This study reviews 87Sr/86Sr depth profiles of formation waters sampled by Sr residual salt analysis (Sr RSA) from >100 oil/gas wells and research sites, including reservoirs with clastic and carbonate host rocks and with gas, oil and water as the continuous fluid phase. Globally, the water data form a smooth trend between low seawater-like 87Sr/86Sr ratios (~0.706) at shallow depths and high (~0.724) ratios in deeply buried rocks, where water-rock interaction dominates.

We test the hypothesis that 87Sr/86Sr depth profiles in individual wells could be influenced by diffusional mixing processes by developing 1D diffusion mixing equations to simulate compositional patterns through time and comparing them with observed profiles. Different combinations of boundary and initial conditions generate various patterns characteristic of diffusion, including non-steady-state curves relating to incomplete mixing and steady-state patterns (such as vertical or inclined straight lines) where initial heterogeneities have fully mixed. The dataset yielded 193 occurrences of these patterns. Steady-state patterns are more common and longer in water zones, while non-steady-state patterns are more common and longer in oil and gas zones. The detection of diffusional mixing patterns in hydrocarbon-saturated rocks suggests that diffusion is active, although on average a factor of ~13–18 slower, than in comparable water-saturated rocks.

Pattern generation and equilibration times were modelled for each non-steady-state pattern and compared with the time since reservoir filling with oil/gas, revealing that 90% of them could have been generated since filling, but 60% of them would already have mixed to steady state had the initial compositional heterogeneities arisen during or before reservoir filling. This is critical evidence that at least some of the initial heterogeneities must have arisen, and subsequently partially mixed, after filling; these patterns tend to be short (<40 m, usually <20 m). We conclude that it is essential to consider post-filling processes when interpreting 87Sr/86Sr depth profiles. This may enable the effects of post-filling processes to be stripped back to reveal larger-scale patterns inherited from oil/gas filling. This approach provides a new framework for identifying and quantifying barriers to fluid communication in petroleum reservoirs, which could be applied to help optimize oil/gas production and water or gas injection, including CO2 injection for enhanced oil recovery and/or subsurface storage.

The evidence for post-fill water-rock interaction and diffusive transport has important implications for porosity/permeability prediction, indicating that key diagenetic processes like quartz cementation may be inhibited by hydrocarbon filling but not stopped altogether. Compositional differences of water within oil/gas zones reveal the folly of using one aquifer water sample as the basis for interpreting water saturations from well resistivity logs for in-place resource estimation. Sr RSA studies may thus be useful in designing water sampling strategies.