湘中地区早寒武世牛蹄塘组黑色岩系地球化学特征与有机质富集机理
来源期刊:中南大学学报(自然科学版)2020年第8期
论文作者:郭建华 吴诗情 王玺凯 李智宇 刘辰生 焦鹏 陈广
文章页码:2049 - 2061
关键词:湘中地区;牛蹄塘组;黑色岩系;地球化学;有机质富集
Key words:Central Hunan Province; Niutitang formation; black rock series; geochemistry; organic matter enrichment
摘 要:中国南方扬子板块寒武纪早期广泛发育一套富有机质黑色岩系,为了研究其有机质富集特征及控制因素,对湘中地区AY-1井牛蹄塘组32件岩芯样品总有机碳质量分数w(TOC)、主微量和稀土元素质量分数等地球化学特征参数进行测试。研究结果表明:研究区牛蹄塘组黑色页岩有机质丰度高,与热液相关的Ni和Sb等微量元素富集以及热液指标w(Co)+w(Ni)较高;w(P生物)和w(Ba生物)显示牛蹄塘组生产力在中—高等水平,w(U)/w(Al),w(V)/w(Al),w(Ni)/w(Co),w(V)/w(Cr)和w(U)/w(Th)共同指示水体基本处于缺氧、还原环境;从底部至顶部,页岩受热液作用强度、生产力水平和水体还原性变化均表现为由弱—强—弱的趋势,且与有机质丰度呈现正相关;顶部生产力水平虽然较高且趋于稳定,但沉积时海平面下降使水体还原性减弱,有机质无法良好地保存。强热液活动、高古生产力和强还原环境使牛蹄塘组中部形成高有机碳丰度的页岩层系,成为页岩气勘探的有利层段。
Abstract: During the early Cambrian period, in the Yangtze plate of South China, there widely developed a set of organic-rich black rock series. In order to study its features and controlling factors of organic matter(OM) enrichment, 32 core samples of Niutitang formation from well AY-1 in Central Hunan were selected for geochemical tests such as total carbon mass fraction of TOC, major elements,trace elements and rare elements. The results show that the mass fraction of TOC of black shale in Niutitang formation is high. The trace elements related to hydrothermal fluid(HF) such as Ni and Sb were accumulated, while w(Co)+w(Ni) as the HF indicators is high.w(Pxs) and w(Baxs) show a middle-high productivity level of Niutitang formation. w(U)/w(Al),w(V)/w(Al), w(Ni)/w(Co), w(V)/w(Cr) and w(U)/w(Th) jointly indicate that the water is basically in anoxic and reducing environment.From bottom to top of Niutitang formation, hydrothermal activity intensity, productivity level and the change of water’s reducibility all show a weak-strong-weak tendency. And this tendency has a positive correlation with mass fraction of TOC. Although the productivity of the top of Niutitang formation is relatively high and tends to be stable, sea-level falling weakens the reduction of water during deposition, which makes the OM not able to well preserved. In summary, intense hydrothermal activity, high paleoproductivity and strong reducing environment result in the high TOC shale series in the middle part of Niutitang formation which is also a favorable exploration layer of shale gas in the study area.
