No significant hydrocarbon source-rock potential was identified in Hole 1108B. In contrast, zones within the coaly interval encountered in Hole 1109D displayed good to excellent hydrocarbon source-rock potential. At the appropriate levels of thermal maturity, these organic-rich intervals would yield largely gas. Limited oil potential was identified in one of the samples (180-1109D-38R-2, 29-31 cm).
The coaly interval at Site 1109 is thermally immature, based on the two pyrolysis thermal maturation indexes, the relative abundance and gross composition of the bitumen, and the nature of the saturated hydrocarbon-fraction gas chromatograms.
No clear trend with depth in methane stable-carbon isotope composition was detected, suggesting that the isotope values are not simply reflecting a thermal maturation trend. It appears that the isotopically heavier values are not the result of the introduction of thermally mature gas but are an artifact of fractionation during sample handling and storage.
The in situ origin and limited hydrocarbon-generation potential of Site 1108 suggest that the location can be safely revisited, but because of the elevated geothermal gradient, careful monitoring will be required, especially if a stratigraphic equivalent to the coaly interval observed at Site 1109 is penetrated.