We imaged an oil-wet sandstone at residual oil saturation (S,) conditions using X-ray micro-tomography with a nominal voxel size of (9 mu m)(3) and monochromatic light from a synchrotror, source. The sandstone was rendered oil-wet by ageing with a North Sea crude oil to represent a typical vrettability encountered in hydrocarbon reservoirs. We measured a significantly lower S-or for the oil-wet tore (18.8%) than for an analogue water-wet core (35%). We analysed the residual oil cluster size distribution and find consistency with percolation theory that predicts a power-law cluster size distribution. We measure a power-law exponent tau = 2.12 for the oil-wet core which is higher than tau for the water-wet system (tau = 2.05), indicating fewer large clusters in the oil-wet case. The clusters are rough and sheet-like consistent with connectivity established through layers in the pore space and occupancy of the smallei pores; in contrast the clusters for water-wet media occupy the centres of the larger pores. These results imply less trapping of oil, but with a greater surface area for dissolution. In carbon storage applications, this suggests that in CO2-wet systems, capillary trapping is less significant, but that there is a large surface area for dissolution and reaction. (C) 2012 Elsevier Inc. All rights reserved.