Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases
Gas hydrate systems at Hydrate Ridge offshore Oregon inferred from molecular and isotopic properties of hydrate-bound and void gases
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DOI：
10.1016/j.gca.2004.08.021
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发表时间：
2005-02
影响因子：
5
通讯作者：
A. Milkov;G. Claypool;Youngjoo Lee;R. Sassen
中科院分区：
文献类型：
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作者：
A. Milkov;G. Claypool;Youngjoo Lee;R. Sassen
本文报道并讨论了大洋钻探计划（ODP）第204航次在俄勒冈州海伯利脊近海采集的55个样品和494个样品的水合物结合气体的分子和同位素性质。天然气水合物在两种端元气源（深部异地气源和原位气源）的沉积物中结晶为不同比例的混合物。在海脊南端的一个高气体通量区（1248-1250号站点），浅层（海底以下0-40米）天然气水合物主要由外来混合微生物和热成因甲烷以及一小部分热成因C2+气体组成，它们从2- 2.5公里深处垂直和横向迁移。相比之下，南部峰顶（1248和1250号站点）和海脊侧翼（1244-1247号站点）的深层（50-105百万立方英尺）天然气水合物主要由微生物甲烷和乙烷结晶而成，这些甲烷和乙烷主要在原地生成。在地质和构造环境有利于从更大深度集中垂直气体迁移的地点（例如，1244和1245号站点）。水合物样品中CO2、H2S等非烃类气体含量不高。新的天然气地球化学数据与早期的模型不一致，这些模型表明，海参崴海天然气水合物的形成在南部峰会解释的模型中，天然气从深层沉积物中迁移，并可能被困在天然气水合物稳定带（GHSZ）的基础上的天然气水合物密封。当气柱中的超压超过上覆沉积物的封闭能力时，游离气运移进入GHSZ，主要在浅层沉积物中以天然气水合物的形式沉淀。天然气水合物聚集在山顶的蘑菇状三维形状可能是由围绕主要运移通道的天然气扩散光环、浅层沉积物中天然气溶解度的降低以及海底附近的碳酸盐和天然气水合物盖层对天然气的重新聚焦到局部背斜构造的顶部所限定的。
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 CO2and 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.