Most basement reservoirs occur either on platforms or in intermontane basins. They are rare in foredeep basins. Almost all basement reservoirs occur below a regional unconformity. All source beds of basement reservoirs lie above the reservoir rock, but all source beds lying above the reservoir rock do not necessarily form basement reservoirs. Although most source beds lie above an unconformity, some lie below it and form a so-called second crop. The unconformity surface provides evidence that basement rocks have undergone weathering, erosion, solution, and leaching for so long a time that porosity and permeability have increased greatly, and the accumulation of petroleum was facilitated. Basement reservoirs are characterized by thick reservoir rocks. Porosity and permeability are irregular. Production from basement reservoirs is usually high and reserves are large.--Modified journal abstract.