The clay mineralogy of the Upper Triassic Skagerrak Formation, Central North Sea

Sandstones from the Upper Triassic Skagerrak Formation from the Gannet oil field in the North Sea contain authigenic, grain-coating corrensite, chlor ite-smectite and chlorite. The corrensite occurs as an irregular network of crinkled and folded sheets that typically contain more Al and Si and less Mg and Fe than reported corrensite analyses. Whilst the chlorite-smectite occurs as plates 5 jjm in length, perpendicular to the detrital grain surface, the chlorite occurs as poorly developed "crinkly" flakes. Both chlorite-smectite and chlorite are more ferroan than reported authigenic chlorites. It is proposed that the clays formed by direct precipitation from porewaters containing Al, Si, Mg and Fe derived in part from the dissolution of detrital albites and pre¬ existing authigenic dolomite. The clays have a major effect upon the reservoir quality ; corrensite reduces the permeability more than chlorite-smectite and chlorite.Purvis Kévin. The clay mineralogy of the Upper Triassic Skagerrak Formation, Central North Sea. In: Proceedings of the 9th international Clay Conference, Strasbourg, 1989. Vol IV : Clays in sediments. Diagenesis and hydrothermalism. Strasbourg : Institut de Géologie – Université Louis-Pasteur, 1990. pp. 125-134. (Sciences Géologiques. Mémoire, 88