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热带海洋学报 >

2023 , Vol. 42 >Issue 5: 134 - 143

DOI: https://doi.org/10.11978/2022204

海洋地质学

海底冷泉渗漏油气泡原位定量测定——以墨西哥湾GC600为例

  • 邸鹏飞 , 1 ,
  • 李牛 1 ,
  • 陈多福 2 ,
  • Ian R MacDonald 3
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  • 1. 边缘海与大洋地质重点实验室(中国科学院南海海洋研究所), 广东 广州 510301
  • 2. 上海深渊科学工程技术研究中心, 上海海洋大学海洋科学学院, 上海 201306
邸鹏飞。email:

邸鹏飞(1982—), 男, 副研究员, 主要从事海底冷泉原位观测与甲烷生物地球化学研究。email:

收稿日期: 2022-09-28

  修回日期: 2022-11-22

  网络出版日期: 2023-03-14

基金资助

国家自然科学基金项目(41676046)

国家重点研发计划项目(2017YFC0307704)

广东省自然科学基金项目(2019A1515011809)

海南省重点研发计划项目(ZDYF2021SHFZ060)

收起

In situ quantification of oil-gas bubble seep flux from cold seeps at the seabed ― a case study of GC600 in the Gulf of Mexico

  • DI Pengfei , 1 ,
  • LI Niu 1 ,
  • CHEN Duofu 2 ,
  • Ian R MacDonald 3
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  • 1. CAS Key Laboratory of Ocean and Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
  • 2. Shanghai Engineering Research Center of Hadal Science and Technology, College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China
  • 3. Florida State University, Tallahassee, Florida USA 32306
  • 3. Florida State University, Tallahassee, FL 32306, USA
DI Pengfei. email:

Received date: 2022-09-28

  Revised date: 2022-11-22

  Online published: 2023-03-14

Supported by

National Natural Science Foundation of China(41676046)

National Key R&D Program of China(2017YFC0307704)

Natural Science Foundation of Guangdong Province(2019A1515011809)

Key Research and Development Plan of Hainan Province(ZDYF2021SHFZ060)

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摘要

海洋环境中冷泉渗漏是甲烷等有机化合物和其他温室气体进入海洋和大气的重要来源。准确定量确定冷泉渗漏的甲烷气泡通量对于评估它们对全球甲烷预算和气候变化有着重要的研究意义。采用高分辨率视频成像系统原位观察了墨西哥湾GC600冷泉渗漏区Mega Plume 2喷口, 并获得了油气泡连续释放的视频片段。通过采用半自动气泡计数算法估算了Mega Plume 2 喷口释放的油气泡个数及其释放速率。而通过图像处理技术确定了Mega Plume 2释放的油气泡的大小和尺寸分布。Mega Plume 2混合喷口释放的油气泡平均直径2.56mm±1.01mm, 油气泡释放速率为80.25个·秒-1。Mega Plume 2喷口年释放通量为177.7m3·a-1 (19.55~106.62t·a-1)。然而, Mega Plume 2 喷口的气泡释放速率与潮汐无关, 可能与其他因素如水合物, 油气藏增压及沉积层差异加载有关。只有对冷泉渗漏系统开展长期的原位在线观察, 我们才能准确获取冷泉渗漏系统活动特征及其释放通量。

关键词： 水下摄像系统; 油气泡; 释放速率; 气泡通量; 墨西哥湾

本文引用格式

邸鹏飞 , 李牛 , 陈多福 , Ian R MacDonald . 海底冷泉渗漏油气泡原位定量测定——以墨西哥湾GC600为例[J]. 热带海洋学报, 2023 , 42(5) : 134 -143 . DOI: 10.11978/2022204

