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天然气水合物是在低温高压环境下由水和天然气(主要是甲烷)形成的笼形结晶化合物[1]。由于其潜在的巨大规模储量(约3000万亿m3)和清洁的燃烧产物,天然气水合物被视为很有前景的未来能源[2-4]。另一方面,天然气水合物中的甲烷是一种温室效应高于CO2二十多倍的温室气体,因此由于温压条件变化而导致的天然气水合物分解释放出的大量甲烷会对全球气候造成影响[5-8]。因此对于天然气水合物的研究具有重大的意义。
沉积物孔隙水地球化学研究作为海洋沉积研究最主要的手段之一,在天然气水合物的研究当中发挥了非常重要的作用,在天然气水合物异常识别、成藏流体来源、流体运移过程以及流体组分在浅表层微生物地球化学过程的响应等方面取得了大量研究成果[2, 9-15]。如用氯离子和氧同位素异常来识别水合物赋存[2, 16-18],用硫酸根梯度(或硫酸盐还原带深度)结合溶解无机碳(Dissolved Inorganic Carbon,DIC)含量及其同位素组成来识别AOM(Anaerobic Oxidation of Methane,AOM)过程等[10, 12, 19-21]。在南海北部陆坡天然气水合物勘探研究中,孔隙水地球化学在水合物成藏对浅表层沉积物早期成岩作用过程的影响的研究中扮演了非常重要的角色,并取得了很多成果[15, 16, 19, 22-24]。南海北部陆坡潜在水合物远景区均表现出可能与天然气水合物相关的地球化学异常特征,主要表现为浅表层沉积物中较浅的SMI(Sulfate-Methane Interface,SMI)界面、相对偏负的溶解无机碳的碳同位素组成以及其他主微量元素特征等[17, 19, 20, 22, 23, 25-28]。但由于取样限制,大部分报道的工作集中于浅表层,主要的分析结果均是间接证据,真正与水合物直接相关的孔隙水地球化学研究较少。而南海北部地区GMGS1-GMGS3三次的水合物钻探计划为我们的研究提供了基础。
本次研究选取了GMGS2航次的GMGS2-09站位作为研究对象,分析直接与水合物相关的孔隙水地球化学特征,利用地球化学异常识别水合物的赋存并计算水合物在孔隙介质之间的饱和度。
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南海是西太平洋最大的边缘海之一,其北部陆坡区地形复杂,许多大中型新生代沉积盆地(台西南盆地、珠江口盆地、琼东南盆地和莺歌海盆地等)跨越了陆坡区,最大沉积厚度超过10000m。晚更新世和全新世地层沉积速率分别为17.9~19.6cm/ka和9.6~14.6cm/ka[29]。北部陆坡的水深为200~3400m,水温为2~5℃,较快的沉积速率和适宜的温压条件都非常有利于天然气水合物的形成[30]。
中国自20世纪90年代开展天然气水合物的调查研究以来,已在南海北部陆坡圈定了一系列成矿远景区[17, 18, 31-34]。在南海北部的神狐海域,广州海洋地质调查局于2007年实施了首次天然气水合物钻探航次GMGS1,钻获分散状天然气水合物实物样品[17],并于2015年的GMGS3钻探航次中再次钻获多种类型水合物[35]。2013年,在神狐海域东北部的珠江口盆地东南海域,实施了GMGS2钻探航次,首次钻获了纯块状天然气水合物岩芯样品,并且还有大量层状、块状、结核状及分散状等多种形态的天然气水合物实物样品[18]。2015年在琼东南海域海马冷泉区,发现并采获了浅表层水合物[36]。最终于2017年,中国地质调查局在神狐海域成功实施了天然气水合物试开采,也是国际上首次对细粒沉积物储层实现试采[37]。
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本文研究的GMGS2钻探位置位于南海北部陆坡珠江口盆地与台西南盆地间(图 1)。其中在GMGS2-05、GMGS2-07、GMGS2-08、GMGS2-09、GMGS2-16等5口井发现水合物实物样品。测井曲线结果显示除了GMGS-08、GMGS-16站位有浅层和深部两层水合物外,其他站位仅发育单层水合物[38]。GMGS2-09井水深664m,钻井深度近110 m,在9~21m层段发育有结核状水合物[38]。GMGS2-09井位在钻探现场采用压榨法提取孔隙水样品13份,样品从8.65m到100.16m连续分布,每份孔隙水取样岩心厚度约3cm。
图 1 南海北部陆坡地质图(a)和GMGS2-09站位位置(b)
Figure 1. Geological map of northern slope of South China Sea(a), and the location of site GMGS2 (b)
针对研究需要对GMGS2-09井的孔隙水进行了氯离子含量、氢氧同位素和阳离子的分析,所有分析均在南京大学内生金属矿床成矿机制研究国家重点实验室完成。
氯离子含量采用滴定法测定,使用0.1M硝酸银溶液为滴定液,监测标准为IAPSO国际海水标准,使用仪器为瑞士Metrohm的916 Ti-Touch全自动点位滴定仪,电极为复合银电极,分析相对误差低于1%。硫酸根含量采用离子色谱法,使用瑞士Metrohm公司790-1型通用离子色谱仪,Metrosep A Supp4-250型阴离子柱, 相对标准偏差小于3%。
