Separation of minerals from reservoir sandstones into grain size classes, and chemical and mineralogical analysis, allows accurate description of relations between Th occurrence and mineralogy. Detrital heavy minerals, and particularly monazite, are found to account for the majority of the Th. Some heavy minerals occur as inclusions within detrital quartz which, because of its abundance, may account for a substantial proportion of the Th content in individual samples. Minor concentrations of Th are identified in diagenetic rare earth element (REE) phosphates. Kaolinite is not associated with Th-enrichment. Determination of Th content is not recommended as a guide to the kaolinite content of sandstones. Th content may be useful for identification of monazite and lithostratigraphic applications.