施振生, 王红岩, 赵圣贤, 周天琪, 赵群, 祁灵. (2023). 川南地区上奥陶统—下志留统五峰组—龙马溪组快速海进页岩特征及有机质分布* [J]. 古地理学报, 25(4): 788-805.
SHI Zhensheng, WANG Hongyan, ZHAO Shengxian, ZHOU Tianqi, ZHAO Qun, QI Ling. (2023). Rapid transgressive shale characteristics and organic matter distribution of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in southern Sichuan Basin,China[J]. Journal Of Palaeogeography, 25(4): 788-805.   
Doi: 10.7605/gdlxb.2023.04.065
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川南地区上奥陶统—下志留统五峰组—龙马溪组快速海进页岩特征及有机质分布*
施振生1,2, 王红岩1,2, 赵圣贤3, 周天琪1,2, 赵群1,2, 祁灵1,2
1 中国石油勘探开发研究院,北京 100083
2 国家能源页岩气研发(实验)中心,河北廊坊 065007
3 中国石油西南油气田分公司页岩气研究院,四川成都 610051

第一作者简介 施振生,男,1976年生,高级工程师,博士生导师,主要从事细粒沉积学与储层地质学研究。E-mail: shizs69@petrochina.com.cn。
收稿日期:2023-01-23 修回日期:2023-06-17
基金资助:*中国石油天然气股份有限公司科技部“十四五”重大专项(编号: 2021DJ1901)项目资助
摘要

快速海进页岩的特征及形成机理是细粒储层地质学研究的重点之一。地震层位追踪、连井地层对比、 X衍射全岩分析、总有机碳测试和场发射扫描电镜分析表明,川南地区五峰组—龙马溪组快速海进页岩位于龙马溪组最底部,对应于笔石带 LM1。该页岩石英平均含量 49.3%(其中黏土级石英含量 85%),方解石平均含量 10.5%,白云石平均含量 8.4%,黏土矿物平均含量 23.4%。向盆地方向,石英含量增加,黏土矿物含量降低。快速海进页岩形成于相对海平面快速上升阶段,页岩厚度 0.5~2.8 m,由盆地边缘向盆地中心逐渐增厚。页岩平均 TOC含量 5.4%,由盆缘向盆地中心逐渐降低,纵向 TOC含量剖面呈现 4种叠置样式。该套页岩的矿物组成和厚度分布与沉积时期的快速海进、生物及火山作用密切相关。快速海进导致陆源供给减少,故页岩厚度较小; 生物及火山作用导致页岩以微晶石英为主,盆地中心厚度较大。该套页岩的高 TOC含量与水体缺氧、低沉降速率和高初级生产力有关。水体缺氧导致有机质保存能力增强,低沉积速率可减弱有机质的稀释,而初级生产力高可增加有机质的供给。该套页岩 TOC含量平面变化及叠置样式与水深有关。随着水深增加,有机质沉降过程中的降解和再循环增加,故 TOC含量降低。同时,随着水深增加,沉积物可容空间增大,从而形成不同的 TOC含量叠置样式。

关键词: 海进页岩; 分布模式; 富集机理; 五峰组; 龙马溪组; 川南地区
中图分类号:P512.2 文献标志码:A 文章编号:1671-1505(2023)04-0788-18
Rapid transgressive shale characteristics and organic matter distribution of the Upper Ordovician-Lower Silurian Wufeng-Longmaxi Formations in southern Sichuan Basin,China
SHI Zhensheng1,2, WANG Hongyan1,2, ZHAO Shengxian3, ZHOU Tianqi1,2, ZHAO Qun1,2, QI Ling1,2
1 PetroChina Research Institute of Petroleum Exploration and Development,Beijing 100083,China
2 National Energy Shale Gas R & D(Experiment)Center,Hebei Langfang 065007,China
3 PetroChina Southwest Oil & Gas Field Company,Shale Gas Research Institute,Chengdu 610051,China

About the first author SHI Zhensheng,born in 1976,senior engineer,doctoral supervisor,is engaged in fine-grained sedimentary geology and reservoir geology. E-mail: shizs69@petrochina.com.cn.

