Petrography and mineralogy of greensand in the Nini area including mineral reactions in CO₂ flooding experiments. Greensand Project Phase 1 WP2

Glauconitic sandstones from the Nini field in the Siri Canyon, Danish North Sea, were described mineralogically and petrographically to document typical compositional variation besides changes induced by CO₂ flooding experiments. Mineralogical quantification was obtained by petrographic point counting, automated quantitative mineral mapping (AQM), and X-ray diffraction (XRD). The results show that quartz and glauconite are the dominant components in the greensand, whereas feldspars, mica minerals, rock fragments, and heavy minerals are minor to rare constituents. The glauconite clasts consist of mixed-layer Fe-smectite/illite. Many types of partly of wholly glauconized clasts are present. Cementing phyllosilicates in the Nini cores comprise berthierine and mixed-layer Fe-rich illite/smectite that occur as thin rims on glauconite clasts and pore-fillings. Minor diagenetic phases include siderite rhombs, patchy calcite, clinoptilolite, pyrite, microquartz, and K-feldspar overgrowths. Greensand samples from other fields in the Siri Canyon have been analysed to investigate the regional variation. The results show that the glauconite clast composition changes with increasing depth due to illitization, chloritization, and dissolution.

Only few mineralogical changes have been identified by petrographic observations of samples before and after CO₂ experiments. Siderite shows clear dissolution features after the experiments, however, the low amount and scattered occurrence of siderite crystals means the dissolution must have inconsiderable effect on rock stability. No dissolution phenomena were observed in the framework including glauconite clasts, and the sandstones did not disintegrate during the experiments, although they were only loosely consolidated beforehand. However, for some minerals it is difficult to distinguish if dissolution has occurred. A slight but statistically significant increase in Fe-content in glauconite after the experiments is documented by AQM, along with decrease in Si, Al, and K. Halite precipitations were found as a very thin layer on all surfaces after one of the CO₂ flooding experiments. Petrographical investigation of material from the internal traps showed that it consists of crumbled sandstone, and material from the external traps consists of metal flakes of Fe or Fe+Cr, organic fibres, tiny sandstone fragments (quartz, K-feldspar, albite, glauconite, clay, phyllosilicates).

In the Nini field, the glauconitic clasts are smectite-rich due to the rather shallow burial, so special attention should be paid to the reactivity. Although only minor ion exchange occurred during the experiments, it cannot be ruled out that the smectite-rich glauconitic clasts may swell and produce fines when exposed to increased number of cyclicity or changes in salinity and pH, so this is recommended to be tested in future experiments. Experiments with longer duration involving more volumes than in the present study would better constrain the cation exchange.