Variations and mechanisms of the Asian summer monsoon revealed by stalagmite δ18O records
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摘要:
亚洲夏季风（ASM，Asian Summer Monsoon）是全球气候系统的重要组成部分，也是目前地球系统科学的热点问题。亚洲石笋δ18O揭示的亚洲古季风变化，加深了我们对其时空演化机制的理解。然而，目前东亚夏季风区和印度夏季风区石笋δ18O记录在不同时间尺度上的相互关系和主控因素仍存在一些争议。针对这一问题，本文综述了东亚夏季风（EASM，East Asian Summer Monsoon）区和印度夏季风（ISM，Indian Summer Monsoon）区的高分辨率石笋δ18O记录，发现两者在轨道尺度上都受控于岁差驱动的北半球夏季日照量的变化，其机制涉及海陆热力差异强度、辐合强度和上升气流强度以及夏季时长的变化。这些机制共同导致了在北半球夏季日照高值期EASM和ISM区夏季降雨量增加以及降雨和石笋的δ18O值负偏。在千年尺度上，EASM区和ISM区的石笋δ18O记录则响应于北大西洋区的气候变化，具体响应机制主要有热带辐合带南移导致的印度洋水汽分馏减弱（主要影响ISM区和EASM区），以及西风带调节的水汽来源和降雨季节性变化（主要影响EASM区）。在百年及以下尺度，EASM区和ISM区石笋δ18O记录都受到与厄尔尼诺-南方涛动有关的大尺度大气环流影响。未来在关键区域构建更多高分辨率的石笋δ18O记录及相关的降雨指标，将有助于理解大气环流变化与东亚夏季风区降雨的关系。
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关键词:
- 石笋氧同位素 /
- 东亚夏季风 /
- 印度夏季风 /
- 北半球日照 /
- Heinrich事件
Abstract:The Asian summer monsoon is an important part of the global climate system and a hot issue of the earth system science. The variation of the Asian paleo-monsoon revealed by δ18O data from the Asian stalagmites deepens our understanding of the mechanisms of its spatiotemporal evolution. However, the main controlling factors of the stalagmite δ18O records in the East Asian and Indian summer monsoon regions in different time scales remain controversial. Aiming at this problem, we reviewed the high-resolution stalagmite δ18O records in the East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) regions and revealed that they are in response to the variation in the Northern Hemisphere summer insolation driven by the precession on orbital scale. The specific mechanism involves the variation of zonal sea-land thermal contrast, seasonal humidity changes and cycles in low-level convergence, and the variation of summer length. These mechanisms jointly lead to increased summer rainfall with depleted δ18O in the EASM and ISM regions during high Northern Hemisphere summer insolation. On millennial scale, the stalagmite δ18O records in EASM and ISM regions are in response to the abrupt North Atlantic climate changes consistently, by mainly the weakening of water vapor fractionation in the Indian Ocean due to southward migration of the intertropical convergence zone (mainly influenced the ISM and EASM regions) and seasonal