The U.S. Bureau of Mines (USBM), developed by Donaldson et al. in 1969,  is a method to measure wettability of petroleum reservoir rocks. In this method, the areas under the forced displacement Capillary pressure curves of oil and water drive processes are denoted as  and  to calculate the USBM index.
USBM index is positive for water-wet rocks, and negative for oil-wet systems.