Abstract

Natural cold seeps in the marine environment are important sources of organic compounds, such as methane and other greenhouse gases, to the ocean and atmosphere. Accurate quantification of methane bubbles flux at hydrocarbon seeps is therefore necessary to evaluate their influence on the global methane budget and climate change. A deep-sea high-definition video time-lapse camera was used to observe the gaseous and oily bubbles released from Mega Plume 2 vent in the GC600 cold seep in the Gulf of Mexico and obtained the video clips of continuous oil bubble release. A semi-automatic bubble counting algorithm was used to estimate the oil bubble number and release rates of Mega Plume 2 from video data. Image processing techniques were used to determine the bubble type (oily, mixed, and gaseous), and size distribution. The Mega Plume 2 vent at GC600 released a mixture of oily and gaseous bubbles with an average diameter of 2.56 mm ±1.01 mm at a rate of 80.25 bubbles·s-1. The oil-gas bubbles flux released from Mega Plume 2 is 177.7 m3·yr-1 (19.55~106.62 T·yr-1). However, the bubbles release rate was not correlated with tidal effects and may be linked with other factors, such as hydrate, pressurization of oil and gas in reservoirs, and differential loading of sedimentary layers. Only through long-term in-situ observations of cold seeps over a significant spatial extent, we will be able to adequately obtain their activity characteristics and released flux in marine environments.

Key words： underwater camera system; oil bubble; release rates; bubbles flux; Gulf of Mexico

冷泉是海洋环境中广泛分布的自然现象, 全球几乎所有的大陆边缘都有发育(Judd et al, 2002; Judd 2003; Campbell et al, 2006)。海底冷泉是天然气, 尤其是甲烷, 从沉积圈中进入到水圈和大气圈的重要方式。甲烷是种强烈的温室效应气体, 其温室效应是同质量二氧化碳的23倍, 是全球气候变化的一个重要的影响因子(Dimitrov, 2003; Etiope et al, 2004; Etiope et al, 2008)。目前, 每年通过冷泉渗漏释放到海洋及大气中的甲烷初步估计约为25Tg·a-1 (Kvenvolden et al, 2005), 大约占全球陆地和海洋环境释放甲烷通量的一半(40~60Tg·a-1)(Etiope et al, 2004, 2008; Etiope, 2009) 。然而, 由于技术和方法的限制, 目前通过这种方式释放到海洋和大气中甲烷的量很难以量化, 造成相当大的不确定性(Di et al, 2014a)。因此, 有必要准确估算冷泉释放甲烷的量, 这对于对认识冷泉在全球气候变化及全球碳循环中的作用有着重要研究意义。
近年来, 国际上已经对许多典型发育的海底冷泉渗漏区开展了原位观测, 如墨西哥湾布什山、圣芭芭拉海峡Coal Oil Point, 水合物脊、黑海、新西兰Hikurangi Margin等(Roberts et al, 1999; Bole et al, 2001; Tryon et al, 2001, 2002; Torres et al, 2002; Leifer et al, 2005; MacDonald et al, 2005; Vardaro et al, 2006; Solomon et al, 2008; Sahling et al, 2009; Krabbenhoeft et al, 2010; Leifer, 2010; Linke et al, 2010; Römer et al, 2012)。观测结果表明, 海底冷泉渗漏系统在不同沉积截面的能量和物质在时间上和空间上是多变的, 主要受潮汐、涌浪、构造、高盐对流、生物泵和海底气藏短暂排空等多种因素的控制(Henry et al, 1992; Wang et al, 1996; Wallmann et al, 1997; Suess et al, 1998; Tryon et al, 1999; Boles et al, 2001; Forrest et al, 2005; Leifer et al, 2005; Talukder, 2012)。
墨西哥湾位于被动大陆边缘, 广泛发育盐底辟和丰富的油气藏构造, 被认为是研究冷泉渗漏流体活动特征的天然实验室(Kvenvolden et al, 2003)。盐底辟不仅可以形成油气藏的圈闭结构, 同时也能破坏储层形成渗漏通道使油气藏中烃类物质向海底渗漏。冷泉在墨西哥湾广泛发育, 它将油气藏中原油和气体沿着裂隙通道释放到海洋水体, 形成多个海底冷泉渗漏喷口(Garcia-Pineda et al, 2010; Brun et al, 2011; MacDonald et al, 2015)。冷泉渗漏改变了海底沉积物性质造成了如水合物丘和自生碳酸盐岩等硬底质发育, 同时还伴有化能合成的生物群如管状蠕虫、贻贝等(MacDonald et al, 1994; Roberts et al, 1994; Sassen et al, 1994; De Beukelaer et al, 2003; MacDonald et al, 2004)。
海底冷泉渗漏的一些特征可以被卫星和水声数据探测到。如合成孔径雷达(synthetic aperture radar, SAR)图像可显示海表面持续存在的浮油层, 回声探测仪可探测到冷泉渗漏在水体中形成羽状流(Merewether et al, 1985; De Beukelaer et al, 2003; Klaucke et al, 2005; Greinert et al, 2006; Garcia-Pineda et al, 2010; MacDonald et al, 2015)。此外, 还可以通过浮油层和羽状流追踪到海底的渗漏喷口(Johansen et al, 2017), 这为寻找海底冷泉渗漏区与量化冷泉释放甲烷通量提供了依据(Kvenvolden, 1993, 2002; Maslin et al, 2003; MacDonald et al, 2015)。目前, 很多学者通过对海底冷泉喷口释放气泡的视频记录估算了渗漏气泡的大小分布、行为与归宿(Valentine et al, 2001; Grant et al, 2002; Leifer et al, 2003; Sahling et al, 2009; Greinert et al, 2010; Leifer, 2010; Römer et al, 2012, 2014; Sahling et al, 2014; Johansen et al, 2017; Di et al, 2020)。摄像技术的不断创新可以获取高清视频记录从而更加准确地定量计算气泡大小和释放频率, 而且长时间视频记录表明冷泉渗漏气泡释放速率随时间有明显的变化(Boles et al, 2001; Leifer et al, 2005; Thomanek et al, 2010; Schneider von Deimling et al, 2011; Bian et al, 2013; Bayrakci et al, 2014; Di et al, 2014a, 2014b; Wang et al, 2015; Römer et al, 2016; Wang et al, 2016)。因此, 有必要对海底冷泉渗漏喷口开展长期原位观测, 这对于认识海底冷泉渗漏流体活动特征及定量估算冷泉渗漏甲烷的量有着重要的研究意义。
本文重点介绍采用水下高分辨率成像系统对位于墨西哥湾北部Green Canyon 600 (GC600) 的冷泉渗漏喷口Mega Plume 2开展了原位观察, 获得了Mega Plume 2 渗漏喷口释放油气泡的几段视频数据集, 估算了Mega Plume 2 喷口释放的油气泡尺寸大小、释放频率及其释放通量, 并探讨了其控制因素。

