Many subsurface technologies—including geologic CO₂ storage—require a quantitative understanding of the mechanical and hydraulic behavior of geologic faults. To minimize earthquake hazard and associated leakage hazard^1-4 , it seems prudent to target geologic formations in extensional sedimentary basins away from the crystalline basement². Here, we address the multiphase flow aspects in a setting with these features by means of a 3D computational flow model of CO₂ migration in faulted, unlithified sediments.

Importantly, faults developed in granular materials with low cohesion tend to display an internal permeability structure that differs from the low-permeability core surrounded by a highly-connected damage zone model typical of mature faults in lithified rocks^5-9. Hence, we incorporate a detailed, physics-based, probabilistic representation of clay and sand smearing to populate the flow properties of a large-scale (throw ≈ 100m) normal fault. At the scale of tens of kilometers, our flow model is representative of the Miocene sedimentary section in shallow waters offshore Texas, which offers excellent potential for storing CO₂ at scale¹⁰. We employ the computer code MRST¹¹ to simulate CO₂ injection into a saline aquifer, and track the plume migration within the injection layer, along the fault zone, and in overlying formations. The multiphase flow properties of the fault depend on the displaced stratigraphic sequence. We consider different stratigraphic cases in the top seal formation, ranging from a thick, clay-rich, partially offset layer to an interbedded succession of finer, totally offset sands and clays.