第一作者简介 施振生,男, 1976年生,高级工程师,博士生导师,主要从事细粒沉积学与储层地质学研究。 E-mail: shizs69@petrochina.com.cn。

Fund:Financially supported by the 14th Five-Year Plan of the Ministry of Science and Technology of PetroChina(No.2021DJ1901)
Abstract

The characteristics and formation of the MF(maximum flooding)black shale are one of the focuses of fine-grained reservoir geology research. Seismic interpretation,well correlation,X-ray diffraction whole rock analysis,total organic carbon(TOC)test and field emission scanning electron microscope analysis show that the MF black shale of the Longmaxi Formation in the southern Sichuan Basin is in the basal part of the Longmaxi Formation,corresponding to the graptolite belt LM1. The shale has an average content of 49.3% quartz(85% clay-sized),10.5% calcite,8.4% dolomite and 23.4% clay minerals. The quartz content increases while the clay mineral content decreases basinward. The MF black shale formed during the stage of rapid relative sea level rise,with a thickness of 0.5-2.8 m and gradually thickening basinward. The average TOC content is 5.4%,which exhibits a gradual decrease towards the basin and forms four distinct stacking patterns in the vertical TOC content profile. The mineral composition and thickness distribution of the shale are closely related to the rapid transgression,biology and volcanism during the sedimentary period. The rapid transgression has led to a decrease in terrestrial sediment input,resulting in a reduction in shale thickness. Additionally,biological activity and volcanic influences have resulted in a prevalence of microcrystalline quartz and an increase in shale thickness towards the basin. The high TOC content of this shale is related to anoxic water,low sedimentation rate,and high primary productivity. Anoxic water body enhances preservation of organic matter. Low sedimentation rates can weaken the dilution of organic matter,while high primary productivity can increase the supply of organic matter. The planar variation and stacking style of TOC content in this set of shale are related to water depth. With increasing water depth,there is an amplified degradation and recycling of organic matter during sedimentation,leading to a decline in TOC content. Simultaneously,as the water depth rises,the sediment accommodation space also increases,resulting in distinct stacking patterns of TOC content.

Key words: transgressive black shale; distribution pattern; enrichment mechanism; Wufeng Formation; Longmaxi Formation; southern Sichuan Basin
1 概述

黑色页岩是指粒径小于62.5μ m的沉积岩, 其常形成于缺氧的底层水体中, 并聚集在深水盆地中(Demaison and Moore, 1980; Wignall and Newton, 2001)。黑色页岩在不同沉积环境均可发育。现代沉积环境包括大型受限盆地环境(如黑海和波罗的海)和开放环境(如受中层水氧最低值影响的秘鲁海岸、纳米比亚海岸和加利福尼亚湾)(Demaison and Moore, 1980; Arthur and Sageman, 1994)。其他环境包括热带堤岸、沿海沼泽、大型河口湾、峡湾和受盐度分层影响的斜坡盆地等。一般来说, 现代黑色页岩的形成多受局部因素控制, 如受限盆地或构造孤立盆地等。然而, 在地质历史时期, 黑色页岩多形成于广阔的陆表海环境, 具有分布范围广、时间跨度长等特征(Arthur and Sageman, 1994)。

黑色页岩有机质富集有高生产力和强保存2种解释模式。高生产力模式包括“ 上升洋流” 模式(Demaison and Moore, 1980)、“ 氧含量最低值区(OMZ)” 模式(Jenkyns, 1980, 1988, 2010)和“ 近岸透光带缺氧(PZE)” 模式(Sł owakiewicz et al., 2015)。强保存模式包括“ 受限盆地” 模式(Demaison and Moore, 1980)、“ 改进的受限盆地” 模式(McArthur et al., 2008)、“ 不规则底形” 模式(Hallam and Bradshaw, 1979; Wignall and Newton, 2001)、“ 水洼扩张” 模式(Wignall, 1991)、“ 海侵化学跃层” 模式(Rö hl and Schmid-Rö hl, 2005)和“ 近岸海侵(TN)” 模式(Wignall and Newton, 2001; Leonowicz, 2016)。本质上, 高生产力模式强调高初级生产力是有机质富集的根本原因。有机质在降解过程中造成溶解氧消耗量增加, 从而导致底层水体缺氧。该模式认为高有机质含量造成了水体缺氧, 而不是缺氧造成有机质含量升高(Pedersen and Calvert, 1990)。相反, 强保存模式认为, 在初级生产力不高的情况下, 水体缺氧也可造成有机质富集(Demaison and Moore, 1980)。在水体分层的情况下, 由于水体上下对流受阻, 溶解氧很难补给到底层水体, 从而导致底层水体缺氧。

海平面快速上升常造成黑色页岩的形成和有机质的富集。一方面, 海平面上升会造成浮游生物勃发, 从而造成水体缺氧和有机质富集(Jr. Coveney et al., 1991; Middelburg et al., 1991); 另一方面, 海平面上升会造成底层缺氧水体向浅水区扩张, 从而造成富有机质页岩大面积分布(Heckel, 1977)。海平面上升期, 富有机质页岩常发育于最大海泛面(MF)附近或海侵体系域底部(BT)。MF型富有机质页岩记录了随着海平面上升、水深增大, 盆地边缘沉积条件逐渐变得与深水盆地相似, 而BT型富有机质页岩仅限于盆地位置或局部地形低洼地区(Luening et al., 2000; Wignall and Newton, 2001), 海平面波动形成不同的总有机碳(TOC)含量叠置样式。一些学者认为, 高位体系域早期紧临凝缩段位置页岩TOC含量通常最高(Myers, 1996)。Palsey等(1991)、Myers(1996)则认为, TOC含量最大值通常出现在凝缩段下部的海侵体系域中。