1 研究区域

GC600是一个水深约1200m的油气藏区, 海表面有大量的油膜出现, 它已成为开展海底冷泉渗漏活动研究的重要试验区(图1a)(Garcia-Pineda et al, 2010; Roberts et al, 2010; Wang et al, 2015; D’souza et al, 2016; Wang et al, 2016)。研究区域位于 GC600 一个5km×5km的地块上, 发育有两个大的海脊, 它们之间相距1km, 而通向山脊的两条断层可能是油气向上渗漏的通道, 导致有多个冷泉渗漏喷口正在活动(Roberts et al, 2010; Johansen et al, 2017)。此外, 这两个海脊上的海底沉积物中有着大量的水合物聚集, 聚集至一定程度后, 它们可能会冲破沉积物出露在海底, 且在沉积物上分布着大量的白色菌席。位于较大海脊的北西末端发育有2个典型冷泉渗漏喷口, 分别是Mega Plume 1和Mega Plume 2, 它们释放出大量的油气泡。
图1 墨西哥湾GC600渗漏区位置图(a)和海底地形图(b)

图a中红色方框为研究位置; 图b中红色方块为Mega Plume 2喷口位置, 白色线表示海脊边缘, 黑色等值线表示海底数据空白区

Fig. 1 The location (a) and seafloor topographic map (b) of GC600 seep area in the Gulf of Mexico. The red box in Figure a is the study location. The red box in Figure b is the location of Mega Plume 2 seep vent. The white line indicates the edge of the ridge and the black contour line indicates blanking zone in subbottom data

