Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized core
Co-existence of gas hydrate, free gas, and brine within the regional gas hydrate stability zone at Hydrate Ridge (Oregon margin): evidence from prolonged degassing of a pressurized core
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DOI：
10.1016/j.epsl.2004.03.028
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发表时间：
2004-06
影响因子：
5.3
通讯作者：
A. Milkov;G. Dickens;G. Claypool;Youngjoo Lee;W. Borowski;M. Torres;Wenyue Xu;H. Tomaru;A. Tréhu
中科院分区：
文献类型：
--
作者：
A. Milkov;G. Dickens;G. Claypool;Youngjoo Lee;W. Borowski;M. Torres;Wenyue Xu;H. Tomaru;A. Tréhu
海洋钻探计划 (ODP) 第 204 段的标准科学操作记录了俄勒冈州近海水合物岭南峰以下 0-20 m 处存在大量天然气水合物和高盐度孔隙水的地平线。沉积物区位于活跃的海底气体喷口附近，增加了游离气体与天然气水合物共存于浅表层的可能性，其中孔隙水的盐度太高，无法沉淀更多的天然气水合物。在这里，我们讨论一个解决这个重要概念的独特实验。在 1249 号站点海底约 14 m 处取回 1 米长的加压岩心，并在实验室中约 0 °C 下缓慢脱气约 178 小时，以确定大量天然气水合物层段的原位盐度和气体浓度。堆芯释放了约 95 升气体（主要是甲烷），这是迄今为止在船上环境压力和温度条件下测量的 1 m 堆芯的最大气体量。地球化学质量平衡计算和初始气体释放压力（4.2 MPa）均表明孔隙水的原位盐度接近或超过 105 g kg−1，即不含天然气水合物的气卤水系统所需的近似盐度。岩心脱气开始时相对较高浓度的丙烷和高级烃气体也表明存在原位游离气体。天然气水合物、游离气和盐水可能共存于水合物岭的浅层沉积物中。天然气水合物快速结晶提取大量水时产生的近海底盐水会影响该地区甚至其他地方的天然气和天然气水合物的分布和循环。
Standard scientific operations on Ocean Drilling Program (ODP) Leg 204 documented a horizon of massive gas hydrate and highly saline pore water ∼0–20 m below the southern summit of Hydrate Ridge offshore Oregon. The sediment zone lies near active seafloor gas venting, raising the possibility that free gas co-exists with gas hydrate in shallow subsurface layers where pore waters have become too saline to precipitate additional gas hydrate. Here we discuss a unique experiment that addresses this important concept. A 1-m-long pressurized core was retrieved from ∼14 m below sea floor at Site 1249 and slowly degassed at ∼0 °C in the laboratory over ∼178 h to determine in situ salinity and gas concentrations in the interval of massive gas hydrate. The core released ∼95 l of gas (predominantly methane), by far the greatest gas volume ever measured for a 1 m core at ambient shipboard pressure and temperature conditions. Geochemical mass-balance calculations and the pressure of initial gas release (4.2 MPa) both imply that pore waters had an in situ salinity approaching or exceeding 105 g kg−1, the approximate salinity required for a gas hydrate–free gas–brine system. Relatively high concentrations of propane and higher hydrocarbon gases at the start of core degassing also suggest the presence of in situ free gas. Gas hydrate, free gas and brine likely co-exist in shallow sediment of Hydrate Ridge. Near-seafloor brines, produced when rapid gas hydrate crystallization extracts large quantities of water, impact the distribution and cycling of gas and gas hydrate in this region and perhaps elsewhere.