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泥质岩的地球化学特征能够反映物源区属性(McLennan et al., 1993)、地形地貌(Goudie et al., 2012)、古气候(Yang Jianghai et al., 2017;Varela et al., 2018)以及沉积环境(Hofer et al., 2013;Lash et al., 2014;Zhang Xianguo et al., 2017)等信息。泥质岩通常具有良好的均质性和沉积后的低渗透性,后期的成岩作用过程中某些微量元素和稀土元素的含量及比值受影响较小,因而具有指示沉积环境、古气候以及物源区性质的功能(Hofer et al., 2013)。西湖凹陷已被证实油气勘探潜力大,但所探明的大中型油气田并不多,平湖组煤系烃源岩不仅是油气生成的主要物质基础,同时平湖组也作为目前主要的勘探领域之一(Zhou Xinhuai, 2020),而物源区属性和古环境特征则直接影响到平湖组砂岩储层的质量、砂体和煤系的分布以及烃源岩的生烃潜力。利用泥质岩地球化学特征分析西湖凹陷物源区属性及古环境特征,有助于深化西湖凹陷烃源岩及油气储层分布规律的认识。
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西湖凹陷位于东海陆架盆地东部浙东坳陷带内,是中国海域重要的含油气凹陷之一(Huang Zhichao et al., 2010),平湖组煤系烃源岩是区内主力烃源岩层系(Zhang Gongcheng et al.,2013;Liu Jinshui et al., 2020a, 2020b; Kang Shilong et al., 2020; Shen Yulin et al., 2021)。前人通过对地震剖面、钻井岩芯、古生物组合以及地球化学等资料的分析,认为平湖组沉积时期为半封闭的海湾环境,平湖组从早期到晚期海侵规模逐渐减小,主要发育受潮汐影响的三角洲和潮坪沉积体系(Li Shunli et al., 2018; Abbas et al., 2018; Jiang Yiming et al., 2020),但对于水体介质受河流和海洋影响程度如何,还存在较多争议。前人通过碎屑锆石U-Pb定年、重矿物组合及重矿物ZTR指数等资料的分析,认为西湖凹陷西侧隆起区(虎皮礁隆起和海礁隆起)是平湖组沉积期主要物源区(Li Ning et al., 2017;Zhao Ke et al., 2020),但隆起区基底研究较少且缺乏钻探验证,平湖组沉积期物源还存在一些争论。沉积环境和物源区属性的争议,严重限制了平湖组沉积砂体的精细刻画及有利勘探目标的搜寻。
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针对研究区存在的种种争议,利用沉积地球化学的方法对东海陆架盆地西湖凹陷A-1、A-2和B-1三口钻井采集的泥质岩样品进行分析,并据实验结果对研究区的物源区岩石性质、源岩风化作用、沉积背景、古构造背景及古环境演化进行分析,为西湖凹陷平湖组物源区、古构造、古气候及沉积环境演化研究提供依据,从而为深入了解砂岩储层的质量以及砂体和煤系的分布提供理论基础。
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1 地质背景
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西湖凹陷地处东海陆架盆地东部坳陷带中部偏北,东接钓鱼岛隆褶带,西邻海礁隆起、虎皮礁隆起和渔山东低隆起,南部和北部分别与福江凹陷和钓北凹陷相邻(图1a),呈北东向展布,面积约5.18万km2。西湖凹陷基底主要为NNE向和NW向两组断裂系统所控制,在NNE向断层控制下西湖凹陷呈北北东向长条形,在NW向基底断裂控制下西湖凹陷呈现南北分块的特点。西湖凹陷可划分为五个构造单元,包括西部斜坡区、西次凹、中央洼陷区、东次凹及东部断阶区(图1b)。
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图1 西湖凹陷构造区划图(a、b,据Liu Jinshui et al., 2020c)和新生代地层柱状图(c,据Chen Zhongyun et al., 2013)
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Fig.1 Map showing the division of tectonic units of the Xihu depression (a, b, after Liu Jinshui et al., 2020c) and the stratigraphic subdivisions of the Cenozoic in the Xihu depression (c, after Chen Zhongyun et al., 2013)
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西湖凹陷同中国东部许多盆地构造演化一样,总体上经历了断陷、拗陷和整体沉降三个演化阶段,古新世—始新世为断陷期,渐新世—中新世为拗陷和构造反转期,上新世至今为整体沉降期(Liu Jinshui et al., 2020c)(图1c)。
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综合前人对西湖凹陷地层划分成果(Chen Zhongyun et al., 2013),并根据钻井资料,西湖凹陷地层由老到新为古新统、始新统(八角亭组、宝石组、平湖组)、渐新统(花港组)、中新统(龙井组、玉泉组、柳浪组)、上新统(三潭组)和第四系(东海群)(图1c)。本文重点研究始新统平湖组,平湖组与下伏宝石组和上覆花港组地层呈不整合接触。