variation of water vapor sources and rainfall amount regulated by the westerly (mainly influenced the EASM region). Finally, at the centennial or shorter scale, the stalagmite δ18O records in both EASM and ISM regions are influenced by large-scale atmospheric circulation associated with EI Niño-Southern Oscillation. In the future, the development of more high-resolution stalagmite δ18O and other rainfall indicators records in key areas would facilitate our understanding of the relationship between atmospheric circulation and rainfall changes in the Asian summer monsoon region.
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西湖凹陷勘探面积逾5万km2，古近系的花港组、平湖组以及宝石组地层巨厚，具有良好的油气勘探前景，是我国天然气勘探的重要战场[1-2]，其中平湖组既是主力烃源岩发育层系，又是重要的勘探目的层，油气勘探潜力巨大。有大量学者对西湖凹陷平湖组储层的古地理背景进行了研究，总体认为平湖组沉积时期处于海陆过渡相的半封闭海湾背景，以广泛分布的薄煤层为特色[3-10]。西湖凹陷始新统平湖组以砂岩、泥岩、碳质泥岩、薄煤层互层为特征，由于平湖组分布面积广，古地貌背景差异大，沉积水动力条件复杂，不同部位与不同层序的沉积构造与沉积微相类型多样，导致不同学者对不同地区平湖组的砂体成因一直存在争议，严重影响了平湖组的储层评价预测与勘探开发部署。
近年来，平湖组的油气勘探主要集中在平湖构造带北部，对不同钻井平湖组沉积相划分存在一定分歧，其争议主要集中于平湖组的沉积作用是以河流作用为主，还是以潮汐作用为主。有些学者综合微体古生物、地球化学、岩芯、录井、地震等资料分析认为，西部斜坡平湖构造带平湖组砂体为辫状河三角洲成因，以河流作用为主，局部存在短期的海侵，受潮汐作用的影响较大 [11]；煤层含硫量高，薄且分散，与近岸潟湖沉积类似[9]；平湖组中代表淡水环境的盘星藻等数量较多，而代表海洋沉积的钙质超微化石等古生物少见，平湖组沉积期以淡水沉积环境为主[12]；纵向上平湖组早中期潮汐作用影响大，晚期发育受潮汐影响较弱[13-14]。前人基于沉积构造，结合测井、分析化验数据及地震等资料的综合分析，认为平湖构造带主要发育受潮汐影响的三角洲－潮坪相沉积，发育三角洲与潮坪两大类沉积体系[15-17]；通过频谱分析等方法，分析了平湖构造带平湖组潮汐韵律与潮汐周期[18]；基于平湖构造带平湖组岩芯、测井及分析化验资料，建立了平湖构造带平湖组海侵体系域潮控三角洲-潮坪沉积特征及模式。
尽管不同学者所用资料不同，对西湖凹陷平湖组沉积背景存在不同认识，但都可以归纳到半封闭海湾海陆交互相的古地理背景中[19-24]。由于河流与潮汐相互作用复杂，潮汐影响范围广，潮汐作用对三角洲的改造程度差异较大，导致目前还没有统一的潮汐影响三角洲的相模式[25-27]。现代的潮汐三角洲多位于中低纬度的河口湾附近，受潮汐作用改造，分支流河道加宽而成漏斗形，并形成一系列垂直岸线排列的线状砂体，如中国的长江三角洲、墨西哥湾的科罗拉多河三角洲、伊朗-伊拉克的底格里斯-幼发拉底河三角洲、西非卡萨芒斯河三角洲、印度-孟加拉恒河-布拉马普特拉河三角洲等[28]。潮汐改造三角洲的典型沉积标志包括双向交错层理、向岸倾斜的交错纹层、低角度砂质纹层中发育的泥质纹层（泥披盖）、交错纹层倾角频繁发生变化、脉状层理、波状层理、透镜状层理与生物扰动构造等。与典型的潮汐三角洲沉积特征相比，平湖构造带平湖组中除复合层理与生物扰动构造较常见外，其他典型的潮汐成因沉积构造少见，而曲流河的天然堤与前三角洲同样发育脉状层理、波状层理、透镜状层理等[28-32]，这也是西湖凹陷不同地区平湖组砂体成因分析存在争议的主要原因。
平湖组煤层具有厚度薄、层数多、分布散等特点，不同钻井中薄煤层的辨识度较低，测井相差异不明显，给砂体的岩相分析带来困难。目前平湖组沉积相的研究成果主要侧重于平湖组砂岩的沉积作用、水动力与沉积成因分析，对薄煤层与碳质泥岩的沉积学与岩石学分析较少。由于海上岩芯资料少，取芯井段短，很难覆盖平湖组一个完整的沉积旋回；煤与泥岩岩屑颗粒普遍较砂岩岩屑颗粒大，能够保存岩石的微观结构与矿物学特征，纵向上能够连续取样。开展煤与泥岩的单岩屑颗粒微观沉积学分析，可以丰富平湖组的沉积相研究方法，有效解决钻井岩石学分析中样品不足的难题，为井下潮坪交互相的沉积学分析提供新的地质信息。
本文从岩芯与泥岩岩屑的微观岩石学分析出发，对平湖构造带北部平湖组中碳质纹层与菱铁矿进行成因分类，提出了基于砂岩、煤与碳质泥岩微观岩石学特征的“三煤三铁”水动力分析与古环境分析方法，明确细粒沉积物及其岩屑颗粒微观岩石学分析对砂岩成因及岩相划分、砂体微相编图与预测砂体的辅助作用。
