Crude oils of the Powder River basin are of two major types. One has low sulfur content and occurs in younger formations the other has high sulfur content and occurs in older formations. The low sulfur oils are generally light colored and waxy, and may be predominately either paraffinic or naphthenic. The high sulfur oils are usually dark colored and asphaltic; only their low boiling fractions are paraffinic. Alteration appears to account for the anomalous properties of some oils found at shallow depths. Shape and slope of correlation index curves classify the oils into the same two types as their sulfur contents, with gently sloping curves representing the low sulfur type and steep curves representing high sulfur, asphaltic oils.