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2 样品采集和实验方法
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本次研究样品取自西湖凹陷平湖组北部A-1(3480~4325m)、中部A-2(3060~4010m)和南部B-1(3800~4800m)3口探井,共采集39个泥质岩岩屑样品,具体取样井位分布如图1b所示,样品位置及编号如图2所示。样品均选取能够较好地保存原始沉积环境信息、风化蚀变和成岩作用影响较弱的泥质岩,平湖组泥岩颜色为灰色或深灰色。样品的前期处理包括超纯水清洗,烘干及研磨,均在中国矿业大学(北京)煤炭资源与安全开采国家重点实验室完成。样品的化学处理及测试在核工业北京地质研究院分析测试中心完成,分析精密度优于5%。利用飞利浦PW2404X射线荧光光谱仪(XRF)进行主量元素分析测试,测试方法依据GB/T14506.28—2010;微量元素分析测试仪器为Element XR,测试方法依据GB/T14506.30—2010。
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3 实验结果
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3.1 主量元素特征
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样品的主量元素以SiO2和Al2O3为主,其中SiO2含量介于28.46%~57.10%之间,平均52.08%,除个别样品外,含量差异不大;Al2O3含量介于12.64%~23.15%之间,平均为18.61%,差异较小。其次为Fe2O3、K2O和MgO;TiO2、CaO、Na2O、P2O5及MnO含量极低(表1)。
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3.2 微量元素特征
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样品的微量元素特征表现为亲铁性元素Co、Ni和Cr整体接近大陆上地壳,部分样品Cr在北部(A-1)表现为轻微的富集。高场强元素Th、Zr等整体表现为接近大陆上地壳,部分样品Zr在南部(B-1)有亏损。亲硫性元素Cu和大离子亲石元素Ba含量相对于大陆上地壳较为富集。
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大离子亲石元素Sr含量相对于大陆上地壳有亏损。Sc、V含量接近大陆上地壳(表2、图3)。
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3.3 稀土元素特征
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稀土元素在不同的井区及同一钻井的不同深度表现出不同的分布特征,在北部(A-1)和南部(B-1)轻稀土元素(LREE:La、Ce、Pr、Nd、Sm、Eu)相对富集,而在中部(A-2)三个样品呈相对亏损特征,其余样品含量相当;重稀土元素(HREE:Gd、Tb、Dy、Ho、Er、Tm、Yb、Lu)在A-1和B-1井区除少量呈相对富集,其余均含量相当,而在A-2井区三个样品呈相对亏损特征,其余样品含量相当(表3、图3)。
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4 讨论
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4.1 构造背景讨论
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不同构造背景的母岩被风化后,经搬运、沉积所形成的碎屑岩具有特定的沉积地球化学特征(Bhatia et al., 1986;Roser et al., 1988;McLennan et al., 1993)。本文根据Bhatia et al.(1986)提出的La-Th-Sc和Th-Sc-Zr/10图解进行构造背景的判识,从图4中可以看出,除了少数泥质岩样品投点落在了被动大陆边缘和活动大陆边缘区域,大部分投点落在了大陆岛弧区域。西湖凹陷位于欧亚板块、菲律宾板块和太平洋板块汇聚的活动大陆边缘,侏罗纪时期西湖凹陷受到东亚多向汇聚构造体系影响,处于多向挤压环境,但未遭受强烈挤压变形,此时古亚洲特提斯构造体制处在大陆边缘拗陷演化阶段;白垩纪以来西太平洋板块俯冲和后撤,使凹陷进入太平洋构造体制的弧后裂陷演化阶段(Liu Jinshu et al., 2020c),从而造成西湖凹陷具有活动大陆边缘构造背景之外,还具有大陆岛弧构造背景。
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4.2 物源区属性分析
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Th/U、Al2O3/TiO2、Cr/Zr及稀土元素可以作为定性或定量化指标来分析母岩类型及来源(Bhatia, 1985;Taylor et al., 1985;McLennan et al., 1993;Hayashi et al., 1997)。