DOI: 10.11817/j.issn.1672-7207.2020.08.001
吴诗情1, 2,郭建华1, 2,王玺凯1, 2,李智宇1, 2,刘辰生1, 2,焦鹏1, 2,陈广1, 2
(1. 中南大学 地球科学与信息物理学院,湖南 长沙,410083;
2. 中南大学 有色金属成矿预测与地质环境监测教育部重点实验室,湖南 长沙,410083)
摘要:中国南方扬子板块寒武纪早期广泛发育一套富有机质黑色岩系,为了研究其有机质富集特征及控制因素,对湘中地区AY-1井牛蹄塘组32件岩芯样品总有机碳质量分数w(TOC)、主微量和稀土元素质量分数等地球化学特征参数进行测试。研究结果表明:研究区牛蹄塘组黑色页岩有机质丰度高,与热液相关的Ni和Sb等微量元素富集以及热液指标w(Co)+w(Ni)较高;w(P生物)和w(Ba生物)显示牛蹄塘组生产力在中—高等水平,w(U)/w(Al),w(V)/w(Al),w(Ni)/w(Co),w(V)/w(Cr)和w(U)/w(Th)共同指示水体基本处于缺氧、还原环境;从底部至顶部,页岩受热液作用强度、生产力水平和水体还原性变化均表现为由弱—强—弱的趋势,且与有机质丰度呈现正相关;顶部生产力水平虽然较高且趋于稳定,但沉积时海平面下降使水体还原性减弱,有机质无法良好地保存。强热液活动、高古生产力和强还原环境使牛蹄塘组中部形成高有机碳丰度的页岩层系,成为页岩气勘探的有利层段。
关键词:湘中地区;牛蹄塘组;黑色岩系;地球化学;有机质富集
中图分类号:P595 文献标志码:A 开放科学(资源服务)标识码(OSID)
文章编号:1672-7207(2020)08-2049-12
WU Shiqing1, 2, GUO Jianhua1, 2, WANG Xikai1, 2, LI Zhiyu1, 2, LIU Chensheng1, 2, JIAO Peng1, 2, CHEN Guang1, 2
(1. School of Geosciences and Info-Physics, Central South University, Changsha 410083, China;
2. Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring, Ministry of Education, Central South University, Changsha 410083, China)
Abstract: During the early Cambrian period, in the Yangtze plate of South China, there widely developed a set of organic-rich black rock series. In order to study its features and controlling factors of organic matter(OM) enrichment, 32 core samples of Niutitang formation from well AY-1 in Central Hunan were selected for geochemical tests such as total carbon mass fraction of TOC, major elements,trace elements and rare elements. The results show that the mass fraction of TOC of black shale in Niutitang formation is high. The trace elements related to hydrothermal fluid(HF) such as Ni and Sb were accumulated, while w(Co)+w(Ni) as the HF indicators is high.w(Pxs) and w(Baxs) show a middle-high productivity level of Niutitang formation. w(U)/w(Al),w(V)/w(Al), w(Ni)/w(Co), w(V)/w(Cr) and w(U)/w(Th) jointly indicate that the water is basically in anoxic and reducing environment.From bottom to top of Niutitang formation, hydrothermal activity intensity, productivity