The alkane composition of oils from noncoeval deposits of some oil fields in Tatarstan and Vietnam was geochemically investigated. Their generation, degree of catagenic alteration, and thermal maturity were studied. The general and specific features of the composition of petroleum alkanes were correlated with the extent of catagenic alteration and thermal maturity of the oils. A linear relationship between the amount of alkanes with the normal and isoprenoid structure in the oils and the thermal maturity of the oils was revealed. Based on the revealed relation, a method for determining the paleotemperatures of oils was proposed.