STYLOLITES, POROSITY, DEPOSITIONAL TEXTURE, AND SILICATES. RELATIONSHIP TO BURIAL-STRESS AND TEMPERATURE OF ONTONG JAVA PLATEAU CHALK – COMPARISON TO GORM FIELD, NORTH SEA

In the Ontong Java Plateau (OJP) chalk-sediments mechanical compaction causes a porosity-decline from 70% to 50% in the uppermost 600 m. Wackestones and mudstones tend to have similar porosity because the wall and cavity of the microfossils contribute equally to pore-space and to solid phase. Initial pore-filling cementation can be correlated to the occurrence of the first stylolites at 830 m accompanied by a decrease in bulk sediment δ¹⁸O. Temperature is around 20^oC and effective burial stress 6 MPa.

Stylolites may originate in primary diffuse green bands enriched in smectite-chlorite, probably of volcanic origin. Below 490 m the bands are replaced by more distinct flaser structures enriched in smectite. The onset of pressure dissolution thus corresponds to an interval of waning mechanical compaction.

Down to 790 m the chalk contains biogenic opal-A, but then opal-CT gradually takes over, and below 1130 m quartz is the only silica-phase. This coincides with the cementation front, below which depth pore-filling calcite cementation causes porosity to drop. Silica-Ca complexes in the pore water may become unstable when the temperature and age cause quartz to become the stable silica phase. Pore-filling cementation tends to fill microfossils completely, so that textural differences are now clearly visible on the porosity log.

In the stylolite-bearing chalk of the Gorm field, in the Central North Sea, porosity is 20%-40%, lower than OJP chalk but higher than OJP limestone. The difference is reflecting cementation of microfossils whereas the matrix porosity is similar to the OJP chalk. The difference between OJP and Gorm field chalk cannot be explained alone by early introduction of hydrocarbons, which does not explain the cementation of microfossils. Another factor may be the overpressure in the chalk of the central North Sea: the ratio between temperature and effective burial stress is high. Silica may thus have equilibrated to quartz before stylolites formed, so that calcite cementation was not retarded by silica diagenesis.