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稀土元素球粒陨石标准化分布模式可用于判断物源区类型(McLennan et al., 1993),A-1和B-1井稀土元素分配模式与上地壳十分相似(图3),A-2井除59号、52号和49号样品,其余均与上地壳稀土分配模式相似。从稀土元素球粒陨石标准化分布模式可以得出,研究区平湖组泥质岩母岩主要是由上地壳岩石演化而来,部分来自于地幔。因稀土元素在风化、成岩和沉积过程中相对比较稳定,不易迁移,其含量特征可作为判断母岩类型的良好指标 (Taylor et al., 1985, 1986;Bhatia, 1985)。例如:铁镁质岩通常具有较低的LREE/HREE和(La/Yb)N值,且无Eu负异常;然而长英质岩却有较高的LREE/HREE和(La/Yb)N值及明显的Eu负异常(Taylor et al., 1985)。本次分析的样品表现出LREE富集(LREE/HREE=7.46~12.70,LaN/YbN=7.02~24.17),HREE分布较为平缓(GdN/YbN=0.88~4.18),且具有明显的Eu负异常(平均值=0.75),样品的上述特征指示其母岩很可能是长英质岩。
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图2 西湖凹陷平湖组地层柱状图及采样位置(钻井A-1、A-2、B-1)
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Fig.2 Stratigraphic column showing sampling positions of the Pinghu Formation in the Xihu depression (wells A-1, A-2, and B-1)
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注:ICV—成分变异指数(Index of compositional variability); CIAcorr—校正后的化学蚀变指数(Chemical index of alteration)。
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Th/U值可作为确定物源区类型的参数之一(Taylor et al., 1985;McLennan et al., 1993),现今地壳Th/U平均值为4.25~4.30,上地壳与上地幔分界值为3.8,上下地幔的分界值为2.6(Paul et al., 2003)。西湖凹陷A-1、A-2和B-1井泥质岩的Th/U平均值分别为4.55、4.26和4.28,与上地壳存在明显的亲缘关系。微量元素Th/U-Th判别图可更好确认母岩来源(Taylor et al., 1985;McLennan et al., 1993),北部(A-1)大部分样品投点落在上地壳区域,仅19号样品和60号样品投点落在地幔区域;中部(A-2)除底部59号、52号和49号样品投点落在上地幔区域,其余样品投点落在上地壳区域;南部(B-1)全部样品投点落在上地壳区域(图5a)。因此可得出平湖组泥质岩母岩主要由上地壳岩石演化而来,少部分为地幔来源。在微量元素La/Th-Hf判识母岩类型图解中(Cox et al., 1995),大部分样品投点落在酸性岩浆弧物源区,极个别落在长英质、基性岩混合物源区,仅A-1井53号和61号样品投点落在老沉积物组分区(图5b)。综上可得出研究区平湖组泥质岩母岩以上地壳岩浆岩为主,少量老沉积物组分和地幔岩浆岩。研究区中部多口探井钻遇一套以火山岩为主夹碎屑岩的地层,根据同位素地质年龄推断其岩浆活动为早、中始新世(Yang Chuansheng et al., 2012),推测中北部(A-1和A-2)平湖组沉积早期,由于早、中始新世岩浆活动导致地幔物质涌出地表,使平湖组底部泥质岩母岩部分表现为地幔来源的特征。
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图3 西湖凹陷始新统平湖组泥质岩微量元素PASS标准化模式图(标准化数据引自Taylor et al., 1985) 和稀土元素配分图(标准化数据引自McLennan, 1993)
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Fig.3 The distribution patterns of the trace elements (standardized data from Taylor et al., 1985) and REE (standardized data from McLennan, 1993) of the Eocene argillaceous rocks in the Xihu depression
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图4 西湖凹陷平湖组泥质岩La-Th-Sc and Th-Sc-Zr/10判别图解(底图据Bhatia et al., 1986)
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Fig.4 La-Th-Sc and Th-Sc-Zr/10discriminatory plots of the Pinghu Formation argillaceous rocks in the Xihu depression (base map after Bhatia et al., 1986)
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注:Bc—校正后的B含量。
