Siderite occurrence in petroleum systems and its potential as a hydrocarbon-migration proxy: A case study of the Catcher Area Development and the Bittern area, UK North Sea

We report a detailed magnetic study of Tertiary sandstone cores from the Bittern field, West Central Graben, and Catcher Area Development (CAD), West Central Shelf, UK North Sea that identified siderite as a potential magnetic proxy for the differentiation of vertical and lateral hydrocarbon migration. Magnetic hysteresis experiments revealed increased presence of paramagnetic minerals in the oil-stained sandstones of the fields compared to the nearby dry Tertiary (unstained) sandstone of offset wells. Within the oil-stained sandstones, the paramagnetic proportion is highly variant, with low and high paramagnetic sections present. Detailed experiments including thermomagnetometry between 10 K and 900 K and XRD analysis, combined with Mössbauer spectroscopy confirmed that this increase in paramagnetic response was primarily due to the formation of siderite. An increase in the quantity of paramagnetic clays and occasionally pyrite also contributed to the increased response. Siderite and iron sulphides have been interpreted to form during hydrocarbon migration and potentially oil biodegradation. We propose that hydrocarbon migration pathways may be indicated by the distribution of siderite. We argue that as hydrocarbons migrate vertically and the equilibrium partial pressure of CO₂ with the environment reduces, essential HCO₃⁻ is produced which reacts with available Fe²⁺ to form siderite if all the other diagenetic requirements for its formation are met. The distribution of siderite and magnetic susceptibility values along the oil-stained layer provides a tool for the determination of the migration pathways in low sulphur hydrocarbon environments containing reactive iron.