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Heavy Mineral Stratigraphy of the Upper Clair Group in the Clair Field, West of Shetland


The Clair Field is located west of the Shetland Islands on the UKCS (United Kingdom Continental Shelf) and contains more than a kilometer of biostratigraphically barren red bed deposits of Late Devonian to Early Carboniferous age. Heavy mineral analysis using changes in sediment provenance is an alternative approach to microfossils for hydrocarbon reservoir stratigraphy and intra-reservoir correlation. This project aims to establish a heavy mineral stratigraphic framework for the Upper Clair Group (UCG) and correlate the results from a well located in the Core Area (well 206/8-5) with published information from the Ridge Area (well 206/8-2) located 7 km away. A total of 20 ditch cutting samples and five core samples were processed for heavy mineral analysis following standard heavy mineral processing methods with cleaning, washing and gravity-setting using bromoform before they were mounted on slides for quantitative analysis. Petrophysical data (gamma ray) and sedimentology (300 meters of logged core) were integrated with the heavy mineral analysis in order to enhance the stratigraphic framework for the UCG. The sedimentology data suggest a fluvial depositional environment with medium to coarse grained sandstones with several fining upwards cycles and sporadic deposits of low energy fine grained lacustrine mud- and siltstones. Excellent porosity and permeability values are encountered in some intervals. The heavy mineral data show a major change in heavy mineral assemblage from the Lower Clair Group (LCG) where the samples are dominated by garnet, apatite and zircon and the UCG, the samples of which mainly contain garnet and to a minor extent staurolite. Based on the ratio of apatite to tourmaline, rutile to zircon and staurolite to zircon, a further subdivision of the UCG into three subunits is proposed. Using heavy minerals it was possible to establish a stratigraphic framework for the UCG Core Area and to correlate it with published data from the Ridge Area. By integrating sedimentology, petrophysics and heavy mineral data it was possible to place the results into a broader regional context. The successful correlation between the Ridge Area and the Core Area has also demonstrated that heavy mineral analysis is a suitable tool for intra-field correlation.