Sandstone-hosted concretionary cements of the Hebrides, Scotland.

The geometries of the sandstone-hosted calcite concretions of the Jurassic Valtos Sandstone Formation and Bearreraig Sandstone Formation are described and related to the processes operating during concretion growth. As concretionary bodies analogous to those studied form potential permeability barriers within some North Sea petroleum reservoirs; the relationship between the growth processes and permeability barrier formation is examined. The growth times for model spherical concretions are calculated for the complex carbonate-water system. Two growth processes are modelled, solute transport and surface reaction. Growth times for a 1m diameter concretion forming under geologically reasonable conditions are predicted to be 22.3Ma, which reduces to 8.8Ma in porewaters flowing at 1m/year. The depth of formation of the concretions is assessed, through an examination of depth dependent properties of both the host-sediment and the calcite cement, and is found to be less than 500m. Concretion formation preceeded the Paleocene igneous activity which affected the Hebrides. The majority of the concretions examined formed at burial depths which were too great to allow effective contact between the concretions and seawater. The major source of carbonate was the dissolution of aragonitic shell material from within the host sandbody. The nature of the porewaters from which the concretions formed is assessed. The majority were meteoric in origin, though some marine influence is noted. The minor element contents of the cements cannot be used to calculate porewater compositions, as disequilibrium between the porewaters and the cements can be demonstrated. A model is proposed to account for the minor element patterns. Crystal breeding can be demonstrated to have occurred during concretion growth. A hypothesis is presented to explain the petrographic features of a typical Valtos Sandstone Formation concretion.