Characterisation of Gas Transport Properties of the Opalinus Clay, a Potential Host Rock Formation for Radioactive Waste Disposal

The Opalinus Clay in Northern Switzerland has been identified as a potential host rock formation for the disposal of radioactive waste. Comprehensive understanding of gas transport processes through this low-permeability formation forms a key issue in the assessment of repository performance. Field investigations and laboratory experiments suggest an intrinsic permeability of the Opalinus Clay in the order of 10(-20) to 10(-21) m(2) and a moderate anisotropy ratio < 10. Porosity depends on clay content and burial depth; values of similar to 0.12 are reported for the region of interest. Porosimetry indicates that about 10-30 of voids can be classed as macropores, corresponding to an equivalent pore radius > 25 nm. The determined entry pressures are in the range of 0.4-10 MPa and exhibit a marked dependence on intrinsic permeability. Both in situ gas tests and gas permeameter tests on drillcores demonstrate that gas transport through the rock is accompanied by porewater displacement, suggesting that classical flow concepts of immiscible displacement in porous media can be applied when the gas entry pressure (i.e. capillary threshold pressure) is less than the minimum principal stress acting within the rock. Essentially, the pore space accessible to gas flow is restricted to the network of connected macropores, which implies a very low degree of desaturation of the rock during the gas imbibition process. At elevated gas pressures (i.e. when gas pressure approaches the level of total stress that acts on the rock body), evidence was seen for dilatancy controlled gas transport mechanisms. Further field experiments were aimed at creating extended tensile fractures with high fracture transmissivity (hydro- or gasfracs). The test results lead to the conclusion that gas fracturing can be largely ruled out as a risk for post-closure repository performance.

Resolving Cl and SO4 Profiles in a Clay-Rich Rock Sequence

The chloride and sulfate concentration profiles in a 260 m thick clay-rich Mesozoic sediment sequence have been analyzed by various methods. Chloride data generally indicate a good consistency between different methods if anion exclusion is accounted for in leaching tests. For sulfate, however, there is an apparent inconsistency between leaching data and those obtained from the other methods, which points to the dissolution of a sulfur-bearing mineral. Traces of diagenetic gypsum seem to be a likely source, but other sulfur minerals cannot be ruled out.

(Table 1) Characteristics of silicified sediments at DSDP Site 61-462 according to host-rock lithology, opal-CT-to-quartz ratio, and the origin of the quartz phase

This study deals with the mineralogical variability of siliceous and zeolitic sediments, porcellanites, and cherts at small intervals in the continuously cored sequence of Deep Sea Drilling Project Site 462. Skeletal opal is preserved down to a maximum burial depth of 390 meters (middle Eocene). Below this level, the tests are totally dissolved or replaced and filled by opal-CT, quartz, clinoptilolite, and calcite. Etching of opaline tests does not increase continously with deeper burial. Opal solution accompanied by a conspicuous formation of authigenic clinoptilolite has a local maximum in Core 16 (150 m). A causal relationship with the lower Miocene hiatus at this level is highly probable. Oligocene to Cenomanian sediments represent an intermediate stage of silica diagenesis: the opal-CT/quartz ratios of the silicified rocks are frequently greater than 1, and quartz filling pores or replacing foraminifer tests is more widespread than quartz which converted from an opal-CT precursor. As at other sites, there is a marked discontinuity of the transitions from biogenic opal via opal-CT to quartz with increasing depth of burial. Layers with unaltered opal-A alternate with porcellanite beds; the intensity of the opal-CT-to-quartz transformation changes very rapidly from horizon to horizon and obviously is not correlated with lithologic parameters. The silica for authigenic clinoptilolite was derived from biogenic opal and decaying volcanic components.

