Molecular and stable isotope composition of headspace and total hydrogencarbon gases of ODP Leg 104 sites

Molecular and isotope compositions of headspace and total (free + sorbed) hydrocarbon gases from drilled cores of the three ODP Leg 104 Sites 642, 643, and 644 of the Voring Plateau are used to characterize the origin and distribution of these gases in Holocene to Eocene sediments. Only minor amounts of methane were found in the headspace (0.1 to < 0.001 vol%). Although methane through propane are present in all of the total gas samples, different origins account for the concentration and composition variations found. Site 643 at the foot of the outer Voring Plateau represents a geological setting with poor hydrocarbon generating potential, (sediments with low TOC and maturity overlying oceanic basement). Correspondingly, the total gas concentrations are low, typical for background gases (yield C1 - 4 = 31 to 232 ppb, C1/C2+ = 0.6 to 4; delta13C(CH4) -22 per mil to -42 per mil) probably of a diagenetic origin. Holocene to Eocene sediments, which overlie volcanic units, were drilled on the outer Vdring Plateau, at Holes 642B and D. Similar to Site 643, these sediments possess a poor hydrocarbon generating potential. The total gas character (yield C1 - 4 = 20 to 410 ppb; C1/C2+ = 1.7 to 13.3; delta13C(CH4) ca. -23 per mil to -40 per mil) again indicates a diagenetic origin, perhaps with the addition of some biogenic gas. The higher geothermal gradient and the underlying volcanics do not appear to have any influence on the gas geochemistry. The free gas (Vacutainer TM) in the sediments at Site 644 are dominated by biogenic gas (C1/C2+ > 104; delta13C(CH4) -77 per mil). Indications, in the total gas, of hydrocarbons with a thermogenic signature (yield C1 - 4 = 121 to 769 ppb, C1/ C2+ = 3 to 8; delta13C(CH4) = -39 per mil to -71 per mil), could not be unequivocally confirmed as such. Alternatively, these gases may represent mixtures of diagenetic and biogenic gases.

Molecular and isotopic properties of gases from ODP Holes of ODP Leg 204

We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248-1250), shallow (0-40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C2+ gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50-105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244-1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO2 and H2S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure.