Clay Minerals (1984) 19, 309-321 EARLY CARBONATE DIAGENESIS WITHIN PHANEROZOIC SHALES AND SANDSTONES THE NW EUROPEAN SHELF OF J. G. GLUYAS BP Petroleum Development Ltd, Farburn Industrial Estate, Dyce, Aberdeen AB 2 0PB, UK (Received 4 November 1983; revised 13 February 1984) ABSTRACT: Continuous nodular limestone beds in north-west European shale sequences developed during early diagenesis via anaerobic, bacterial catabolysis of organic matter which yielded both carbonate and sulphide. The high carbonate content of the nodular limestones (50-90%) demonstrates that they developed in the upper tens of metres of the sediment column. In addition, each of the nodular limestones displays a centrifugal decrease in carbonate, indicating that growth was synchronous with compaction of the host ooze. The chemistry of the carbonate changed with time, becoming more rich in magnesium and iron and reflecting a progressive change in the porewater chemistry from its initially marine composition. The earliest precipitate was calcite containing < t mole% MgO; later precipitates contained several percent MgO and ultimately dolomite was precipitated. Similarly, iron content of the carbonate increased as precipitation continued and sulphate reduction, and thus pyrite production, declined. The early diagenetic cementation of many marine sandstones followed a pattern similar to that of nodular limestones and much of the carbonate was also derived from organic matter via bacterial oxidation. Thus, early diagenetic bacterial oxidation of organic matter may not only reduce the source potential of shales but also reduce the reservoir potential of associated sandstones, or those more distant which lie along compactional-fluid pathways. Rhythmical alternations of nodular limestone and shale are common in marine sediments (Fig. 1) and sedimentologists have long debated their genesis (Day, 1865; Cox, 1923; Hallam, 1957; Bjorlykke, 1973; Henningsmoen, 1974). Are the nodular limestones within shale nodular limestones of primary depositional or secondary diagenetic origin? There is little sedimentological evidence which is not ambiguous, and both the same and different features have been interpreted as depositional and diagenetic products. The present sedimentological and diagenetic study of nodular limestones from the Jurassic shales of SW England, Carboniferous shales of SW Scotland. Silurian shales of central Sweden and Ordovician shales of the Oslo district Norway, clarifies not only their origin but also the origin and thus source of early diagenetic carbonate cements within associated, potential reservoir, sandstones. MORPHOLOGY Careful examination of the sedimentary structures within, and the morphology of, nodular limestones indicates that most such limestones are concretionary, having developed during diagenesis. Inextensive nodular beds occur both in the Lias of Lyme Regis and Carboniferous of 9 1984 The Mineralogical Society