<p>Geological storage is expected to play an important role in the energy transition, in various forms spanning across CO₂ sequestration and nuclear waste storage to that of short-term energy (H₂ or compressed air energy storage, CAES). The efficiency of geological storage relies on the low permeability of clayrocks, whose integrity might be compromised if they exhibit brittle behavior. Therefore, identifying the geological conditions that lead to brittleness is essential to guide storage projects toward the safest options. Here, we show that diagenesis is the main control on clayrock brittleness and that it can be predicted using first order parameters accessible from exploration borehole data. We evidence a strong correlation between clayrocks unconfined compressive strength and burial-induced diagenesis, driven by compaction and cementation resulting from clay-mineral transformation. We suggest that mechanically-consolidated clayrocks may retain ductile, low-permeability behavior if they remain confined, while cementation favors their brittle behavior. The trend we present enables the estimation of brittleness and permeability behavior of clayrocks based on their burial history. We propose that this trend can help guide the selection of safe storage sites when integrated into a dedicated exploration framework.</p>