ABSTRACT

Isotopically distinct pore waters remained stratified in the Upper Jurassic Magnus Sandstone, reservoir for the Magnus oil field in the northern North Sea, throughout diagenesis. A petrographic and isotopic study of the reservoir sandstones in three wells indicates that a similar suite of diagenetic quartz overgrowths, kaolinite, magnesian siderite, ankerite, and illite developed across the field. However, the oxygen isotope ratios (δ18O) recorded in these diagenetic minerals from the crest of the field are consistently more negative (by about 3‰) than those from the downdip flank. We propose that this consistent difference in δ18O values records the persistence of isotopically different and stratified pore waters in the Magnus sandstones. The sandstones were deposited as marine turbidites, but were subaerially exposed during periods of structural uplift and tilting during the Early and middle Cretaceous. Meteoric water entered the structural crest, resulting in pore-water stratification. Deposition of thick Upper Cretaceous mudstones and marls sealed the Magnus structure during subsequent rapid burial.

Diagenetic minerals in the crest of the oil field grew in a pore water with significantly more meteorically derived water than those downdip, assuming that each diagenetic phase developed at a similar temperature in each well. The pore waters remained stratified for at least 35 m.y., from middle Cretaceous (~ 90 Ma) subaerial erosion of the sandstone until reservoir charging at 55 Ma. The sandstones and mudstones of the Kimmeridge Clay Formation behaved essentially as a closed system during diagenesis, in contrast to the open-system concept envisaged in most other diagenetic reconstructions.

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