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1 海底泥火山钻探科学问题的背景
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1.1 泥火山研究意义
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泥火山是地下泥质流体喷发到地表或海底所形成的丘状地质构造,因其喷发的形式和喷发后形态与火山相似,故而得名(图1)(Milkov, 2000; Dimitrov, 2002; Etiope et al.,2004; Kopf, 2008; 阎贫等,2014;万志峰等,2021a)。泥火山发育较为普遍,主要分布于特提斯缝合带和环太平洋带,全球已在40多个陆地地区和20多个海域地区发现2000多座泥火山(Milkov, 2000; Somoza et al.,2012; Kholodov, 2013; Feseker et al.,2014; Wu Tingting et al.,2020, 2021)。我国至少已有13处海域(南海北部台西南盆地、珠江口盆地、中建南盆地及东海等)和陆地(准噶尔盆地、羌塘盆地、四川盆地及台湾南部等)区域发现了泥火山,总计400多座(Yin Ping et al.,2003; 何家雄等,2010;戴金星等,2012;阎贫等,2014;Chen Songchuen et al.,2014)。
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泥火山具有重要的研究意义。泥火山喷发出的油气、泥质沉积物、地下水等可作为深层沉积和油气成藏研究与评价的依据,是研究泥火山物质来源和形成机制的基础。泥火山活动表明该区曾发生过油气的生成、运聚和破坏,是盆地早期油气评价的重要标志,全球近40%的油气田是在地表有油气渗漏的地区发现的(Link, 1952; Abrams, 2005; Rovere et al.,2014)。海底泥火山是连接深层油气与表层天然气水合物的通道,泥火山中心的水合物形成是水和甲烷渗滤驱动的低温热液过程,因此海底泥火山的发现是天然气水合物存在的活证据,是寻找深水油气与天然气水合物的重要标志(Ginsburg et al.,1984; 陈多福等,2005;沙志彬等,2005; Egorov et al.,2010;Franek et al.,2014; Wan Zhifeng et al.,2020; 万志峰等,2021a, 2021b)。泥火山喷发的甲烷气体占泥火山排放气体体积的90%以上,全球泥火山每年甲烷排放量达500万t,大量甲烷气体会加剧温室效应和气候变化(Dimitrov, 2002; Kopf, 2003; Sauter et al.,2006)。另外,泥火山是现代地壳运动、新构造活动的显著标志,泥火山活动影响油气钻井与开发和管道铺设,会引发工程事故,在大陆边缘的泥火山活动易引发边坡失稳而造成海底基础设施的损害(Mellors et al.,2007; Normile, 2008; Manga et al.,2009; Kholodov, 2013; Wan Zhifeng et al.,2022)。
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图1 泥火山地质模型图
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Fig.1 Geological model of mud volcano
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ρw—海水密度;ρs—砂岩密度;ρm—物源层泥浆密度;ρc—泥火山通道密度;P0—深部超压;D —海水深度;h —泥火山高度
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ρw—Sea water density; ρs—sandstone density; ρm—mud density of provenance; ρc—density in mud volcano channel; P0—deep overpressure; D —depth of sea water; h —height of mud volcano
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1.2 泥火山形成机制与演化规律
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前人开展了诸多泥火山相关的研究工作,多以喷出物(天然气、沉积物、水)地球化学测试分析为主(Kopf, 2003; Sauter et al.,2006; Etiope et al.,2009; 戴金星等,2012),辅以野外地质构造分析与地球物理解释(Yin Ping et al.,2003;Mellors et al.,2007; Kopf, 2008; Rudolph et al.,2012; 杜建国等,2013; Wu Tingting et al.,2019),也有部分学者开展泥火山喷溢过程的数值模拟研究(Murton et al.,2003;Zoporowski et al.,2009)。另外在泥火山油气渗漏及其与水合物成藏的关系方面亦开展了一些研究(Milkov, 2000; 吴时国等,2010;Nuzzo et al.,2012; Zhang Wei et al.,2020)。然而,泥火山的形成机制特别是其触发因素问题一直悬而未决,到底是由于深层超压流体泥底辟活动刺穿上覆岩层并突破地表或海底形成,还是在挤压、走滑和拉张等构造应力作用下深层泥质流体沿断裂喷出而成,或是由于地震活动等其他因素触发,存在较多争议(Mazzini et al.,2007, 2009; Manga et al.,2009; Feseker et al.,2014; Wan Zhifeng et al.,2019; Zhong Siling et al.,2019)。同时,对于泥火山活动过程中油气渗漏的“聚”和“散”周期规律,及其对水合物成藏的贡献与破坏等方面,有待深入探究。
