Fluidisation and injection in the deep-water sandstones of the Eocene Alba Formation (UK North Sea)

The Nauchlan Member of the Late Eocene Alba Formation (UK North Sea) consists of a deep-water channel fill that was extensively modified by post-depositional sand remobilization and injection. Sandstone textures, facies associations and the geometry of the channel fill were affected. A suite of sand-rich facies was produced by large-scale fluidization and injection within the channel fill and above it. These facies, termed here unstratified facies, are characterized by the absence of stratification surfaces and by discordant relationships with bedding in the adjacent succession. They reflect variable degrees of disruption of the primary sedimentary structures caused by escaping pore fluid, the velocity of which is estimated at least in the order of 0.1 ms(-1). Adjacent mudstones were severely disrupted by hydraulic fracturing, and fragments of fractured mudstone were incorporated into the fluidized sand. Average porosity was decreased in the sandstones affected by fluidization. Two main phases of sand injection are inferred to occur at different burial depths. A shallow burial phase (below 100 m) produced thin dykes with ptygmatic folds. The second phase occurred at the boundary between Eocene and Oligocene (approximate to 300 m burial depth) and resulted in large-scale tabular wing-like dykes that project from the edges of the channel fill. The significant pore-fluid overpressure, which was required to hydraulically fracture the thick mudstone seal and to fluidize the large volume of sand, was likely to be built up by static liquefaction of the source sand and was possibly enhanced by hydrocarbon gas influx.