increase in ferromagnetic magnetite and paramagnetic vivianite at magnetic enhancements, suggesting they are a universal characteristic of magnetic OWCs. The iron sulphide greigite has previously been suggested to form at OWCs, however, thermodynamic modelling presented here predicts greigite is more likely to be stable in iron-rich, anoxic and alkaline and low-temperature conditions (< 25 °C), suggesting that it is unlikely to be preserved at the OWC at burial depths above 900 - 1000 m.
Comparing magnetic observations with petroleum systems modelling shows a strong depth correlation with Late Cretaceous and present-day OWCs, indicating magnetic OWCs can be used to calibrate petroleum systems models. The Wytch Farm reservoir was charged in the Cretaceous via Creech, Bushey Farm and offshore conduits from the Bridport carrier bed.
Cenozoic tilting to the east of ≤ 1◦ caused significant westerly remigration, particularly in
offshore areas, and drastically reduced the size of the Wytch Farm Bridport structure. A deepening of the OWC and multiple magnetic enhancements at Wareham indicate the tilting
was periodic, causing pulses of westerly spill from Wytch Farm.