The Early to Middle Jurassic of the Norwegian Sea shows local preservation of high porosity reservoirs (>30%) at great burial depths (>5000m) where thorough silica cementation is to be expected. The deposits are littoral facies with a strong tidal influence. The reservoir sandstones display thick chlorite coatings around quartz grains, whereas the cemented intervals show discontinuous to no chlorite coatings. The thick coatings are shown to have prevented quartz overgrowth during burial.
The development of chlorite is proven to have been a synsedimentary to extremely early diagenetic process. Modern analogues suggest that optimal chlorite growth conditions are reached where freshwater and marine waters mix. Core observations show that all facies containing a significant amount of chlorite correspond to mixed tide- and flood-influenced deposits, although the converse is not true. Some of the mixed tidal-fluvial facies display poorly developed chlorite, leading to further over-cementation.
A detailed sequence stratigraphic approach has shown that chlorite-rich levels are located in the aggradational stages of genetic units, whereas the facies with lower chlorite content are characteristic of progradational stages. It is suggested that during progradation the water-mixing zone was shifted seawards, with lower potential for infiltrating the littoral sands. Periods of increasing accommodation were characterised by better mixing of marine and meteoric waters, leading to precipitation of chlorite within broad sand-flat areas. This distribution is observed at several scales of stacking of genetic units, and allows detailed stratigraphic prediction of good reservoir occurrence.