level and the change of water’s reducibility all show a weak-strong-weak tendency. And this tendency has a positive correlation with mass fraction of TOC. Although the productivity of the top of Niutitang formation is relatively high and tends to be stable, sea-level falling weakens the reduction of water during deposition, which makes the OM not able to well preserved. In summary, intense hydrothermal activity, high paleoproductivity and strong reducing environment result in the high TOC shale series in the middle part of Niutitang formation which is also a favorable exploration layer of shale gas in the study area.
Key words: Central Hunan Province; Niutitang formation; black rock series; geochemistry; organic matter enrichment
震旦纪—寒武纪是一段重要的地质历史时期,在此时期内伴随着全球构造板块变化及火山作用的发生[1],全球古海洋地球化学环境[2]、生物数量及种类均发生了较大变化[3]。震旦纪结束后,由于区域性拉张裂陷,海平面快速上升为最大海侵,形成广泛的南方扬子地区下寒武统牛蹄塘组黑色岩系[4]。该套岩系主要由富有机质黑色页岩、黑色硅质岩及底部的磷块岩和含磷质结核页岩组成[5],因其具有长英质岩石和基性—超基性岩石的元素富集特征,且表现出总有机碳质量分数w(TOC)高的特点(平均大于2%)[6]而被研究者广泛研究。前人针对牛蹄塘组黑色岩系有机质富集机理的研究主要集中在扬子台地[7-9]及扬子台地—转换带上[10-12],且在有机质富集的影响因素上存在争议,如:久凯等[13-14]认为有机质的富集受多因素控制,海底热液活动及上升洋流为富有机质页岩提供了物质基础,底部缺氧水体为有机质提供了良好的富集条件;梁钰等[15]认为海底热液活动为黑色岩系中的有机质提供了必要的物质基础和保存条件,是有机质富集的主控因素。扬子台地—转换带的东南缘与扬子台地构造、沉积环境不一致,同时人们对有机质富集规律的研究较少,为此,本文作者以湘中地区安化AY-1井下寒武统牛蹄塘组黑色岩系为研究对象,对其进行主量元素、微量元素及稀土元素和TOC的质量分数进行测试分析,建立高分辨率地球化学剖面,恢复寒武纪扬子板块东南缘深水盆地黑色页岩形成的氧化还原条件,研究热液影响强度,探讨湘中地区下寒武统牛蹄塘组黑色岩系有机质富集控制因素。
1 地质背景
中国南方扬子地台的基底是中深变质的太古界和早元古界以及浅变质的中元古界,自基底形成后,从震旦纪开始,扬子地区进入相对稳定的地台发展阶段[16],扬子板块东南缘由陆内裂谷逐渐演化为被动大陆边缘[17],与开阔大洋连通性较好并广泛接受沉积充填。湘中地区位于扬子板块东南缘向华夏陆块的过渡区,早寒武世湘中地区基本继承了震旦纪构造格局,由北西向南东形成了稳定的碳酸盐台地相、台地—盆地过渡相、盆地相[18]。随着泛大陆的解体,寒武纪初期广泛海侵[19],扬子板块留茶坡组硅质岩、灯影组白云岩之上普遍沉积一套富有机质细粒岩系—牛蹄塘组,发育黑色富有机质页岩、硅质岩、硅质页岩及磷块岩。区域地质背景图见图1。
图1 区域地质背景图
Fig. 1 Regional geological setting
AY-1井位于湖南省安化县,开孔层位为下奥陶统桥亭子组,钻至震旦系南沱组终孔,终孔深度为997.4 m,目的层为下寒武统牛蹄塘组,厚度为166.3 m;底部为黑色泥岩夹深灰色硅质泥岩、含磷质结核黑色页岩,中部为炭质页岩与硅质页岩互层,顶部为黑色泥质页岩。以底部磷块岩及赋存磷质结核的黑色页岩作为地层对比的标志层[20],下覆地层为震旦系留茶坡组深灰色、灰黑色硅质岩,可见大量穿层方解石脉(图2)。AY-1井留茶坡组—牛蹄塘组岩性组合与湘西北龙鼻嘴[21]、广西泗里口剖面[22]的岩性组合相似,均以黑色炭质页岩、硅质页岩为主,为深水盆地沉积。
图2 湘中地区AY-1井综合柱状图