2 仪器与方法

2.1 水下高分辨率视频成像系统与布放

通过采用高分辨率视频成像系统(图2a)可以连续拍摄海底冷泉渗漏喷口释放气泡的视频片段。固定在高密度聚乙烯架上的高分辨率成像系统主要由高分辨率视频成像相机和水下灯两个部分组成, 高分辨率视频成像相机(像素分辨率: 1080×1920, 视角: 120°, 29帧·s-1)安装在压力舱内, 耐压舱内装有一组为高分辨率视频成像相机和水下灯供电的24V的锂电池, 它们通过水下电缆连接(图2a)。高密度聚乙烯架通过无人遥控潜水器(Remotely Operated Vehicle, ROV)的机械手将高分辨率视频成像系统准确地布放在海底渗漏喷口前30~50cm处, 同时将带有刻度的不锈钢棒插入渗漏喷口处。布放前, 高分辨率视频成像系统通过连接电脑上控制程序进行设定(包括设定工作时长, 工作开始时间和结束时间以及采集时间间隔等), 以固定采样频率记录冷泉释放气泡的视频片段, 并将采集的视频数据存储在32GB 存储卡。当对高分辨率视频成像系统布放完成后, 它将按照预先设定的时间自动运行。
图2 高分辨率视频延时相机(a)和GC600的Mega Plume 2 喷口(b)

喷口周围可见原油污染的水合物和白色菌席

Fig. 2 The high-definition video time-lapse camera (a) used to measure the Mega Plume 2 at GC600 (b). Oil-stained gas hydrate and white bacteria mats are visible around the vent

2014年3月搭载R/V鹈鹕号(航次PE14-14)到达位于GC600 的水深1222m的Mega Plume 2喷口, 通过ROV Global Explorer将高分辨率视频成像系统布放在Mega Plume 2喷口前约50cm处以及将刻度棒插入渗漏喷口处, 高分辨率视频成像系统每隔30 min记录一次12s视频数据(图2b), 26d之后, 高分辨率视频成像系统通过R/V亚特兰蒂斯号上的DSV Alvin号回收。我们从所有获得的视频片段中选取了几段进行分析。

2.2 半自动气泡计数算法

我们采用改进后半自动气泡计数算法(semi-automatic bubble counting algorithm, SABCA)(Johansen et al, 2017)对墨西哥湾GC600 Mega Plume 2喷口释放的气泡个数和释放速率进行了估算。在运行半自动气泡计数算法前, 视频数据的某些变量必须通过以下步骤手动设置: 1) 将每个视频片段全部转换成帧图像, 且对每个视频片段中所有帧图像的气泡释放速率进行平均, 得到平均气泡释放速率; 2) 确定帧图像中用于气泡计数裁剪窗口的最佳位置(图3a), 它是由视频片段中静止帧图像中气泡计数的最佳区域确定的; 3) 在确定的裁剪窗口内指定计数区域(图3b~3d), 界定了用于算法计数和减少重叠气泡、离开帧图像的气泡与阴影造成的误差的水平线(图3e中的白线); 4) 估算计数区域内气泡的上升速度和高度(单位: 像素), 以确定气泡在计数区域内的移动速度; 5) 赋予合适的阈值。待这些变量确定之后, 必须在整个视频数据集中对这些变量进行调整和交叉检查以确保它们在气泡计数程序运行前与视频片段中多个图像的一致性, 以达到确定自动算法分析图像间的间隔。
图3 图像处理方法示意图

a. 将初始照片中已确定的裁剪窗口转换成灰度; b和c. 连续图像中已确定的裁剪窗口相减; d. 去掉恒定背景并识别出气泡; e. 采用高、中、低3个阈值分别用来代表气泡大小、气泡运动模糊和油含量

Fig. 3 Schematic diagram of the image processing method. (a) Original image from which a cropped window is determined and converted to a gray image; (b) and (c) The determined cropped window subtraction in the consecutive image; (d) Removing the constant background and identify bubbles; (e) Three thresholds (high, medium, and low) are determined to account for bubble size, motion blurring, and oil content