1. 地质背景
西湖凹陷位于东海陆架盆地东部的浙东坳陷内，东邻钓鱼岛隆褶带，西部以海礁凸起、渔山凸起为界，是一个呈NNE向展布的古近系含油气凹陷，面积约5.9×104 km2。研究区平北地区位于西湖凹陷平湖斜坡带北部（图1），是西湖凹陷重要的油气聚集区。近年来，平北地区的天然气勘探开发不断获得新突破，为平湖组沉积背景分析提供了新的地质信息。
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图 1 西湖凹陷构造单元与平湖构造带北部构造位置Figure 1. The structural units of Xihu Sag and the structural position of the northern part of the Pinghu Slope西湖凹陷新生代主要经历了6次构造运动，分别是雁荡运动（T100）、瓯江运动（T40）、平湖运动（T30）、花港运动（T20）、龙井运动（T12）和冲绳海槽运动（T0），形成了凹陷内5个区域性不整合面，将西湖凹陷新近系自下而上分为五大构造层系（表1）：裂陷构造层（T100—T40）、断拗转换构造层（T40—T30）、拗陷构造层（T30—T20）、反转构造层（T20—T12）和区域沉降构造层（T12—T0）。平湖组位于断拗转换构造层，是本文研究的目的层系。
表 1 西湖凹陷新生代地层层序与演化阶段Table 1. Table1 The Cenozoic stratigraphic sequence, tectonic evolution stage of the Xihu Sag
平湖组沉积期西湖凹陷处于断拗转换期，平北地区构造活动强烈，NE-NEE向生长断层持续活动，断层多断穿至平湖组顶界，且呈断阶状组合样式[31-33]。复杂多变的沉积水动力与有利的成煤环境使得平北地区平湖组沉积了一套含薄煤层的砂泥层序。目前，对于平湖斜坡带平湖组的沉积相划分主要依据西湖凹陷平湖组海陆过渡相的沉积背景，如三角洲、潮坪与潟湖等，对于平湖斜坡带的古地貌特征以及与西湖凹陷其他地区沉积背景与沉积作用的差异性分析还没有系统展开，砂体成因分析存在模式化与碎片化等问题。
平湖斜坡带油气勘探已进入构造岩性圈闭勘探开发阶段，需要对砂体成因与微相类型进行精细研究，但由于取芯井较少，取芯进尺有限，揭露的沉积学信息不足以支撑详细的微相类型划分与砂体分布规律预测研究。在海上油气田岩芯资料较少的情况下，本文通过泥岩、碳质泥岩、薄煤层等细粒沉积物岩屑样品的微观岩石学分析，弥补砂岩岩芯资料少与沉积学信息碎片化的缺陷，总结不同沉积环境下的泥岩、碳质泥岩与煤层的岩石学特征、沉积过程与沉积物搬运方式，进一步厘清平湖斜坡带平湖组的沉积模式，为西湖凹陷平湖组的沉积背景分析提供新的信息。
2. 平湖组储集砂体发育特征
多数文献认为西湖凹陷平湖组总体上为海陆过渡相沉积[4-8, 23-24]，但不同构造带的古地理环境与沉积特征差异较大，有些地区以河流作用为主[11, 34-36]，有些地区则处于潮汐作用为主的潮坪环境[16, 19]。平湖组沉积时期，西湖凹陷西缓坡带以河流作用为主，发育物源来自虎皮礁隆起、海礁隆起—渔山隆起的三角洲-陆棚体系；西湖凹陷东陡坡带，以阵发性水流沉积为主，发育物源来自钓鱼岛隆褶带的扇三角洲或近岸水下扇砂砾质-陆棚体系；西部斜坡带三角洲体系外缘，受潮汐影响较大，发育线状潮汐沙脊砂体；西次洼、中央反转构造带与东次洼以浅水陆棚的潮汐作用为主[7-8]。平湖斜坡带处于西湖凹陷西部斜坡带，靠近凹陷西部的海礁隆起与渔山隆起，其外侧还发育宝云亭古隆和宁波14-5古隆，受河流影响较大，斜坡带内发育宁波19洼与宁波8洼沉降中心等地貌单元。
从岩性组合上看，平湖斜坡带平湖组以砂泥互层组合为主，夹多个薄煤层。平湖斜坡带平湖组普遍发育箱形、钟形、指状测井相的砂体，底部发育冲刷面，冲刷面上有滞留沉积的砂砾岩，砂体中发育特殊的菱铁矿纹理交错层理，具有典型水道砂体的正旋回序列（图2）。上述这些砂体中，典型的羽状交错层理少见，与砂岩互层的粉砂岩、泥质粉砂岩中普遍发育复合层理与生物扰动构造，其潮汐作用或潮汐改造的典型标志还不明确。
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图 2 KB8井平湖组岩性组合及其典型沉积构造与菱铁矿类型Figure 2. Lithologic association, typical sedimentary structure, and siderite type of the Pinghu Formation in Well KB8KB8井平湖组主要岩性为箱状或钟形砂岩与深灰色泥岩互层，砂岩中发育底砾岩，含菱铁矿交错纹层。平湖组砂岩中普遍发育交错层理，交错层理中普遍发育菱铁矿纹层，而在背闪射图片中，菱铁矿呈薄膜状覆盖在石英或长石碎屑颗粒表面，体现了强水动力条件下的菱铁矿特征。与菱铁矿交错层理砂岩互层的泥岩中普遍发育菱铁矿结核，表明研究区水体中菱铁矿含量较高，指示相对封闭水介质环境[37]。平湖组泥岩中普遍富含碳质，依据碳质含量可分为含泥薄煤层、碳质泥岩与粉砂质纹层泥岩，碳质表明陆源有机质供给丰富，并受河水的搬运与改造。平湖组砂岩以块状正旋回砂岩为主，为多期水道砂体叠置的复合水道砂砾岩与砂岩。平湖斜坡带平湖组地层厚度变化不大，砂岩以低角度前积反射为特征，表明沉积坡度平缓（图3）。
