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.本研究报道并探讨了俄勒冈州近海水合物脊（Hydrate Ridge）大洋钻探计划（Ocean Drilling Program, ODP）第204航次采集的55件水合物包裹气体样品与494件孔隙气体样品的分子与同位素特征。天然气水合物似乎由两种端元气源（深部异地源与原位源）的气体以不同比例混合后，在沉积物中结晶形成。在该脊南部峰顶（站位1248~1250）的高气体通量区域，浅部（海底以下0~40米，mbsf）的天然气水合物主要由异地源混合的微生物成因与热成因甲烷，以及少量热成因C2+烃类气体组成，这些气体从2~2.5千米的深部垂向与侧向运移而来。与之相反，南部峰顶（站位1248与1250）及该脊侧翼（站位1244~1247）的深部（50~105 mbsf）天然气水合物则主要由原位生成的微生物成因甲烷与乙烷结晶形成。在地质与构造环境有利于气体从更大深度集中垂向运移的站位（如站位1244与1245），可能也存在少量异地源气体的贡献。采样的水合物中，二氧化碳（CO₂）与硫化氢（H₂S）等非烃气体含量并不丰富。本研究获得的新气体地球化学数据与早期模型相悖，该早期模型认为水合物脊的海底天然气水合物由深部更古老的水合物分解产生的气体形成。南部峰顶的天然气水合物形成可用下述模型解释：气体从深部沉积物运移而来，可能被气体水合物稳定带（Gas Hydrate Stability Zone, GHSZ）底部的水合物封层所捕获。当气柱超压超过上覆沉积物的封闭能力时，游离气体便会运移进入GHSZ，并主要在浅部沉积物中以天然气水合物的形式析出。该峰顶处天然气水合物藏的蘑菇状三维形态，可能由主运移通道周围的气体扩散晕、浅部沉积物中气体溶解度的降低，以及海底附近碳酸盐与水合物封层对气体的重新聚焦作用，共同限定于局部背斜构造的顶部。