Diagenesis of carbonate cements in Permo-Triassic sandstones in the Wessex and East Yorkshire-Lincolnshire Basins, UK: a stable isotope study

Summary The 13 C/ 12 C and 18 O/ 16 O ratios of carbonate cements in Permo-Triassic fluvio-deltaic sandstones differ between the Wessex and East Yorkshire-Lincolnshire Basins. The isotopic data indicate different conditions of deposition and early post-depositional carbonate precipitation which supports petrological and mineralogical studies of the same sandstones focussed on the development of reservoir properties. Isotopic compositions of carbonate cements vary across the Wessex Basin; they indicate early derivation of dissolved carbonate during recrystallization of detrital limestone. Within the basin, calcite cements show little variation in δ 13 C but considerable differences in δ 18 O between early calcrete calcites and isotopically-light later calcites; δ 18 O decreases to values close to isotopic equilibrium with present water while δ 13 C is not in isotopic equilibrium with bicarbonate in present-day brines. Two distinct generations of replacive dolomite, early non-ferroan and later ferroan, are found in the basin centre and have different δ 18 O but similar δ 13 C values. 34 S/ 32 S and 18 O/ 16 O ratios of anhydrite cements could indicate a remote Zechstein source of sulphate or a mixed marine sulphate-sulphide source. The Permo-Triassic sandstones of the East Yorkshire-Lincolnshire Basin, in contrast, contain dominantly dolomite cement. δ 18 O and δ 13 C values are relatively heavy and indicate early dolomitization of calcite over large areas. δ 18 O values of minor calcites indicate that dedolomitization is continuing in the present-day groundwater regime. The isotopic differences between carbonate cements in the two basins is tentatively attributed to the differing early post-depositional conditions and to the differences in carbonate chemistry between calcite dissolution/precipitation reactions and dolomitization reactions.