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图 3 平湖斜坡北段平湖组的地震反射特征Figure 3. Seismic reflection profiles of the Pinghu Formation in the northern section of the Pinghu Slope平湖斜坡带不同部位砂岩的粒度差异较大，但其测井相多以箱形与钟形为主，与砂岩互层的薄煤层的测井响应差异小。平湖斜坡带砂岩岩芯少，取芯井段长仅几米，而且较分散，典型的羽状交错层理等典型潮汐作用的沉积构造少见，局部粉砂岩中见脉状层、波状层理、透镜状层理等复合层理类型，目前主要依据沉积模式将最大海泛面附近的P7砂组中的中厚层泥岩划分为潮坪沉积。P7砂组以泥岩与薄煤层为主，夹薄层粉砂岩，厚层砂体不发育，目前普遍认为其是最大海泛面附近的沉积产物[16,18-19]。平湖组中厚层箱形与钟形砂岩是以河流作用为主还是以潮汐作用为主，是目前岩相分析中的难点难题。因此，需要通过对砂岩互层泥岩、碳质泥岩及煤层微观岩石学特征差异性的对比分析，揭示研究区不同地区沉积作用的差异性，为砂体岩相划分与成因评价提供新的依据。
3. 泥岩与煤层的微观岩石学分析
研究区平湖组薄煤层与泥岩的岩屑颗粒由于具有较强的韧性，普遍比砂岩的岩屑颗粒大，并保留了原始岩石结构（图4），可以用于制作岩石薄片，为泥岩与薄煤层岩屑颗粒的微观岩石学分析提供了可能。通过岩芯-岩屑一体化、砂泥一体化分析，提高样品的覆盖率与控制面，可以解决钻井条件下沉积学分析中样品取样难的问题。由于粒度小、观察难度大以及受实验条件的限制，占沉积记录2/3的细粒物质的沉积特征及成因等问题成为沉积学界乃至地质学界的薄弱研究领域。由于研究区岩芯资料有限，取芯井段短，难以覆盖完整的沉积序列，平湖组砂体成因主要是依据西湖凹陷总的沉积模式与零星的岩芯展开，导致平湖斜坡带沉积学研究普遍存在碎片化与模式化的问题。论文拟采用岩芯-岩屑一体化、砂-泥一体化的技术思路，对重点层段进行密集的泥岩与薄煤层岩屑取样，增加样品的控制密度，展开详细的沉积学分析，以建立连续样品控制的垂向沉积演化序列，并对细粒沉积物单岩屑颗粒的沉积学分析进行了探索。目前普遍采用PDC钻井技术，导致岩屑颗粒细小，不利于岩石微观结构分析。通过研究区平湖组细粒沉积物（泥页岩）岩屑颗粒的不同尺度微观结构分析，表明煤层、泥岩的单颗岩屑能够保存原始的岩石结构，结合泥岩碎屑颗粒排列方式、粒度分布、典型金属硫化物、有机质类型、黏土矿物类型等分析实验，能够提供大量的沉积学与地球化学信息，辅助解决砂岩沉积作用中的难点问题。
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图 4 KB8井平湖组典型薄煤层与泥岩岩屑图片Figure 4. Typical pictures of coal seams and mudstones debris in the Pinghu Formation3.1 岩屑挑选与制样
研究区岩芯资料较少，论文对重点层段展开重点井岩屑的挑样与分析工作。通过岩屑样品的预处理分析，发现泥岩、碳质泥岩与煤层的岩屑颗粒普遍比砂岩岩屑大，岩屑颗粒完全可以满足普通光学薄片、电镜的制样要求，而且透射光、电子显微镜的微观岩石学特征能够揭示岩石结构与沉积学方面的大量信息，为井下沉积学分析提供了新的思路，可以有效解决井下沉积学分析中样品少的难题。采样与样品预处理是煤层与泥页岩屑颗粒微观岩石学分析的关键，需要严格岩屑颗粒的挑选与预处理，最大限度地减少泥浆、岩屑混合、掉渣对样品质量的影响。泥岩岩屑样品现场采集、室内预处理、精细定位制样与砂岩样品差异较大，沉积矿物与隐晶质矿物识别是细粒沉积物测试分析中的关键环节，分析流程如下：（1）岩屑分类与岩性归位；（2）现场岩屑的鉴别、描述与取样，尽量减少混样、掉渣与泥浆的影响；（3）室内精细处理：为了防止岩屑破裂，不能用淡水冲洗，只能用毛刷剥离岩屑颗粒表面泥浆；（4）精细制片与定位制样：盐水磨片、典型纹层、矿物等的精细定位制样；（5）微观岩石学观察：注意碎屑颗粒排列、隐晶质矿物（碎屑黏土）与金属硫化物识别。
3.2 微观岩石学分析与“三煤三铁”划分
通过平湖斜坡带平湖组岩屑的微观岩石学特征与菱铁矿形态特征分析，结合岩芯中的宏观沉积构造，将研究区的菱铁矿划分为3种类型：交错层理纹层状菱铁矿、结核状菱铁矿、纹层状与凝胶状条带菱铁矿（图5）。平湖组碳质泥岩中的菱铁矿可能主要由同生期成岩过程中的生物异化铁还原作用（DIR）形成，也可能出现在砂岩的交错纹理中。澳大利亚、西非铁矿主要赋存在太古代—元古代碳酸盐岩中，其中菱铁矿主要为泥晶与微晶菱形体，它的形成与生物作用紧密相关，是从海水中直接沉淀而形成，其沉积背景主要是弱氧化-弱还原的浅水环境，通过生物作用将Fe3+在缺氧、低硫、富铁的较浅水海洋环境中发生反应析出FeCO3。菱铁矿广泛分布在湖泊沼泽环境中，除了DIR外，还有细菌硫酸盐还原反应（BSR）或甲烷厌氧氧化反应（AOM）参与到有机碳转换成无机碳的过程中[37-41]。