对于缓慢上升的油气泡来说, 若要避免了重复计数或连续帧图像之间多次采样计数, 须要选择合适的图像间的间隔, 这样就可以分析连续图像间全新的一组气泡(图3b、3c)。对于Mega Plume 2喷口的视频片段的采样频率为29帧·s-1, 足以满足采集到快速移动的气泡。最后, 将裁剪的图像转换成连续灰度图像, 连续灰度图像间相同的像素值相减可去除图像中不变的背景如水合物、沉积物、细菌席和水体。而图像中亮点(气泡运动)作为连续帧图像之间的变化(图3d), 由于受到气泡大小、运动模糊、含油量或阴影等多种因素引起的比较差异, 处理后的图像中气泡的亮点强度是不同的。当视频片段被转换成静止的帧图像时会造成运动模糊出现, 而且较大气泡的边缘比小气泡的边缘更加不明显。因此, 为了验证气泡计数和去噪数据, 基于亮点的像素值定义了3个阈值: 高、中、低(图3e)。这3个阈值取决于视频中的采光, 还需在每个站点进行手动调整。
当所有步骤全部处理后, 运行自动气泡计数算法程序(公式1)(Johansen et al, 2017):
$B=\left[ \sum\nolimits_{r=1}^{R}{(\sum\nolimits_{i=1}^{I}{{}_{t}{{c}_{ri}}})} \right]\times {}^{F}/{}_{(R\times I)}$
式中: B为给定视频片段在阈值t处的每秒平均气泡数; R为计数区水平像素线数; I为视频帧数; tcri为图像i中r行上的气泡个数; F是每秒视频帧数。为了测试算法的准确性, 对一部分数据进行手动计数并与自动结果比较。

2.3 气泡尺寸计算

每台相机显示的气泡成像都在距镜头小于50cm的二维平面上。由于受到视差、原位摄像位置和ROV在海底有限时间的限制, 气泡尺寸的测定存在一定程度的误差。在Mega Plume 2, 将带有刻度棒插入到气泡喷口处(图2b), 用此来计算像素与毫米的转换关系。在其转换后, 采用ImageJ®软件来获得所有气泡的长轴和短轴以此来计算气泡的球状半径。为降低测量误差, 气泡尺寸测量均选在位于渗漏喷口上方约6cm处。我们总共分析了Mega Plume 2喷口的345张图像, 获得了1050个气泡直径, 最小的气泡直径约为1mm。依照Sam等(1996)所给出的公式(2), 球状半径可以通过计算椭球气泡的长轴和短轴而得出:
$r=\sqrt[3]{{}^{({{a}^{2}}b)}/{}_{8}}$
式中: a为半长轴(单位:cm), b为半短轴(单位:cm), r为球状气泡的半径(单位:cm)。
我们采用公式(2)计算气泡的球状半径(r), 而气泡的体积(V)计算则采用公式(3):
$V={4}/{3}\;\pi {{r}^{3}}$
尽管天然气中含有其他高分子碳氢化合物, 但通常在计算过程中假定天然气100%都是CH4。如在墨西哥北部, 高分子碳氢化合物含量通常只占气体体积的10%以下(Brooks et al, 1974; Bernard et al, 1976; Kvenvolden, 1995), 而在南海北部, 特别是莺歌海盆地, 高分子碳氢化合物含量非常少(Huang et al, 2009; Zhang et al, 2011; Di et al, 2016; Zhang et al, 2017)。基于这个假设, 我们采用公式(4)确定了体积V中气体数量n, 且公式(4)还修正了在已知深度和压力下甲烷压缩系数:
PV=nRTZ
式中: P是绝对压力(单位: MPa); V是气泡体积(单位: cm3); R是理想气体常数(取值为8.314J·mol-1·K-1); T是水的温度(单位: K); Z是在已知温度和压力下基于范德华稳态方程的甲烷压缩系数, Mega Plume 2渗漏喷口处平均水温为4.5℃, 则Z=0.79 (Johansen et al, 2017)。
气泡的上升速度受到各种水动力性质的影响, 如气泡大小、自然气泡振荡、海流、释放速率、上涌浮力和气泡表面洁净度(Grant et al, 2002; Leifer et al, 2002; Leifer, 2010)。此外, 气泡一旦离开渗漏喷口, 气泡在压力变化、气泡振荡和气体交换作用下会逐渐变大(Greene et al, 2012)。因此, 为了降低差异, 我们选择在喷口上方6cm处对气泡进行测量, 同时为了减少气泡计数的附加误差, 气泡释放速率的确定与气泡大小无关。

3 结果与讨论

3.1 Mega Plume 2 渗漏喷口特征

通过对获取的Mega Plume 2 渗漏喷口视频分析后, 发现Mega Plume 2喷口处是由成群的小孔组成, 油气泡则是通过出露在海底的水合物中的小孔释放至海底(图2b)。Mega Plume 2有时释放油-气混合气泡, 有时也仅释放油气泡或气体气泡(Johansen et al, 2017)。在渗漏通道形成之前水合物逐渐生长迫使这些小孔被改造成烟囱状通道, 而且在水合物孔洞中存在有大量的冰虫, 可能会影响海底浅表层沉积物水合物长期稳定(MacDonald et al, 2005; Johansen et al, 2017)。