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Al2O/TiO2值能很好地指示母岩类型,Hayashi et al.(1997)认为Al2O3/TiO2值介于3~8之间,母岩为镁铁质火成岩;介于8~21之间,母岩为中性火成岩;介于21~70,母岩为长英质火成岩。研究区样品的Al2O3/TiO2值介于19.99~34.62,平均值为25.39(表2),表明平湖组泥质岩的母岩主要为长英质火成岩。仅在研究区中部A-2井中8号样品Al2O3/TiO2值为19.99,指示中部地区平湖组一、二段泥质岩母岩可能有少量中性岩浆岩。K2O/Al2O3值通常被用来定性判断沉积物母岩中碱性长石的含量,Cox et al.(1995)指出当K2O/Al2O3值大于0.5时,沉积母岩中含有较高含量的碱性长石,研究区泥质岩样品的K2O/Al2O3值范围为0.15~0.25,平均值为0.22(表1),表明母岩中碱性长石的含量较低,推测母岩可能为酸性花岗闪长岩或英安岩,并有少量中性安山岩或闪长岩。
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前人通过二维地震资料和重磁资料的处理解释,认为海礁隆起基底以燕山期花岗岩、花岗闪长岩等侵入岩以及安山岩、英安质凝灰岩等喷发岩和火山碎屑岩为主,在中生界火山岩系之下可能存在古生界变质岩系(Yang Chuansheng et al., 2012;Zhu Lixin, 2016)。因此本文认为海礁隆起区为西湖凹陷西部斜坡区平湖组沉积期主要物源区,母岩整体以上地壳的长英质火成岩为主,北部(A-1)平湖组五段部分泥质岩母岩受到少量来自上地壳古生界变质岩系影响,中部(A-2)平湖组四段部分泥质岩母岩受幔源岩浆的影响,母岩类型主要为花岗闪长岩和英安质凝灰岩,少量安山岩和变质岩。西湖凹陷变质岩母源来源的砂体相对于岩浆岩母源的砂体具有更好的物性条件(Li Ning et al., 2017),因此本文认为西湖凹陷北部(A-1)平湖组五段可能分布有物性条件较好的砂体。
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图5 西湖凹陷平湖组泥质岩Th/U-Th (a)(底图据McLennan et al., 1993)和La/Th-Hf判别图(b)(底图据Cox et al., 1995)
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Fig.5 Plot of Th/U versus Th (a) (base map after McLennan et al., 1993) and La/Th-Hf discriminant diagram (b) (base map after Cox et al., 1995) for the Pinghu Formation argillaceous rocks in the Xihu depression
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4.3 物源区化学风化作用及其反映的源区古气候特征
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物源区的化学风化强度受源区岩石类型、气候特征、构造背景、地形特征、植被条件等因素的影响(Berner, 1992;Oliva et al., 2003)。化学蚀变指数CIA(Nesbitt et al., 1982)可用于定量评价化学风化强度,计算公式为:
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其中氧化物为摩尔百分数,CaO*是岩石中硅酸盐所含的CaO。
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由于碎屑岩在成岩过程中的钾交代作用及搬运、沉积过程都会造成钾的富集,因此在使用CIA指数时必须首先进行钾交代作用校正。本文采用Nesbitt et al.(1984, 1989)提出的A-CN-K(Al2O3-(CaO*+Na2O)-K2O)三角图解进行校正,结果如图6所示。其中L和L’为预测未发生钾交代作用泥质岩风化趋势线,l和l’为实际泥质岩风化趋势线,从图6可以看出泥质岩样品存在一定程度的钾交代作用,A-1和B-1井的泥质岩风化趋势线偏向K2O端元的程度要强于A-2井。从K定点出发穿过实测数据点与L和L’的交点即为剔除钾交代作用后泥质岩CIAcorr值。
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另外在使用CIA值判断源区风化程度时必须考虑到泥质岩是否受到再旋回作用的影响,本文利用Cox et al.(1995)提出的成分变异指数ICV来判断,计算公式为:
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式中氧化物以摩尔百分数为单位。当ICV>1时,指示样品中含有少量的黏土矿物,属于构造活动背景下的首次沉积;ICV<1时,指示样品中含有大量的黏土矿物,代表可能经受了再旋回作用或首次沉积条件下经历了强烈的风化作用。从本次研究的样品的ICV值(表2)可知,大多数样品的ICV值均接近1或大于1,表明为活动构造带的首次沉积;少数A-2井样品的ICV值位于0.55~0.57,表明其可能经历了再循环沉积或首次沉积时经历了强风化作用。Zr/Sc是常用的判断沉积物再旋回的指数(Hassan et al., 1999),而Th/Sc可作为判断化学分异的指标(McLennan et al., 1993),因此采用Zr/Sc-Th/Sc图解(McLennan et al., 1993;Mongelli et al., 2006),来判断低ICV值的样品沉积物是否经历再旋回作用的影响。从图7可以看出所有样品投点与成分趋势线非常接近,可初步得出样品并未遭受再旋回作用的影响,而低ICV值的样品所对应的CIAcorr值分布范围77.34~83.33,因此可得出低ICV值的样品是首次沉积时强风化作用所导致的。