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图 5 平湖组三种典型菱铁矿的形态特征Figure 5. Morphological characteristics of three typical types of siderite in the Pinghu Formation交错层理纹层状菱铁矿对应分支流水道，结核状菱铁矿对应分支流间湾，纹层状菱铁矿与凝胶状菱铁矿和变形纹理的出现对应前三角洲和局限洼地。交错层理纹层状菱铁矿泥岩与碳质层纹层泥岩共生，该类泥岩与粉砂质薄煤层共生，煤层中含大量惰质组组分，表明水动力强度大，煤层受冲刷改造明显；结核状菱铁矿泥岩与碳质层纹层泥岩、薄煤层共生，泥岩中碳质纹层较发育，薄煤层较纯，但草莓状黄铁矿少见；纹层状菱铁矿、凝胶状菱铁矿与碳质泥岩共生，主要分布在宁波19洼内部，泥岩中碳质纹层丰富，薄煤层含藻质体等水生显微组分，富含草莓状黄铁矿（图6）。通过微观岩石学分析，将平湖组煤及碳质泥岩划分为3种类型，分别与菱铁矿3种类型相对应，形象称为“三煤三铁”，为研究区细粒沉积物的岩相划分提供了新的线索。
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图 6 平湖组典型煤层的显微岩石结构特征与硫化物a：草莓状黄铁矿与薄煤层，KB1井，P8砂组，背闪射；b：泥质煤中草莓状黄铁矿，KB1井，P8砂组，背闪射；c：泥质与碳质互层，KB1井，P8砂组，背闪射；d：颗粒表面的菱铁矿包壳，KB8井，P5砂组，背闪射；e：碳质粉砂岩，KB8井，P5砂组，背闪射；f：碳质纹层，KB9井，P3砂组，背闪射。Figure 6. Microstructures and sulfide types in typical coal seams of the Pinghu Formationa: Strawberry-shaped pyrite and coal seam in P8 sand unit of Well KB1; back flash; b: strawberry-shaped pyrite in argillaceous coal in the P8 sand unit of Well KB1; back flash; c: argillaceous and carbonaceous interbed in the P8 sand unit of Well KB1, back flash; d: siderite cladding on particle surface in the P5 sand unit of Well KB8; back flash; e: carbonaceous siltstone in the P5 sand unit of Well KB8; back flash; f: carbonaceous lamina in the P3 sand unit of Well KB9; back flash.4. 煤与碳质泥岩的古环境意义
研究区平湖组中煤层普遍较薄并分散，平均厚度1～3 m。单煤层的地震预测难度大，如果把含薄煤层地层单元作为一个整体，称之为含煤层系，在地震上可以预测：当厚层的含煤层系与中厚层砂岩互层时，其地震反射特征表现为低频连续强反射。同时，通过“三煤三铁”的古环境分析，明确了不同类型薄煤层的成因与古环境意义，有效区分了三角洲平原、三角洲前缘与洼陷等亚环境薄层、菱铁矿微观岩石结构的差异性，为潮河联控复杂水动力条件下优势相带识别提供了地质依据，为薄煤系地层单元的沉积构型分析提供了新的思路，为平湖组不同沉积相类型中砂体成因解释与薄煤层的区分提供了新的依据。
4.1 泥岩与煤层的岩相分析
通过对平湖斜坡带平湖组薄煤层的微观岩石学分析，发现研究区薄煤层除少数为纯煤层外，绝大多数为含泥煤层、含粉砂质纹层煤层和碳质泥岩与碳质粉砂岩（图7）。表明煤层受到分支流水道频繁冲刷，薄煤层往往与碳质泥岩、碳质粉砂岩一起构成“煤系层系”，多位于分支流水道砂体之上，代表分支流水道改道，或三角洲废弃后的产物。
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图 7 平湖组典型煤层的显微岩石结构特征a：纯煤层，KB3井，P9砂组，单偏光；b：纯煤层，含草莓状黄铁矿，KB3井，P9砂组，反射光；c：泥质煤，KB3井，P7砂组，单偏光；d：粉砂质纹层煤，KB2井，P4砂组，单偏光；e：粉砂质纹层煤，KB8井，P4砂组，单偏光；f：碳质条带，KB7井，P9砂组，单偏光。Figure 7. Microstructures of typical coal seams in the Pinghu Formationa: Pure coal seam in P9 sand unit of Well KB3; single polarized light; b: pure coal seam, strawberry-shaped pyrite in P9 sand unit of Well KB3; reflected light; c: argillaceous coal in P7 sand unit of Well KB3; single polarized light; d: silty laminated coal in P4 sand unit of Well KB2; single polarized light; e: silty laminated coal in P4 sand unit of Well KB8l; single polarized light; f: carbon strip in P9 sand unit of Well KB7; single polarized.