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天然气水合物是由甲烷等相对分子质量较低的气体和水在低温高压条件下结合形成的似冰状固态化合物,具有分布广、储量大、热值高、能量密度大等特点,作为能源资源已引起世界各国的广泛关注(Sloan,2003; Chong et al.,2016; You Kehua et al.,2019)。其全球资源总量接近3000×1012m3甲烷(Boswell and Collett,2011),2019年度全球天然气消耗量约为2×1012m3,中国则达到约0.3×1012m3(周庆凡,2020),估算水合物资源可供人类使用数百年(Dudley,2018)。不仅如此,天然气水合物还含有丰富的有机碳,约占世界可移动有机碳库的三分之一,对全球碳循环有着深远的影响(Beaudoin et al.,2014)。研究天然气水合物的碳储-碳汇机制,也是实现碳达峰、碳中和目标不可或缺的一部分。因此,无论是从资源潜力还是环境效应来看,天然气水合物的研究都具有重要意义。
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天然气水合物的成藏成矿受多种因素的影响,包括气源、气体运移、储层以及温度和压力条件等(Collett et al.,2009)。大量研究表明,水合物的形成需要高通量的甲烷气体运移至天然气水合物稳定带(GHSZ)内,仅靠原位微生物成因气显然很难形成大规模高饱和度的水合物矿藏(Kuang Zenggui et al.,2018; Lai Hongfei et al.,2023)。因此,深部热成因气作为天然气水合物成藏的气源之一就显得格外重要。气体运移通道作为天然气水合物系统的重要组成部分,其研究同样受到了大量学者的关注(Davies et al.,2014; Fu Xiaojing et al.,2020; Santra et al.,2022)。水合物的聚集成藏与断层有着重要的联系,断层是深部热成因气向天然气水合物稳定带垂向运移的重要输导通道(Milkov and Sassen,2002; Hui Gege et al.,2016; 丛晓荣等,2018)。此外,气体垂向输导通道还包括气烟囱、泥底辟等,侧向运移路径则主要为高渗透性砂层等,均是天然气水合物气体运移输导体系不可忽视的组成部分(苏明等,2014; Fraser et al.,2016; Hillman et al.,2017; Su Ming et al.,2017; Ren Jinfeng et al.,2022; Slowey et al.,2022; 万志峰等,2022; Zhang Boda et al.,2023)。
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近几十年来,俄罗斯、加拿大、美国和日本等国都对天然气水合物进行了试采试验,并取得了成功(Chong et al.,2016; You Kehua et al.,2019)。我国天然气水合物的研究以南海北部为主,近些年来已取得了巨大进展,特别是2017年及2020年神狐海域(图1)两轮天然气水合物试采成功,标志着我国在天然气水合物领域的理论研究和勘探实践水平已经处于国际先进行列,实现了从“探索性试采”向“试验性试采”的重大跨越(Li Jinfa et al.,2018; Ye Jianliang et al.,2020)。神狐海域已发现深部热成因气对天然气水合物的形成具有重要贡献(Zhang Wei et al.,2017; Wang Dongdong et al.,2021; Lai Hongfei et al.,2022; Liang Qianyong et al.,2022),因此,深部热成因气向浅层运移的过程和机制就成为了研究的关键。地震资料研究表明,气烟囱和泥底辟是神狐海域天然气水合物气体垂向运移的主要输导通道(Chen Duanxin et al.,2013; Liang Jinqiang et al.,2017; Su Ming et al.,2017; 万志峰等,2022)。除此之外,多边形断层对气体输导也起到了重要的促进作用(Chen Duanxin et al.,2013)。由于地震数据的采集时间较早,深部地层地震反射特征不明显,在地震剖面上观察到大量模糊带。为进一步加强对神狐海域天然气水合物的研究,中国地质调查局广州海洋地质调查局于2018年开展了高分辨率三维地震数据的采集工作。此后,Zhang Boda et al.(2023)认为峡谷脊部沉积物厚度大,导致了深部地层超压的形成,为气烟囱的大量发育提供了条件。深部断层也被认为是气体垂向运移的重要输导通道,极大促进了天然气水合物的聚集成藏(Jin Jiapeng et al.,2020; Wang Dongdong et al.,2021)。随着越来越多的研究发现气烟囱的发育和水合物的分布具有一定的关联,有关气烟囱等气体输导通道的发育特征以及它们对天然气水合物成藏影响的讨论就显得尤为重要。
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图1 南海北部珠江口盆地神狐海域地理位置
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Fig.1 Location of the Shenhu area in the Pearl River Mouth basin, northern South China Sea
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2020年,中国地质调查局广州海洋地质调查局进一步对高分辨三维地震资料进行了处理,解决了深部地震反射模糊的问题,为气体运移通道的刻画奠定了基础。本文综合运用地震、测井资料以及气体地球化学数据,精细刻画了神狐海域气烟囱发育特征和天然气水合物赋存特征,并分析了气烟囱对天然气水合物分布的影响,探讨了深部热成因气对天然气水合物成藏的贡献,最后建立了神狐海域天然气水合物成藏模式,以期为天然气水合物的勘探开发提供地质依据。
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1 区域地质概况
