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

The ¹³C/ ¹²C and ¹⁸O/ ¹⁶O 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 ¹³C but considerable differences in δ ¹⁸O between early calcrete calcites and isotopically-light later calcites; δ ¹⁸O decreases to values close to isotopic equilibrium with present water while δ ¹³C 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 ¹⁸O but similar δ ¹³C values. ³⁴S/ ³²S and ¹⁸O/ ¹⁶O 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 ¹⁸O and δ ¹³C values are relatively heavy and indicate early dolomitization of calcite over large areas. δ ¹⁸O 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.