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泥火山的形成机制是当前研究的热点与难点。Milkov(2000)总结了泥火山形成的原因,可以分成4类,包括地质原因、构造原因、地球化学原因、水文地质原因。总结起来,主要有以下两个基本要素:① 深部具有丰富的泥质地层,且具有较高的超孔隙流体压力(超压),这是泥火山形成的物质基础与动力源;② 需要有触发泥火山喷发的动力机制,包括构造运动、断层活动、地震作用等(Pitt et al.,1982; Davies et al.,2008; Feseker et al.,2014)。
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泥火山喷发的触发因素一直是地质学家争论的焦点。以著名的印尼东爪哇岛泥火山(LUSI)为例,该处泥火山于2006年5月开始爆发,最大喷发速率达180000m3/d,堆积的泥浆厚度大约20m,致使超过7.8km2的地区受影响,4万人流离失所,然而其触发因素问题一直悬而未决(Normile, 2008; Istadi et al.,2009)。Davies et al.(2008)认为钻井破坏了地下的岩层结构,导致泥浆喷涌出来。Normile(2008)认为一口探测性的钻井对地下结构的影响微弱,不足以触发泥火山。Manga et al.(2009)认为距离泥火山以西300km处曾发生了里氏6.3级地震,导致地下岩层发生断裂,封闭在地下的超压流体由此喷发出来。Zoporowski et al.(2009)指出,此次地震的震中距离泥火山较远,是否能让几百千米之外的岩层发生断裂,尚存疑虑。Mazzini et al.(2009)认为超压流体背景下,走滑断层活动触发了印尼泥火山的喷发。此外,还有一些泥火山的形成与火山活动有关,如美国的黄石公园泥火山,不仅数量多,而且规模大(Pitt et al.,1982)。
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近年来,我国学者主要基于地质地球物理、地球化学和数值模拟等方法对我国区域内泥火山开展研究,涉及其地质特征描述、喷出物地球化学特征分析及矿物学研究、与油气及天然气水合物的关系分析、与中强地震的关系分析及形成机制的推测等方面,对海底泥火山的研究则主要涉及多波束测深/地震等地球物理资料解释、与天然气水合物赋存关系分析及泥火山形成机制推测等方面,但是缺乏针对我国近海典型泥火山流体渗漏规律与形成机制的定量研究与科学钻探分析。
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1.3 台西南盆地泥火山发育特征
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台西南盆地位于南海北部大陆边缘东侧,整体呈NEE向,具有“两坳夹一隆”的盆地构造格局。盆地海水总体北浅南深,台西南盆地南部跨越陆坡至深海区,南端最大水深可达3000m。盆地的形成演化过程受太平洋板块运动、菲律宾海板块俯冲等作用的影响(薛友辰等,2012),盆地整体演化具有明显的“先张后挤”特征(丁巍伟等,2004)。台西南盆地主要发育NE向、NEE向、近SN向及NW向断裂,其中,NEE向断裂为盆地主要控盆断裂。新生代后期,南海北部大陆边缘南北两侧构造相对稳定,但是东端构造活动显著增强,走滑断裂构造活跃,台西南盆地发生强烈的构造变形及盆地沉降(朱伟林等,2007)。
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台西南盆地新生代沉积厚度平均5000m,最厚可达8000m,其中上新统—更新统泥岩层厚达4000~5000m(丁巍伟等,2004;薛友辰等,2012)。台西南盆地渐新世末期以来长期处于构造挤压的应力环境之中,大量的陆源碎屑及高的沉积充填速率造成了盆地中新统及上新统巨厚泥页岩等细粒物质的快速沉积充填而产生欠压实作用,加上有机质成熟生烃作用,最终导致了该区异常高温超压环境的形成,为台西南盆地泥火山、泥底辟等流体渗漏的形成奠定了重要的物质基础和动力源基础(Chen Songchuen et al.,2014)。
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台湾岛西南部和东部沿海、台西南盆地均广泛发育泥火山构造,泥火山的分布与断裂在空间上有很好的相关性(莊文星,2010;Sung Quocheng et al.,2010; 何家雄等,2012;Chen Songchuen et al.,2014)。台西南盆地海域泥火山主要分布在南部坳陷带陆坡深水区,平面上可分为高雄海岸带、靠近高屏的海底峡谷带、枋寮海底峡谷带及永安线形构造带等四个泥火山展布带(Chiu Juikun et al.,2006)。泥火山总数高达94个,直径几十米至4km,高度几十米至300m(何家雄等,2012;Chen Songchuen et al.,2014)。其中,发育在高屏斜坡的泥火山在地震剖面上体现出了多期次的泥浆活动,泥火山多发育在泥底辟之上(图2)。
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2 台西南盆地泥火山钻探的科学目标
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目前对于泥火山形成机制、油气渗漏周期规律等方面存在一定的不足,主要表现在:
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(1) 泥火山喷发的诱发因素问题。构造应力(伸展、挤压、走滑)、地层流体(异常高压)、沉积作用(欠压实)以及外界动力因素(地震、钻井)等对泥火山喷发的贡献如何?是什么因素破坏了深层沉积与流体的动态平衡,导致泥火山携带深层沉积物、水与油气的喷发?