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南海是西太平洋最大的边缘海,为中国近海面积最大、水最深的海区,平均水深1212 m,最大深度5559 m。南海北部发育于太平洋板块、印度-澳大利亚板块以及欧亚板块等多个板块的交汇地带,发育了台西南盆地、珠江口盆地、琼东南盆地、莺歌海盆地以及北部湾盆地等多个新生代盆地。其中,珠江口盆地是南海北部最大的含油气盆地(图1),面积约为19.38×104 km2,位于陆架陆坡区(He Min et al.,2017)。盆地自新生代以来经历了多期构造运动,形成了一系列断层、底辟和气烟囱,盆地构造演化可划分为裂陷期和裂后期两大阶段(Shi Hesheng et al.,2014; Pang Xiong et al.,2018; Zhang Gongcheng et al.,2021)(图2)。珠江口盆地受太平洋板块、印度洋板块以及欧亚板块交汇作用影响,处于复杂的大陆动力学背景下,现今的构造格局以北东向构造带的发育为特征,平面上呈“三隆两坳”的格局,具有东西分块、南北分带的构造特征,形成了断陷和坳陷两层结构特征(Sun Zhen et al.,2014; Zhao Yanghui et al.,2021)。盆地由北向南可划分为5个构造单元,包括北部隆起带、北部坳陷带、中央隆起带、南部坳陷带及南部隆起带。
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珠江口盆地地层沉积充填特征复杂,自下而上主要发育的地层为:古新统神狐组、下始新统文昌组(Tg~T80)、上始新统恩平组(T80~T70)、渐新统珠海组(T70~T60)、下中新统珠江组(T60~T40)、中中新统韩江组(T40~T32)、上中新统粤海组(T32~T30)、上新统万山组(T30~T20)以及第四系(T20至今)(图2)。盆地新生代地层沉积厚度大于11 km,沉积物供应充足(Huang Chunju et al.,2005)。文昌组和恩平组是珠江口盆地的主力烃源岩,多为粗粒辫状河三角洲相和湖相沉积(Mi Lijun et al.,2018)。珠海组、珠江组、韩江组及粤海组沉积环境主要从三角洲相和滨海相过渡到半深海相(Xie Hui et al.,2013)。第四纪地层作为天然气水合物储层,岩性主要表现为黏土、粉砂质黏土以及黏土质粉砂,为半深海—深海相。
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神狐海域构造上位于珠江口盆地白云凹陷,地处陆坡区,水深500~1700 m,发育多条海底峡谷,地形变化明显(图3)。研究区是南海天然气水合物勘探开发的第一个先导试验区,海底温度在4℃左右,热流值和地温梯度分别约为62~98 mW/m2和44~68℃/km,具有水合物成藏成矿的良好地质条件,且至今也已取得了大量研究成果(Liang Jinqiang et al.,2014; 郭依群等,2017; 杨胜雄等,2017)。
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2 数据和方法
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高精度三维地震资料覆盖范围约800 km2(图3),频宽为40~70 Hz,目标层(第四系)频率约为60 Hz,采集面元为12.5 m×6.25 m,采样间隔为2 ms,地震剖面以SEG负极性显示。随后,采用Geoframe软件对地震资料进行了解释并提取了包括相干切片属性在内的地震属性,通过利用地震信号相干值的变化来描述地层和岩性的横向非均质性,用于刻画断层和气烟囱(Bahorich et al.,1995)。
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中国地质调查局广州海洋地质调查局在神狐海域已获取了大量钻井数据,本文主要利用2007年和2015年天然气水合物钻探航次期间获取的SH5井、W17井、W18井以及W25井的随钻测井数据进行分析,包括自然伽马(GR)测井、电阻率(RES)测井、P波速度(VP)测井、密度(DEN)测井以及电阻率成像(RES_BD_IMG)测井,可用于识别天然气水合物和游离气(Wang Xiujuan et al.,2014; Ren Jinfeng et al.,2023)。
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除此之外,我们还获得了W17井和W18井的天然气水合物气体地球化学数据,主要包括甲烷碳同位素δ13C-C1值、烃类气体成分甲烷与乙烷和丙烷之和的比值(R=C1/(C2+C3))以及甲烷氢同位素δ13H-C1值(Zhang Wei et al.,2019),通过气体成因模式图版来判别天然气水合物气体来源(Milkov and Etiope,2018)。
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图2 珠江口盆地综合柱状图(据庞雄等,2008; Xie Hui et al.,2013; Shi Hesheng et al.,2014; He Min et al.,2017修改)
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Fig.2 Comprehensive stratigraphic column of the Pearl River Mouth basin (modified after Pang Xiong et al., 2008; Xie Hui et al., 2013; Shi Hesheng et al., 2014; He Min et al., 2017)
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3 气烟囱发育特征
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3.1 气烟囱识别
