Reactivity of glauconitic clasts during burial diagenesis

Palaeogene glauconitic reservoir sandstones in the produced Nini West oilfield in the Siri Canyon, Danish North Sea, are chosen as the first pilot CO₂ storage site in Denmark. Since glauconitic clasts comprise up to 35 vol% of the sandstones, their stability is crucial for the reservoir permanence. The likely reactivity of glauconitic clasts during CO₂ injection is evaluated from their changes during burial diagenesis (long-term reactions) as a contribution to experimental testing (short-term reactions) of sandstone resilience towards CO₂. Cored wells penetrate the Palaeogene gravity-flow sandstone units in depth intervals of 1700 m (<52 °C) to 3100 m (107 °C). The depth range allows a detailed microscopic and geochemical investigation of diagenetic changes in mineralogical composition with increasing burial depth. Although the glauconitic clasts show a large variation in chemical composition, clear average trends of increasing K₂O and decreasing Fe₂O₃ in glauconitic clasts are recorded with increasing burial depth down to 2600 m. This trend is caused by the gradually reduced number of expandable layers (smectite) in the mixed-layer glauconitic mica/Fe-smectite. This shift in smectitic content is accompanied by a change in authigenic minerals from berthierine, siderite, microquartz and opal coatings at shallow burial depth to chlorite, calcite and quartz overgrowths at burial > 2200 m. Chloritisation of glauconitic clasts is intensive in the central parts of the gravity-flow sandstone units in the deepest (2800–3000 m) and most distal settings. Consequently, authigenic chlorite precipitation in pores occurs at shallower burial depth than chloritisation of glauconitic clasts. The microporous structure of the glauconitic clasts controls the slow gradual illitisation during increased burial. Abrupt lowering of pore-water salinity is considered the main reason for expansion of smectite components in the mixed-layer glauconitic mica/Fe-smectite, opening of the micropores and thereby intense chloritisation of glauconitic clasts in the deep and distal gravity-flow units. Hence, glauconitic clasts are potentially highly reactive, although slow to react due to their microporous structure unless the water composition is distinctly changed.