Fig. 2 Integrated column chart of well AY-1 in central Hunan
2 样品采集与分析测试
从AY-1井岩芯牛蹄塘组自下而上系统地采集32个新鲜样品用于主量(32个)、微量(32个)、稀土元素(16个)质量分数及总有机碳(25个)质量分数分析。样品分析测试均在国土资源部重庆矿产资源监督检测中心完成。主量元素、微量元素质量分数使用荷兰帕纳科Axios mAx PW4400/40 X射线荧光光谱仪进行检测,稀土元素质量分数使用电感耦合等离子体质谱仪(ICP-MS)X seriesⅡ测试。计算时,采用澳大利亚太古代平均页岩(PAAS)的稀土元素质量分数(即w(PAAS稀土元素))进行标准化。为减小ICP-MS测试中Ba元素富集对Eu异常造成的影响,采用DULSKI等[23-24]提出的稀土元素异常值公式计算。总有机碳质量分数w(TOC)使用德国耶拿Multi N/C 3100总碳分析仪进行检测。
3 实验结果
3.1 主量元素
牛蹄塘组页岩样品的主量元素中w(SiO2)最高,平均为64.83.%,与澳大利亚后太古宙平均页岩w(SiO2)(62.8.%)较接近[24],Al2O3(6.50%),CaO(3.41%),TFe2O3(2.46%),TiO2(0.29%)和P2O5(1 218×10-6)平均质量分数依次减少。SiO2从底部至顶部不断增加,w(Al2O3),w(TFe2O3),w(K2O)和w(TiO2)由下而上呈现先降低后升高的趋势(表1)。w(P2O5)在牛蹄塘组中部磷块岩样品(AH-37)中出现最大值,其平均值由底部1 669 μg/g降至顶部301 μg/g。
表1 AY-1井牛蹄塘组页岩样品主量元素测试结果
Table 1 Test results of major elements in Niutitang formation shale samples of well AY-1
3.2 微量元素
微量元素富集系数的高低可以表征富集程度的强弱[25],富集系数(XEF)采用如下公式计算:
XEF={[w(X)]/[w(Al)]}/{[w(X)]/[w(Al)]}UCC
其中:[w(X)]/[w(Al)]指样品中某微量元素质量分数和Al质量分数的比值,{[w(X)]/[w(Al)]}UCC指该在上地壳中某微量元素和Al的质量分数比值[26]。
将得到的微量元素富集系数取平均值,可获得不同层段相应微量元素的富集特征,见图3。由图3可知:湘中地区AY-1井牛蹄塘组样品微量元素表现出一定富集规律,总体而言,V,Ni,Zn,Cu,Mo,Ag,Ba,U,Sb和B等元素相较上陆壳出现不同程度的富集,Mo,Ag,U和Pb等元素富集程度较高,Mn,Sr和Th等元素则在大部分样品中与上陆壳相比表现亏损。除Mn和Pb以外,牛蹄塘组中部微量元素富集程度普遍比顶部和底部的高,且Mo,Ag,U和Sb等元素质量分数远超上陆壳中质量分数,富集系数均在100左右。牛蹄塘组顶部和底部微量元素的富集程度差异不大。
3.3 稀土元素
牛蹄塘组总稀土元素质量分数(Σw(REE))差异较大,在61.15~260.04 μg/g之间,平均值为127.74 μg/g,Σw(LREE)/Σw(HREE)范围为0.174~4.701(见表2)。稀土元素在牛蹄塘组顶部、中部和底部呈现不同的配分模式(图4)。牛蹄塘组底部稀土元素配分模式为左倾型,表现出Ce负异常和不显著的Eu正异常(图4(a));与之类似,牛蹄塘组中部稀土元素也呈左倾型配分模式(图4(b)),w(LaN)/w(YbN)平均值(0.378)比牛蹄塘底部(0.289)的略大,且牛蹄塘组中部δw(Eu)比底部的略高,但不同的是,牛蹄塘组中部大部分样品有显著的负Ce异常和正Y异常;与底部、中部不同,牛蹄塘组顶部呈平坦型稀土配分模式(图4(c)),除样品AH-80呈显著Eu正异常外,其他样品均表现为微弱的Eu负异常,无显著Ce异常。
图3 AY-1井牛蹄塘组页岩样品微量元素富集系数
Fig. 3 Trace elements enrichment factor in Niutitang formation shale samples of well AY-1
图4 AY-1井牛蹄塘组页岩样品稀土元素配分模式图
Fig. 4 REE distribution pattern of Niutitang formation shale samples in well AY-1
表2 AY-1井牛蹄塘组样本AH-101稀土元素特征值计算结果(质量分数)
Table 2 Calculation results of rare earth elements characteristic values of sample AH-101 in Niutitang formation of well AY-1
