Examination of conodont colour alteration (CAI) in samples from more than 40 petroleum exploration wells and extensive outcrop collections along the northern margin of the Lennard Shelf forms the basis for a study of the thermal maturation and geothermal history of the Canning Basin of Western Australia. The thickness of the measured CAI intervals is variable and does not conform to the 1200 m standard of the Appalachian Basin. The CAI interval 1 is thick and indicates a low geothermal gradient in the basin but CAI intervals 1.5 and 2 are thin and indicate higher geothermal gradients. A major thermal event of Miocene Age, associated with the intrusion of the Fltzroy Lamproites in the Fitzroy Graben and Lennard Shelf, may be the source of the increased heat flow and also explain an area of high heat flow in some parts of the Graben and shelf.Using the vertical and horizontal distribution of trends of the CAI intervals it is suggested that over large areas of the basin the oil generation window is restricted to an interval about 1100 m thick and, except where migration has taken place, that liquid hydrocarbons will be restricted to the interval between 1600 and 3000 m. In areas affected by the intrusion of the Fitzroy Lamproites, the top of the oil generation window may be as shallow as 800 m.
