Diagenetic ferroan carbonates grew in the Upper Jurassic reservoir sandstones of the Magnus oilfield in porewaters which differed in composition across the field. These porewaters remained compositionally different and stratified for at least 35 M.y. Variations in carbonate chemistry across the field are attributable to these porewater variations, which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late Cimmerian subaerial exposure. Original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution. Rare diagenetic calcite occurs as discrete rhombic crystals. Diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains. Poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones. Three compositional growth zones in siderite crystals are observed across the field from crest to flank. In all three wells studied, a similar trend of compositional evolution through time is observed in both two and three zoned rhombs. First-formed siderite is relatively magnesian, intermediate zones are more ferroan, and outer zones are at least as magnesian as the first stage. These individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples (up to 87 mol % Fe + Mn) and more magnesian downdip (up to 58 mol % Mg). This reflects the greater influence of relatively Fe-rich meteoric-derived water in the crest and the greater influence of marine-derived Mg-rich porewater downdip. Ankerite shows a similar variation in Fe and Mg abundances across the field (crest and flank maximum 25 and 17 mol % Fe + Mn respectively, 27 and 43 tool % Mg) and developed due to release of Mg, Fe, Ca and HCO (sub 3-) ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions. Both siderite and ankerite have lower delta 18 O at the crest of the oilfield than downdip (respectively siderite 16.0 per thousand and 17.6 per thousand SMOW; ankerite 17.7 per thousand and 21.4 per thousand ). These differences in delta 18 O reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity, whereas downdip porefluid contained a larger marine-derived component. Strong organic influence on delta 13 C (-8.0 to -14.6 per thousand PDB for magnesian siderite; -7.7 to -13.6 per thousand for ankerite), closed system delta 34 S values (up to 15.7 per thousand for late cubic pyrite), and stratified delta 18 O from crest to flank of the field argue against large scale porewater movements. Diagenetic porewater stratification is strongly supported by the parallel, but distinct, geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field.