Identifying the depths of the hydrocarbon-fluid contacts in a reservoir is important for determining hydrocarbon reserves and production planning. Using core samples from the Tay Sandstone reservoir in the central North Sea, we show that there is a magnetic enhancement at the hydrocarbon-fluid contacts that is detectable through both magnetic susceptibility measurements and magnetic hysteresis measurements. We observed this magnetic enhancement at both gas-oil and oil-water contacts, which have been independently identified using nonmagnetic methods; we did not consider gas-water contacts in this study. We demonstrate that this magnetic enhancement is caused by the precipitation of new nanometric iron oxide (magnetite) and iron sulfide (greigite) phases. The magnetic enhancement may be caused by diagenetic changes or preferential biodegradation at the top of the oil column during early filling and at the oil-water contact. Our findings have the potential to be used to identify paleo-hydrocarbon-fluid contact in both structurally modified fields and failed wells. The technique can also be used to infer the fill history of a basin and calibrate petroleum systems models. Magnetic susceptibility measurements have the advantage that they can easily and quickly be measured in the field on whole core material.