Grain-rimming kaolinite in Permian Rotliegend reservoir rocks

Upper Rotliegend sediments of Permian age from the northeast Netherlands show moderate to good reservoir
qualities. The predominant control is by the presence of authigenic grain-rimming kaolinite, which has a
negative, but in some parts also a positive, effect on reservoir quality. To better understand the formation and
distribution of grain-rimming kaolinite, reservoir rocks were studied in terms of composition and diagenetic
processes. Petrographic evidence, summarized as a paragenetic sequence, is integrated with geochemical
modeling results to identify early mesodiagenetic water–rock interactions under the participation of gases,
i.e., CO2 and H2S, released from underlying Carboniferous source rocks. The sediments investigated were
deposited at varying distance from the southern flank of the Southern Permian Basin. Sediments near the
basin margin are mainly attributed to a fluvial environment and comprise mediumto coarse-grained sandstones
and conglomerates. There, vermicular kaolinite occurswith a lath-like structure. Distal to the basinmargin, mainly
in sandstones intercalated with fine-grained playa sediments, comparatively high amounts of grain-rimming
kaolinite occur. There, the presence of this mineral has a significant influence on the rock properties and the
reservoir quality. Geochemical modeling suggests that the formation of such kaolinites cannot be explained
exclusively by in situ feldspar dissolution. The modeling results support evidence that kaolinite can be formed
from precursor clay minerals under the presence of CO2-rich formation waters. Such clay minerals could be
corrensite, smectite–chlorite mixed-layer minerals, or chlorite that is potentially present in Rotliegend sediments
during early diagenesis. Furthermore, the geochemical modeling can reflect several mineral reactions that were
identified from petrographic analysis such as the formation of illite and kaolinite at the expense of feldspar
dissolution and consequent silica precipitation.Under the presence of sulfate (SO4 2 −), partial hematite dissolution
and the reduction of Fe3+ to Fe2+ occur, where the latter is further incorporated in Fe–Mg-rich carbonates. The
sulfate is bonded in anhydrite. The results of this study are relevant for reservoir quality predictions in kaolinitedominated
systems, as well as for CO2 storage projects and the prediction of long-term fluid–rock interactions
under the participation of carbon dioxide and/or other gases