上奥陶统— 下志留统黑色页岩在全球广泛分布(Brett, 1983; Munnecke et al., 2010; Paris et al., 2015)。川南地区五峰组— 龙马溪组作为该套黑色页岩的重要组成部分之一(图 1), 其形成分布与该时期全球海平面升降密切相关(Wang et al., 2020a; Chen et al., 2021)。该套页岩具有高TOC含量、高含气量、高孔隙度、高石英含量、层理和裂缝发育等特征, 是目前中国南方页岩气勘探开发的关键层段(董大忠等, 2018; Qiu and Zou, 2020; Shi et al., 2021, 2022a, 2022b)。然而, 关于该黑色页岩的特征、分布模式和成因机制, 目前认识仍相对较薄弱。本研究借助钻井和露头资料, 通过系统的层序地层学、矿物学和地球化学分析, 明确了该套页岩的特征和分布, 并初步探讨其形成机理。

图 1 四川盆地及周缘五峰组— 龙马溪组页岩沉积古地理背景(据Chen et al., 2004; Shi et al., 2021; 有修改)
a— 志留纪早期全球古地理; b— 川南地区早志留世扬子陆表海古地理; c— 上奥陶统— 下志留统黑色页岩分布Fig.1 Paleogeographic background of the Wufeng-Longmaxi shale of Sichuan Basin and its surrounding areas (modified from Chen et al., 2004; Shi et al., 2021)

2 地质背景

华南板块包括扬子板块和华夏板块, 川南地区位于扬子地块东南部(图 1)。晚奥陶世, 华南板块仍附着于冈瓦纳大陆的边缘。扬子地块是前寒武纪华南板块的重要组成部分, 早古生代与塔里木板块和华北板块分离。五峰组— 龙马溪组形成于华南盆地消亡和华南造山带形成时期(Yu et al., 2008; Yao et al., 2015; Zhang et al., 2015a)。寒武纪末期, 由于广西造山运动的影响, 华夏地块和扬子地块发生会聚, 扬子板块东南和江南盆地相继抬升(Wu, 2000; Yu et al., 2008; Zhou et al., 2017), 扬子地区进入被动大陆边缘发展阶段(Yu et al., 2008)。早奥陶世至中奥陶世, 扬子地区主要发育碳酸盐岩台地沉积(Munnecke et al., 2010; Zhang et al., 2010)。从晚奥陶世开始, 扬子地区碳酸盐岩— 碎屑岩混合沉积广泛发育。奥陶纪— 志留纪转折期, 全球海平面大幅波动(Chen et al., 2004; Haq and Schutter, 2008)。早志留世, 伴随着华夏板块扩张和华南大部分地区的隆升, 华南古陆和海洋分布发生重大变化(图 1-b)。区域构造作用形成古陆和水下高地, 上扬子地区变成半封闭的海洋(Chen et al., 2004; Rong et al., 2020)。由于古陆和水下高地的阻隔, 上扬子地区水体整体处于缺氧停滞状态(Chen et al., 2004; Yan et al., 2011; Zou et al., 2018a)。

四川盆地及周缘五峰组— 龙马溪组分布广泛, 总厚度超过500m(图 1-c)。页岩分为五峰组、观音桥层和龙马溪组(图 2)。五峰组与下伏宝塔组呈平行不整合接触, 与上覆观音桥层呈平行整合接触(图 2)(Wang et al., 2020), 龙马溪组与观音桥层呈平行整合接触。龙马溪组分为龙一段和龙二段, 龙一段细分为龙一1亚段和龙一2亚段, 龙一1亚段进一步分为龙一 11至龙一 144个小层(Wang et al., 2020)。基于地震、测井、笔石带和沉积构造综合分析认为, 五峰组— 龙马溪组沉积时期发生2次二级海平面升降旋回和4次三级海平面升降旋回(Wu et al., 2018)。龙一1亚段具有TOC含量高、层理和微裂缝发育特点(董大忠等, 2018; 施振生等, 2018, 2020, 2022), 是页岩气勘探开发的“ 甜点” 段(Shi et al., 2022a)。

图 2 川南地区五峰组— 龙马溪组地层综合柱状图(据Shi et al., 2021; 有修改)
WF1: Dicellograptus complanatus, WF2: Dicellograptus complexus, WF3: Paraorthograptus pacificus, WF4: Metabolograptus extraordinarius, LM1: Metabolograptus persculptus, LM2: Akidograptus ascensus, LM3: Parakidograptus acuminatus, LM4: Cystograptus vesiculosus, LM5: Coronograptus cyphus, LM6: Demirastrites triangulates, LM7: Lituigraptus convolutes, LM8: Stimulograptus sedgwickii, LM9: Spirograptus guerichiFig.2 Comprehensive stratigraphic column of the Wufeng-Longmaxi Formations in southern Sichuan Basin(modified from Shi et al., 2021)

