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海域天然气水合物主要赋存在大陆边缘的斜坡地带、隆起等水深大于300 m的深水环境[1]。海底水合物在特定的温度、压力和盐度条件下保持稳定[2],通常可以通过直接的原位观测或随钻测井来探测[3, 4],或者通过地震勘探方法——似海底反射(BSR)来识别[5-7]。
冻胀丘是由于土中水分冻结所造成的地表局部隆起现象。根据生长期长短,分为季节性冻胀丘和多年生冻胀丘(Pingo)。多年生冻胀丘是指具有一个冰核的多年生冷丘体,是多年冻土区所特有的冰缘地貌现象之一。它的直径由几米到上百米,高度通常达数十米,大多成圆丘形,也有的成拉长丘形。在满足天然气水合物形成的冻胀丘发育区,可以形成含天然气水合物层的海底丘形体(submarine hydrate “pingo-like structures”)。这种特殊丘形体的形成与冻胀丘的形成具有一定的相似性,常伴随着海底气体渗漏、沉积物失稳、丘形体内冰核和天然气水合物的周期性发育[8-11]。不同的学者从不同的角度采用各种术语描述了这种特殊的局部地形特征,如水合物堆(hydrate mounds)[12-16]、巨型气体堆(giant gas mounds)[17, 18]、垮塌的火山口(disruption craters)[19, 20]、沉积物滑塌(sediment slides)[21, 22]、以及大型倒塌地物(large collapse features)[23]等。国内尚未对这些术语进行统一,本文采用水合物冰丘(submarine hydrate pingo)这一术语。
海底水合物冰丘的形成与岩性、浅层水动力循环、含气流体的运移及热传导作用等存在着紧密的相互作用[24-27]。水合物冰丘的大小、形态及地球物理特征,反映了其不同的发育阶段,也可以间接表征浅海底水合物生成与分解情况。同时海底水合物冰丘对海底作业的危害也使其成为海底工程必须面对的挑战[28]。开展海洋水合物冰丘的研究,有助于加深浅层水合物分布、汇聚型流体运移以及下部油气藏的认识,并可以对海底工程建设的灾害防治等提供参考。因此本文综述了海底水合物冰丘的地形地貌特征、内部结构特征、其与BSR的关系,最后提出了海底水合物冰丘可能的演化模式。
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陆域冰丘是多年冻土区所特有的冰缘地貌现象之一。这些冰丘多在山间谷地、坡地及断裂带附近成群出现,一般高数米到数百米,直径可达上百米。典型的陆域冰丘为圆形、圆穹形或圆锥形结构[29],其表面具有植物生长和火山口(溶蚀槽),在气候温暖时期,当下表面的冰核融化,在表面将残留湿土壤或者椭圆形湖。陆域冰丘形成于低渗土壤中,由于地下热量和地表热量相差较大,地下水的热量便向热量低的冻结方向传递[30, 31]。地下水的集中运移和不断汇聚产生聚冰层。随冻结面向下发展,当冻结层上水的压力大于上覆土层强度时,地表就发生隆起,便形成冻丘。
海底水合物冰丘已在油气渗漏的深水区域被发现,如加利福尼亚近海[32]、尼日利亚[33]、日本[34]、加拿大卡斯卡迪亚大陆边缘[16]以及波弗特海[35]。但与冰川环境中发现的冰丘相比,海底水合物冰丘形成过程认识还较局限,概念限定条件还很不充分。一般认为,海底水合物冰丘的形成过程中,较高的流体通量维持了海底沉积物和海底表层水中的高甲烷饱和度,有效地抑制了水合物的分解[36-38]。对于开放型(渗漏型)由于其能够与深部高渗漏通道沟通,亦即存在深部高通量汇集型烃类气源的供给,故可以形成规模较大的水合物冰丘;而在缺少深部高通量汇集型烃类气源供给系统,只存在扩散型原地运聚低通量气源供给时,则只能形成规模微小的水合物冰丘。
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海底水合物冰丘,外观表现从平滑浑圆的峭壁丘状体变化到粗糙不平的缓斜丘状体,其周边多发育有麻坑,并且底栖生物非常发育[26, 39]。海底水合物冰丘在地震剖面上表现为空白反射带,并与BSR紧密相关。对于较强BSR反射的近圆形冰丘,随着水合物在沿着断裂和倾斜地层这类高渗高孔介质中成核,浅层水合物将不断地形成和富集,从而引起上覆地层的隆起;而对于较弱BSR反射的粗糙且较缓倾斜的冰丘,一般与不充足的流体运移或者流体对流作用有关,其关键是温度变化导致的异常,这也将引起浅表层海底水合物的分解及气体逸散。
