I-S precipitation in pore space as the cause of geopressuring in Mesozoic mudstones, Egersund Basin, Norwegian continental shelf

Effect of diagenesis on pore pressures in fine-grained rocks in the Egersund Basin, Central North Sea

“…Furthermore, the newly formed microquartz crystals may decrease the size of the pore throats significantly and strongly influence the permeability (Nadeau et al, 2002;Thyberg et al, 2010). In addition, the neoformed illite will be oriented parallel to the bedding (normal to the principal stress axis), decreasing the vertical permeability.…”

“…The Egersund data points show higher Vp compared to the Tau data points due to a lower TOC conIn fine-grained sediments, illitisation is found to be a rather slow process with increasing depth/temperature. The nature of illitisation in clay-dominated sedimentary sections triggers a gentle pressure increase termed overpressure ramp by Nadeau (2011) and Nadeau et al (2002). Progressive illitisation and quartz cementation is reflected by changes in petrophysical properties.…”

“…This shows how uncertain pre diction of pore pressure in organic-rich shales and mudstones is due to the complicated interplay between various parameters influencing variations in pore pressure. Besides the level of organic maturation, the porepressure development in organic-rich, fine-grained sediments can be directly affected by the nature and progress of chemical compaction and the microfracture frequency (Vernik & Landis, 1996;Berg & Gangi, 1999;Osborne & Swarbrick, 1999;Nadeau et al, 2002;Nadeau, 2011). The potential for pressure build-up may be compensated by microfracturing (Fig.…”

Compaction of diagenetically altered mudstones – Part 1: Mechanical and chemical contributions

“…When mudstones have attained a temperature of ~120 C through burial, the proportion of expandable 'smectitic' interlayers containing hydrated cations in mixed-layer illite/smectite crystals has generally reduced to about 20 % with all the smectite 2:1 layers having dissolved, so that the remaining expandable interlayers separate illite fundamental particles (Środoń et al, 2000). Details of other diagenetic reactions that take place in mudstones in the temperature range 70-120 C are given by Hower et al (1976), Freed andPeacor (1989), Bjørlykke (1998), Nadeau et al (2002), Nadeau (2011) and Thyberg and Jahren (2011). Above 120 C, kaolinite starts to transform to illite, provided that potassium is still available, with further release of water (Giorgetti et al, 2000;Nadeau et al, 2002).…”

“…Details of other diagenetic reactions that take place in mudstones in the temperature range 70-120 C are given by Hower et al (1976), Freed andPeacor (1989), Bjørlykke (1998), Nadeau et al (2002), Nadeau (2011) and Thyberg and Jahren (2011). Above 120 C, kaolinite starts to transform to illite, provided that potassium is still available, with further release of water (Giorgetti et al, 2000;Nadeau et al, 2002).…”