A thick calcareous shale of Early/Middle Kimmeridgaiann age is the principal source rock for the oils discovered in the Jeanne d'Arc Basin of the Grand Bankks. This Upper Jurassic source rock forms a geological marker throughout the basin, but is not a stratigraphic marker. Its regional distribution is controlled by the tectonic and sedimentary history and the depositional enviornment.Rift tectonics in the Grand Banks during Oxfordian/Kimmeridgian times formed a series of silled, anoxic basins, which received low input of terrigenoous material from the adjacent continent. This favoured the accumulation of a thick sequence of calcareous shales rich in oil‐prone, amorphous organic matter. The principal source‐rock unit is generally 200‐300m (656–984 ft) thick in the southern and western part of the basin and thickens in the NE to more than 700m (2,296ft). Anoxic conditions were maintained in the NE area throughout the Late Kimmeridgian. While the SW area underwent uplift and was flooded by terrigenous clastics.Detailed geochemical analysis of oils and condensates discovered in many wells allows distinction of two principal oil families of different origins: one of Jurassic source, and the other of a probable Teritary origin. Most oils found in the basin belong to the Jurassic oil family.Distinct regional variations in the maturity and composition of the Jurasssic oil family are evident. These can be explained by the regional maturity pattern of the principal source rock, and by the co‐contribution of a secondary, slightly younger source‐rock interval, which contains a predominantly terrestrial type of organic matter. This secondary source input is recognized in areas where the younger source rock reaches fully‐mature conditions. Condensates in the centre of the basin are derived from the deeply‐buried source beds at present in late‐ to overmature conditions.The Adolphus oil in the centre of the basin represents the second oil family, and is probably derived from a Teritary source rock. No oil‐source correlation could be established due to lack of data and proper cutting samples. However, geological and heat‐flow considerations suggest a Teritary source. Heat‐flow values are high in the Adolphus structure as a result of salt diapirsm and the base of the Tertiary shale section is