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Serié等[27]描述了安哥拉近海宽扎盆地海底水合物冰丘与汇聚型流体运移及BSR间的相互关系。该区海底地貌特征为较缓的西-西南向斜坡(坡度1°~2°),水深850~1 000 m(图 1A),发育许多宽阔的海底高地,下伏发育有大量盐底辟和盐推覆体。在海底高地上,发育有一系列小型隆起,这便是海底水合物冰丘,它们的长度约为80~300 m,高度约为5~40 m(图 1B、图 1D、表 1)。由于晚白垩世-古近纪超压岩石在海底的突然出现,使得开阔的海底高地在地震振幅上表现为中等强度,与之对应,在其周围的小型盆地内,由于伴随着第四纪沉积物的填充,海底振幅表现为相对低振幅(图 1C)。同时,局部亦出现特殊的、中-高振幅异常,通常与水合物冰丘的位置一致,这可能暗示了坚硬地层、自生碳酸盐、化能群生物壳骸或与冷泉伴随的水合物的存在[40]。
图 1 安哥拉近海宽扎盆地海底水合物冰丘发育情况
Figure 1. Developmental state of hydrate pingoes in Kwanza basin, offshore Angola
在该区近海底(海底以下40~70 m)地震剖面上,出现不连续的强负极性及穿层反射异常,与常规BSR特征相仿[41],然而,以相似的深水环境为条件(水深850~1 000 m)进行考量,BSR深度通常出现在更深的位置,约为海底以下250~300 m[33, 42]。浅部不连续BSR的出现,可能代表着离散的气体聚集,这些气体被圈闭在含水合物冻结层下方、高渗沉积层或高破碎断裂带内[6, 43]。水合物稳定带厚度的减薄可能与下部水温变化相关,也可能与浅表层地温梯度异常相关,但考虑到同样环境下可能不变的底水温度(约4℃),水合物稳定带厚度的减薄主要原因,可能是与沿着盐底辟两翼发育的断裂直接相关,这些断裂与深源输导体系相通,盐及热流体通过对流作用提高了海底地层的热传导率,即导致近水合物地层温度升高[42, 44](图 1B和图 1C)。海底水合物冰丘的出现可能指示了大陆边缘汇聚型流体的运移,深部的热流和高盐度水随着汇聚型运移通道向上移动,从而导致了区域内BSR的深度变浅。
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Serié等[27]描述的安哥拉近海宽扎盆地海底水合物冰丘,平面上通常表现为圆形-椭圆形,其地形从平滑的、浑圆的、陡峭丘(9°~16°)(M1、M2、M6)变化到粗糙的、不平的、缓坡丘(3°~10°)(M4、M5、M7、M8)(表 1)。这些水合物冰丘在地震剖面上具有以下共同特征:缺乏内部构造、以及对应海底的地震反射缺失。此外,在M1和M7周围还发育有麻坑、断层崖和海沟等(图 1B)。
表 1 安哥拉近海宽扎盆地海底水合物冰丘地形地貌及地球物理特征(修改自文献[27])
Table 1. Geomorphology and geophysical characteristics of submarine hydrate pingoes in Kwanza basin, offshore Angola (after reference [27])
水合物冰丘序号 地形地貌 高度/m 宽度/m 长度/m 平均坡度/(°) 海底振幅 BSR振幅 M1 平滑、浑圆的陡峭丘 35 230 270 16 高 高 M2 平滑、浑圆的陡峭丘 10~15 150 180 9 中等 中等 M3 粗糙、不平的缓坡丘 5 160 250 3 高 低 M4 粗糙、不平的缓坡丘 5~10 220 270 4 高 低 M5 粗糙、不平的缓坡丘 5~10 80 150 7 高 中等 M6 平滑、浑圆的陡峭丘 40 270 290 16 中等 高 M7 粗糙、不平的缓坡丘 5~40 280 300 9 高 低 M8 粗糙、不平的缓坡丘 5~40 190 200 10 高 中等 Hovland和Svensen[26]描述了挪威海Nyegga区域复合麻坑G11内部的水合物冰丘,通常具有4个方面的共同特征(图 2):(1)都拥有正向地形(圆形堆和锥形);(2)丘体表面部分被生物薄膜所覆盖(指示正在进行中的流体流动);(3)都是部分被一片小型管虫(多毛类,以甲烷为食)覆盖;(4)在各自表面都有明显的溶蚀槽,显示上覆地层的液态化(可能是由局部的浅表层水合物分解造成)。
图 2 海底水合物冰丘形态及表面生物发育情况
Figure 2. geomorphology of submarine hydrate pingo and the development of its surface organism