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(2) 泥火山油气渗漏周期规律问题。泥火山喷发伴随着油气的逸散,而泥火山停止活动时渗漏通道封闭,有利于油气的聚集。那么泥火山活动的周期性规律如何?海底泥火山形成演化过程中油气如何分布,油气渗漏的通量有多少?以及此过程中温度、流体压力等物质与能量的变化交换规律如何?
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(3) 海底泥火山喷发对水合物成藏的影响问题。为什么并非所有的海底泥火山周围都存在天然气水合物呢?泥火山型天然气水合物成藏主要受哪些因素控制?泥火山活动在水合物成藏过程中扮演怎样的角色?泥火山周期性活动将深层热液(油、气、水等)运移到表层,是否会造成天然气水合物分解和甲烷的大量释放而引起全球灾难性的气候变化?
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针对这些科学问题,本文建议选取台西南盆地典型泥火山,开展海底泥火山科学钻探,结合泥火山地质背景、喷发特征、油气渗漏规律等综合研究,分析泥火山的形成机制与主控因素,揭示泥火山油气渗漏规律。研究成果对于超压沉积盆地油气勘探与开发、深水区泥火山渗漏型水合物研究与勘探、自然过程碳排放以及新构造与地质灾害研究等诸多领域均具有重要的指导意义。
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3 台西南盆地泥火山科学钻探的重要意义
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3.1 海底泥火山发育的物质基础与动力条件
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泥火山/泥底辟的发育需要一定的物质基础(丰富的泥源物质)和动力源(沉积地层深部超压)。台西南盆地新生代沉积厚度平均5000m,最厚可达8000m(张伟,2016),其中上新统—更新统泥岩层厚达4000~5000m(丁巍伟等,2004;朱婷婷等,2009;薛友辰等,2012)。台西南盆地渐新世末期以来长期处于构造挤压的应力环境之中,大量的陆源碎屑及高沉积充填速率造成了盆地中新统及上新统巨厚泥页岩等细粒物质的快速沉积充填,从而产生欠压实作用,加上有机质成熟生烃作用,最终形成了该区异常高温超压的环境,为台西南盆地泥火山、泥底辟等流体渗漏的形成奠定了重要物质和动力源基础(Chen Songchuen et al.,2014)。
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图2 台西南盆地地震剖面图(据Chen Songchuen et al.,2014修改)
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Fig.2 Seismic profiles of southwest Taiwan basin (modified from Chen Songchuen et al.,2014)
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(a)—发育在高屏斜坡上的泥火山的地震剖面(主测线);(b)—发育在高屏斜坡上的泥火山的构造剖面(主测线);(c)—发育在高屏斜坡上的泥火山的地震剖面(联络线);(d)—发育在高屏斜坡上的泥火山的构造剖面(联络线);KC—高屏峡谷;SB—陆坡盆地;STR—Ahell Tomb脊;FC—枋寮峡谷;FR—枋寮海脊;MV—泥火山;MD—泥底辟
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(a)—Seismic profile of mud volcanoes developed on Gaoping Slope (inline); (b)—structural profile of mud volcanoes developed on Gaoping Slope (inline);(c)—seismic profile of mud volcanoes developed on high screen slope (crossline); (d)—structural profile of mud volcanoes developed on Gaoping Slope (crossline); KC—Gaoping Canyon; SB—slope basin; STR—Ahell Tomb ridge; FC—Fangliao Canyon; FR—Fangliao Ridge; MV—mud volcano; MD—mud diapir
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图3 南海北部泥火山形成演化过程物理模拟实验(据Wan Zhifeng et al.,2021c)
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Fig.3 Physical simulation experiments on the formation and evolution of mud volcano in the northern South China Sea (after Wan Zhifeng et al.,2021c)
