An apparatus and a method is disclosed for storage of solar energy in a subsurface geologic reservoir. The method includes transferring concentrated solar thermal energy to a fluid, thereby generating a supercritical fluid. The supercritical fluid is then injected into the subsurface geologic reservoir through at least one injection well. The subsurface geologic reservoir may be a highly permeable and porous sedimentary strata, a depleted hydrocarbon field, a depleting hydrocarbon field, a depleted oil field, a depleting oil field, a depleted gas field, or a depleting gas field. Once charged with the supercritical fluid, the subsurface geologic formation forms a synthetic geothermal reservoir.