3 方法和样品
3.1 样品采集

本次研究共观察W203井岩心52m、W211井岩心42m、W214井岩心4 9m、Z202井岩心4 0m、Z203井岩心52m、Z205井岩心61m、Z207井岩心35m、L206井岩心50m及长宁双河剖面取心19m(具体井位见图 1-c), 研究层位为五峰组— 龙马溪组底部。

研究样品取自W203井、W211井、W214井、Z202井、Z205井、Z207井、L202井、L206井和长宁双河剖面岩心(图 1-c), 取样层位为五峰组— 龙马溪组底部。所有岩心均开展了总有机碳(TOC)含量、X衍射全岩(XRD)和偏光显微镜分析。另外, W211井岩心开展了20块氩离子抛光片SEM成像分析, 长宁双河剖面开展18块氩离子抛光片SEM成像分析和主微量元素分析, L202井岩心开展10块氩离子抛光片SEM成像分析。氩离子抛光片尺寸为10mm× 10mm× 5mm。TOC含量、XRD和主微量分析测试在西南油气田分公司实验研究中心进行, 氩离子抛光片SEM成像分析在中国石油勘探开发研究院实验研究中心进行。

3.2 有机地化和岩石学分析

利用LECO CS-200硫碳分析仪进行TOC含量分析, 实验前先用盐酸去除样品中无机碳成分, 有机碳含量通过高温燃烧称重直接测得。利用日本理学RINT-TTR3型X射线衍射仪进行XRD实验, 采用Cu靶(单色), 旋转角度3° ~45° , 管压45 kV, 管流100 mA。定量分析采用步进扫描, 扫描速度4° /min, 采样间隔0.02° 。按照标准(SY/T 5163-2010)《沉积岩中黏土矿物和常见非黏土矿物X射线衍射分析方法》对矿物成分进行定量分析。

3.3 氩离子抛光片成像分析

为了获得高精度和大视域的矿物和有机质图像, 采用了氩离子抛光片制作、图像采集和拼接、矿物和有机质分析等研究步骤和方法。氩离子抛光片尺寸为10mm× 10mm× 5mm, 图像采集选用携带冷排放的Hitachi场发射扫描电镜, 并配备有低到高的二次电子探针和X射线能谱仪(EDS)。扫描电镜放大倍数为30000倍(单张照片最大分辨率为9nm)。图像采集区域垂直于纹层面, 累积采集面积60μ m× 40μ m。图像采集完成后, 选用微软公司HD View软件进行矿物成分和有机质分析。

3.4 主量和微量元素分析

主量元素主要通过XRF光谱仪测定, 分析误差不大于3%。先将页岩样品研磨至200目, 放置干燥箱中烘干, 然后称量1.2 g样品, 加入6g溶剂(Li2B4O7)充分混合后, 将样品移入坩埚中, 并在110 0℃的高温下溶解。然后, 将溶剂均匀涂到玻璃板上, 通过XRF-1500光谱仪测定其元素成分。

通过电感耦合等离子体质谱(ICP-MS)测定微量元素浓度。先将100 mg样品用105℃高温干燥, 然后用0.5 mL HClO4+2.5 mL HF+0.5mL HNO3试剂溶解, 然后再干燥。然后再用1mL HNO3+3mL H2O净化样品, 直到获得澄清的溶液。将溶液稀释至 1︰1000 浓度, 并在VG PQ2 Turbo电感耦合等离子体源质谱仪(ICP-MS)上开展分析, 测量微量元素组成。

4 结果
4.1 快速海进页岩垂向分布

测井、录井、矿物成分、地球化学和古生物资料等综合分析表明, 五峰组— 龙马溪组发育1个二级层序、5个三级层序(图 3)。二级层序划分为海进(TST)和高位(HST)2个体系域, TST对应于笔石带WF1-4至LM5下部, HST对应于LM5上部至LM9, LM5中部为最大海泛期(MFS)。三级层序Seq1对应于五峰组和观音桥层, 可划分为TST和HST, TST对应于笔石带WF1-2, HST对应于笔石带WF3-4; 三级层序Seq2对应于笔石带LM1-4, 可划分为TST和HST, TST对应于笔石带LM1, HST对应于笔石带LM2-4; 三级层序Seq3对应于笔石带LM5, 可划分为TST和HST, TST对应于笔石带LM5下部, HST对应于笔石带LM5上部; 三级层序Seq4对应于笔石带LM6-8, 可划分为TST和HST, TST对应于笔石带LM6-8下部, HST对应于笔石带LM6-9上部; 三级层序Seq5对应于笔石带LM9。

