Heavy-mineral stratigraphy of the Clair Group (Devonian-Carboniferous) in the Clair Field, West of Shetland, U.K.

The Devonian-Carboniferous Clair Group reservoir succession in the Clair Field, located west of Shetland on the UK continental shelf, comprises over 1000 m of clastic sediment deposited in a range of fluvial, lacustrine and aeolian environments. Owing to the unfavourable depositional conditions, palynomorphs and microfossils are almost entirely absent, precluding development of a high-resolution biostratigraphic framework for reservoir correlation. An alternative approach to reservoir subdivision and correlation is therefore necessary in order to establish a viable reservoir model prior to field development. Heavy mineral analysis, which subdivides clastic successions on the basis of changes in provenance and sediment transport history, has proved successful in establishing a high-resolution correlation framework for the Clair Field. This paper concentrates on the heavy mineral stratigraphy of the Lower Clair Group, which is the target for the first phase of the field development. The key parameters that have been used to erect the correlation framework are provenance-sensitive ratios of heavy minerals (notably garnet:zircon, rutile:zircon and apatite:tourmaline), grain morphology (apatite roundness) and mineral chemistry (garnet composition). The Lower Clair/Upper Clair boundary is a first-order heavy mineral event related to a major change in provenance. Six major units (I-VI) and a number of subunits have been recognised within the Lower Clair Group, boundaries being related to more subtle changes in provenance and sediment transport history.