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为了得到每一个样品的准确CIA值,本文采用Panahi et al.(2000)提出的校正公式进行校正:
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式中氧化物以摩尔百分数为单位,CaO*是岩石中硅酸盐所含的CaO, K2Ocorr是指未发生钾交代作用的泥质岩中的K2O的摩尔百分数;m值代表母岩中的K2O的比例,研究区地处海域,无法得到母岩的信息,因此根据图6中平行于A-CN连线L和L’的延长线与CN-K轴的交点即为m值。校正后CIAcorr值如表1所示,其中A-1井CIAcorr分布范围为63.34~83.41,平均值为75.63,A-2井CIAcorr分布范围为61.40~83.33,平均值为76.15,B-1井CIAcorr分布范围为78.40~83.49,平均值为81.52。Fedo et al.(1995)指出CIA=50~60,反映物源区遭受弱风化作用影响;CIA=60~80,指示了中等风化作用;CIA=80~100则反映了强烈风化作用。因此可得出西湖凹陷平湖组泥质岩的母岩经历了中等—强烈的风化作用。
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图6 西湖凹陷平湖组泥质岩样品A-CN-K三角图解
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Fig.6 A-CN-K ternary diagram for the Pinghu Formation argillaceous rocks in the Xihu depression
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L和L’—未发生钾交代作用的泥质岩风化趋势线;l和l’—代表发生钾交代作用的泥质岩风化趋势线
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L and L’ are the initial progressive chemical weathering trends; l and l’ are the K-metasomatism trend that resulted from the diagenetic K addition to the saprolith
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图7 西湖凹陷平湖组泥质岩Zr/Sc-Th/Sc图解 (底图据McLennan et al., 2003)
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Fig.7 Plot of Th/Sc versus Zr/Sc for the Pinghu Formation argillaceous rocks in the Xihu depression (base map after McLennan et al., 2003)
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探讨化学风化作用强弱时应首先考虑其直接控制因素古气候(温度、湿度等)(Gislason et al., 2009)。物源区风化作用强弱的定量指标,可用来反演沉积期的古气候条件(Goldberg et al., 2010;Yang Jianghai et al., 2017)。始新世—渐新世时期全球古气候由温室期逐步过渡到冰室期(Kraatz et al., 2010;Anagnostou et al., 2016;Herman et al., 2017)。古生物特征(Herman et al., 2017;Rivero-Cuesta et al., 2018),大气中二氧化碳含量(Pearson et al., 2009)、碳同位素以及氧同位素特征(Jicha et al., 2009)均记录了此次显著的变冷事件。始新世平湖组泥质岩样品的CIA值分布范围为61.40~84.23,平均值为76.99,根据Hao Lewei et al.(2018)报道的渐新世花港组泥质岩样品的主量元素,计算CIA值分布范围为67.54~79.35,平均值为74.95(表4)。对比分析不同时期的CIA值,始新世平湖组泥质岩CIA值要明显高于花港组,说明始新世平湖组沉积期风化作用要强于花港组,这与始新世到渐新世时期由温室期向冰室期过渡有着很好的对应关系。Sr/Cu值是判断古气候的重要指标,通常认为Sr/Cu值位于1.3~5.0指示潮湿气候,而大于5.0指示干旱气候(Wang Suiji et al., 1997),平湖组泥质岩Sr/Cu值位于0.5~8.2之间,平均3.21(表2、图8),且Sr/Cu值与CIA值有很好的对应关系。综上可推断出西湖凹陷在始新世平湖组沉积期古气候较为温暖湿润。
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4.4 古环境恢复