图 3 川南地区五峰组— 龙马溪组快速海进页岩纵向分布
BT=宝塔组, WF=五峰组, KYQ=观音桥层, SB=层序边界, TST=海进体系域, HST=高位体系域, MFS=最大海泛面; 笔石带划分据陈旭等, 2015; 同位素年龄数据来自Gradstein, 2006Fig.3 Vertical distribution of rapid transgressive black shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

快速海进页岩分布于龙马溪组最底部, 对应于笔石带LM1(图 3)。二级层序TST可细分为早期海进、快速海进和晚期海进3个阶段, 快速海进页岩形成于快速海进的阶段(Wignall and Newton, 2001)。该套页岩是现阶段川南页岩气勘探开发的“ 甜点” , 具有高石英含量、高孔隙度、高渗透率、高有机碳含量、高水平渗透率与垂直渗透率之比的特征(Shi et al., 2022b)。

4.2 快速海进页岩平面分布

快速海进页岩在川南地区大面积分布。该套页岩直接覆盖于笔石带WF1-4之上, 并被笔石带LM2-3、LM4、LM5和LM6覆盖。在连井地层对比剖面W210井— L201井上(图 4), L201井至W206井之间均有快速海进页岩分布, 其完全覆盖于笔石带WF1-4之上, 其上被笔石带LM2-3、LM4和LM5逐渐超覆。在地震剖面a-a’ 和b-b’ 上(图 5), 快速海进页岩与上覆层段LM2-3、LM4和LM5一起形成连续的强波峰反射, 整条剖面均可进行追踪对比, 而下伏层段分布局限, 仅局部可以追踪对比。

图 4 过W210井— L201井连井剖面显示川南地区五峰组— 龙马溪组快速海进页岩分布Fig.4 Transection from Well W210 to Well L201 showing distribution of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

图 5 地震剖面展示川南地区五峰组— 龙马溪组快速海进页岩分布模式(地震剖面位置见图1)Fig.5 Seismic profiles showing distribution of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin(seismic profiles location in Fig.1)

快速海进页岩厚度为0.5~2.8 m, 由古隆起核部向中心逐渐增大(Wignall, 1991)。在连井地层对比剖面W203井— L202井上(图 6), 页岩厚度由古隆起核部的1.2 m增至盆地中心的2.3 m, 自303井厚度最大(2.3 m)。在过WJ1井— JYT1井连井地层对比剖面上(图 7), 页岩厚度0.9~2.8 m, 大安2井厚度最大(2.8 m)。

图 6 过W203井— L201井连井剖面显示川南地区五峰组— 龙马溪组快速海进页岩分布Fig.6 Transection from Well W203 to Well L202 showing distribution of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

图 7 过WJ1井— JYT1井连井剖面显示川南地区五峰组— 龙马溪组快速海进页岩分布Fig.7 Transection from Well WJ1 to Well JYT1 showing distribution of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

4.3 快速海进页岩的矿物组成

快速海进页岩纹层和层理发育(图 8), 主要矿物成分有石英、黏土矿物、方解石和白云石等, 含有少量黄铁矿和长石(表 1), 少数样品可见磷灰石和重晶石。

图 8 川南地区五峰组— 龙马溪组快速海进页岩岩心照片
a— W203井(样品号: W-1); b— W211井(样品号: W-2); c— Z201井(样品号: Z4-8); d— Y101H3-8井(样品号: Y-1)Fig.8 Core photographs of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

表 1 川南地区五峰组— 龙马溪组快速海进页岩矿物组成及TOC含量 Table1 Mineral compositions and TOC contents of rapic transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

快速海进页岩石英含量27.9%~68.2%(平均值49.3%)。石英按粒径可分为微晶石英和粉砂级石英: 微晶石英粒径小于3.9μ m, 粉砂级石英粒径3.9~31.2μ m。微晶石英含量可达85%(图 9); 方解石含量4.2%~25.4%(平均值10.5%), 扫描电镜下颜色相对较浅, 形状不规则, 表面可见溶蚀孔; 白云石含量4.2%~17.0%(平均值8.4%), 扫描电镜下相对较暗, 常以规则自形晶体出现; 黏土矿物含量12.6%~38.5%(平均值23.4%), 以伊利石、绿泥石和伊/蒙混层为主, 高岭石含量较少(Shi et al., 2022b)。

图 9 SEM照片展示川南地区五峰组— 龙马溪组快速海进页岩矿物成分特征
a— Z201井, 3666.34m; b— Z201井, 3666.34m; c— 长宁双河剖面; d— Y101H3-8井, 3684.65m; e— Y101H1-8井, 3682.22m; f— 长宁双河剖面; Cal=方解石, Qtz-c=微晶石英, Qtz-s=粉砂级石英; Py=黄铁矿, F=长石, Dol=白云石Fig.9 SEM photos showing typical mineral compositions of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

