The Impact of Meteoric Water Flushing on Diagenesis of Tertiary Deep-Marine Turbidite Sandstones in the Northern North Sea? - Case Study: Frigg vs. Grane Field

Abstract

Meteoric water flushing into deep-marine environments is a topic of debate since the actual transport mechanism is still an enigma. To investigate a potential early diagenetic impact of meteoric water on deep-marine sand, two submarine fan sandstone reservoirs, the Eocene Frigg sandstones (Frigg oilfield) and the Palaeocene Heimdal sandstones (Grane field) were compared that share the same source area. Pet rographic studies reveal that both sandstone reservoirs are today at the eodiagenesis stage (<70℃). However, the Eocene Frigg sandstones are characterized by potential features of extensive meteoric water diagenesis, such as dissolution and kaolinization of the silicate grains while the Palaeocene Heimdal sandstones show negligible alteration. The δ 13 C V-PDB and δ 18 O V-PDB values of siderite in the Eocene Frigg sandstones are +5.2‰ to +16.7‰, and -8.2‰ to -6.6‰, respectively, which indicate siderite formation during methanogenesis in meteoric pore water. Results from generic hydrogeochemical modelling scenarios also support the hypothesis of meteoric water flushing. Thus we suggest that massive meteoric water might have been brought into the Frigg turbidite sands by basinward migration of the meteoric water. Palaeogeographically the East Shetland Platform experienced two stages of relative sea-level fall (during deposition and at the end of the Eocene, Priabonian), and flushing might have taken place via connected incised canyons. The location of the Frigg sands deposited immediately below the Shetland escarpment and the narrow palaeo-shelf area may have facilitated such meteoric water incursions. Our hypothesis suggests meteoric water intrusions as possible diagenetic control of deep-marine sandstones, and, therefore, meteoric water diagenesis should be considered in the prediction of properties of deep-marine sandstone reservoirs.

Keywords: Meteoric water, turbidite, Diagenesis, kaolinite, Tertiary, North Sea