平湖斜坡北段宁波19洼内的煤层通常较纯，或含泥质，碳质泥岩内含丰富的草莓状黄铁矿与重晶石，粉砂质含量较低，受分支流水道的影响较弱。宁波19洼碳质泥岩中还见到碳屑团块与旋转纹理，是陡坡背景下的准同生滑塌变形产物（图8）。平湖组沉积早期，断裂活动强度大，宁波19洼水体较深，部分达到浪基面之下，充填薄煤层与碳质泥岩，为前三角洲—局限浅海沉积。KB1井P8砂组见重力断层、旋转纹理的碳质泥岩与泥质粉砂岩，为典型的滑塌变形构造，表明其为沉积陡坡。同时在上述旋转纹理与重力断层内见纹层状的草莓状黄铁矿与碳质纹层互层，表明其为水体相对较深的还原环境。
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图 8 KB1井泥岩中典型的滑塌变形构造与菱铁矿a：重力断层P8砂组，b：旋转纹层。Figure 8. Typical structure of slumping deformation and siderite in mudstone of Well KB1a: Gravity fault in P8 sand unit, b: rotated laminae.通过薄煤层与泥岩岩屑的微观岩石学与沉积作用分析可以弥补砂岩取芯段短、零星分布的局限性。通过砂泥岩一体化分析，能够为砂岩沉积作用与成因分析提供更丰富的古地理信息。KB3井位于宁波19洼内，平湖组沉积早期P9砂组发育三角洲分支流水道前缘箱形砂岩，与砂岩互层的泥岩岩屑中富含碳质纹层，碳质泥岩中见到大量草莓状黄铁矿，表明宁波19洼受到陆源碎屑供给的影响较大，同时还存在相对封闭的静水环境；平湖组沉积晚期P3砂组泥岩中见到丰富的植干化石碎片，碎片呈长条状分布在碳质泥岩中，表明宁波19洼已被浅沼泽化，碳质沉积后还受到后期水流的改造（图9）。
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图 9 KB3井单井相与泥岩岩屑颗粒的微观岩石学特征Figure 9. Microfacies and micro-petrological characteristics of mudstone debris in Well KB34.2 泥岩与煤层的古地理背景探讨
通过分析细粒沉积物碎屑黏土矿物，可以有效地揭示砂岩沉积的水介质环境。海水介质主导的细粒沉积物中，泥岩中碎屑黏土矿物以伊利石、绿泥石为主；以淡水介质主导的细粒沉积物中，泥岩中碎屑黏土矿物以高岭石为主。分支流间湾泥岩中，富含植物碎片与碳质；而潮坪泥岩中，植物碎片与碳质含量较低。相对封闭水体中，如潟湖、分支流间湾，隐晶质的菱铁矿含量较高，富含草莓状黄铁矿与重晶石；而在开放潮坪环境，或开放的浅海背景中，以海绿石为主。平湖斜坡区由于斜坡外围宝云亭古隆、NB14-5古隆，以及反向断阶等的分割作用，研究区与西湖凹陷海湾区处于半隔离状态，使得研究区平湖组沉积时期处于一个相对封闭水介质环境，具有典型淡化潟湖的沉积特点。平湖斜坡北段平湖组泥岩的黏土矿物以高岭石为主，混合少量伊利石，体现了淡水主导近海潟湖的水介质特点（图10）。现有钻井揭示的沉积构造中，与潮汐作用相关的典型沉积构造不多。因此，平湖组沉积时期，尽管西湖凹陷整体处于海湾背景，但由于平湖斜坡带靠近海礁隆起与渔山低隆起，受陆源供给的影响大，水介质明显淡化，再加上外围宝云亭等古隆的限制作用，海水对陆源碎屑的搬运改造不明显（图11）。
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图 10 KB3井平湖组碳质泥岩中的黏土矿物能谱图a：碳质泥岩的背散射图片，X1为碳质纹层，X2为黏土质纹层；b：黏土质纹层的能谱。Figure 10. Energy spectrum diagram of clay minerals in carbonaceous mudstone of the Pinghu Formation in Well KB3a: The backscatter image of carbonaceous mudstone; X1: carbonaceous laminae, X2: clay laminae; b: energy spectrum of clayey laminae.![]()
图 11 平湖斜坡北段平湖组沉积模式图Figure 11. Sedimentary model of the Pinghu Formation in the northern section of the Pinghu Slope在大量岩屑颗粒微观岩石学分析基础上，辅助岩芯观察，新发现了交错层理纹层状、结核状、纹层状3种类型的菱铁矿，以及碳质粉砂岩、粉砂质纹层煤、泥岩煤和纯煤层3类薄煤层，分别对应不同的古地理背景（表2），揭示了三角洲平原分支流水道夹层煤、分支流间湾粉砂质纹层煤及局限洼地薄煤层的差异性，反过来为互层的箱状、钟形砂体的岩相划分提供了新线索。这些碳质泥岩中富含完整的植物碎片与菱铁矿，表明平湖斜坡带平湖组沉积时期海水对泥岩的改造较弱，海水相对局限，古地理背景为相对封闭的潟湖环境。