由古隆起核部向中心方向, 快速海进页岩石英含量增加、黏土矿物含量降低。例如, 由盆缘向盆地中心方向, W211井石英含量29.1%, Z205井53.1%, L202井则增至68.2%(表 1; 图 10)。相应的, W211井黏土矿物含量23.7%, Z205井26.1%, L202井降至12.6%。

图 10 川南地区五峰组— 龙马溪组不同井快速海进页岩矿物组成特征Fig.10 Mineral compositions of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

4.4 TOC含量和分布模式

快速海进页岩TOC含量为4.6%~6.4%(平均值5.4%), 由古隆起核部到中心逐渐降低。在过W203井— L202井连井对比剖面上, W214井TOC平均含量6.4%, Z205井4.7%, L202井降至4.6%(图 11)。

图 11 过W203井— L202井连井剖面显示川南地区五峰组— 龙马溪组TOC含量及变化Fig.11 Transection from Well W203 to Well L202 showing TOC content of rapid transgressive shale of Wufeng-Longmaxi Formations in southern Sichuan Basin

快速海进页岩发育4种TOC含量叠置剖面: (1)由下至上, TOC含量从笔石带LM2-3底部突然升至最大值, 然后逐渐下降至背景值(图12-a); (2)由下至上, TOC含量从笔石带WF1-4底部快速增大, 笔石带LM1底部跃至最大值, 然后向上逐渐降低至背景值(图 12-b); (3)由下至上, TOC含量在笔石带WF1-4底部快速增大至某个固定值, 然后一直保持稳定, 在笔石带LM1底部突然跳到最大值, 然后向上逐渐降低至背景值(图12-c); (4)由下至上, TOC含量向上逐渐增大, 在笔石带LM1中部达到最大值, 然后向上逐渐降低至背景值(图 12-d)。

图 12 川南地区五峰组— 龙马溪组页岩TOC含量典型叠加样式(笔石带代码含义见图2)
a— W203井; b— W204井; c— Z201井; d— W206井Fig.12 Typical stacking patterns of TOC content of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

4.5 地球化学特征

微量元素U/Th、V/Cr和Ni/Co值是指示水体氧化还原条件的重要指标。U/Th< 0.75为氧化环境, U/Th介于0.75~1.25为贫氧环境, U/Th> 1.25为还原环境(Mo et al., 1973; Spirakis, 1996; McManus et al., 2005)。同样, V/Cr< 2.00为氧化环境, V/Cr介于2.00~4.25之间为贫氧环境, V/Cr> 4.25为还原环境; Ni/Co< 5.00为氧化环境, Ni/Co介于5.00~7.00之间为贫氧环境, Ni/Co> 7.00为还原环境。五峰组— 龙马溪组页岩U/Th值为0.27~4.19, 平均值1.55(表 2); V/Cr值为2.32~7.64, 平均值4.86; Ni/Co值为6.26~22.58, 平均值11.80。纵向上, 快速海进页岩的U/Th、V/Cr和Ni/Co值均最高, 分别达到4.19、7.64和11.80(图 13)。

表 2 川南地区五峰组— 龙马溪组快速海进页岩地球化学特征 Table2 Geochemistry of the rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

图 13 川南地区五峰组— 龙马溪组沉积速率、TOC含量、古氧化还原条件和古生产力特征(笔石带代码含义见图2)Fig.13 General trends of sedimentation rate, TOC content, paleoredox, and paleoproductivity of the Wufeng-Longmaxi Formations in southern Sichuan Basin

生源钡(Babio)含量是估算古生产力的一个重要指标, 其计算公式如下(Schroeder et al., 1997): w(Babio)=w(Ba样品)-w(Ba/Al)PAAS。式中, w(Ba样品)为页岩样品实验分析的Ba质量分数, 单位: μ g/g, (Ba/Al)PAAS为澳大利亚后太古代平均页岩中的Ba含量与Al2O3含量比值。五峰组— 龙马溪组w(Babio)为225.53~566.28μ g/g, 平均值为443.29μ g/g(表 2)。快速海进页岩的w(Babio)值最大, 平均值达515.66μ g/g(图 13)。