4 讨论
4.1 热液作用
重力、磁性等地球物理资料显示雪峰山地区有慈利—保靖断裂带、安化—溆浦断裂带,加里东早期研究区进入拉张裂陷阶段,这些断裂带成为了热液上升的通道,形成了凤凰县潘公潭铀矿床和湘贵渝交界处松桃地区铅锌矿床等[27]。
OLIVAREZ等[28-31]研究表明:受还原性极强的高温热水流体(>200 ℃)影响的沉积物稀土配分模式常呈右倾型,具有显著的Eu正异常。另外,海水中热液活动还可以造成沉积物中Cu,Sb,As和Hg等亲铜元素及Co和Ni等铁族元素明显富集[32-33]。本文利用稀土元素配分模式、微量元素富集特征和w(Co)+w(Ni)指标综合分析热液活动影响强度。
AY-1井牛蹄塘组底部稀土配分模式呈左倾型,可见不明显的Eu正异常和Ce负异常(图4(a)),沉积物呈现热液及海水混合的稀土配分模式[30]。在与热液活动相关的微量元素中,Sb的富集程度较高,其富集系数的平均值高达30.30(图3),w(Co)+w(Ni)平均为35.63 μg/g,且有向上递增的趋势。由此判断,牛蹄塘组底部沉积时受到了热液作用的影响,但热液对研究区的影响较小。
牛蹄塘组中部稀土元素配分模式呈左倾型,HREE富集程度比LREE的高,呈现Ce负异常、Y正异常和轻微Eu正异常,平均δw(Eu)(1.068)略比底部(0.998)的高,稀土元素配分没有表现出明显热液作用沉积物的特征[20]。然而,牛蹄塘组中部Co,Ni,Pb和Sb等与热液相关的微量元素显著富集,分别为上地壳的1.203,10.405,1.734和331.919倍,Ni和Sb微量元素的富集系数远比底部的高(图3)。w(Co)+w(Ni)平均值为144.35 μg/g,明显比底部以及湘中地区不受热液影响的烟溪组的平均值(13.624 μg/g)高。由此推断,牛蹄塘组中部受到了热液活动的影响,且中部接受深部物源供应较底部更充足。牛蹄塘组底部和中部页岩稀土元素配分模式与典型热液沉积物配分不一致的原因可能是沉积物在海底暴露时间较长,且在沉积时有不同比例的海水参与循环等因素的影响[34]。
牛蹄塘组顶部稀土元素配分模式表现出平坦型稀土元素配分模式(图4(c)),除样品AH-80的δw(Eu)较高外(2.032),其他样品均表现为Eu负异常,w(Co)+w(Ni)平均为86.06 μg/g,热液作用影响明显比中部的弱。
除石煤样品外(AH-66和AH-31),热液作用强弱的元素w(Ni),w(Sb),w(Co)+w(Ni)和w(TOC)之间存在良好的相关性,说明热液活动对牛蹄塘组黑色岩系有机质的富集产生了积极影响,在牛蹄塘组沉积时期,热液为水体提供了热能[35]并携带了大量微量元素和营养物质到地表[20],使水中生物更加繁盛,有助于富有机质黑色岩系的发育。
图5 牛蹄塘组Ni和Sb富集系数、Co+Ni元素质量分数与w(TOC)相关性
Fig. 5 Correlation between richment coefficient of Ni,Sb,mass fraction of Co+Ni and TOC in Niutitang Formation
4.2 古生产力
古生产力即海洋的初级生产力,是指地质历史时期海洋单位面积、单位时间内所产生的有机质的总质量分数[36]。湘中牛蹄塘组样品干酪根碳同位素丰度δw(13C)为-29.7‰~31.3‰,为I型干酪根,主要为藻类堆积物,因此,海洋有机质的供给量表征了牛蹄塘组页岩的古生产力。
海洋古生产力主要受到海水中营养元素可利用程度的控制[37],营养物质越丰富,生物越繁盛,水体中的浮游植物的造碳能力越强,相应的古生产力越高。P元素是生物代谢过程中重要的营养元素之一,也是许多海洋生物骨骼的组成部分,可随着生物体死亡后进入沉积物中[38],且Al标准化后的w(P)与古生产力之间具有良好的相关性,因此,被广泛应用于判别古生产力强弱。AH-18磷质结核赋存页岩样品和AH-37磷质页岩样品因磷灰石的吸附作用造成微量元素大量富集,w(P)出现异常高值,不适用于古生产力的恢复[12]。牛蹄塘组w(P)/w(Al)为27.78×10-4~449.67×10-4,平均为116.75×10-4;底部至中部生产力水平增加,至牛蹄塘组顶部沉积时期古生产力保持较高水平且趋于稳定。
