Sand injection or deposition, a perspective from mineral-chemical stratigraphy

Mineral-chemical stratigraphy and provenance analysis are applied to aid diagnostics of a sandstone body of uncertain origin in the northern North Sea. The main Gamma discovery sandstone (well 24/9-3), hosted by fine-grained strata in the Balder Formation (early Eocene), has many characteristics of sandstone intrusions, such as discordant upper and lower surfaces and significant jack-up of overlying strata. However, the gross volume and high net-to-gross in boreholes (75% and 85% in 24/9-3 and 24/9-14S, respectively) are unusually high compared with sandstone intrusions known from the subsurface or outcrop examples. To constrain the origin of intra-Balder Fm sandstones in the Gamma area, their heavy mineral assemblages (HMAs) and garnet chemistry are compared with those preserved in depositional Heimdal, Hermod S2, Odin and Frigg Member sandstones found in stratigraphic and geographic proximity. Mineral-chemical characteristics in the Gamma Sandstone are similar to those in depositional sandstone of the Odin Member, a sandstone unit in the Balder Formation. All other depositional sandstone units analysed have dissimilar mineral-chemical features, or other factors preclude them as parent units for the Gamma Sandstone. We conclude that the intra-Balder sandstones in the Gamma discovery occupy their original stratigraphic position and can be assigned to the Odin Member, despite the intense changes to internal and external features caused by in situ remobilisation and sand fluidisation.