The late Carboniferous ‘Barren Red Bed’ play of the Silver Pit area, Southern North Sea

Abstract

A multidisciplinary study of the gas-bearing Westphalian red bed sequence of the Southern North Sea is presented. Comparison with analogous fades in Carboniferous sequences in the UK onshore and the eastern Netherlands suggests that two distinct alluvial red bed units are present, one of Westphalian C and one of Westphalian D age. Development of red pigment in these units was largely the result of contemporaneous pedogenesis. Interpretation of the stratigraphy is complicated by the widespread occurrence of reddening caused by penetrative weathering, both during the Carboniferous and related to the formation of the sub-Permian unconformity. Gas reservoirs are present in sandstones in the Westphalian C red bed unit. This unit has higher net:gross ratios and better poroperm characteristics than the underlying Coal Measures. The better reservoir quality in the red bed unit is largely influenced by late diagenetic alteration related to hydrocarbon migration. A sedimentological and diagenetic model for the formation of the Westphalian C red bed unit implies that this unit is likely to have a distinctive reservoir architecture, which, together with better reservoir qualities, distinguishes it from the hydrocarbon play in the underlying Coal Measures.