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(a)~(d)—纯超压作用下泥火山形成过程的物理模拟结果;其中(d)中进行横切时出现了塌陷,为呈现完整形态用红色标示补齐了塌陷部位
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(a)~(d)—Physical simulation results of mud volcano formation process under pure overpressure; in (d), collapse occurred during crosscutting, and the collapse site was marked in red to present a complete shape
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丰富的泥质物为泥火山或泥底辟等流体渗漏构造的发育奠定了极好的物质基础(Zhong Siling et al.,2021),同时沉积厚度又影响着流体渗漏构造发育的类型。本课题组曾利用砂箱物理模拟实验探讨盖层厚度对泥火山形成的影响,实验使用60目和80目的白色石英砂模拟泥岩和砂岩,在两层白砂之间用一层极薄的褐色石英砂作为地层活动的指示层,并采用流体注入的方式模拟底辟过程(Wan Zhifeng et al.,2021c)。综合南海北部盆地的构造活动及各个盆地主要岩性为泥岩和粉砂岩且泥岩多于粉砂岩的沉积地层特性,结合相似性原理来设置物理模拟模型的厚度。结果表明,在纯超压背景下,当盖层厚度为78mm时,流体喷出表层所需的压力为0.3MPa;盖层厚度为104mm时,压力为0.4MPa;而当盖层厚度为130mm时,压力为0.46MPa。说明在流体黏度、岩性等条件相同的情况下,沉积盖层的厚度与底辟形成所需的压力大小具有正相关关系,上覆沉积盖层越厚,深部流体向上运移至地表(海底)所需的压力越大(图3)。
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根据上述分析,丰富的泥岩层是泥火山形成的物质基础,但是含水泥质物向上运移形成泥火山需要深部动力的作用,影响地层超压形成的因素包括较高的沉积速率导致的欠压实及有机质生烃等,而泥源层(物源层)之上的沉积盖层厚度又将影响流体渗漏构造的形成。通过对目标泥火山开展大洋钻探,揭示泥火山发育区沉积过程,分析其地层压力特征,有效探讨泥火山形成的物质基础与动力条件(Wan Zhifeng et al.,2018)。
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同时,通过泥火山钻探,揭示泥火山活动期次与规律。受泥火山流体活动影响的沉积物、孔隙流体、自生碳酸盐和冷泉动物群等可成为重要的研究对象,对揭示泥火山形成发育机理方面起着重要作用。通过对泥火山钻井岩心孔隙水分析,快速反映沉积物中生物化学反应的变化,记录泥火山流体特征并提供流体来源信息。通过自生碳酸盐岩分析揭示碳氢化合物渗漏活动记录,包括对碳的来源和通量、碳酸盐形成的条件以及关于流体沉积物和岩石相互作用等信息。利用泥火山钻探发现的冷泉生物群落判断甲烷渗流通量与演变规律。
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3.2 探索海底泥火山喷发的触发机制
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地震活动被认为是触发泥火山喷发的重要因素(Manga et al.,2009; Mazzini et al.,2009; Feseker et al.,2014; Bonini et al.,2016)。从构造上来说,全球的泥火山主要分布在喜马拉雅-特提斯缝合带和环太平洋地震带(Kopf, 2002; Sun Chihhsien et al.,2010),与全球地震带的分布关系密切。在世界许多区域的研究结果证实了地震对触发泥火山活动的作用,Mellors et al.(2007) 认为地震发生之后的几分钟之内或几个月甚至几年之后,泥火山的活动性会增强。例如2013年Balochisatan地震之后的短时间内在距离震中383km的Gwadar海岸形成了一个200m宽,20m高的小岛(Bonini et al.,2016)。Bonini et al.(2016) 通过分析位于阿塞拜疆、日本、罗马尼亚、巴基斯坦及安达曼群岛等地的大地震和泥火山喷发的潜在关系发现,地震发生之后,泥火山的活动会显著增强。
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台湾位于一个复杂的构造活跃区(Ma Kuofong et al.,2013),我国东部大陆边缘及吕宋弧分别以11~12mm/a和70mm/a的速度向SEE及NW方向运动,导致整个台湾地区处于一种高度压缩的应力状态。台湾地区属于环太平洋地震带,区内构造活动强烈,断层发育错综复杂。在台湾周边海域,由于受弧陆碰撞造山运动的影响,许多断裂仍较活跃,其中台南盆地及冲绳海槽发育正断层,而在台湾东部海域发育走滑断层与逆断层(燕云鹏,2002)。
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根据台湾地区板块构造、地震活动和地壳运动特征、新构造和现代构造运动、应力场特征及地震构造类型、地震断层性质和方向等因素,燕云鹏(2002)将台湾地区划分为2个地震区,6个地震带。台湾地区地震具有地震密度东大西小、地震活动性东强西弱、震源深度东深西浅和活动周期东短西长的特点,且地震带的分布与区域断裂具有较大的相关性,多位于断裂之间(燕云鹏,2002)。