氢氧同位素采用TC/EA-IRMS法测定,分析仪器为FLUSH2000元素分析仪和MAT253型稳定同位素质谱(ThermoFisher, Germany),使用标准为VSMOW、SLAP、GISP国际氢氧同位素标准,氧同位素分析精度优于0.2‰,氢同位素分析精度优于1‰。
孔隙水的阳离子含量采用离子色谱法,使用瑞士Metrohm公司790-1型通用离子色谱仪,Metrosep C 2-150型阳离子柱。相对标准偏差小于3%。
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GMGS2-09站位沉积物孔隙水阴阳离子浓度随深度变化特征如图 2所示。从图中可以看到,氯离子含量从顶到底在正常层位其含量基本稳定,分布在580mM左右。而在水合物赋存层位A区则表现出了较为明显的负漂移,另外在B、C区也出现了明显负漂移迹象。
钾和钠离子随深度变化相较于氯离子变化较大,但在A、B、C等水合物存在或者疑似存在的层位也都出现了较为明显的浓度降低情况。除3个异常层位外,整体上钠和钾离子呈现出浓度随深度降低的趋势。镁、钙离子与钾、钠离子表现基本相同,其中钙离子在10m以下含量基本稳定。
GMGS2-09站位氢氧同位素组成如图 3所示,氢氧同位素随深度变化较大,氧同位素值普遍较高,相较以往珠江口盆地东南海域浅表层沉积物孔隙水的值高0.1‰~0.4‰[40],但在阴阳离子异常区域仍然可以看出氧同位素相对于上下层位有较为明显的高异常,氢同位素值变化也具有类似特征。
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由排盐作用引起的盐度异常和水合物结晶引起的氢氧同位素分馏是天然气水合物研究最直接的地球化学异常现象。氯离子浓度负异常耦合氧同位素正异常是天然气水合物形成后分解的最佳证据[2, 41],或者反之氯离子正异常、氧同位素负异常则被认为是天然气水合物形成的证据,总体上来说在有天然气水合物存在的区域两者应该呈负相关关系。
GMGS2-09站位沉积物孔隙水地球化学特征显示了较为明显的盐度异常以及氢氧同位素异常。如图 4所示,部分出现氯离子含量低异常的层位,其氢氧同位素也表现出了较为明显的异常。但是从图中也可以看出,氧同位素与氯含量并没有呈现出理想的负相关关系,可能是由于沉积物原位孔隙水中氯含量以及氧同位素值初始含量或组成存在差异。
图 4 GMGS2-09站位孔隙水氯含量以及氢氧同位素相关关系
Figure 4. Correlation of chloride concentration, hydrogen and oxygen isotopes
沉积物孔隙水的地球化学异常是相对于相邻层位正常沉积物孔隙水而言,不是与海水比较。由于沉积环境不同(如沉积速率、沉积厚度、热流以及构造等),不同区域的沉积物孔隙水特征会有不同程度的差别,也会使其偏离海水的数值[2, 42, 43]。如ODP 164航次994、997站位,其氯含量与氧同位素组成均随着深度呈现下降趋势,该现象受制于深部流体的地球化学特征影响[44]。因此我们在对氯离子浓度和氢氧同位素异常进行判别时首先需要对其背景进行校正,先使用非水合物储层的数据进行曲线拟合,然后根据水合物层的拟合值与真实测定值来求出该区域的地球化学异常值并进行图解判别。
通过对GMGS2-09站位的沉积物孔隙水各离子浓度曲线进行了基线拟合(排除图 2所示的含量异常点),然后根据拟合值对其差异进行校正,我们得到氯含量以及氧同位素的异常值(图 5)。
图 5 GMGS2-09站位孔隙水氯含量以及氧同位素随深度分布图
Figure 5. Profile of original chloride concentration and oxygen isotope with depth (on the left), and corrected profile (on the right)
对比数据校正前后的变化可以看出校正后前文所述的三层异常层表现出明显的氯低异常和氧同位素正异常的耦合关系,确证了GMGS2-09井在9~17、47以及100m处水合物的存在,其中9~17m层位与实际取样结果完全一致,也与测井曲线结果吻合[38]。中下两层对应的测井曲线并未出现异常,可能与这两层水合物的赋存形态及含量有关。浅表层水合物以块状形式存在,在电阻率伽马等物理指标上会有较明显的显示,中下两层可能是以浸染状形式赋存,含量较低,测井曲线上特征不明显。
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基于水合物晶格的排盐机理,取样时所观察到的氯离子负异常均来源于水合物分解产生的淡水稀释所致,因此可以用氯离子淡化程度来估算埋藏时水合物的饱和度[45] (公式1)。
$ S_{\mathrm{h}}=\left[\beta\left(C_{\mathrm{b}}-C_\text{s}\right)\right] /\left[C_\text{s}+\beta\left(C_{\mathrm{b}}-C_{\mathrm{s}}\right)\right] $
(1) 公式中Sh为水合物饱和度;Cb为沉积物原始层位孔隙水离子的背景浓度,一般采用原位测量来获取,但是由于原位测量存在困难,可以用基线拟合的背景浓度来代替;Cs为各层位的实测浓度;β为常数,用于校正水合物分解产生的流体密度变化,采用Malinverno等[45]的方法推算出的数据为1.257。