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通过对研究区高分辨率三维地震资料进行解释,可在地震剖面上识别出多个长柱状异常反射区,即气烟囱(图4)。气烟囱主要表现为底部杂乱反射带、体部振幅空白带以及顶部强振幅反射带。其中,顶部强振幅反射带还可观察到似海底反射(bottom-simulating reflector),简称BSR(图5),是地震波穿过高速的含水合物层及其下方低速的含气或含水层时,由于速度差异造成的强反射界面(Boswell et al.,2016)。分析发现,研究区BSR与气烟囱在平面上表现出良好的匹配关系(图3)。此外,在部分气烟囱上部,可以观察到由于含气所造成的同相轴“下拉”异常,但其是否真实存在还需进一步厘定。并且气烟囱底部一般还可以识别出杂乱反射带,这些地震异常反射特征是识别气烟囱的有效手段(图4)。
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借助地震属性同样可以较好地识别气烟囱,如果沉积物中存在流体运移,则会在瞬时频率属性剖面上表现为异常的低频区,因为油或气的存在会对高频地震能量产生较强的衰减影响,这将导致瞬时频率的降低(Coren et al.,2001; Yang Rui et al.,2015)。瞬时频率属性剖面低频区与气烟囱发育的位置具有一致性,因此也可以用来识别气烟囱(Cheng Cong et al.,2020)。
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图3 神狐海域天然气水合物钻探区海底多波束地形图及钻井位置
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Fig.3 Multi-beam map of the seafloor and the locations of deep-water drillings in in the Shenhu gas hydrate drilling area
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3.2 气烟囱顶部形态
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根据气烟囱内部反射特征和顶部形态,可以在研究区识别出4种顶部形态各异的气烟囱,分别是花冠状、囊状、穹顶状和蘑菇状(图5和表1)。气烟囱C1顶部为花冠状,表现为强振幅反射特征,可能是游离气聚集所形成(图5a)。气烟囱C2顶部则表现为囊状,在气烟囱体部还可观察到同相轴“下拉”异常,证明了游离气的存在(Qian Jin et al.,2018; Cheng Cong et al.,2020)(图5b)。气烟囱C3顶部为穹顶状,可见强振幅反射带和BSR,也可观察到同相轴“下拉”异常,整体呈长柱状(图5c)。蘑菇状气烟囱在研究区发育广泛,数量较多,覆盖面积广。气烟囱C6是典型的蘑菇状气烟囱,呈长柱状,内部地震反射从底到顶依次表现为杂乱反射带、振幅空白带以及强振幅反射带,也可见同相轴“下拉”异常(图5d)。
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3.3 气烟囱发源层位及发育时间
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通过分析气烟囱顶部的终止层位可基本确定气烟囱的发育时间,据此厘定天然气水合物成藏的关键时刻。在地震剖面上,气烟囱根部则普遍表现为杂乱反射,结合其根部延伸层位,可将研究区气烟囱根部发源层位分为两大类:古近系和新近系。其中,气烟囱C1、C5和C6发源于古近系文昌组—恩平组,是研究区主力烃源岩层,以热成因气为主(Zhu Weilin et al.,2021)(图4、图6,表1)。气烟囱C3和C4则发源于新近系珠江组,气体来源可能以微生物成因气为主(Zhu Youhai et al.,2013)。
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图4 典型气烟囱地震反射特征(据Sun Luyi et al.,2020; 匡增桂等,2024修改)
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Fig.4 Seismic reflection characteristics of typical gas chimneys (modified after Sun Luyi et al., 2020; Kuang Zenggui et al., 2024)
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(a)—过气烟囱典型地震剖面;(b)—过气烟囱C6和W17井典型地震剖面
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(a) —typical seismic profile across the gas chimneys; (b) —typical seismic profile across the gas chimney C6 and Well W17
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图5 不同类型顶部形态的气烟囱(图5c、d据匡增桂等,2024修改)
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Fig.5 Gas chimneys with different types of top morphology (Fig.5c, d are modified after Kuang Zenggui et al., 2024)
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(a)—花冠状气烟囱C1;(b)—囊状气烟囱C2;(c)—穹顶状气烟囱C3;(d)—蘑菇状气烟囱C6;图5a、b位置可见图3
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(a) —corolla-shaped gas chimney C1; (b) —capsule-shaped gas chimney C2; (c) —dome-shaped gas chimney C3; (d) —mushroom-shaped gas chimney C6; the locations for Fig.5a, b can be seen in Fig.3