在挪威海Nyegga区域复合麻坑G11内部的水合物冰丘中,Ice1显著特征为独特的圆形正向地形和相应的生物薄膜及密集管虫群(图 2A)。冰丘为明显圆形,且圆形边缘抬升,底部直径约0.5 m,相对于周缘隆起海底,圆形或椭圆形边缘凸起高度约0.25 m。水合物冰丘周围的沉积物表面有着密集的管虫群落,可能为须腕蠕虫类,抬升构造的内部,仅分布有斑块状生物薄膜,其余空间则鲜有观察到生物存在(管虫缺乏)。Ice4处的冰丘差不多为位于平坦的海床上的标准对称圆锥形,与周围海底相比,高度抬升约1 m(图 2B)。其底部为圆形,直径约1.5 m。同样,这个冰丘顶部覆盖着管虫和生物薄膜。在其侧面,存在显见的溶蚀现象,根据源于该处的液化沉积物(崩坏作用)证据,解释为溶蚀槽。Ice 6处发现的冰丘构造,形状类似于鲸鱼背部,整体表现为从陡峭倾斜坡过渡到缓慢弯曲的脊部(图 2C),平面上呈不规则状,尺寸为4.0 m×2.0 m×1.0 m,表面绝大部分被管虫和生物薄膜覆盖,具有明显凹痕和溶蚀证据,并可见一大型篮状海星,直径约1 m。
Chapman等[16]描述的卡斯卡迪亚Barkley峡谷中单个水合物冰丘规模为高3 m,长约4 m,其出露地表的水合物可见黄色或棕黄色条痕(图 2D),这与水合物中出现轻质油的特征相一致,在水合物冰丘附近发育有相应的蛤类等化能聚居群,这些现象均暗示了该区存在下部油气渗漏的可能。
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根据海底水合物冰丘形成的气体来源及地层水的补给方式,可将海底水合物冰丘分为开放型和封闭型。海底水合物冰丘的不同形态和地球物理特征,一定程度上代表了水合物动态成藏(形成与分解)的不同阶段,包括水合物成核、聚集/冰丘型、以及分解/冰丘垮塌等[27](图 3)。封闭型海底水合物冰丘模式中地层水补给以原地为主,气体的来源主要为原地微生物成因气体。这类水合物丘通常规模较小,饱和度较低,因此不在重点考虑之列(图 3Ⅰ-Ⅱa-Ⅲ)。马更些三角洲部分地区和班克斯岛地区可能发育这类海底水合物丘。开放型海底水合物丘模式中参加反应的水和气体都来自于“深部”沉积物渗滤的外部流体, 水合物可以以高饱和度的形式赋存于埋藏浅的海底沉积物之中(图 3Ⅰ-Ⅱb-Ⅲ)。开放型海底水合物丘是在形成过程中由外来的含气流体不断补给水合物形成所必须的气体和地层水,因而可形成较厚的水合物层,发生较强烈的膨胀而发育成的规模较大的丘形体,如挪威海Nyegga区域。
基于挪威海Nyegga区域复合麻坑G11的证据,推测水合物冰丘能在气体流量较大的地方形成,并且在该地区海水能够较容易地进入到海底沉积物中,从而交换和补充水合物形成过程中所消耗的海水(图 4A)。由于水合物在海底浅表层聚集并向上扩张,因此需要在较长的一段时间内保持一定的气体流量。同一个地区,持久且高通量的局部区域更容易形成水合物,从而在近海底表面沉积物中沿最小阻力形成水合物丘状体。Nyegga复合麻坑G11为一个开放型运移系统,其通道或空隙中,充填有来自下部的气体和周围沉积物中的海水。Clennell等[45]认为对于这样的开放系统,至少应满足下面两个条件:(1)水和气体的供给可视为充足,因此水合物能够在冰丘内部聚集;(2)水合物形成需要消耗自由水,“新的”孔隙水从邻近沉积物中吸收进来,并通常会导致海水流入冰丘中(图 4B)。随着海水进入到冰丘,海水将接触到冰丘沉积物表面及其下部水合物,而由于周围海水中的烃类浓度明显偏低,这将导致在正常海水盐度下的水合物处于非化学相平衡条件,诱发水合物分解边界上出现点式分解,从而引起溶蚀槽的形成,这一过程,可能造成了所覆盖沉积物出现局部流化的现象[26]。此外,Pecher等[46]主要强调了温度变化对冰丘形成的影响,认为由于海底温度变化等因素,水合物周期性的分解-生成,水合物的“冻融”循环即导致含水合物层及周缘地层中的裂缝周期性的开启和闭合,这使得上覆地层力学性质变弱,从而利于气体逃逸以及地层向上膨胀而形成丘状体。
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(1) 形成较大规模的海底水合物冰丘需要相对高通量汇聚型的烃类流体运移条件,因而海底水合物冰丘的出现可能指示了大陆边缘汇聚型流体运移以及下部可能赋存的油气藏。同时,海底水合物冰丘与BSR及其位置表现出较大的相关性,在深部的热流和高盐度水随着汇聚型运移通道向上运移的过程下,区域BSR表现出深度变浅的特征。
(2) 海底水合物冰丘的形态以正向地形为主,冰丘表面被大量嗜甲烷生物所覆盖,并发育溶蚀槽,反映海底水合物冰丘附近富含甲烷等烃类气体的存在,显示了水合物及水合物冰丘的形成与发育为一动态过程,且目前仍处于演化过程中。
(3) 根据气体来源和地层水的补给方式不同,海底水合物冰丘的演化分为开放型和封闭型。水合物丘的形成过程可分为水合物成核、聚集/冰丘型以及分解/冰丘垮塌3个阶段。
(4) 为了更好地了解海底水合物冰丘的分布、与汇聚型流体运移通道及BSR的关系,需要进一步针对重点区域开展海底调查和高精度多道地震调查,并结合地球化学等方法。此外,海底水合物冰丘被视为超压释放阀,其对大型边坡的稳定性影响是有利还是有害需要进一步进行研究。
THE CHARACTERISTICS AND IMPLICATIONS OF SUBMARINE HYDRATE PINGO