3.2 气泡尺寸和释放速率

海底冷泉渗漏喷口释放的气泡直径与上升速率成反比关系, 即其释放气泡直径越小, 其上升速率就越快; 反之亦然, 这与气泡表面的洁净度有关(Leifer, 2010; Johansen et al, 2017)。而Mega Plume 2渗漏喷口释放的是油气泡, 其表面洁净度较差, 上升速率很慢。通过对Mega Plume 2所有气泡进行统计分析发现其释放的油气泡直径尺寸分布在1~8mm之间, 平均直径为2.56mm±1.01mm, 且气泡直径呈正态分布 (图4)。而且通过与其他冷泉喷口释放的气泡直径和尺寸分布比对发现, Mega Plume 2释放的气泡平均直径明显比其他渗漏喷口如Sleeping Dragon、Confetti、Rudyville、Birthday Candle、Mega Plume 1、F站位和海马冷泉的气泡直径小(图5)(Wang et al, 2016; Johansen et al, 2017; Di et al, 2020)。此外, Mega Plume 2喷口处是成群的孔洞组成, 因此, 海底冷泉喷口释放的气泡直径可能与渗漏通道的喷口大小有关, 同时也可能与其释放的成分有关。
图4 Mega Plume 2气泡尺寸分布和释放个数与其他冷泉渗漏喷口的结果对比

Fig. 4 Bubble size distribution and releasing number of Mega Plume 2 compared to that of other cold seeps

图5 Mega Plume 2气泡的平均直径与其他地区冷泉喷口结果比较

Confetti和Sleep dragon站位数据来自Wang等(2016); Birthday Candle、Mega Plume 1和Rudyville站位数据来自Johansen等(2017); F站位喷口1、喷口2 和海马喷口1站位数据来自Di等(2020)

Fig. 5 Bubble size average diameter from Mega Plume 2 compared with that of cold seeps at other regions. Data of Confetti and Sleep dragon stations were obtained from Wang et al (2016); data of Birthday Candle, Mega Plume 1 and Rudyville stations were obtained from Johansen et al (2017); data of site F 1, 2 and the Haima cold seep 1 were obtained from Di et al (2020)

为了准确确定Mega Plume 2的气泡释放速率, 需考虑采用3个不同的阈值来处理噪声(图6)。由于噪声增加, 宽松的阈值会导致获得气泡释放速率存在较大的标准偏差, 因此, 在手动气泡计数交叉检查中, 严格阈值和中等阈值的气泡释放速率才是最可靠的。Mega Plume 2油气泡释放速率明显要快于Birthday candle、F站位和海马冷泉区(Johansen et al, 2017; Di et al, 2020)。由此可知, 海底喷口释放油气泡尺寸越大, 其释放速率就越慢, 反之亦然。然而这个结论不适用于只释放气体的喷口, 如Rudyville释放的气泡更小, 其释放速率也很快。
图6 不同的阈值(严格、中等、宽松)和手动交叉检查计数Mega Plume 2的平均气泡释放率与标准误差

Fig. 6 Average bubble release rates and standard error for Mega Plume 2 considering the different thresholds (strict, medium, loose) and manual cross-check count