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图6 典型气烟囱地震反射特征
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Fig.6 Seismic reflection characteristics of typical gas chimneys
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(a)—过气烟囱C4和SH5井典型地震剖面;(b)—过气烟囱C5和W18井典型地震剖面;(c)—过W25井典型地震剖面;图6a~c位置可见图3
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(a) —typical seismic profile across the gas chimney C4 and Well SH5; (b) —typical seismic profile across the gas chimney C5 and Well W18; (c) —typical seismic profile across Well W25; the locations for Fig.6a~c can be seen in Fig.3
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研究区气烟囱顶部均终止于中中新统以上,以万山组和第四系为主(图4、图5、图6和表1)。气烟囱C2、C4、C5和C6均刺穿T20界面,终止于第四系,是天然气水合物的主要富集层段(Liang Jinqiang et al.,2017);气烟囱C1和C3则终止于上新统,据此可判断气烟囱发育时间分别为第四纪以来和上新世以来,这为天然气水合物成藏关键时刻的研究奠定了基础。通过对气烟囱C1至C6的发源层位和终止层位进行研究发现,气烟囱根部发源层位越深,其顶部终止层位越浅;反之,发源层位越浅,终止层位约深,这可能与形成气烟囱的超压强度有关(Cheng Cong et al.,2020; Zhang Boda et al.,2023)。例如,气烟囱C6发源于文昌组—恩平组,终止于第四系;气烟囱C3发源于珠江组,终止于万山组。
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4 天然气水合物空间分布特征
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4.1 天然气水合物赋存特征
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本文主要利用地震和测井资料来识别天然气水合物,地震剖面上可通过BSR来判断水合物的赋存,测井则主要是通过RES曲线和VP曲线来识别。SH5井水深约1424 m,完钻深度约为海底以下250 m,可获取该井GR测井曲线、RES测井曲线以及VP测井曲线(图7a)。其中,RES曲线值变化较小,未观察到明显的天然气水合物和游离气的赋存。
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W17井水深约1252 m,完钻深度约为海底以下300 m(图7b)。RES曲线和VP曲线在海底以下约210~260 m处急剧增大,RES_BD_IMG也表现出高亮特征,反映了天然气水合物的赋存。水合物层厚度约为43.1 m,平均饱和度约为19.4%(表1)。天然气水合物层下方也可见RES曲线值明显增大,且VP曲线值有降低的趋势,显示了游离气层的存在。游离气层厚度约为12.0 m,平均饱和度约为30.1%( Ren Jinfeng et al.,2023)。
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W18井水深约1282 m,完钻深度约为海底以下250 m(图7c)。RES曲线和VP曲线在海底以下约150 m处急剧升高,而DEN曲线值则下降。RES_BD_IMG清晰地显示了天然气水合物层的存在,其下还伴有游离气层。天然气水合物层厚度约为11.6 m,游离气层厚度约为28.9 m(表1)。水合物平均饱和度约为30.5%,而游离气饱和度则为14.1%( Ren Jinfeng et al.,2023)。
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W25井水深约1522 m,完钻深度约为海底以下300 m(图7d)。RES曲线值和VP曲线值变化较小,未观察到明显的天然气水合物的赋存。
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4.2 气烟囱对天然气水合物分布的影响
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为了分析气烟囱对天然气水合物分布的影响,本文利用相干切片属性,将气烟囱分布范围与BSR分布范围叠合,可发现二者具有高度重合性,气烟囱发育区基本都有BSR的存在(图3,图8)。由于BSR的发育并不能完全证明天然气水合物的赋存,本文进一步结合随钻测井数据,对典型钻井进行了分析,识别了天然气水合物和游离气(图7)。SH5井钻遇气烟囱C4,并未发现天然气水合物的赋存(图7a)。气烟囱发源于新近系,终止于第四系,为穹顶状气烟囱,可见BSR(图6a)。W17井钻遇气烟囱C6,可发现明显水合物与游离气赋存(图7b)。气烟囱发源于古近系文昌组—恩平组,终止于第四系,顶部形态为蘑菇状,可见强振幅反射带以及BSR(图4b)。同样地,W18井钻遇蘑菇状气烟囱C5,与W17井类似,地震剖面上从底到顶依次为杂乱反射带、振幅空白带以及强振幅反射带。根部发源于文昌组—恩平组,顶部终止于第四系,为蘑菇状气烟囱,顶部也可见明显BSR(图6b),也可发现水合物与游离气的赋存(图7c)。W25井则并未钻遇气烟囱,测井也未见有水合物与游离气的赋存,但在地震剖面上可见BSR(图6c和图7d)。由此可见,气烟囱的发育对天然气水合物的分布具有强烈的控制作用(Zhang Wei et al.,2019; Cheng Cong et al.,2020; Wan Zhifeng et al.,2022; Zhang Boda et al.,2023)。不发育气烟囱的区域就不会有天然气水合物的赋存,发育气烟囱的区域才可能会有水合物的赋存。其中,发源于深部古近系文昌组—恩平组并终止于第四系的气烟囱其上部才会生成水合物,且顶部形态为蘑菇状的气烟囱可形成高饱和度水合物。
