A process is disclosed using a dispersion of supercritical fluid and oil to upgrade a hydrocarbon feedstock such as a heavy oil into an upgraded hydrocarbon product or synthetic crude with highly desirable properties such as low sulfur content, low metals content, lower density (higher API), lower viscosity, lower residuum content, etc. The process utilizes a capillary mixer to form the dispersion. The process does not require external supply of hydrogen nor does it use externally supplied catalysts.