在用氯含量进行估算的同时,我们也用钾和钠含量进行了水合物饱和度的估算(图 6),在靠近海底的位置,三者的估算结果几乎完全一致,从17m处的20%左右到8.5m处的58%左右。在47m处,氯和钠离子估算值较为接近,均为13%左右,钾离子估算值为31%。在底部100m处,钾和钠离子相当,为20%左右,而氯离子估算结果为48%。从估算结果可以看出,从9m到17m左右深度的水合物含量最高,这与实际采样时在该层位发现结核状水合物的情况相吻合。而估算结果也表明,47m及100m处可能存在相当含量的水合物赋存,尤其是100m处。
图中三者对水合物饱和度的估算也表现出了一定程度的差异,可能来自三种离子自身地球化学性质的差异。钾和钠离子相对于氯离子稳定性稍差,其含量仍会受到深部黏土矿物变质的影响。由于变化不同,在用数学方法拟合时,其精度和准确度也有差异。这个差异会表现在最终结果上。浅表层位置孔隙水的原始含量主要受控于海水,因此其拟合数据的准确度较高,因此三者之间的一致性也比较理想。
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GMGS2-09站位位于珠江口盆地东南海域,在该站位钻探时多个层位发现有疑似水合物埋藏的迹象。通过对该站位孔隙水地球化学研究发现,在9、47以及100m三个层位存在水合物,三个层位均表现出氯含量的相对低异常和氢氧同位素的相对正异常。根据氯离子含量以及钠离子含量,我们对该站位的水合物饱和度进行了估算,浅表层最高约为50%,中间以及底层约为20%。浅表层相对较高的水合物饱和度估算结果也与钻探采样过程中在此深度发现结核状水合物的分布相吻合。
Geochemistry of sediment pore water from Well GMGS2-09 in the southeastern Pearl River Mouth Basin, South China Sea: An indication of gas hydrate occurrence
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摘要: 沉积物孔隙水地球化学是天然气水合物勘探与研究的重要手段。为了探究珠江口盆地东南海域GMGS2-09钻孔的沉积物孔隙水地球化学特征及其对埋藏的天然气水合物的指示意义,我们在前人的研究和认知基础上,通过测试该钻孔沉积物孔隙水的氯离子含量、氢氧同位素和阳离子组成来识别天然气水合物的赋存层位。结果表明GMGS2-09钻孔在9~17、47以及100m处存在氯离子浓度的负异常耦合氧同位素的正异常,指示相应的天然气水合物赋存,其中9~17m层位指示结果与实际取样情况完全一致。此外,采用基于水合物晶格的排盐机理推导的经验公式计算显示水合物饱和度在浅表层(17m)最高约为50%,中间以及底层约为20%。Abstract: Geochemistry of pore water of marine sediments playes an important role in gas hydrate research. In order to detect the gas hydrater-bearing layers, geochemical characteristics and their implications for buried gas hydrates are studied with pore water collected from the Site GMGS2-09 in the southeastern part of the Pearl River Mouth Basin. Cations, anions, hydrogen and oxygen isotopic compositions are analyzed and studied upon previous researches. Chloride anomalies coupled with increased δ18O in pore water are observed at the depths of 9~17 meter, in which gas hydrate is recovered, 47 meter and 100 meter, which may indicate the presences of gas hydrate-bearing layers. Besides, gas hydrate saturation calculated indicates that it is 50% in depth of 17 m, and about 20% in depth of 47 m and 100 m.
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Key words:
- gas hydrate /
- pore water /
- geochemistry /
- Pearl River Mouth Basin
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