海底冷泉渗漏甲烷气泡的释放速率是受到如潮汐、运移路径、水合物形成、孔口大小、生物扰动、油气藏、渗透率和孔隙率等多种因素所控制(Leifer et al, 2005; Liu et al, 2009; 苏正 等, 2009; Krabbenhoeft et al, 2010; Netzeband et al, 2010; Kannberg et al, 2013; Di et al, 2014a, 2014b)。前人研究认为潮汐是影响气泡释放速率的主要因素(Krabbenhoeft et al, 2010; Netzeband et al, 2010; Di et al, 2014a, 2014b), 这是由于潮汐作用引起的静水压力变化改变着海底裂隙的毛细孔压力和气泡表层张力, 即涨潮时, 气泡流速减少; 落潮时, 气泡流速增加(Boles et al, 2001; Leifer et al, 2005; Liu et al, 2009; Schneider von Deimling et al, 2010; Bayrakci et al, 2014; Di et al, 2014a, 2014b; Johansen et al, 2017), 但仅适用海底冷泉喷口释放的含气气泡, 而Mega Plume 2是油-气混合喷口释放油气泡, 因此油气泡的释放速率与潮汐作用引起的静水压力无关(Johansen et al, 2017), 而与深部油气藏向上运移形成的增压有关(苏正 等, 2009)。
墨西哥湾GC600海底深部11km或14km处发育有大量的烃源岩, 而且在烃源岩的上部发育一个很大的盐层, 且在盐层中存在一个约10km2的空洞并在盐层中形成一个裂隙, 这个裂隙可能是深部油气藏中油气向上运移最有效路径, 油气先沿着盐层-沉积物界面向上运移, 然后沿着浅层裂隙进入渗漏区(Hood et al, 2002; Johansen et al, 2020)。根据地震剖面上似海底模拟反射(bottom simulating reflector, BSR)分布发现GC600海底处于水合物稳定带内(Johansen et al, 2020), 因此, 沿着裂隙渗漏通道向上运移的烃类化合物在合适的温度和压力条件下与水结合形成水合物(MacDonald et al, 2005)。通常海底渗漏的甲烷与水结合形成的水合物呈雪白色, 但Mega Plume 2 出露在海底水合物呈灰黑色, 表明此地区形成的水合物受到原油的污染(图2)。而且, 向上运移的油气在海底沉积物中不断生成水合物且促进其生长(Torres et al, 2004)。此外, 水合物还作为盖层阻碍油气向海底运移, 使得聚集在水合物层下的油气不断增压并可能导致水合物盖层破裂, 使得油气直接向海水中喷溢(MacDonald et al, 2005)。因此, 油气泡的释放速率可能主要受油气藏增压因素控制, 同时可能受沉积层差异加载等作用影响(Clayton et al, 1994; Leifer et al, 2005)。

3.3 Mega Plume 2渗漏油气泡通量估算

由于Mega Plume 2油气混合喷口, 其年释放通量不能采用通过渗漏喷口的气泡释放速率和每个气泡的气体含量来估算(未计算原油), 因此必须将原油和甲烷全部计算在内, 但由于目前没有原油与天然气的百分比值, 只能以纯原油(密度: 600kg·m-3)(Smith et al, 2014)或纯甲烷(密度: 110kg·m-3)( Johansen et al, 2017)的质量通量范围来估算, 通过计算得到Mega Plume 2喷口的油气渗漏质量通量为177.7m3·a-1 (19.55~ 106.62t·a-1), 比Mega Plume 1 (62m3·a-1)、F站位 (43.6m3·a-1)、Confetti (79~121m3·a-1)和海马冷泉区(110m3·a-1)的高, 与Sleep Dragon (105~158m3·a-1)的释放通量相当, 但比Rudyville释放通量略低(188m3·a-1) (Wang et al, 2016; Johansen et al, 2017; Di et al, 2020)。由于海底冷泉渗漏的气泡上升过程中受到微生物有氧氧化和气体交换作用的消耗, 到达海表面的通量很少, 然而由于Mega Plume 2释放的是油气泡, 气泡表面的原油降低了微生物的消耗和减少气体交换作用, 最终会有部分气体被释放到大气中(Patro et al, 2002)。因此, 为更加准确地获得海底冷泉喷口油气泡释放通量, 需要采用不同类型的观测装置以及对不同渗漏区的多个喷口进行长期的原位观测, 才能够更加准确地获得其迁移过程与释放通量。

4 结论

本文通过采用高分辨率视频成像系统获得了位于墨西哥湾北部GC600的Mega Plume 2渗漏喷口高清视频数据集, 结果研究表明Mega Plume 2 喷口油气释放速率可能与油气藏增压及沉积层差异加载有关, 而和静水压力不相关。而Mega Plume 2 喷口释放的油气泡平均直径为2.56mm±1.01mm, 油气泡释放速率80.25个·s-1。根据Mega Plume2 喷口处油和CH4的密度, 估算其释放油气通量为177.7m3·a-1 (19.55~106.62t·a-1), 表明墨西哥湾海底有大量的油气释放到海洋中。只有通过对海底渗漏喷口开展长期的原位在线观测, 才能准确地获知海底渗漏喷口流体的活动特征及其释放通量。

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