Stable isotope ratios measured on carbonate tests from the Kara Sea

River discharge of Ob and Yenisei to the Kara Sea is highly variable on seasonal and interannual time scales. River water dominates the shallow bottom water near the river mouths, making it warmer and less saline but seasonally and interannually more changeable than bottom water on the deeper shelf. This hydrographic pattern shows up in measurements and modelling, and in stable isotope records (delta18O, delta13C) along the growth axis of bivalve shells and in multiple analyses of single benthic foraminiferal shells. Average isotope ratios increase, but sample-internal variability decreases with water depth and distance from river mouths. However, isotope records of bivalves and foraminifera of a sediment core from a former submarine channel of Yenisei River reveal a different pattern. The retreat of the river mouth from this site due to early Holocene sea level rise led to increasing average isotope values up core, but not to the expected decrease of the in-sample isotope variability. Southward advection of cold saline water along the palaeo-river channel probably obscured the hydrographic variability during the early Holocene. Later, when sediment filled the channel, the hydrographic variability at the core location remained low, because the shallowing proceeded synchronously with the retreat of the river mouth.

Mg/Ca variability in Globigerinoides ruber tests

Laser ablation inductively coupled plasma-mass spectrometry microanalysis of fossil and live Globigerinoides ruber from the eastern Indian Ocean reveals large variations of Mg/Ca composition both within and between individual tests from core top or plankton pump samples. Although the extent of intertest and intratest compositional variability exceeds that attributable to calcification temperature, the pooled mean Mg/Ca molar values obtained for core top samples between the equator and >30°S form a strong exponential correlation with mean annual sea surface temperature (Mg/Ca mmol/mol = 0.52 exp**0.076SST°C, r**2 = 0.99). The intertest Mg/Ca variability within these deep-sea core top samples is a source of significant uncertainty in Mg/Ca seawater temperature estimates and is notable for being site specific. Our results indicate that widely assumed uncertainties in Mg/Ca thermometry may be underestimated. We show that statistical power analysis can be used to evaluate the number of tests needed to achieve a target level of uncertainty on a sample by sample case. A varying bias also arises from the presence and varying mix of two morphotypes (G. ruber ruber and G. ruber pyramidalis), which have different mean Mg/Ca values. Estimated calcification temperature differences between these morphotypes range up to 5°C and are notable for correlating with the seasonal range in seawater temperature at different sites.

Consolidation tests of ODP Leg 113 samples

Examination of the geotechnical characteristics of Weddell Sea, Maud Rise, and South Orkney microcontinental margin sediments recovered during ODP Leg 113 reveals that the reduction in porosity (consolidation) of the siliciclastic, calcareous, and diatomaceous sediments is primarily a process governed by vertical stresses created by overburden. The initial porosity of the sediments in these areas is governed by the amount of diatoms present. The more diatoms, the higher the porosity. Surficial diatom-rich sediments are everywhere overconsolidated. This is attributed to the strong microfabric created by the diatoms, calcareous and clay particles. The deeper diatom-free sediments of Maud Rise range from slightly underconsolidated to normally consolidated. The silty clays and clays of the Weddell Sea and South Orkney margin are underconsolidated. The degree of underconsolidation of these sediments is similar to that determined in a number of different locations throughout the world's oceans. The very low permeability of the Weddell Sea and South Orkney margin sediments appears to account for this underconsolidation.

Stress tests of ODP Site 131-808 sediments

Sediments undergoing accretion in trench-forearc systems are subjected to conditions of large lateral thrusting. This stress regime controls the mechanism of faulting as well as the yield and strength properties of the sediment. Understanding them is therefore crucial for the construction of quantitative models of sediment dynamics in convergent margin settings. For this purpose triaxial and oedometer tests were performed on six whole-round core samples recovered from Site 808 from depths between 173 and 705 mbsf. Samples from five depth intervals were subjected to a triaxial test program that was primarily designed to define yield and strength behavior. Test specimens were cut parallel and normal to the core axis. Additional five oedometer tests with similarly prepared specimens were performed on samples from four depth intervals to evaluate the directional state and degree of sediment compaction. Test results show that the degree of sediment compaction is higher than expected from overburden. This overcompaction increases with depth. A well-developed mechanical anisotropy is evident in all samples tested, regardless of their depth and lithology. Values of yield limit, stiffness, and shear strength are up to 40% higher in the horizontal direction compared to the vertical direction. In addition the test data demonstrate that the axis of the volumetric yield loci have rotated into extensional stress field. This verifies that the mechanical state of sediment in the accretionary wedge is controlled by in-situ stress conditions of extensional nature. The coefficients of lateral stress inferred suggest that the extensional stress regime becomes increasingly effective with depth.