Prudhoe Bay is a mixed-wet reservoir where about half the oil recovery is attributable to gravity drainage. Gas/oil relative permeability data show that gravity-drainage recovery efficiency is poorer for more fine-grained sandstone and increases as the grain size increases. Gravity-drainage efficiency also increases with connate-water saturation. Dependence of recovery efficiency on grain size is related to changes in sorting. An effective grain size, defined by inverting the Carman-Kozeny relation, provides a useful parameter for correlating recovery efficiency. This estimate correlates well with visual estimates and direct measurements on disaggregated core. Grain size is also found to be a more effective parameter for correlating trapped gas than porosity, a common alternative. Lithology impacts trapped-gas level with finer-grained, more poorly sorted rock having higher trapped gas. Trapped gas decreases with increasing microporosity. Because little gas is trapped in microporosity, a zero-slope generalization of the Land curve better represents trapped-gas data.