5 讨论
5.1 快速海进页岩的分布模式

快速海进页岩厚度整体较小, 一般为0.5~2.8 m, 其超覆于下伏层段之上, 并被上覆层段超覆(图 14)。这种分布模式与快速海进有关。五峰组— 龙马溪组沉积时期, 川南地区发育乐山— 龙女寺水下古隆起。快速海进之前, 古隆起核部由于古地形相对较高、可容空间有限, 故页岩厚度可忽略不计。快速海进期间, 由于海平面上升、可容空间增大, 故发生页岩沉积。快速海进造成海岸线向陆收缩, 从而导致陆源碎屑供给减少、页岩沉积速率降低, 故页岩厚度较小。海侵作用之后, 由于陆源供给增加、海岸线向盆地方向迁移, 沉积物逐渐覆盖快速海进页岩。

图 14 川南地区五峰组— 龙马溪组快速海进页岩分布模式(笔石带代码含义见图2)Fig.14 Distribution pattern of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

盆地中心位置快速海进页岩厚度最大, 向古隆起核部逐渐减小(图 14)。这种分布模式与细粒物质的生物和火山成因密切相关。XRD全岩分析表明, 五峰组— 龙马溪组黑色页岩主要由硅质、方解石、白云石、黏土矿物和少量黄铁矿组成, 盆地中心位置硅质含量高达60%~80%。黑色页岩中的硅质主要为生物成因, 硅质生物主要为放射虫、硅质海绵等(赵建华等, 2016; 卢龙飞等, 2018)。

较为清洁的水体有利于放射虫和硅质海绵的生长(Zhang et al., 2015b), 盆地中心位置由于受陆源影响小、水体较为清洁, 故放射虫和硅质海绵等生物发育, 黑色页岩厚度大。而古隆起核部位置由于陆源影响大、水体较为混浊, 故放射虫和硅质海绵等生物不发育, 页岩厚度较小。同时, 五峰组— 龙马溪组页岩组成与分布受火山喷发作用影响(Hu et al., 2009; Su et al., 2009; Yang et al., 2019; Shi et al., 2022a)。盆地中心位置由于水体能量低, 细粒凝灰物质容易沉降, 从而页岩厚度较大。古隆起核部位置由于水体能量较强、细粒物质不容易沉降, 故页岩厚度较小。

5.2 快速海进页岩的TOC含量

快速海进页岩高TOC含量可能是水体还原性强、沉积速率低和初级生产力高共同作用的结果(图 15)。前人研究表明, 有机质的保存与海洋初级生产力、水体封闭性、沉积速率和水深密切相关(Myers, 1996)。川南地区快速海进页岩沉积时期强烈的火山作用导致大气中二氧化碳含量迅速增加、大气温度突然升高, 从而引起大范围海侵(Jenkyns, 2010; Ozaki et al., 2011; Blumenberg and Wiese, 2012; Wu et al., 2018; Yang et al., 2019; Bond and Grasby, 2020; Zhao et al., 2021)。物源分析表明, 沉积物源岩主要来自活动大陆边缘和大陆岛弧的酸性火成岩(Su et al., 2009; Shi et al., 2022a), 这为该时期火山活动提供了有用的证据。全球快速变暖加速了陆地风化作用和营养物质向海洋的输入, 从而引发海洋富营养化和全球贫氧/缺氧(Blumenberg and Wiese, 2012)。根据对Fe、U和Mo元素的研究, 该层段主要形成于水体分层导致的缺氧水体中(Chen et al., 2004; Yan et al., 2015, 2021; Li et al., 2017a, 2021; Wu et al., 2018; Zou et al., 2018a, 2018b)。水体由于含氧量低, 故有机质保存能力增强(Demaison and Moore, 1980; Oschmann, 1988; Wignall, 1991)。此外, 快速海进作用会导致盆地碎屑沉积物供应减少, 从而页岩TOC含量急剧增加。前人研究(Shi et al., 2021, 2022a)表明, 该时期海进页岩的沉积速率仅为1.7~7.5 m/Ma, 这非常有利于有机质的大量聚集。除了保存能力增强和低沉积速率外, 高初级生产力也发挥了重要作用。晚奥陶世至早志留世, 藻类、放射虫、笔石和其他生物广泛分布于扬子陆架海(Li et al., 2017b), 地表水营养元素如Ba、P、Ni、Zn等含量高(Qiu and Zou, 2020), 均表明快速海进页岩具有高生产力背景。

图 15 川南地区五峰组— 龙马溪组快速海进页岩TOC分布及成因Fig.15 TOC distribution and gensis of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

快速海进页岩的TOC含量通常从古隆起核部向中心逐渐降低(图 15)。水深的增加和黏土矿物含量的降低是TOC含量向盆地方向降低的主要原因。海相页岩中分散有机质主要来源于海洋浮游植物, 藻类有机质向海洋沉积物的输入是透光带初级生产力和水深的函数(Calvert, 1987)。底层沉积物能接收的碳含量受水深影响较大。

