Impact of meteoric water flushing on diagenesis of deep-marine turbidite sandstones: A case study from the Tertiary sandstones of Frigg and Grane fields, northern North sea

Abstract

Meteoric water flushing into deep-marine environments is a topic of debate since the actual transport mechanism remains elusive. In order to evaluate the potential impact of meteoric water on early diagenesis of deep-marine sandstones, two submarine fan sandstone reservoirs, the Eocene Frigg (Frigg oilfield) and the Palaeocene Heimdal (Grane field) that have the same source area, were compared and contrasted. Petrographic studies reveal that both sandstone reservoirs are at the eodiagenesis stage (<70 °C) at the present day. However, the Eocene Frigg sandstones are characterized by geological features suggestive of potential meteoric water diagenesis, such as dissolution and kaolinization of the silicate grains, whereas the Palaeocene Heimdal sandstones show negligible alteration. The δ13CV-PDB and δ18OV-PDB values of siderites in the Eocene Frigg sandstones range from +5.2‰ to +16.7‰, and from −8.2‰ to −6.6‰, respectively, indicating siderite formation during methanogenesis in meteoric pore water. Results from generic hydrogeochemical modelling scenarios lend further support to 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. This occurred because the East Shetland Platform experienced two stages of relative sea-level fall (during 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 study suggests meteoric water intrusions as a 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.