Hydrocarbon source and migration pathway identification through basin & petroleum systems modelling and mineral magnetism: a case study on the Beatrice Field, Inner Moray Firth, UK North Sea

Abstract

This research builds on past studies analysing the relationship between the presence, formation and alteration of magnetic minerals with hydrocarbons, by investigating the poorly understood petroleum system of the Inner Moray Firth. This research shows that the Inner Moray Firth differs from the Central North Sea through both its petroleum system and its magnetic mineral assemblage.

Magnetic mineral analysis provides extensive evidence of magnetite, goethite and siderite, alongside more minor lepidocrocite and iron sulphides, differing from the iron sulphide-rich magnetic mineral assemblages of the Central North Sea. Furthermore, this research shows two magnetic mineral behaviours around oil-water contacts in wells of the Inner Moray Firth, resulting from varying magnetic mineralogy. Most wells show continuous presence of goethite, which alters into magnetite towards the oil-water contacts, creating a strong magnetic enhancement directly below the contact. In contrast, wells bearing extensive iron sulphides experience magnetic enhancement directly above the oil-water contact, produced by the iron sulphides.

Basin and petroleum systems modelling shows basin-wide maturity of Type I, Devonian-age and Type II-III Mid Jurassic-age source rocks, whilst dispelling the hypothesis of extensive Kimmeridge Clay Formation-sourced hydrocarbons, due to limited maturation. Modelling shows evidence of wide-spread lateral and vertical hydrocarbon migration, and that extensive faulting, alongside the late-stage, Cenozoic tilting of the basin, combine to heavily constrain and drastically reduce the trapping success of the petroleum system.

Connections between the magnetic mineral analysis and modelling shows direct correlations between vertical migration and siderite presence, alongside the preferential formation of iron sulphides over other magnetic minerals when sulphur is present. When sulphur is not present, magnetite typically forms through the alteration of goethite around oil-water contacts. Furthermore, this study has shown direct connections between iron sulphide presence and Type II source rocks, and between iron hydroxide presence and Type I and Type II-III source rocks.