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泥质岩的微量元素(B、Sr、Ba、Ga、Cu等)和稀土元素(Ce等)的地球化学特征可作为定量化的指标来判断不同的古环境条件(Hofer et al., 2013;Lash et al., 2014;Zhang Xianguo et al., 2017;Wang Tong et al., 2020)。
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4.4.1 沉积背景分析
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硼元素(B)在沉积物中的富集主要取决于沉积环境的盐度,在高盐度的海水中可达4440 μg/L,而淡水中溶解度则为18 μg/L(Martin et al., 1983)。因此B常被用来作为判断海相、海陆过渡相和陆相的指标之一(Hofer et al., 2013;Lash et al., 2014;Zhang Xianguo et al., 2017)。B吸附于黏土矿物中,伊利石通常具有较高的B吸附能力,而碎屑岩粒度的变化与B的富集呈负相关。本次研究样品全部为泥质岩,因此可排除粒度变化对B富集的影响。为了减小黏土矿物组成等对B富集程度的影响,优化B对沉积背景的指示作用,本研究采用Walker(1968)提出的相当硼法进行校正,其计算公式如下:
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式中,Bc为校正后的B含量(×10-6);Bt为实际测试的硼含量(×10-6);K2O为K2O含量(%)。
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校正后B含量如表3所示,A-1井Bc含量分布范围137×10-6~195×10-6,平均值为165×10-6;A-2井Bc含量分布范围178×10-6~252×10-6,平均值为204×10-6;B-1井Bc含量分布范围133×10-6~168×10-6,平均值为145×10-6。Walker(1968)指出相当硼<200×10-6时,水较淡;介于200×10-6~300×10-6时,为半咸水;介于300×10-6~400×10-6时,为正常海水;>400×10-6时,为过咸和超咸水。据上述指标对西湖凹陷A-1、A-2和B-1井进行沉积背景分析,结果表明:平湖组五段沉积期,北部(A-1)相当硼含量呈增加趋势,但沉积水体整体上为淡水;平湖组四段沉积期,北部(A-1)相当硼含量呈增加的趋势,但沉积水体整体上为淡水,中部(A-2)相当硼含量整体上呈减小的趋势,从相当硼含量变化可推断出在该时期沉积水体表现为淡水和半咸水交替演化的特征;平湖组三段沉积期,北部(A-1)相当硼呈减小的趋势,沉积水体整体上为淡水,中部(A-2)除少数样品相当硼含量低于200×10-6,沉积水体为淡水,大部分样品相当硼含量大于200×10-6,但小于300×10-6,为半咸水;平湖组一、二段沉积期,北部(A-1)和南部(B-1)水体含盐度较低,为淡水,而中部(A-2)为半咸水(图8)。
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从古地貌角度来看,研究区在北部(A-1)的古地形高程要明显高于中部(A-2)和南部(B-1)(Cai Hua et al., 2019),古地形上的差异可能导致平湖组沉积期中部和南部更易受到海水的影响;从古生物特征来看,西湖凹陷南部发育钙质超微、有孔虫、介形虫、沟鞭藻等低丰度海相化石,且由南向北海相化石丰度逐渐降低,纵向上平湖组中下部发育低丰度海相化石,到平湖组上部发育有指示淡水的盘星藻、腹足类、蚌类、双壳类等化石(Zhou Xinhuai, 2020)。
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研究区内平湖组以三角洲沉积体系为主,多发育三角洲平原和三角洲前缘两种亚相类型(图2)。前人认为西湖凹陷平湖组沉积期主要发育受潮汐影响的三角洲和潮坪沉积体系(Li Shunli et al., 2018; Abbas et al., 2018; Jiang Yiming et al., 2020)。通过古盐度指标判断,并综合古地貌及古生物特征分析,认为西湖凹陷平湖组沉积期自北向南沉积背景有明显的地区差异性,北部主要发育河控三角洲沉积体系,中部和南部则主要发育受潮汐影响三角洲沉积体系。
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4.4.2 氧化还原条件分析
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泥质岩微量元素可用来说明古水体的氧化还原特征(Hatch et al., 1992;Jones et al., 1994;Cullers, 2002;Lash et al., 2014)。Mo和U元素在沉积物中含量很低,Mo元素在2.7×10-6左右,而U元素在3.7×10-6左右(Taylor et al., 1985);且这两种元素在海水中具有较长的滞留时间(Mo大约780ka,U大约450ka),Mo和U在海洋浮游生物中含量很低,沉积物中的Mo和U主要在缺氧条件下富集(Algeo et al., 2009)。Th元素在富氧环境中通常比较稳定,并且趋向于富集在富含黏土的沉积物中(Wignall, 1988)。Jones et al.(1994)认为含氧环境中的沉积物U/Th<0.75,研究区泥质岩样品U/Th值范围为0.12~0.60,平均值为0.24,因此可得出研究区泥质岩样品形成于含氧环境中。
