Fracture cements and cementation processes in the Devonian-Carboniferous Clair group and underlying Lewisian basement, West of Shetland
Clair Groups and underlying basement contain cataclastic faults, cement-sealed faults/veins and open fractures. The dominant fracture cements are calcite and pyrite, although fluorite (with sulphides and native silver) occurs in minor amounts in some Clair Group fractures, marcasite is often associated with Clair Group fracture-hosted pyrite, and some basement fractures contain epidote/chlorite and quartz/feldspar/hematite. Calcite cements in many Clair Group fractures may have crystallised from porewaters whose calcium and bicarbonate were mostly remobilised from Clair Group calcretes. Many of these cements may predate Late Cretaceous/Early Tertiary oil charge. Some may have crystallised from Early Tertiary meteoric porewaters, the fluorite with which they are associated perhaps having crystallised from porewaters hydrothermally affected by local igneous activity. Calcite cements within some basement fracture may have crystallised from porewaters whose calcium and bicarbonate came from Clair Group calcretes, as may most non-ferroan calcite cement within Clair Group sandstones. Calcite cements in some Clair Group fractures contain a high proportion of non-calcrete carbonate. These cements may have crystallised after oil charge began, from Early Tertiary meteoric porewaters or meteoric/basinal-derived brine mixtures. Non-calcrete bicarbonate may have been generated via bacterial activity (organic matter oxidation and methanogenesis) within the Clair Group. Bacteria could have utilised hydrocarbons, substrates generated via aerobic hydrocarbon oxidation, or externally-sourced substrates such as acetate. Authigenic iron sulphides often predate the calcite cements. The sulphur within these was probably externally-sourced, H2S either having come direct from external sources, or having been generated in situ via bacterial sulphate reduction. Sulphur may have undergone redox cycling through contact with oxygenated meteoric water in some fractures and marcasite genesis may have resulted from this. Calcite cements within some basement fractures may have crystallised from Early Tertiary porewaters containing bicarbonate generated in situ via the oxidation of hydrocarbons, and these cements are often predated by authigenic pyrite.