Mixed-layer illite/smectite (I-S) is an important semi-quantitative geothermometer for the study of thermal histories of sedimentary basins. The purpose of this research is to examine the relationship between smectite illitization and hydrocarbon formation and migration in the Cretaceous Mowry shale of the Wyoming Powder River Basin. The I-S (R≥1) and the measured K-Ar ages of I-S ranging from 49 Ma to 64.9 Ma were observed in the clay fractions of the Mowry Shale. The measured ages of I-S correspond to burial temperatures ~110 C. While maximum burial occurred later in the Miocene Epoch for the Mowry Shale. The extent of illitization in the Mowry Shale bentonites was limited by the availability of potassium.