Carbonate rocks are the cornerstone to both pure and applied Earth sciences. They chronicle Earth's early evolution, host most of the planet's hydrocarbon resources, and yet offer promising storage options for anthropogenic CO2. Physical properties, such as porosity, are the major factors controlling the production quality of hydrocarbon reservoirs, the ability of carbonate reservoirs to accurately preserve climatic and biologic signals, and determine the feasibility of a particular carbonate reservoir for geologic CO2 storage. Traditional measurements cannot capture the complexity of the scale-dependent heterogeneities present within carbonate rocks. We are proposing to answer the questions: 1) What controls the formation, evolution, and hydraulic accessibility of pores in the size range 800 nm - 10 microns within carbonate reservoirs? and 2) Which of these nanoscale pores are physically accessible to flowing reservoir fluids (i.e., hydrocarbons)? By performing measurements both in air and saturated with a contrast-matched solvent, we will be able to observe the relative proportion of pores that are actually connected.