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图7 神狐海域不同钻井天然气水合物赋存特征
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Fig.7 Gas hydrate occurrences in different drillings in the Shenhu area
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(a)—SH5井测井特征;(b)—W17井测井特征;(c)—W18井测井特征;(d)—W25井测井特征;钻井位置可见图3
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(a) —logging characteristics of Well SH5; (b) —logging characteristics of Well W17; (c) —logging characteristics of Well W18; (d) —logging characteristics of Well W25; the drilling locations can be seen in Fig.3
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4.3 天然气水合物成藏模式
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基于地震资料、测井资料、激光拉曼技术以及盆地模拟技术的分析发现,神狐海域已有Ⅱ型水合物的存在的证据,深部热成因气是天然气水合物气体来源的重要组成部分,对天然气水合物成藏具有重要的贡献(Qian Jin et al.,2018; Su Pibo et al.,2018; Wei Jiangong et al.,2018; Zhang Wei et al.,2019; Sun Luyi et al.,2020; Lai Hongfei et al.,2022)。为了进一步证实研究区深部热成因气的贡献,本文结合W17井和W18井已发表的天然气水合物气体地球化学数据(Zhang Wei et al.,2019),利用Milkov和Etiope(Milkov and Etiope,2018)建立的气体成因模式图,可初步确定天然气水合物气体来源(图9)。其中,W17井5个样品主要显示了生物成因-热成因混合特征,表明其既有生物成因气的影响,也有热成因气的影响。W18井4个样品投点则主要位于热成因气区域内。其中3个样品为晚期热成因气,1个样品可能为次生微生物成因气。由此可见,W17井和W18井天然气水合物气体来源与深部热成因气密切相关,可证实古近系文昌组—恩平组烃源岩所生成的热成因气对天然气水合物成藏的贡献。
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据此,本文建立了研究区天然气水合物成藏模式(图10)。源自文昌组—恩平组等烃源岩的热成因气沿断层向上运移,从而聚集形成深部天然气藏。随着天然气的聚集,深部地层流体压力增大,产生超压。一般认为,东沙运动是超压释放的触发机制,超压释放导致了气烟囱的形成(Guo Xiaowen et al.,2016; Kong Lingtao et al.,2018),且白云凹陷已被证实有幕式超压活动的记录,这也是深部热成因气通过断层和气烟囱垂向运移的重要驱动力。深部热成因气在浅部地层聚集并在天然气水合物稳定带底部之上形成水合物。除此之外,次生微生物气以及原生微生物气同样也是形成天然气水合物不可忽视的气体来源(Lai Hongfei et al.,2022)。
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5 结论
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本文运用高精度三维地震资料,刻画了神狐海域重要的气体运移输导通道——气烟囱,分析了其地震反射特征、发源层位及发育时间。气烟囱从底到顶表现为底部杂乱反射带、体部振幅空白带以及顶部强振幅反射带,根据其顶部形态可划分为花冠状、囊状、穹顶状和蘑菇状。气烟囱主要发源于古近系,少部分发源于新近系,且绝大多数终止于第四系,其顶部与BSR相连。结合测井资料,可发现气烟囱的发育对天然气水合物的分布具有明显的控制作用,发育气烟囱的区域才可能赋存有天然气水合物,其中发源于古近系文昌—恩平组且终止于第四系的气烟囱之上才有天然气水合物的存在。最后借助气体地球化学数据,证实了研究区热成因气对天然气水合物成藏的贡献,表明了断层和气烟囱作为垂向气体运移输导通道的重要性,并据此建立了天然气水合物成藏模式,为早日实现天然气水合物的商业化开采提供了地质依据。
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图8 叠合天然气水合物与游离气赋存特征、断层、气烟囱和BSR的T32界面沿层相干切片图
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Fig.8 Coherence slice map of Horizon T32 superimposed gas hydrate and free gas occurrences, faults, gas chimneys, and BSRs
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图9 天然气水合物气体成因判别模式图
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Fig.9 Genetic discrimination diagram for hydrate gas
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图10 神狐海域天然气水合物成藏模式图
