Lower Jurassic Cook Formation is an important hydrocarbon reservoir in northern North Sea due to its anomalously high porosity at depths around 3000 meters. This study presents information about the depositional environment, petrographic and petrophysical properties, and reservoir quality of Cook Formation in Oseberg and Veslefrikk area. Reservoirs of Brent and Dunlin groups have been studied by examination of cores from three wells. Reservoir intervals are studied by electrical logs, sedimentological core description, quantitative bulk and clay XRD analysis, optical point counting and SEM analysis. The main goal has been to study the relation between depositional environment, diagenesis, quartz cementation and formation of chlorite precursor. The Cook Formation consists of several depositional settings because of its extensive lateral distribution. In Veslefrikk area it has been interpreted as prograding delta deposit, while in Oseberg are, it is observed as upper shoreface deposit. Sandstones in Cook Formation have overall quartz dominant mineralogy with some carbonate cemented intervals. Kaolinite, chlorite and illite are major clay minerals found. Well developed and extensive chlorite grain coats were observed in Veslefrikk area causing quartz inhibition and porosity preservation. Reservoir quality of Cook Formation is moderate to good in Veslefrikk area. In Oseberg area, sandstones are observed to be more porous and cleaner indicating good reservoir quality. The formation of iron-rich clay precursor in Veslefrikk area has been suggested mainly by mechanical infiltration of clay suspensions and by minor possible contribution from bioturbation. Chlorite coating or precursor was not observed in Oseberg area, possibly due to the erosion during reworking. Prediction of chlorite coating in deeply buried reservoirs is difficult. If the relation between depositional environment and formation of chlorite precursor is understood, then prediction and mapping of chlorite coatings in intermediate to deeply buried reservoirs can become possible.