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摘要: 海底水合物冰丘是一种特殊的海底构造,外观表现从平滑浑圆的陡峭丘状体变化到粗糙不平的缓斜丘状体。地震剖面上,内部常表现为空白反射,并与似海底反射(BSR)紧密相关。归纳了前人对海底水合物冰丘的研究。综合分析认为规模较大的海底水合物冰丘通常形成于高通量汇聚型流体运移通道附近;海底水合物冰丘的出现可能指示了下部水合物及油气藏的赋存和大陆边缘汇聚型流体的运移;同时,在汇聚型流体运移条件下,伴随着流体向上运移的深部热流和高盐度水可能导致BSR深度变浅;此外,水合物冰丘的形成与发育为动态过程,其形成过程可分为水合物成核、聚集/冰丘型以及分解/冰丘垮塌3个阶段。开放型流动系统中形成的水合物丘的气体来源于“深部”沉积物渗滤的外部流体,可形成规模较大、高饱和度的水合物矿产,具有勘探价值。为更好地认识海底水合物冰丘演化史以及其与汇聚型运移通道、BSR分布位置等之间的关系,需要进一步开展海底调查、高精度多道地震调查,并结合地球化学等方法进行综合评价研究。Abstract: Submarine hydrate pingo is a special kind of seabed topographic features, whose appearances vary from smooth, well-rounded, steep-sided mounds to rough, uneven, gently dipping mounds. Submarine hydrate pingo usually emerges as blank reflection in seismic profiles, and is closely related with bottom-simulating reflection (BSR). This paper summarizes the previous researches on the submarine hydrate pingo. On this basis, we believe that, the relatively large-scale submarine hydrate pingo is usually formed adjacent to high-flux focused fluid migration pathway; the occurrence of submarine hydrate pingo possibly indicates the occurrence of hydrate and hydrocarbon reservoir under the pingo, and the continental margin focused-fluid migration. At the same time, in the function of focused-fluid migration, upward migration of heat flow and highly saline fluids may result in the shoaling of BSR. Furthermore, the formation and evolution of submarine hydrate pingo is a dynamic process, conducted in an open flow system, which is divided into three phases, including hydrate nucleation, hydrate aggregation/pingo formation, and hydrate dissociation/pingo collapse. The gases of hydrate pingo formed in the open flow system are derived from external fluid after "deep" sediment percolation, which can form hydrate deposits with larger-scale and high saturation, and have the exploration value. To better understand the evolution history of submarine hydrate pingo, and the relationship between submarine hydrate pingo and focused migration pathway as well as the location of BSR distribution, more seabed survey and high-resolution multi-channel seismic survey should be carried out, and a comprehensive evaluation should be conducted combined with geochemical methods.
