CONCLUSIONS

Sediments of Miocene to Pleistocene age were recovered from Holes 994C, 995A, 997A, and 997B, and their gaseous, soluble, and insoluble organic matter contents were investigated. The downhole variations of the amount and composition of the organic matter are virtually parallel in all three holes. The sediments are somewhat enriched in TOC in comparison to most deep-sea sediments. The liptinites are dominated by conifer pollen in the upper sections and by marine algae in the lower ones. Overall, however, an immature terrestrially dominated, mixed type of the particulate organic matter prevails with an increase of the vitrinite with depth. These downhole changes in the organic facies are mirrored by changes in the composition of the soluble organic matter. Its low stage of maturity is confirmed by the occurrence of unsaturated triterpenoids and 17ß, 21ß-pentacyclic triterpenoids.

The organic matter of the drilled sections has a ¹³C_org = -21, which should indicate a prevailing marine facies. However, optical and chemical investigations suggest the prevalence of a mixture of the organic matter: the marine one isotopically heavier, the terrestrial one lighter.

The combined hydrocarbon gases are a mixture of predominantly microbial methane and thermal hydrocarbon gases. Indications of the location of the hydrate stability zone were only found in the data of the gas ratios C₁/(C₂+ C₃). Ethane and propane are inferred to have a predominantly thermal origin. On the basis of their carbon isotope ratios and assuming a ¹³C_org = -21, the corresponding source rock should have a marine facies and a maturity in the oil window. Therefore, the bulk of the ethane and propane have migrated from below.

The amount of organic matter that may have been present in the sediments originally would have been sufficient to produce enough methane to fill the pore space to the observed level. An additional supply of methane from deeper sections, however, is inferred from the isotope data.