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台湾陆地及台西南盆地海域发育有大规模的泥火山构造,且该区地震诱发条件较好,由于大型泥火山喷发可能对人类生命财产造成损失,并且中强地震可能会对泥火山的活动造成一定影响,所以有必要利用现已发生的中强地震,判断可能对泥火山喷发起到触发作用的地震的震中距大小及震级范围。
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通过USGS地震检索目录选取了发生在台湾西南侧及其附近海域的几次中强地震进行库伦应力模拟,探讨台西南盆地及台湾陆地泥火山发育区周缘的中强地震所产生的的静态应力变化值对泥火山活动的影响。2006年12月26日的一场6.9级地震,震源深度10km,地震矩为3.05×1019N/m2、断层破裂长度为51.05km,破裂面积为647.14km2,滑移量为1.57m,根据USGS的震源机制解“沙滩球”,判断此次为走滑兼正断层型地震,且以走滑作用为主。本研究选用节面Ⅱ的破裂断层产状:走向61°,倾向90°,滑移角138°。假设垂直接收断层走向为45°和135°,倾向为90°, 分别计算各个不同走向的接收断层上的正应力的变化值(图4)。
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台西南盆地及其周缘区域部分中强地震库伦应力模拟结果显示,地震震级和震源深度对正应力的变化值起决定性作用,震级越大、震源深度越小,在1km深度上引起的正应力变化值越大。在震中附近的正应力变化值最大,随着震中距增大,正应力的变化值 Δσn迅速衰减。当地震引起的正应力的变化值超过1×104 Pa时,可能会对泥火山的活动具有一定的触发作用。
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台湾周缘地质构造较为活跃,中强地震频发,海底泥火山等流体渗漏构造规模大。地震作为触发泥火山等喷发的重要因素,将影响泥火山的活动,为尽量减少由于泥火山的喷发诱发的地质灾害(如天然气水合物分解、海底滑坡、浊流等)造成的破坏,在海底工程建设时应尽量规避泥火山发育区及其可能波及的范围。当海底中强地震发生时,应根据震级及震中距,估算其影响范围,判断短期内泥火山喷发等地质灾害发生的可能性,以便采取必要的补救措施。
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3.3 揭示海底泥火山发育区水合物动态成藏机理
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海底泥火山是连接深层油气与表层天然气水合物的重要通道,可为水合物形成提供充足的甲烷气体,有利于渗漏型水合物的形成(Dimitrov, 2002; 沙志彬等,2005;苏正等,2006;陈多福等,2005;张光学等,2006)。Ginsburg et al.(1984)第一次提到了天然气水合物与海底泥火山的关系问题。Milkov(2000)认为,泥火山中心天然气水合物的形成是水和甲烷渗滤驱动的低温热液过程,在泥火山的边缘,气体水合物由甲烷扩散与地层水的混合所驱动的交代作用形成。泥火山型天然气水合物资源潜力巨大,Milkov(2000)估算了全球与海底泥火山相关的气体水合物中的甲烷量为1010~1012 m3。
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另一方面,在泥火山喷发过程中,深层温度较高的流体渗漏到表层会改变水合物赋存层的温度场,从而抑制水合物的形成,并导致已形成的水合物分解(Brown et al.,1988; Crutchley et al.,2014; Di Pengfei et al.,2014; Smith et al.,2014)。巴伦支海、巴巴多斯近海、黑海、地中海以及我国南海神狐海域(SH-5井)水合物钻探结果表明,部分泥底辟/泥火山发育区未发现水合物样品(Brown et al.,1988; Milkov, 2000; Bohrmann et al.,2003; 王力峰等,2010)。前人于1999年和2007年在西南巴伦支海Håkon Mosby泥火山赋存海域实施了沉积物取芯。水合物钻探结果表明在泥火山喷口处未钻遇水合物样品,而在泥火山圆丘区的多个站位均发现了天然气水合物样品,水合物呈环带状分布于泥火山周缘(Ginsburg et al., 1999;Jerosch et al., 2007)。部分站位没有成功钻获水合物样品的原因是泥火山通道内的温度过高,致使原本形成的水合物分解,迅速释放出甲烷气体,且当温度高于当地水合物赋存的临界温度,不利于水合物的再次形成(Ginsburg et al.,1999)。
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图4 台西南盆地及其周缘区域2006年12月26日6.9级地震所引起的正应力的变化值
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Fig.4 Normal stress changes caused by the Mw 6.9earthquake in southwest Taiwan basin and its surrounding area in December 26, 2006
