Fault risk remains a key parameter in evaluating the potential for trapping CO2 in the subsurface, yet very little is known about the conditions under which CO2 and CO2/hydrocarbon mixtures are retained by faults. The project will investigate the roles and properties of faults in their capacity to retain CO2. Natural and engineered accumulations of hydrocarbon and CO2-hydrocarbon mixtures will be examined across a wide self-similar province (to minimize geological variability) to develop a knowledge base of fault flow properties. Fault geometries, orientations, seismic attributes, proven vertical trapping and lateral pressure retention values and column-heights will be documented. High-quality data-rich examples will be selected for analysis with established software tools to predict and calibrate CO2 column height and pressure retention. Differences between prediction and observation will be reconciled by checking site-specific geology and optimising the petrophysical property values assigned to the faults, reservoir, seals and fluids (within realistic ranges) to produce an understandable pragmatic and calibrated fit. The fault properties knowledge-base and the newly calibrated tools will be applied to selected key reservoirs from the ETI UK Storage Assessment Project (UKSAP). This will provide improved and evidence-based assessment of storage in regional UK North Sea aquifers such as the Bunter Sandstone, Forties, Tay and Captain. These are some of the largest and promising early developments for storage and are vital to reducing storage costs via multi-user storage. The Bunter Sandstone has 8Gt CO2 unrisked capacity - but only 1Gt may be considered viable because of fault risk. The Captain, Forties and Tay sandstones total 11.5Gt CO2 unrisked capacity, of which only 1Gt may be currently considered viable. The impact of the research will be to upgrade parts of the UKSAP assessment and to assist the development of the large capacity element in these formations that does have perceived fault risk. Grant number: UKCCSRC-C1-14.