表 2 平湖组泥岩与煤层的微观岩石学类型及古地理指示意义Table 2. Paleogeographic implications of micro-petrology of mudstone and coal seams in the Pinghu Formation菱铁矿与煤层、碳质泥岩类型 微观岩石学特征 古环境 粉砂质纹层泥岩与薄煤层、菱铁矿交错纹理层理 泥质、碳质纹层与粉砂质纹层互层，
长英质碎屑颗粒表面覆盖菱铁矿膜天然堤与分支流水道砂体 富硫化物薄煤层、结核状菱铁矿泥岩 纯的薄煤层、含菱铁矿结核的碳质泥岩 分支流间湾静水沉积环境 富含草莓状黄铁矿薄煤层与碳质泥岩、纹层状菱铁矿 碳质纹层与菱铁矿纹层交互，富含草莓状黄铁矿 浪基面下静水潟湖环境 煤与泥岩中的微古生物组合也能揭示古气候、陆源供给强度与沉积水介质条件。平湖组泥岩、碳质泥岩与煤层的微体化石组合中，孢粉占绝对优势，浮游藻类在组合中所占比例很小，陆源有机质供给占绝对优势。浮游藻类中既有淡水藻类，如盘星藻、环纹藻等，也有海相沟鞭藻；盘星藻含量总体较少，主要分布在平湖组沉积晚期；海相沟鞭藻的总体含量很低，分布较为局限，主要见于平湖组的中、下部[3]。上述微体生物化石组合表明，平湖斜坡带平湖组的海陆过渡环境位于亚热带，发育以河流沉积作用为主导的局限海水背景，且陆源供给丰富[12]。大型沼泽环境与厚煤层需要稳定古地貌环境，地下水位突然升高或下降与海侵等都会影响沼泽的发育，脉动式的频繁海侵不利于沼泽及其植被的稳定发育，因此平湖组薄煤层体现了频繁的海平面升降或洪水侵蚀作用[35-36]。薄煤层与暗色泥岩有机质显微组分包括结构镜质体、均质镜质体、基质镜质体、孢粉体、树脂体、丝质体等，煤层沉积序列主要有障壁-潟湖、三角洲平原分流间洼地及三角洲前缘分流间湾，其中三角洲含煤序列是本区的主要含煤序列。西湖凹陷西部斜坡带平湖组的岩相、粒度、古生物、地球化学以及测井相等综合研究表明，平湖斜坡北段平湖组含煤岩系应为陆相淡水为主的沉积体系，其沉积时期水体较浅，水体具有振荡的特征，煤岩系以还原环境下的湖泊-沼泽相沉积为主，局部区域存在潮汐影响。煤系烃源岩的分子组成以陆源芳烃化合物为主，Pr/Ph比值高达3.5～8.5，指示其形成于弱氧化的沼泽环境[40-41]。
综合平湖斜坡带最新勘探成果，结合碳质泥岩和煤层的煤岩学与微观岩石学特征，建立了平湖斜坡带平湖组的沉积模式（图11）：平湖斜坡带平湖组沉积时期发育近海局限潟湖，主要发育3大物源供给方向，包括孔北轴向物源、宝云亭西轴向物源和武云亭-来鹤亭-孔雀亭一线径向物源。主物源供给对研究区砂体的控制作用也十分明显，主水系与输导方向决定了沉积区砂体规模和砂体展布的差异。轴向物源供给方向有利于大型三角洲的发育，砂体规模较大；而水系规模较小的径向物源区通常对应的三角洲规模也较小；碳质泥岩与薄煤层主要分布在相对封闭的分支流间湾与潟湖局限洼地。
5. 结论
海上油气田钻井岩芯资料少，一直存在样品取样难的问题，难以展开系统的沉积学样品分析，沉积学认识上存在碎片化与模式化的缺陷。本文采用岩芯-岩屑一体化、砂-泥一体化的技术思路，对重点层段进行密集的岩芯-岩屑采样与沉积学分析，建立了连续样品覆盖的垂向沉积演化序列。尽管目前普遍采用PDC钻井技术，导致岩屑颗粒细小，但泥页岩的岩屑颗粒相对较大，通过岩屑颗粒不同尺度微观结构分析，可以揭示岩石微观结构特征与沉积学信息，如泥页岩中碎屑颗粒排列方式、粒度分布、典型金属硫化物、有机质类型、黏土矿物类型等，能够提供大量的沉积学与地球化学信息，辅助解决砂岩沉积作用分析中的难点问题。
（1）薄煤层与泥岩的微观岩石学特征表明，平湖组沉积时期平湖斜坡带处于淡化的潟湖环境，泥岩中黏土矿物以高岭石为主，储集砂体主要为分支流水道砂体，沉积作用以河流作用为主。与前人海陆过渡与河潮交互的认识对比，论文研究成果更突出了平湖斜坡带平湖组河流作用的主导性，强调了西湖凹陷平湖组海陆过渡背景框架下不同地区沉积作用的差异性，为西湖凹陷西部斜坡带近岸古隆背景下的沉积相研究提供了新的研究案例。
（2）细粒物的微观沉积学研究不仅有着重要的沉积学意义，而且有助于更好地精细评价与预测烃源岩，可以有效解决我国近海井下沉积学分析中样品少的难题。泥页岩与煤的微观沉积学分析方法不仅可以有效解决生产中岩芯资料有限的局限性，充分挖掘岩屑资料的研究潜力，还可以辅助解决砂岩水动力与沉积成因分析、缓坡浅水背景的沉积微相编图等难点问题。通过煤层、泥岩岩屑颗粒的微观岩石学分析，探索了砂-泥一体化的沉积相分析技术方法，解决了海上沉积学分析中样品难的问题，增加了沉积序列样品的控制密度，建立了平湖构造带北部平湖组近海潟湖的沉积模式。
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图 2 1981—2020年1、4、7月和10月平均的垂直综合水汽输送图
紫色、绿色和蓝色的点分别表示三宝洞、葫芦洞和董哥洞的位置。
Figure 2. Average vertical synthesis of atmospheric moisture flux in January, April, July, and October from 1981 to 2020
The purple, green, and blue dots indicate the locations of Sanbao Cave, Hulu Cave, and Dongge Cave, respectively.
