Using mineral magnetics and petroleum systems modelling to calibrate hydrocarbon migration: a case study from the UK Central North Sea

Through detailed magnetic and petroleum systems analysis, I have identified a relationship between hydrocarbon migration and reservoir magnetic signatures in Paleogene petroleum fields of the UK North Sea. Hydrocarbons in these fields generally diminished the in-reservoir ferromagnetic signature via the destruction of iron (titano)-oxides, and selectively enhanced their paramagnetic content. The enhancement is due to the formation of siderite and minor hexagonal pyrrhotite, pyrite, lepidocrocite and/or vivianite. The precipitation of hexagonal pyrrhotite and lepidocrocite is centred around the oil water transition zone (OWTZ). Lepidocrocite and greigite are also precipitated within the interbedded shales. Siderite is heterogeneously precipitated within some of the hydrocarbon reservoir sandstones and the contacting shales. The precipitation of siderite in these reservoirs is mainly related to hydrocarbon migration direction. Petroleum systems modelling revealed that the reservoirs with anomalously high concentrations of siderite experienced significant vertical migration of hydrocarbons during filling. This is related to the increasing availability of bicarbonate ions and Fe2+ during vertical migration unlike lateral migration. In-reservoir biodegradation of hydrocarbons via methanogenesis also contributed to the precipitation of siderite. Biodegradation and hydrolysis of hydrocarbon is also related to the precipitation of hexagonal pyrrhotite and lepidocrocite respectively. The relationship between these in-reservoir processes and mineral magnetism suggests an application of magnetic proxies to petroleum system analysis. Magnetic susceptibility measurement can be applied to the determination of the OWTZ and hydrocarbon migration direction. Petroleum systems modelling provided an understanding of the migration history of the complex CAD region, West Central Shelf. It revealed that hydrocarbon was sourced via long distance migration from the deeper graben area. The CAD reservoirs were charged at ~ 10 Ma through a fill-spill process. The system was generally seal limited leading to significant hydrocarbon loss via leaks. Present day, the CAD region is spill limited with prospects further west of the systemOpen Acces