Characterisation of Matrix Pore Water and Fluid Inclusions in Olkiluoto Bedrock from Drilling OL-KR47

Matrix pore water from core samples from drilling OL-KR47 was chemically and isotopically successfully characterised. Matrix pore water that resides in the intragranular and intergranular connected pore space of low permeable crystalline rocks was extracted by indirect laboratory methods applied to naturally saturated core samples. The derivation of the composition of matrix pore water by indirect methods depends on an accurate determination of the connected porosity. Connected porosity values were determined by two independent methods based on the water content of the naturally saturated core samples, i.e. by gravimetric determination of the water content and by diffusive isotope exchange. For the two major lithologies, intercepted by borehole OLKR47, connected porosities vary between 0.26 and 1.37 Vol.% for veined gneiss and between 0.62 and 0.88 Vol.% for pegmatitic granite. Based on a detailed evaluation, it can be inferred that porosity values are representative for in situ conditions within the uncertainty ranges. Variations in connected porosity within the same lithologies along the depth profile are partly attributed to variations in the degree of alteration of the main rock forming minerals. Within this work, solute transport properties of the crystalline rock samples used for pore water investigations were characterised. It could be ascertained that diffusion is the dominant transport process in the connected pore space in the different lithologies. Pore diffusion coefficients are between 1.3 and 6.5·10-11 m/s (20 °C) for veined gneiss and between 4.0 and 7.5·10 m/s for pegmatitic granite. Besides matrix pore water and fracture groundwater, fluid inclusions constitute a third reservoir for liquid and gaseous fluids in crystalline bedrock. These fluids can be released from the host mineral by tectonic processes and changes in the pressure and/or temperature regime of the bedrock system. Fluid inclusions host a considerable reservoir that might influence the chemistry of matrix pore water and fracture groundwater and the gas budget of natural bedrock systems. In addition, the chemical characterisation of the entrapped fluids and gases provides information about the composition of the hydrothermal palaeo-fluids, which have circulated during the formation and later evolution of the bedrock system. Independent of the lithology and depth of the investigated rock, five different secondary fluid inclusion generations could be determined in crystalline bedrock samples from Olkiluoto. Depending on the generation, the salinity of the liquid phase of the fluid inclusions varies between 0.88 and 12.84 mass% NaCleq. The gas phase of the detected fluid inclusion generations contains, depending on the generation and depth, CO2, N2, H2, CH4 and higher hydrocarbons in various amounts. Variations in gas compositions and salinities between the individual generations suggest that the original fluids had different chemical compositions and redox states during entrapment. In crystalline bedrock systems, matrix pore water interacts predominately by diffusion with groundwater flowing in fractures. Due to the slow exchange between the two reservoirs, pore water may act as an archive of past fracture groundwater compositions and therefore the palaeohydrological history of a site. Matrix pore waters, which were characterised by the natural tracers Cl, Br, O, H and Cl, are interpreted in context of the palaeohydrological history of the site by comparing them with fracture groundwater at similar depth. In the shallow bedrock zone (0-150 m.b.s.) matrix pore water is in transient state with respect to Cl and Cl compared to fracture groundwater from similar depths, whereas stable water isotopes suggest apparent steady state conditions between the two reservoirs. Diluted Cl-concentrations and depleted stable isotope ratios in the pore water of the shallow zone indicate the presence of a cold-climate or glacial component, suggesting that pore water in the shallow zone is not entirely overprinted by later marine stages. Combined with the short distances (<3m) between matrix pore water samples and the nearest water-conducting fractures, the tracer data show that the groundwater sampled today from the fractures intersected by borehole OL-KR47 in the shallow zone has circulated in this fracture system only for a short period of time. In the intermediate bedrock zone (150-400 m. b.s.) the trend described above for the shallow bedrock zone continues to a depth of about 210 m.b.s. Below this depth, matrix pore water is in transient state compared to fracture groundwater being diluted in total Cl and enriched in heavy water isotopes compared to present-day fracture groundwater from similar depths, and depleted in heavy water isotopes compared to Littorina seawater. This suggests that meteoric water is also preserved in matrix pore water from such depths. Br/Cl indicate a predominately marine provenance of the Cl preserved in pore water from 150 to 325 m.b.s. and mainly a non-marine one from 355 to 381 m.b.s. The heterogeneities in natural tracers and different distances between pore water and the nearest water-conducting fracture in this bedrock zone indicates that different meteoric low Cl as well as high Cl water components are preserved in the pore water. The transient state of matrix pore water compared to present-day brackish fracture groundwater with respect to total Cl, which is independent to the distances between the two reservoirs, suggests long term circulations of dilute fresh groundwater components in the fractures, considerably longer than the more recent influence of brackish fracture groundwater. In the deep bedrock zone (400-800 m.b.s.), matrix pore water is in a transient state compared to saline fracture groundwater from similar depths with respect to total Cl and stable water isotope compositions. Chlorine isotope ratios and Br/Cl mass ratios suggest a non-marine origin of Cl in the pore water. The dilute Cl concentrations and less negative O and H values of matrix pore water compared to saline type fracture groundwater give evidence for the preservation of a warm climatic meteoric water component in matrix pore water. Based on the given conditions, the preservation of the dilute pore water in the rock matrix with distances 20 m to the next water-conducting fracture indicates very long residence times for the fresh water component, or the occurrence of multiple such fresh water stages in the fracture system.