Spacing and amplitude of IR temperature anomalies and hydrate content of sediments from ODP Leg 304 sites (Table 1)

Large uncertainties about the energy resource potential and role in global climate change of gas hydrates result from uncertainty about how much hydrate is contained in marine sediments. During Leg 204 of the Ocean Drilling Program (ODP) to the accretionary complex of the Cascadia subduction zone, we sampled the gas hydrate stability zone (GHSZ) from the seafloor to its base in contrasting geological settings defined by a 3D seismic survey. By integrating results from different methods, including several new techniques developed for Leg 204, we overcome the problem of spatial under-sampling inherent in robust methods traditionally used for estimating the hydrate content of cores and obtain a high-resolution, quantitative estimate of the total amount and spatial variability of gas hydrate in this structural system. We conclude that high gas hydrate content (30–40% of pore space or 20–26% of total volume) is restricted to the upper tens of meters below the seafloor near the summit of the structure, where vigorous fluid venting occurs. Elsewhere, the average gas hydrate content of the sediments in the gas hydrate stability zone is generally <2% of the pore space, although this estimate may increase by a factor of 2 when patchy zones of locally higher gas hydrate content are included in the calculation. These patchy zones are structurally and stratigraphically controlled, contain up to 20% hydrate in the pore space when averaged over zones ~10 m thick, and may occur in up to ~20% of the region imaged by 3D seismic data. This heterogeneous gas hydrate distribution is an important constraint on models of gas hydrate formation in marine sediments and the response of the sediments to tectonic and environmental change.

天然气水合物(gas hydrates)的能源资源潜力及其在全球气候变化中的作用存在较大不确定性，这一不确定性源于海洋沉积物中所含天然气水合物的储量尚不明确。在大洋钻探计划（Ocean Drilling Program, ODP）第204航次针对卡斯卡迪亚俯冲带增生杂岩体的科考中，我们依托三维地震勘探所界定的差异化地质背景，对从海底到天然气水合物稳定带（gas hydrate stability zone, GHSZ）底部的区域进行了取样。通过整合多种方法的研究结果——包括为本航次开发的多项新技术——我们克服了传统上用于估算岩芯天然气水合物含量的稳健方法所固有的空间采样不足问题，并对该构造系统内天然气水合物的总储量与空间分布差异实现了高分辨率定量估算。研究结果表明，高天然气水合物含量（占孔隙空间的30%~40%，或占总体积的20%~26%）仅局限于构造顶部附近海底以下数十米的浅层区域，该区域存在强烈的流体泄放活动。在其他区域，天然气水合物稳定带内沉积物的平均天然气水合物含量通常不足孔隙空间的2%；但若将局部水合物含量较高的斑块状富集区纳入计算，该估算值可提升一倍。这些斑块状富集区受构造与地层条件控制，在厚度约10米的区域内平均孔隙空间内的水合物占比可达20%，且在三维地震数据覆盖的区域中，此类富集区的占比最高可达约20%。这种非均质性的天然气水合物分布，对海洋沉积物内天然气水合物形成模型以及沉积物对构造与环境变化的响应研究，均构成了重要的约束条件。