水体中有机质的降解和再循环(无论是氧化或缺氧)显著降低了水体中的碳含量(Suess, 1980; Betzer et al., 1984)。一般情况下, 由于表层生产力降低和水体再矿化, 藻类有机质向底部沉积物的供应能力随着水深和距海岸线距离的增加而减少。因此, 海进页岩的TOC含量向盆地方向逐渐降低。此外, 黏土矿物有利于吸收和保存大量有机碳(Zhang, 2021), 从而有利于页岩TOC含量的提高。离海岸线距离的增加导致黏土矿物含量减少, 从而降低了盆地方向页岩的TOC含量。例如, 由威203井至泸202井, 页岩黏土矿物含量从38.5%降至12.6%, TOC含量也相应从6.4%降至4.6%(表 1)。

5.3 TOC含量叠置样式

五峰组— 龙马溪组页岩存在4种TOC含量剖面叠置样式(图12)。其共同特征是, 相对于背景值, 有机质丰度最初突然上升, 随后又逐渐降低。这种叠置样式可能与有机碳聚集的主要控制因素有关。如前所述, 页岩的高TOC含量是有机质保存能力增强、沉积速率低和高初级生产力共同作用的结果。笔石带WF1-4和LM1的TOC含量最初快速增加, 可能是由于碎屑沉积物供应速率的快速降低和海侵期间的保存能力增强造成的, 最大TOC含量常与最大洪泛面相关。随后, 由笔石带LM1至LM5, TOC含量逐渐降低, 这反映了高位期碎屑沉积物供应增加和有机质保存能力逐渐降低。如图 13和表 2所示, Babio的含量以及U/Th、V/Cr和Ni/Co值在穿越笔石带WF1-4和笔石带LM1时突然增大, 表明该时期表层生产力和保存能力突然增大。相比之下, 沉积速率随着这种变化而迅速降低, 从而导致沉积物稀释度相应降低。随后, Babio的含量以及U/Th、V/Cr和Ni/Co值降低, 沉降速率逐渐向上增加, 从而导致TOC含量向上逐渐降低。前人也报道了类似的TOC含量分布模式, 并认为其与缺氧底层水条件下碎屑物沉积速率有关(Stephen and Passey, 1993)。

4种TOC含量叠置剖面的TOC含量从最小值增至最大值的过程存在明显差异, 这可能是与从古隆起核部到盆地中心的位置差异有关(图 16)。第1种叠置样式通常发生在古隆起核部附近。在这种情况下, 海侵体系域和早期高位体系域沉积物由于相对位置较高而缺失沉积。因此, 晚期高位体系域的页岩直接覆盖睛宝塔组灰岩之上, 从而导致宝塔组和龙马溪组边界的TOC含量急剧增加。其他叠置样式通常出现在逐渐靠近盆地中心的位置。在这些位置, 由于地势相对较低, 沉积物容纳量逐渐增加。在海平面上升期间, 这些位置可以连续沉积, 形成TOC含量逐渐增加的黑色页岩。因此, 这些位置的TOC含量早期逐渐增加, 随后逐渐减少。

图 16 川南地区五峰级— 龙马溪组快速海进页岩TOC含量变化成因模式Fig.16 Genesis model of TOC content of rapid transgressive shale of the Wufeng-Longmaxi Formations in southern Sichuan Basin

6 结论

1)川南地区上奥陶统— 下志留统五峰组— 龙马溪组快速海进页岩位于龙马溪组最底部, 对应于笔石带LM1。该套页岩石英平均含量为49.3%(其中黏土级石英颗粒含量85%), 方解石平均含量为10.5%, 白云石平均含量为8.4%, 黏土矿物平均含量为23.4%。向盆地方向, 页岩的石英含量增加, 黏土矿物含量降低。

2)快速海进页岩形成于相对海平面快速上升阶段, 在川南地区广泛分布, 页岩厚度为0.5~2.8m, 由古隆起核部向盆地中心方向逐渐增大。该套页岩TOC含量平均值为5.4%, 由盆缘向盆地中心逐渐降低, 纵向TOC含量剖面呈现4种叠置样式。

3)快速海进页岩的矿物组成和页岩分布与沉积时期的快速海进、生物作用和火山作用密切相关。快速海进导致陆源供给减少, 故页岩厚度较小; 生物作用和火山作用导致黏土级石英含量高、页岩厚度向盆地方向增大。

4)快速海进页岩的高TOC含量与水体缺氧、低沉降速率和高初级生产力有关。水体缺氧有助于有机质保存能力增强, 低沉积速率可减弱有机质的稀释, 而高初级生产力可增加有机质的供给。

5)快速海进页岩TOC含量平面变化及叠置样式与水深有关。随着水深增加, 水体黏土矿物含量减少, 有机质的降解和再循环增加, 故页岩TOC含量降低; 随着水深增加, 沉积物可容空间增加, 从而形成不同的TOC含量叠置样式。

(责任编辑 李新坡; 英文审校 徐 杰)

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