Cu和Ba元素与有机质的沉降量有密切关系,可以定量计算初级生产力的变化[39],但只有与生物成因相关的元素才可以反映古生产力,所以,必须剔除陆源成因,通过公式w(X生物)=w(X总量) -w(Al总量)×[w(XPAAS)/w(AlPAAS)]排除陆源碎屑的干扰(w(XPAAS)为后太古宙澳大利亚页岩中任意元素质量分数,其中,w(CuPAAS)/w(AlPAAS)为0.000 5,w(BaPAAS)/w(AlPAAS)为0.007 3[40])。牛蹄塘组w(Cu生物)为6.13~289.91 μg/g,平均为65.09 μg/g,与安徽巢湖二叠统大隆组黑色页岩的w(Cu生物)平均值60.34 μg/g较接近[41]。w(Ba生物)介于45.19~8 267.93 μg/g,平均为1 415.88 μg/g,通常认为w(Ba生物)在1 000~5 000 μg/g之间时,沉积环境具有高生产力;当w(Ba生物)在200~1 000 μg/g之间时,沉积环境具有中等生产力[42],牛蹄塘组黑色页岩处于中到高等生产力水平。
综合考虑以上不同元素对古生产力恢复的指示,牛蹄塘组黑色页岩古生产力变化表现出相似规律,从底部至中部古生产力水平均不断提高: w(P)/w(Al)由33.11×10-4升至181.19×10-4,w(Cu生物)由5.07 μg/g增加至104.31 μg/g,w(Ba生物)由1 117.6 μg/g增加至1 801.62 μg/g,牛蹄塘组中部至顶部生产力水平降低。
牛蹄塘组w(P)/w(Al)和w(TOC)之间存在显著的正相关关系(见图6),但w(Cu生物),w(Ba生物)和w(TOC)之间的相关性弱。这可能是由于研究区受到了热液作用,导致Cu元素富集,从而影响Cu元素对生产力的指示作用;且AH-18页岩样品中见磷质结核,推测研究区受到了上升洋流的影响,而受到上升洋流的高生产力的海域,所消耗的Cu在水体中得不到补偿。以BaSO4晶体存在的Ba生物元素在还原条件的水体下容易造成BaSO4晶体大量流失,而不能准确反映生产力的变化[40]。
图6 牛蹄塘组古生产力指标和w(TOC)相关性
Fig. 6 Correlation between Paleoproductive index and mass fraction of TOC in Niutitang Formation
4.3 氧化还原条件
当水体为氧化环境时,大量的有机质在发生沉淀过程中被氧化降解,使其中的有机组分向无机组分转化,导致沉积物中的有机质质量分数偏低;而还原性环境则能够将有机质较大程度地保存下来,为富有机质页岩的形成提供了条件,因此,氧化还原条件制约着有机质的富集。
WILDE等[43-45]通过U和V等氧化还原敏感元素来判断底层海水的氧化还原条件。U和V具有较强的氧化还原敏感性并具有相似的沉淀机制:在富氧-贫氧条件下,U和V均以易溶性化合物的形式存在于水体中,沉积物中的U和V质量分数较低;在缺氧条件下,U和V在有机质和微生物的催化、吸附作用下易被还原成低价离子转移到沉积物中。在使用U和V进行氧化还原条件判别时,需将其进行标准化计算以此来校正有机质和成岩过程中的稀释作用,用w(U)/w(Al)和w(V)/w(Al)的高低来判断水体还原性质的强弱[45]。牛蹄塘组样品w(U)/w(Al)和w(V)/w(Al)变化较大,变化范围分别为0.85×10-4~1 000.97×10-4和18.92×10-4~1 725.84×10-4,普遍高于澳大利亚后太古宙平均页岩w(U)/w(Al)(0.31×10-4)与w(V)/w(Al)(15.0×10-4)[23]以及现代黑海沉积物w(U)/w(Al)(3.1×10-4)和w(V)/w(Al)(28.8×10-4)[46],因此,牛蹄塘组沉积时水体基本为缺氧还原环境。牛蹄塘组底部w(U)/w(Al)和w(V)/w(Al)均较低,平均值分别为1.94×10-4和45.64×10-4,呈现向上递增的趋势(图2),即底部沉积时期水体还原性不断增强;至牛蹄塘组中部沉积时期,w(U)/w(Al)和w(V)/w(Al)均达到最高,此时水体还原性最强;顶部w(U)/w(Al)和w(V)/w(Al)比中部的低,平均值分别为2.26×10-4和36.67×10-4,其水体还原性减弱。综上所述,牛蹄塘组沉积时期水体由底部至顶部还原性表现为由弱至强再到弱的变化过程。
w(Ni)/w(Co),w(V)/w(Cr)和w(U)/w(Th)也是判别沉积水体氧化还原环境的主要指标,如表3所示[47]。从表3可见:牛蹄塘组底部样品基本落入富氧环境区间值中,只有靠近中部的AH-27样品为贫氧环境;中部样品的所有指标均远大于缺氧环境中所有指标的最小值,显示为强厌氧环境;顶部w(Ni)/w(Co)为4.40~8.60,w(V)/w(Cr)为2.14~5.15和w(U)/w(Th)为1.14~1.82,表现为贫氧—富氧环境。牛蹄塘组沉积时期水体底部至顶部经历了氧质量分数先降低后升高的规律(图2),这与w(U)/w(Al)和w(V)/w(Al)指示的氧化还原变化规律相一致。
表3 古氧化还原环境判别参数[47]