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(a)—接收断层面的走向为45°,倾向为90°;(b)—接收断层面的走向为135°,倾向为90°;黑色的虚线表示正应力变化值1×104 Pa的等值线,地形图源自Google Earth(据万志峰等,2021a)
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(a)—The strike of the receiving fault plane is 45° and the dip is 90°; (b)—the strike of the receiving fault plane is 135° and the dip is 90°; black dotted lines represent the contour line of 1×104 Pa and the topographic map is derived from Google Earth (after Wan Zhifeng et al.,2021a)
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目前国内外学者对海底泥底辟/泥火山与天然气水合物赋存的关系研究主要在于:地球物理探测分析水合物的分布,探针测试浅表层热流值,钻井测试水合物赋存层孔隙压力、温度与储层物性,取样分析沉积物矿物学与地球化学特征,水合物气源分析等(Ben-Avraham et al.,2002; Bohrmann et al.,2003; Franek et al.,2014; Chen Han et al.,2015)。然而对泥底辟/泥火山流体热力学演化过程及其对天然气水合物成藏的动态影响研究较少。部分学者将泥底辟/泥火山划分为早、中、晚三个演化阶段,认为泥底辟/泥火山早期演化有利于天然气水合物的生成,晚期泥底辟/泥火山高热量流体会引起天然气水合物分解(Milkov, 2000; 王力峰等,2010;Wan Zhifeng et al.,2017)。然而这一认识仅处于概念模型的定性描述与推测阶段,缺少对泥底辟/泥火山流体热力学过程的定量研究,难以有效揭示泥底辟/泥火山发育区水合物的赋存规律,无法针对性地对海域泥底辟/泥火山发育区水合物资源评价与勘探。
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通过对台西南盆地典型泥火山的钻探,并结合原位热流测试与持续观测,可有效细化泥火山发育区流体压力、岩石物性、热力学与沉积动力学等各项参数,优化流体力学与热力学的耦合机制,探究流体热力学的时间与三维空间变化规律,揭示泥底辟/泥火山流体热作用对水合物形成与分布的制约关系,建立泥底辟/泥火山等流体渗漏环境下水合物资源精细评价的热力学指标体系,探索海底泥火山发育区水合物动态成藏机理,为南海水合物研究与勘探提供必要的理论依据。
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3.4 估算海底泥火山碳排放
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Kopf(2003)通过对研究数据的统计分析指出,甲烷气体占泥火山排放气体体积的90%以上,并保持较高的排放速度,是造成全球变暖的因素之一,需引起足够的重视。Dimitrov(2003)估算全球泥火山总的每年甲烷排放量达500万t。Etiope et al.(2004)首次对阿塞拜疆东部泥火山甲烷通量进行了检测,指出阿塞拜疆所有的停止活动的泥火山每年排放0.3~0.9×106 t甲烷。Etiope et al.(2004)对罗马尼亚泥火山群甲烷通量进行了检测,研究区2.3km2每年至少排放甲烷1200t。Yang Tsanyaofrank et al.(2004)对Chung-lun泥火山泥浆池(100m2)气体渗漏进行了检测,估算出台湾地区泥火山静止期每年最少排放甲烷29t。Chang Hungcheng et al.(2010)对台湾地区Chishan断层带的泥火山甲烷排放进行了为期一年的录像和流量检测,该断层带19个泥火山甲烷通量占全球甲烷通量的0.1%~0.2%。Sauter et al.(2006)以最新的水下声学、影像学以及地球化学方法为手段,对挪威-巴伦支-卑尔根海域Håkon Mosby深海泥火山甲烷气体排放进行了检测分析,通过对海底气泡影响分析,估算气体流量平均为0.2mol/s(范围为0.08~0.36mol/s),这就意味着每年至少有几百吨甲烷释放出来。
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泥火山甲烷渗漏是一个重要的温室气体来源,应该引起充分重视。然而,全球泥火山甲烷通量只是一个基于有限数据的保守估计。借助大洋钻探,通过对海底泥火山的解剖,开展甲烷渗漏监测,实现海域泥火山甲烷渗漏研究的突破,对自然过程甲烷通量及其对大气碳收支的贡献研究具有重要的意义。
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4 结论
