Evaluation of δ2H and δ18O of water in pores extracted by compression method-effects of closed pores and comparison to direct vapor equilibration and laser spectrometry method

Abstract Stable isotopes (δ 2 H and δ 18 O) of water can help our understanding of origin, mixing and migration of groundwater. In the formation with low permeability, it provides information about migration mechanism of ion such as diffusion and/or advection. Thus it has been realized as very important information to understand the migration of water and ions in it. However, in formation with low permeability it is difficult to obtain the ground water sample as liquid and water in pores needs to be extracted to estimate it. Compressing rock is the most common and widely used method of extracting water in pores. However, changes in δ 2 H and δ 18 O may take place during compression because changes in ion concentration have been reported in previous studies. In this study, two natural rocks were compressed, and the changes in the δ 2 H and δ 18 O with compression pressure were investigated. Mechanisms for the changes in water isotopes observed during the compression were then discussed. In addition, δ 2 H and δ 18 O of water in pores were also evaluated by direct vapor equilibration and laser spectrometry (DVE-LS) and δ 2 H and δ 18 O were compared with those obtained by compression. δ 2 H was found to change during the compression and a part of this change was found to be explained by the effect of water from closed pores extracted by compression. In addition, water isotopes in both open and closed pores were estimated by combining the results of 2 kinds of compression experiments. Water isotopes evaluated by compression that not be affected by water from closed pores showed good agreements with those obtained by DVE-LS indicating compression could show the mixed information of water from open and closed pores, while DVE-LS could show the information only for open pores. Thus, the comparison of water isotopes obtained by compression and DVE-LS could provide the information about water isotopes in closed and open pores.