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图 3 东亚和南亚石笋δ18O与北半球夏季日照量
a：石笋δ18O数据合成自三宝洞、葫芦洞和董哥洞[10], b：Bittoo洞石笋δ18O数据[18]。 橙色线为7月21日北半球65°N的日照量变化[10,18]。
Figure 3. Stalagmite δ18O records in eastern and southern Asia, and the Northern Hemisphere summer insolation
a: Composited stalagmite δ18O data synthesized from Sanbao Cave, Hulu Cave, and Dongge Cave[10]; b: stalagmite δ18O data of Bittoo Cave[18]. The orange lines show the variation in insolation at 65°N in the Northern Hemisphere on July 21.
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图 4 岁差驱动北半球热带降雨变化的两种机制示意图[61]
a：棕色区域表示陆地，蓝色区域表示海洋；黑色线代表平均径向环流，较粗的线表示较高的输送强度；b：蓝色阴影表示大气湿度，较深的阴影表示较大的湿度。蓝色雨滴代表降水，雨滴越多表示降水越多。
Figure 4. Mechanisms of precession driving tropical rainfall change in the Northern Hemisphere[61]
a: Brown areas indicate land and blue areas indicate sea; the black lines represent the mean meridional circulation; the thicker lines indicate higher transport intensity; b: blue shadows indicate atmospheric humidity, and darker shadows indicate greater humidity; Blue raindrops represent precipitation, and more raindrops mean more precipitation.
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图 5 末次冰期亚洲季风区洞穴石笋δ18O记录所在位置图
Xl：兴隆洞[17]，Dr：Dragon洞/龙洞[16]，Zz：珍珠洞[65]，Sj：宋家洞[66]，Hl：葫芦洞[48]，Sb：三宝洞[9]，Yx：永兴洞[15]，Bi：Bittoo洞[18]，Yz：扬子洞[67]，Fr：芙蓉洞[68]，Yk：羊口洞[69-70]，Sx：三星洞[71]，Wl：Wulu/五路洞[72]，Qx：七星洞[23, 73]，Maw：Mawmluh洞[74]，Dg：董哥洞[49]，Xs：响水洞[75]，Xy：仙云洞[19-20]，Xbl：小白龙洞[76]，Fy：风雨洞[77]。
Figure 5. Locations of stalagmite δ18O records during the last glacial period from different caves in the Asian monsoon region
Xl: Xinglong Cave[17], Dr: Dragon Cave[16], Zz: Zhenzhu Cave[65], Sj: Songjia Cave[66], Hl: Hulu Cave[48], Sb: Sanbao Cave[9], Yx:Yongxing Cave[15], Bi: Bittoo Cave[18], Yz: Yangzi Cave[67], Fr: Furong Cave[68], Yk: Yangkou Cave[69-70], Sx: Sanxing Cave[71], Wl: Wulu Cave[72], Qx: Qixing Cave[23, 73], Maw: Mawmluh Cave[74], Dg: Dongge Cave[49], Xs: Xiangshui Cave[75], Xy: Xianyun Cave[19-20], Xbl: Xiaobailong Cave[76], Fy: Fengyu Cave[77].
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图 6 亚洲季风区石笋δ18O记录的末次冰期千年尺度事件波动
a: NGRIP表示北格陵兰冰芯的δ18O记录[78]，b-h:各个洞穴数据的引用文献与图5中相同。 浅黄色条带指示了Heinrich事件1-7和Younger Dryas事件的持续时间[48,79]。
Figure 6. Millennial-scale events during the last glacial period recorded by stalagmite δ18O in the Asian monsoon region
a: NGRIP represents the δ18O record of the North Greenland ice core[78], b-h: citations of the references are the same as in Fig. 5. The light yellow bands indicate the duration of Heinrich stadials 1-7 and Younger Dryas stadials[48,79].
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