Table 3 Identifying indexex of Paleoredox environment[47]
牛蹄塘组古氧化还原环境指标和w(TOC)相关性见图7。从图7可见:牛蹄塘组w(U)/w(Al),w(V)/w(Al),w(Ni)/w(Co),w(V)/w(Cr),w(U)/w(Th)和w(TOC)之间存在一定的正相关关系,表明缺氧、还原环境有利于有机质的形成或者保存。在缺氧、还原环境下,有机质的分解速率较低,透光层繁盛的生物群落在死亡后能够在沉积物中以有机质的形式有效保存。在牛蹄塘组沉积时期,水体基本处于缺氧、还原环境,为富有机质黑色页岩的形成提供了良好条件。
5 有机质富集机理
扬子板块东南缘被动大陆边缘斜坡带上有深大断裂并伴有热液活动,地壳深部富含营养物质的热液为生物繁育提供能量及生命必须的营养物质,促进了生物数量的增加,提高了牛蹄塘组页岩古生产力水平。在早寒武世牛蹄塘组沉积期(梅树村期和筇竹寺期)全球海平面上升,研究区处在深水沉积环境,但牛蹄塘早期至晚期出现了海平面的相对变化,受上升洋流带来的大量有机质影响,高生产力海域中的微生物活动大量增加,导致水体含氧量急剧减少,呈缺氧、还原环境[40],这为有机质提供了较好的保存条件。热液活动、古生产力水平、水体还原环境共同影响了有机质的富集(图8)。
图7 牛蹄塘组古氧化还原环境指标和w(TOC)相关性
Fig. 7 Correlation between Paleoredox environment index and mass fraction of TOC in Niutitang Formation
图8 湘中地区牛蹄塘组有机质富集模式图
Fig. 8 Organic matter enrichment pattern of Niutitang formation in central Hunan
牛蹄塘组底部安化地区受到微弱热液作用的影响,热液活动提供的生物繁殖所需营养元素使微生物(主要为海生藻类、海洋浮游生物及菌类)在表层海水真光层大量繁殖[9,48],古生物生产力逐步升高(图2),然而,寒武纪初期海侵导致海水底部水动力条件增强[46]以及产生水体富氧—贫氧等不利条件,破坏了有机质的保存,导致w(TOC)偏低(平均为1.40%)。
在牛蹄塘组中部沉积时期,热液活动显著增强,与之相对应的较高的生产力水平为有机质的富集提供了充足的物质来源,同时,较稳定的海平面[49-50]和还原性较强的缺氧深水环境也为有机质的富集提供了良好的保存条件,因此,该时期有机质大量富集,样品中w(TOC)(平均为14.06%)也远比底部的高。
随着热液活动减弱,牛蹄塘组顶部沉积时期古生产力水平略降低且趋于稳定。然而,在此沉积时期,海平面逐渐下降,氧化水体范围扩大[51],沉积水体底部的还原性减弱,制约了有机质的保存,TOC质量分数(平均为4.20%)比牛蹄塘组中部的低。
6 结论
1) 热液活动、古海洋环境和古生产力共同影响了安化地区牛蹄塘组黑色岩系的有机质富集,热液带来生物所需微量元素使得生物大量繁育,对古生产力的提高有积极的影响,有利于有机质的富集;中—高水平的古生产力为有机质富集提供物质基础;氧化还原条件制约有机质的保存,缺氧、还原性水体使有机质较好地保存。
2) 湘中安化地区下寒武统牛蹄塘组从底部—顶部沉积时,热液作用强度、水体还原性先增强后减弱,在强热液活动、高古生产力和强还原环境下,中部发育一套高有机碳质量分数的页岩层系。
3) 虽然顶部古生产力水平较高,但海平面下降,水体含氧量增加,有机质不能较好地保存,有机质丰度比中部的低。
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(编辑 陈灿华)
收稿日期: 2019 -11 -08; 修回日期: 2020 -01 -22
基金项目(Foundation item):中国地质调查局地质调查项目(DD20160183); 湖南省科技创新计划项目(2017GK2233); 湖南省自然科学基金资助项目(2017JJ1034) (Project(DD20160183) supported by the Geological Survey Program of Geological Survey of China; Project(2017GK2233) supported by the Science and Technology Innovation Program of Hunan Provine; Project(2017JJ1034) supported by the Natural Science Foundation of Hunan Province)
通信作者:郭建华,博士,教授,从事沉积学与页岩气研究;E-mail: gjh796@csu.edu.cn