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Fig.10 Gas hydrate accumulation model in the Shenhu area
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摘要
天然气水合物不仅是未来潜在的清洁能源,同时对全球气候变化也有着显著影响,已受到世界各国的广泛关注。神狐海域作为天然气水合物研究热点区域,至今已涌现大量成果,但有关气体运移通道对天然气水合物分布的研究还较为薄弱。为了进一步揭示该区天然气水合物成藏机理,本文综合运用高精度三维地震资料、测井资料以及气体地球化学数据,精细刻画了气烟囱的地震反射特征、发源层位及发育时间,分析了典型钻井天然气水合物赋存特征,阐明了气烟囱发育对水合物分布的影响。气烟囱从底到顶呈现出复杂多样的特征,包括底部杂乱反射带、体部振幅空白带以及顶部强振幅反射带。根据其根部延伸层位,气烟囱主要发源于古近系,部分发源于新近系,并且绝大多数气烟囱顶部终止于第四系。结合测井数据,可发现气烟囱的发育对天然气水合物的分布具有明显的控制作用。天然气水合物赋存在发源于古近系文昌组—恩平组并终止于第四系的气烟囱发育区域。结合天然气水合物气体地球化学数据,证实了研究区热成因气对天然气水合物成藏的贡献,表明了断层和气烟囱在垂向气体运移输导体系中的重要性,并在此基础上建立了天然气水合物成藏模式,为早日实现天然气水合物的商业化开采提供了坚实的地质基础。
Abstract
Gas hydrates represent a promising clean energy source and hold significant implications for global climate change, garnering considerable international attention. Shenhu area, a prominent gas hydrate research hotspot, has yielded a wealth of findings to date. However, a comprehensive under standing of gas migration pathways and their impacts on gas hydrate distribution in this region remains incomplete. To elucidate the mechanisms governing gas hydrate accumulation in the Shenhu area, this study integrates high-resolution 3D seismic data, well logging data, and hydrate gas geochemical data. This multi-faceted approach allows for fine-scale characterization of gas chimneys, analysis of gas hydrate occurrence characteristics in representative drilling wells, and clarification of the influences of gas chimneys on gas hydrate distribution. Gas chimneys exhibit distinct vertical zonation, characterized by a chaotic reflection zone at the bottom, an acoustic blanking zone in the middle, and a high-amplitude reflection zone at the top. Analysis reveals that most gas chimneys originate in the Paleogene, with some originating in the Neogene and the vast majority terminating in the Quaternary. Integration of well logging data reveals that gas chimney development has a significant control on gas hydrate distribution. Gas hydrates are mostly found in areas where gas chimneys originate from the Paleogene Wenchang-Enping formations and terminate in the Quaternary. Geochemical analysis of hydrate gas confirms the contribution of thermogenic gas to gas hydrate accumulation in the study area, highlighting the importance of faults and gas chimneys as conduits in the vertical gas migration pathway system. Based on these findings, a model for gas hydrate accumulation in the Shenhu area is proposed. This model provides a solid geological foundation for informing the future commercial exploitation of gas hydrates in this region.
Keywords
gas hydrates ; gas chimney ; thermogenic gas ; gas migration ; accumulation model ; Shenhu area