The deep marine sandstones in the Siri Canyon (Fig.1) have been reported to import significant amounts of dissolved silica from adjacent Palaeocene shales during early diagenesis (Stokkendal et al. 2009; Weibel et al. 2010). We have studied the diagenesis of one of these shales, the Sele Formation shale to document the diagenetic steps which releases silica and to evaluate the possible timing of the silica export from the shale into neighbouring sandstones. The major silica mobilization occur at shallow depths, related to the transformation of biogenic opal and volcanic ash, and at intermediate depths, related to the dissolution of opal-CT and zeolite. At these processes, the shale may have been active silica exported. Reported sandstone cementation patterns indicate the earlier phase is related to major export of silica, whereas the shale itself was the major consumer during the intermediate phase. Late phase mobilization of silica (smectite to chlorite transformation) is at an initial stage and has not contributed significantly to the massive late-stage quartz cementation of the interbeded sandstones. The Sele Formation in the Siri Canyon has not been influence by illite formation, even at burial depth of approximately 3000 m. Potassium feldspar is not recorded, even at the shallowest depth, and we suggest that the lack dissolvable potassium sources has delayed the smectite transformation.