Large-scale hydrocarbon-driven sand injection in the Paleogene of the North Sea

Abstract

The detailed study of many diagenetic carbonates (concretions, cemented bioturbation burrows etc.) and dykes of a large-scale sandstone injection complex, which are preserved in a cored early Eocene section recovered from the South Viking Graben (North Sea), provides evidence for the dyke emplacement by gas-driven sand injection and coeval methane seepage. The morphology, chemistry of fluid inclusions, isotopic signature (δ13C as low as − 35‰) and petrographic characteristics of the diagenetic carbonates reveal gradual carbonate precipitation with a contribution of carbon derived from methane oxidation. Stratigraphic and cross-cutting relationships reveal that sand injection post-dates the formation of the diagenetic carbonates and, therefore, the beginning of hydrocarbon migration. Fluorescing fluid inclusions, carbonate cement isotopic signature (δ13C as low as − 27‰) and petrographic observations of the injected sandstones indicate the persistence of hydrocarbons (including methane) after the injection process. Gas chromatography of fluid inclusions confirms the presence of methane in both diagenetic carbonates and injected sandstones carbonate cements. We suggest that gas influx in the Eocene deep-water sandstones increased pore fluid pressures during early burial and decreased the grain framework stability, inducing liquefaction and sand injection. Gas involvement in sand injection has been widely invoked to explain the large-scale injection complexes recently recognised in the Paleogene of the North Sea but had previously never been proven. Large-scale injection structures and hydrocarbon seepage appear linked in several localities with obvious great economic significance.