To evaluate the possible changes in petrology within the reservoir sand and across the oil water contact in Rau-1A, Siri Canyon Danish North Sea, 18 samples were selected and studied mainly by electron microscope and XRD. The major diagenetic phases in the well are micro quartz, large syntaxial quartz overgrowths, calcite and chlorite and also minor amount of diagenetic K-feldspar. The investigated materials were core samples of Palaeocene sands referred to the Ty Member from the well Rau 1A. Chlorite was formed as pervasive grain-coating cement, but much porosity was still present. In the oil zone and deeper, porous samples were later cemented by macro quartz, with larger amounts in the water zone. Chlorite is found in varying amounts in Rau 1A samples. It can be recognized in SEM as a platy or bladed precipitate, which is mostly rich in iron. Chlorite is present in most samples, although only in traceable amounts in the samples which are dominated by microquartz cement. There seem to be two chlorite phases: The first phase occurs as rosettes in a grain coating growth pattern. It is partially intergrown with microquartz or forms a dense mixture of small chlorite rosettes and scattered microquartz. Pore-lining chlorite is partially post-compactional and grows into fractures in glauconite grains. Pore lining chlorite is formed prior to macroquartz and post-compactional calcite; whereas early calcite cemented samples did not evolve chlorite. The general impression is, however, that chlorite cement is more abundant and better developed in the lower parts of reservoir units, whereas it may be scarce and poorly developed in upper parts, especially within the oil zone. Below the oil-water contact, the growth of chlorite may have continued for a longer period, resulting in more dense coatings, which are very destructive for permeability.