SOURCE-ROCK QUALITY DETERMINATION FROM OIL BIOMARKERS .2. A CASE-STUDY USING TERTIARY-RESERVOIRED BEAUFORT SEA OILS

Biomarkers (molecular fossils) in 26 Beaufort Sea Mackenzie delta oils reveal three genetic oil groups and suggest descriptions of their pro bable sources. Group 1 (21 Tertiary-reservoired oils) was sourced from Tertiary deltaic sediments. Group 2 (three Cretaceous- and one Devoni an-reservoired oil) and group 3 (one Lower Cretaceous-reservoired oil) derive from two high-quality sources deposited in open-marine environ ments. Geographical variations in the geochemistry of the 21 group 1 o ils suggest that their deltaic source was substantially more oil-prone in the distal deltaic portions than in tile region closer to the pale oshoreline. Although these data do not address source rock volumes, th ey do indicate that poor source quality is not a cause of underfilled traps in the offshore. The mappable lateral variations in group 1 oil compositions illustrate how, in basins with vertically drained sources , lateral source rock facies changes can be inferred from regional var iations in the geochemistry of the overlying oils. Furthermore, the da ta suggest that geochemical studies attempting to deduce genetic relat ionships between geographically separated oils should carefully consid er the potential effects of source rock facies changes on the oil comp ositions.