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泥火山对油气渗漏、水合物成藏、新构造活动、碳排放等方面具有重要的研究意义。南海北部台西南盆地发育大量泥火山,是剖析泥火山的天然实验室。本研究建议选取台西南盆地典型泥火山,开展海底泥火山科学钻探,结合泥火山地质背景、喷发特征、油气渗漏规律等综合研究,获取海底泥火山发育的构造环境、沉积背景、温压场等系列参数,分析泥火山的形成机制与主控因素,揭示泥火山油气渗漏与水合物成藏规律。研究成果对于超压沉积盆地油气勘探与开发、深水区泥火山渗漏型水合物研究与勘探、自然过程碳排放以及新构造与地质灾害研究等诸多方面均具有重要的指导意义。
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摘要
泥火山是地下泥质流体喷发到地表或海底所形成的丘状构造,其形成与油气成藏、天然气水合物成矿有着直接联系,其喷发活动易引发工程事故,喷发的大量甲烷气体会加剧温室效应。国内外学者已在泥火山发育、地球化学、地球物理特征等方面开展了诸多研究。然而泥火山形成机制问题仍存在多种争议,其触发因素可能来自断层活动、地震、超压等。泥火山流体的传热传质及其对水合物形成与动态分布的控制作用还有待深入研究,同时海底泥火山碳排放亦是重要地质问题。因而我们建议选取南海北部台西南盆地典型泥火山开展钻探,获取海底泥火山发育的构造环境、沉积背景、温压场等系列参数,揭示泥火山成因机制,建立海底泥火山油气渗漏模式,探讨泥火山流体传热过程对水合物形成与分布的影响。研究成果对于超压沉积盆地油气勘探与开发、海底渗漏型水合物研究与勘探、新构造活动与地质灾害分析等都具有重要指导意义。
Abstract
Mud volcano is a mound structure formed by the eruption of underground muddy fluid to the surface or seabed. Its formation is directly related to oil and gas accumulation and gas hydrate mineralization. The eruption of mud volcano can easily cause engineering accidents, and with the diffusion of a large amount of methane gasalso aggravates the greenhouse effect. Scholars at home and abroad have carried out many studies on the development, geochemistry and geophysical characteristics of mud volcanoes. However, there are still many controversies about its formation mechanism because the triggering factors may come from fault activity, earthquake, overpressure, etc. The heat transfer law of mud volcanic fluid and the control effect on hydrate formation and dynamic distribution needs to be further studied. At the same time, the carbon emission from mud volcanoes in the sea area is also a geological problem deserving attention. Therefore, we suggest selecting the typical mud volcanoes in the southwest Taiwan basin in the north of the South China Sea to carry out ocean drilling, to obtain a series of parameters such as the tectonic environment, sedimentary background, temperature and pressure field for the development of submarine mud volcanoes, to reveal the genetic mechanism of mud volcanoes, to establish the oil and gas leakage model of submarine mud volcanoes, and to explore the impact of the thermal action of mud volcanic fluids on the formation and distribution of hydrates. The research results have important guiding significance for the exploration and development of oil and gas in overpressure sedimentary basins, the research and exploration of seafloor leakage hydrate, and the analysis of neotectonicactivities and geological disasters.