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图8 西湖凹陷A-1、A-2和B-1井相当硼含量、CIA和Sr/Cu垂向变化趋势图
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Fig.8 B, CIA and Sr/Cu are plotted for wells A-1, A-2, and B-1in the Xihu depression
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Ni和V存在于高度稳定的四吡咯结构(如卟啉)中,这种结构最初来源于叶绿素,并优先在厌氧条件下保存,长期暴露于好氧条件下的有机物具有低的四吡咯含量,因此Ni和V含量低(Lewan et al., 1982)。V也可能吸附于黏土矿物,也可能是成岩后的结果(Breit et al., 1991)。Cr被认为仅与碎屑组分有关(Dill, 1986),不受氧化还原条件的影响,因此高V/Cr值(>2)被认为指示缺氧条件。Ni和Co均存在于黄铁矿中,但高Ni/Co值(>5)与缺氧条件有关(Rimmer, 2004)。V/Cr-Ni/Co和Mo-Ni/Co图解可用来判断碎屑岩形成时的氧化还原条件(Rimmer, 2004)。从图9可以看出所研究的泥质岩样品主要在含氧环境中形成。
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平湖组泥质岩样品有机地球化学特征显示伽马蜡烷含量极低,并且具有较高的Pr/Ph值(Zhu Yangming et al., 2012; Cheng Xiong et al., 2020),表明平湖组沉积期水体处于含氧环境中。原油生物标志化合物反映凹陷自南向北陆源高等植物输入增多,水体变浅,平湖组沉积期整体上为浅水的沉积背景(Zhou Xinhuai, 2020)。V/Cr-Ni/Co和Mo-Ni/Co图解判识结果与煤系和原油有机地球化学判识结果较为一致,因此本文认为研究区平湖组泥质岩主要形成于水体较浅的含氧环境中。
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5 结论
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(1)泥质岩微量元素构造判识结果表明西湖凹陷平湖组沉积期同时具有大陆岛弧和活动大陆边缘构造背景。海礁隆起为西湖凹陷西部斜坡区平湖组主要物源区,母岩主要以上地壳的长英质火成岩为主,并受到少量来自上地壳古生界变质岩系和幔源岩浆的影响,母岩类型主要为花岗闪长岩和英安质凝灰岩,少量安山岩和变质岩。
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(2)CIAcorr计算结果的分析得出平湖组泥质岩的母岩经历了中等—强烈的风化作用。研究区平湖组泥质岩Sr/Cu值和化学蚀变指数CIA对古气候的反演结果较为一致,即该时期古气候条件较为温暖湿润。
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图9 判别古氧化还原条件的微量元素比值交会图(底图据Rimmer,2004)
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Fig.9 Crossplots of trace-element ratios used as paleoredox proxies (base map after Rimmer,2004)
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(a)—V/Cr-Ni/Co判别图;(b)—Mo-Ni/Co判别图;V/Cr和Ni/Co的范围据Jones et al.,1994
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(a)—V/Cr vs Ni/Co; (b)—Mo vs Ni/Co; ranges for V/Cr and Ni/Co are from Jones et al.,1994
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(3)西湖凹陷平湖组沉积期自北向南沉积背景有明显的地区差异性,具体表现为:北部(A-1)沉积水体整体上为淡水,主要发育河控三角洲沉积体系,而中部(A-2)和南部(B-1)沉积水体表现为淡水和半咸水交替演化的特征,主要发育受潮汐影响三角洲沉积体系。微量元素U/Th值,V/Cr-Ni/Co和Mo-Ni/Co图解表明平湖组沉积期水体较浅,以含氧环境为主。
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摘要
西湖凹陷是东海陆架盆地重要的富生烃凹陷之一,平湖组是主要烃源岩和油气富集层系。为了探讨平湖组沉积期构造背景、源区性质及古环境,本文对平湖组泥质岩地球化学特征进行了研究。地球化学分析结果表明,平湖组沉积期同时具有大陆岛弧和活动大陆边缘构造背景。平湖组泥质岩的物源区母岩主要由上地壳岩石演化而来,并通过对比周围隆起区母岩特征,认为西侧的海礁隆起为西湖凹陷西部斜坡区平湖组沉积期的主要物源区,母岩主要以上地壳的长英质火成岩为主,并受到少量来自幔源岩浆和上地壳古生界变质岩系的影响,母岩类型主要为花岗闪长岩和英安质凝灰岩,少量安山岩和变质岩。泥质岩CIA分析表明,平湖组泥质岩的母岩经历了中等—强烈的风化作用,结合Sr/Cu值分析,推断西湖凹陷在平湖组沉积期古气候较为温暖湿润。微量元素和古盐度分析结果以及对盆地古地貌特征分析表明,西湖凹陷平湖组沉积期自北向南沉积背景有明显的地区差异性,北部(A-1)沉积水体为淡水,主要发育河控三角洲沉积体系,而中部(A-2)和南部(B-1)表现为淡水和半咸水交替演化的特征,主要发育受潮汐影响三角洲沉积体系。平湖组泥质岩主要形成于含氧环境中。
