Well-connected basalt sequences as potential reservoirs for large-scale carbon sequestration revealed by strontium isotopes

Well-connected basalt sequences as potential reservoirs for large-scale carbon sequestration revealed by strontium isotopes

Ingar Johansen¹, P. Craig Smalley², Vani N. Devegowda¹, John M. Millett^3,4,5, Marija P. Rosenqvist⁴, Mohamed Mansour Abdelmalak⁴, Sverre Planke^3,4, Stéphane Polteau^1*,

1 –Institute for Energy Technology, Kjeller, Norway

2 –Imperial College London, London, UK

3 – Volcanic Basin Energy Research, Oslo, Norway

4 - University of Oslo, Oslo, Norway

5 - University of Aberdeen, Aberdeen, UK

 

Basalt complexes can cover hundreds to thousands of km2 and be several km thick; with an estimated global capacity of 40 Tt for carbon storage, they represent a solution for the large-scale injection of CO₂ to reach the 2050 emission targets. However, the hydraulic conductivity of basalt sequences is difficult to predict because it has never been the focus in basalt research. In this contribution, we evaluated for the first time the vertical fluid connectivity of a basalt sequence using the Sr isotope composition of pore waters, sampled using the strontium residual salt analysis (SrRSA) method. Thirty-seven samples were collected in the U1571A borehole (IODP Expedition 396 on the Skoll High, offshore Mid-Norway). Sampling of the well targeted the most representative lithologies in the core and hence included vesicular and tight basalt. The SrRSA method measures the ⁸⁷Sr/⁸⁶Sr ratio in the salt residue that precipitated in the pores of core samples after the pore water evaporated, and the value measured in the laboratory should accurately reflect that of the in-situ pore water. However, since the well was drilled using seawater with barite and sepiolite additives, each sample was washed in de-ionized water to reduce potential contamination. The samples were subsequently dried, crushed, the salts were leached, the leachate filtered, and the strontium analyzed using a MC-ICPS-MS. The results show that the Sr concentrations are very low: 1-35 ppb in vesicular basalt samples and up to 2 ppb in tight basalt samples. The ⁸⁷Sr/⁸⁶Sr ratio of the vesicular basalt varies little away from the average value of 0.7085, while the values of tight basalts show more variability with an average of 0.7093. The SrRSA pattern vs. depth for the vesicular basalt samples is smooth, indicating limited contamination from drilling fluids, and further suggesting a good vertical connectivity. On the other hand, the pattern of the tight basalts is shifted towards heavier values similar to modern seawater, suggesting the SrRSA for these samples to be contaminated by the drilling fluids. In addition to the residual salts, we are currently analyzing the ⁸⁷Sr/⁸⁶Sr in carbonate vesicles and in basalt, which together represent the three main reservoirs of strontium in the samples. These additional results should help us constrain the source of the strontium in the pore system (i.e.:  external, flowing or diffusing through the sequence). These results further show that the fluid connectivity of basalt sequences can be characterized using the SrRSA method by focusing the analyses on vesicular basalt samples. Finaly, this study will provide additional constrains to follow-up numerical models simulating the injectivity and capacity of intra- and inter-basalt reservoir units.