Lotsberg Formation: lithological and geochemical constraints for a prime H2 cavern target in Alberta, Canada

Abstract

Abstract Homogenous and thick (approximately 40–170 m) halite intervals in the upper part of the Lotsberg Formation are most favored targets for hydrogen (H2) storage caverns in Alberta. However, repurposing cavern-making technologies for H2 storage must consider higher diffusivity and higher reactivity of H2, including its known dissolving effect on sulfate minerals and intense production of H2S through bacterial sulfate reduction. New core observations, geochemical and XRD data made on a continuous core through the upper informal member of the Lotsberg Formation and overlying red beds elucidate high content of anhydrite nodules and partings in these red beds, whereas the thick (42.9 m in our test well), exceptionally clean and homogeneous upper halite in this succession contains anhydrite only in trace amounts. A dolomarl-rich interval at 1894.0–1899.45 m, traced regionally as the L2 marker, represents a solution-collapse breccia, thus indicating an intraformational unconformity and an episode of meteoric salt removal prior to deposition of the upper-most halite of the Lotsberg Formation. If the cavern roof is made close to the overlying anhydritic dolostone of the Ernestina Lake Formation, reactivity of H2 may cause rapid dissolution of anhydrites leading to roof collapse, as well as accumulation of H2S through bacterial sulfate reduction. We infer that preserving a thick salt roof during cavern making may be a solution to prevent these damaging effects. Reactivity of H2 with carbonates in the caprock should also be considered. Anhydrite nodules also occur in the basal one-third of the upper Lotsberg, the interval containing more non-halite impurities than the upper salt unit of this member. In this part of the section, anhydrites do not seem to represent the same concern as they will be exposed to cavern-floor sump and cushion gas, whereas H2 reservoir can be operated within the limits of the clean and homogeneous upper halite of the upper Lotsberg Formation. Emplacement of horizontally elongated two-well caverns may represent an adequate way to overcome cavern size limitations, especially in overlying, thinner-bedded halites of the Prairie Evaporite Formation.