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Key words:
- submarine hydrate pingo /
- gas hydrate /
- BSR /
- migration pathway /
- heat flow
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图 1 安哥拉近海宽扎盆地海底水合物冰丘发育情况
宽扎盆地位置及水合物冰丘所在区域位置;B:冰丘的地形和分布及其与盐底辟相关的海底高地;C:由小盆地、盐推覆体和盐底辟控制的海底丘的地震剖面;D:显示冰丘的几何形态、不连续型BSR和深部输导体系的二维地震剖面(修改自文献[27])
Figure 1. Developmental state of hydrate pingoes in Kwanza basin, offshore Angola
A: the site of Kwanza basin and submarine hydrate pingoes; B: pingoes morphology and distribution and theirs relation to salt diapir-related bathymetric high; C: two-dimensional seismic profile of bathymetric high formed by mini basins, salt nappes, and salt diapirs; D: two-dimensional seismic profile illustrating pingoes geometries, discontinuous BSR, and deep plumbing system. (After reference[27])
图 2 海底水合物冰丘形态及表面生物发育情况
A:挪威海麻坑G11中的冰丘Ice1;B:挪威海麻坑G11中的冰丘Ice4;C:挪威海麻坑G11中的冰丘Ice6;D:卡斯卡迪亚Barkley峡谷中的冰丘(修改自文献[16, 26])
Figure 2. geomorphology of submarine hydrate pingo and the development of its surface organism
A: the pingo"Ice1" in pockmark G11, in Norwegian Sea; B: the pingo"Ice4" in pockmark G11, in Norwegian Sea; C: the pingo"Ice6" in pockmark G11, in Norwegian Sea; D: the pingo in Barkley Canyon, in the northern Cascadia Margin (After references [16, 26])
表 1 安哥拉近海宽扎盆地海底水合物冰丘地形地貌及地球物理特征(修改自文献[27])
Table 1. Geomorphology and geophysical characteristics of submarine hydrate pingoes in Kwanza basin, offshore Angola (after reference [27])
水合物冰丘序号 地形地貌 高度/m 宽度/m 长度/m 平均坡度/(°) 海底振幅 BSR振幅 M1 平滑、浑圆的陡峭丘 35 230 270 16 高 高 M2 平滑、浑圆的陡峭丘 10~15 150 180 9 中等 中等 M3 粗糙、不平的缓坡丘 5 160 250 3 高 低 M4 粗糙、不平的缓坡丘 5~10 220 270 4 高 低 M5 粗糙、不平的缓坡丘 5~10 80 150 7 高 中等 M6 平滑、浑圆的陡峭丘 40 270 290 16 中等 高 M7 粗糙、不平的缓坡丘 5~40 280 300 9 高 低 M8 粗糙、不平的缓坡丘 5~40 190 200 10 高 中等 -
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