Fig. 3: Change of stable carbon isotope ratios during H2 co-storage in the Lehen test reservoir.
From: Hydrogen storage and geo-methanation in a depleted underground hydrocarbon reservoir

δ13CCH4 signature (‰; δ-notation according to Vienna Peedee Belemnite standard) measured during injection (circles), shut-in (squares) and production (triangles) phase of the field trial. Vertical dashed lines indicate the transition from injection to shut-in (left) and shut-in to production (right).