Abstract
The Xihu depression in the East China Sea basin contains significant oil and gas resources. The Pinghu Formation coal measures are the primary source rock and oil-gas reservoirs. In order to better understand its tectonic setting, provenance characteristics and paleoenvironment, we conduct detailed geochemistry study of the argillaceous rocks of the Eocene Pinghu Formation. La-Th-Sc and Th-Sc-Zr/10 diagrams indicate that the study area was mainly subjected to the continental island arc and active continental margin tectonic background. Argillaceous rocks were predominantly derived from upper crustal felsic igneous rocks based on the values Al2O3/TiO2, K2O/Al2O3, Cr/Zr, LREE/HREE, Eu, LaN/YbN, GdN/YbN and plots of Th/U-Th, and the fairly uniform REE patterns. By comparing the characteristics of the magmatic rocks in the uplift area, it is concluded that the parent rocks are mainly granitic amphibolite and ingenious tuff, with a few andesites and metamorphic rocks. The A-CN-K triangle showed that mudstones were affected by the K metasomatic processes, and in this regard, we corrected the chemical index of alteration (CIA). The Eocene Pinghu Formation mudstones have moderate to high K-corrected CIA (61.40~83.49), indicating moderate to intense chemical weathering conditions. By analyzing and comparing with the CIA of the Oligocene Huagang Formation, the chemical weathering history suggests that a warm and humid climate during the Eocene, whereas a cool and humid to semi-humid climate prevailed during the Oligocene. Using the corrected boron contents (Bc) and integrating the analysis of paleomorphological and paleontological features, it is concluded that there is regional variability in the depositional background of the Pinghu Formation. The northern part (A-1) shows a terrestrial depositional environment as a river-dominated deltaic depositional system. The central part (A-2) and southern part (B-1) were dominated by the marginal marine environments as the tidally influenced deltaic depositional system. Trace element U/Th ratios, V/Cr-Ni/Co and Mo-Ni/Co discrimination diagrams indicate that the mudstones of the Pinghu Formation were formed in an oxic environment.
Keywords
argillaceous rocks ; geochemistry ; provenance analysis ; paleoenvironment ; Pinghu Formation ; Xihu depression