Concentrations of the light hydrocarbon gases methane (C1), ethane (C2), and propane (C3) were routinely measured during Leg 207 to monitor the possible presence of petroleum hydrocarbons and to investigate the existence of active microbial communities in the passive margin sediment sequences of the Demerara Rise. By analogy to Mediterranean sapropels (Coolen et al., 2002), it was anticipated that the organic carbon–rich Cretaceous black shales that were a major objective of Leg 207 coring might support active populations of prokaryotes.
Elevated amounts of gas have previously been reported in Cenozoic and Mesozoic sediments cored during Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) legs (e.g., Kvenvolden and Barnard, 1983; Meyers and Brassell, 1985; Kvenvolden and McDonald, 1986; Meyers and Snowdon, 1993; Meyers et al., 1998; Shipboard Scientific Party, 2002, 2003). Possible origins of these sedimentary gases are (1) residues of relict microbial degradation of sediment organic matter, (2) active generation by existing microbial communities, (3) in situ thermal decomposition of sediment organic matter, and (4) thermal degradation of deeply buried organic matter and migration into the sequences cored by DSDP and ODP legs. In very gassy sediments, gas can exist in the free state or as gas hydrate. However, the hydrostatic and lithologic pressures and temperatures that exist beneath the seabed typically keep gases dissolved in the interstitial fluids of sediment sequences.
If sufficiently abundant, gases exsolve from sediment pore waters during core recovery as pressure is released and sediments are warmed. None of the cores recovered during Leg 207 contained either the large gas bubbles or the gas expansion pockets that are two of the benchmarks of sediments especially rich in interstitial gases. Nonetheless, elevated gas concentrations were consistently measured in the black shale units present at the five sites drilled on the Demerara Rise. Both the amounts and compositions of these gases initially caused concern during drilling operations. This contribution describes the gas contents of these passive margin Cretaceous sedimentary sequences and considers their possible origins.