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On the basis of petroleum geochemical studies of the COST G-1 and G-2 well samples in the George Bank Basin, the Tertiary and Cretaceous sections between depths of 0 and 6,000 ft (1,829 m) are not believed to be prospective for oil or gas because of the thermally immature character of the lignitic and woody type of kerogens and extractable organic matter. The total organic carbon and extractable hydrocarbons of the Jurassic sedimentary rocks below 6,000 ft (1,829 m) indicate that these are poor to fair source rocks.
Compositions of the C1 to C7 light and gasoline-range hydrocarbons, temperatures of maximum pyrolysis (TS2)°C, carbon preference indices (CPI), and thermal alteration indices (TAI) suggest that the shallowest thermally mature rocks are between 16,000 and 18,000 ft (4,877 and 5,486 m) in the G-2 well. High vitrinite reflectance (Ro) percentages may reflect oxidation and recycled organic matter. Estimates of the depth of thermal maturity based on measurements of the present geothermal gradient at the G-1 (1.2°F/100 ft) and G-2 (1.5°F/100 ft) wells are inconsistent with the depth of thermal maturity indicated by temperature-sensitive geochemical characteristics.
The results of this study suggest that further consideration should be given to: (1) a reassessment of thermal-maturation processes in sediments associated with rift basins on passive margins; (2) the possibility that thermal maturation of the organic matter in the basin may have been influenced by a variable heat-flow history; (3) the location of the heat source, which may have been farther east of the present continental shelf than is presently thought.
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