Mineralised fractures (veins) in the Opalinus Clay are found in all boreholes of the TBO pro­gramme (Sections 4.7.4, 4.7.5 and 5.5.4). They are very thin (> 90% are thinner than 2 mm; Sections 4.7.3 and 5.5.4), macroscopically closed and show no detectable effect on the results of hydraulic testing (Section 5.6.3). Larger veins (up to centimetres in thickness) as depicted in Fig. 5‑31 are extremely rare and vary strongly in thickness and continuity even at the core scale.

Veins are observed in clay-rich rocks or shales worldwide (e.g. Cobbold et al. 2013, Gale et al. 2014). They are typically interpreted to have formed by precipitation from supersaturated aqueous solutions in the dilatational fractures (e.g. Bons et al. 2012), with precipitation and mineral growth leading to clogging of voids and essentially acting as a sealing mechanism (Bock et al. 2010; Section 5.7.3). However, mineral growth may also have been concurrent with fracture opening, as has been proposed for bedding-parallel «beef» structures in shales (Cobbold & Rodrigues 2007, Cobbold et al. 2013), that is with no open space for veins to grow into. Irregular, multi-stranded veins have also been considered to represent dewatering structures during early burial of fluid-overpressured mudstones (Lorenz & Cooper 2018). What all the models seem to have in common is that the local effective stress at the time of vein formation was probably quite low to allow for volume expansion (Fig. 5‑24), either with or without fluid overpressure.

Low effective stress conditions can occur during the early stage of burial, especially during rapid sedimentation (Section 4.1) and burial as inferred for the Middle Jurassic (Section 4.3.5). Together with the apparent thermal anomaly in the Late Jurassic to Early Cretaceous, this has probably caused temporary fluid overpressure from disequilibrium compaction. The geochemical signature of vein cements from the Lias and Dogger Groups and formation temperature (Akker et al. 2023) are consistent with such a conceptual model. Also, some of the matrix cement in the Opalinus Clay is attributed to such an early phase of burial (Section 5.2.5). Another phase of rapid burial occurred during the Oligocene and Early Miocene (Section 4.3.5).

Fluid overpressure may also have been triggered from the transformation of mineral reactions. For the Opalinus Clay, this could mainly concern the smectite to smectite-illite transformation with release of structurally bound water (Nadeau et al. 2005).

Tectonic loading may also have increased pore fluid pressure in the Opalinus Clay, provided that the loading rate was fast enough with respect to the drainage capacity or fluid dissipation rate (Section 5.5.3). But, as highlighted in the conceptual figure in Section 4.4, lateral tectonic loading has a stronger effect on stresses in the more competent layers (Malm, Keuper and Muschelkalk Groups). In other words, the stress field in the Opalinus Clay is somewhat shielded by these more competent layers. A certain degree of plastic deformation was also accommodated in the Opalinus Clay during the Miocene to present-day shortening.

There are substantial quantities of calcite in the Opalinus Clay (Section 5.2) and the porewater in the Opalinus Clay today is saturated in calcite and could therefore provide a local source for vein precipitates (Section 4.7). However, it remains unclear whether the existing veins formed in a closed system. Veins in the Opalinus Clay formed at higher or much higher temperatures than the present-day in-situ temperature, with plausible formation ages greater than 5 million years (Section 4.7.4), with most veins probably forming during the Late Jurassic – Cretaceous, and later during Eocene – Miocene times (Section 4.3.5).

Vein formation in the Opalinus Clay at low effective stress is also consistent with a minimum effective stress threshold required for robust self-sealing (see Section 5.7.4). Fracture closure is expected to occur quickly if low effective stress (high fluid pressure) is not maintained, and the rate of swelling is dependent on water supply. If the latter is slow, swelling is slowed down, but this can only occur in a tight ("closed") system, i.e. where water supply from the adjacent rock is low. In contrast, if water supply is ensured, a significant reduction in transmissivity will be fast, i.e. over days in laboratory experiments (e.g. Fig. 5‑43) and weeks to months in field experiments (Nagra 2014d, Lanyon et al. 2014).

The rate of self-sealing by swelling also depends on the width of the fractures generated during deformation. But, as elaborated in Section 5.5, deformation in the Opalinus Clay is characterised by a relatively low brittleness (low softening, low dilation angle), and more distributed deforma­tion with low fracture width. These aspects also favour rapid self-sealing.

In summary, keeping in mind that veins record the integrated burial and tectonic history since the time of deposition, the following conclusions can be drawn:

• Veins in the Opalinus Clay are rare, generally very thin, often discontinuous at core and outcrop scale and, because of soft-linked accommodation of deformation, are poorly interconnected, i.e. they are hydraulically insignificant.

• Their formation is probably associated with low effective stress conditions, which were particularly prevalent during early diagenesis and burial in the Middle and Late Jurassic, and again during the Oligocene and Miocene. As in the clay-rich rocks in the Lias and Dogger Groups, it is probable that many veins in the Opalinus Clay are old (> 5 Ma) to very old structures (> 100 Ma). This interpretation is also consistent with tracer profiles (Section 4.6), suggesting that transport is diffusion-controlled also across intervals of the Opalinus Clay containing veins.

• Today’s porewater is saturated in calcite and could provide a local source of vein formation without the need for an external water source.

• At present depths in the siting regions, the effective stress conditions (Section 4.4) are well above the critical effective stresses of 2 – 3 MPa at which relevant modifications to the self-sealing potential could be expected (Section 5.7.4), and therefore the key mechanisms for robust self-sealing are expected to operate at the rate at which water can be supplied.