Division of high-frequency sequences in black shales of the Wufeng-Longmaxi formations, Upper Yangtze Region

  • Shaoze ZHAO , 1, 2 ,
  • Yong LI , 1 ,
  • Leli CHENG 3 ,
  • Zhou NIE 4 ,
  • Tong WANG 5 ,
  • Chaorong WU 6 ,
  • Maoyu GENG 1
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  • 1. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China
  • 2. Post⁃doctoral Research Station of Geology, Chengdu University of Technology, Chengdu 610059, China
  • 3. Institute of Logging Technology and Engineering, Yangtze University, Jingzhou 434023, China
  • 4. PetroChina Southwest Oil and Gasfield Company, Chengdu 610051, China
  • 5. Sinopec Southwest Oil and Gas Company, Chengdu 610016, China
  • 6. College of Geophysics, Chengdu University of Technology, Chengdu 610059, China

Received date: 2022-04-27

  Revised date: 2022-07-11

  Online published: 2022-11-23

Supported by

The Free Inquiry Fund of the State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Chengdu University of Technology)(SKL2019015)

the Foundation of the State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum, Beijing(PRP/open-2104)

Highlights

The Wufeng-Longmaxi formations in the Upper Yangtze Region are the most successful horizon of shale gas exploration and development in China. The high-frequency sequences division in the Wufeng-Longmaxi formations black shales is helpful to clarify the distribution of organic carbon. The Well N211 black shales were studied to determine Natural Gamma (GR) and total organic carbon content (TOC), and high-frequency sequences were divided based on cyclostratigraphy. The GR value and TOC gradually increase from the bottom of Wufeng Formation to the Guanyinqiao Member (maximum value appears in Guanyinqiao Member), gradually decrease from Guanyinqiao Member to Longmaxi Formation. Using GR spectrum analysis and its filter curve, about six and seven long eccentricity cycles were identified in the Wufeng and lower Longmaxi formations, respectively. The Wufeng and lower Longmaxi formations can be divided into 12 fourth order sequences and one incomplete fourth order sequence. This study can provide a typical case for high-frequency sequences division of black shales, and a reference for shale gas sweet spot prediction in the Wufeng-Longmaxi formations. In the shale high-frequency sequence structure of the Wufeng-Longmaxi formations, the black shales near the sequence boundary are relatively rich in organic matter.

Cite this article

Shaoze ZHAO , Yong LI , Leli CHENG , Zhou NIE , Tong WANG , Chaorong WU , Maoyu GENG . Division of high-frequency sequences in black shales of the Wufeng-Longmaxi formations, Upper Yangtze Region[J]. Natural Gas Geoscience, 2022 , 33(11) : 1808 -1818 . DOI: 10.11764/j.issn.1672-1926.2022.07.004

0 引言

层序地层研究可为沉积相分析、油气储层和烃源岩评价提供等时地层格架1-2。随着页岩油气勘探开发在世界范围内的持续推进,页岩高频层序划分成为国内外的研究热点3-6。页岩高频层序划分与对比有助于明确富有机质页岩的时空分布特征3-6,为页岩油气勘探开发提供依据。但是,沉积在深度较大的稳定水体中的页岩,结构均一,在页岩中识别层序关键界面具有较大的难度6-7。近年来,学者们基于旋回地层学理论来划分页岩高频层序,通过识别地层中的地球轨道参数信息或米兰科维奇旋回,使地层中的沉积旋回具有时间内涵,为页岩层序划分提供了定量依据5-68-9
在划分页岩高频层序的过程中,一般把自然伽马(GR)值和总有机碳含量(TOC)升高作为相对海(湖)平面上升的标志,反之,则作为相对海(湖)平面下降的标志,进而识别层序关键界面并划分层序3-69。在既有砂岩又有泥岩的碎屑岩地层,随着相对海(湖)平面的升高,岩性会由砂岩逐渐变为泥岩,表现为GR值和TOC值变大,因此,可以根据前述指标的变化直接判断相对海(湖)平面的变化及层序地层关键界面。但是,纯页岩的GR值和TOC值受相对海(湖)平面、海洋(湖泊)生产力等多种因素的影响,GR值和TOC值升高可能并不表示相对海(湖)平面升高10,在利用GR值与TOC值开展页岩层序划分时,需注意其指示的相对海平面变化意义。
上扬子地区上奥陶统五峰组—下志留统龙马溪组海相黑色页岩是中国页岩气勘探开发最成功的层位11。学者们利用测井、地震、地球化学等多种资料对五峰组—龙马溪组页岩开展了层序地层研究,主要集中在三级层序关键界面识别、三级层序及体系域的划分与对比12-18。尽管对该套页岩的层序关键界面识别与层序划分还存在分歧,但可以明确富含有机质的黑色页岩集中在五峰组—龙马溪组底部的几十米地层中,此认识对五峰组—龙马溪组页岩气勘探具有重要意义。但是,如何在数十米厚的黑色页岩中寻找厚度更小的、有机质含量相对较高的优质页岩,从而为页岩气高效开发提供科学依据是当前的难题。开展黑色页岩段高频层序划分,并分析富有机质页岩在高频层序结构内的分布特征,可以帮助解决这一问题。JIN等19已在扬子地区五峰组—龙马溪组黑色页岩中识别出了偏心率、斜率、岁差等地球轨道参数,但基于旋回地层学理论的高频层序划分的研究还尚未开展。
针对上述问题,以上扬子地区N211井的五峰组—龙马溪组黑色页岩为研究对象,在分析GR值及TOC特征的基础上,利用N211井GR曲线,识别五峰组—龙马溪组黑色页岩中的地球轨道参数信息,然后,讨论GR曲线对海平面变化的指示意义,并根据GR数据列的轨道参数周期滤波曲线划分高频层序。该研究不仅可为黑色页岩层序地层划分提供典型案例,还对页岩气甜点段预测具有重要意义。

1 地质背景

在晚奥陶世—早志留世,扬子板块与华夏板块共同组成华南板块20,华南板块位于冈瓦纳大陆西缘(原特提斯洋东缘)赤道附近的热带或亚热带区域[图1(a)],华南板块位置相对于现代位置逆时针旋转约90°,即现今上扬子的北缘在晚奥陶世—早志留世是上扬子的西缘21-22图1(b)]。受扬子板块与华夏板块碰撞的影响,上扬子地区由中奥陶世的浅水碳酸盐岩台地转变为晚奥陶世—早志留世的前陆盆地23-25,上扬子地区的西部边缘发育川中水下隆起、东部发育雪峰山隆起和鄂西水下隆起、南部发育黔中隆起,形成大面积低能、欠补偿的沉积环境26-27图1(b)],沉积大面积五峰组—龙马溪组富有机质黑色页岩。
图1 上扬子地区晚奥陶世—早志留世地质背景

Fig.1 Geological setting of the Upper Yangtze Region during the Late Ordovician-Early Silurian

陈旭等31在五峰组—龙马溪组中划分出13个笔石生物带[图2(a)]。富有机质的黑色页岩主要位于五峰组—龙马溪组的下部(上凯迪阶—鲁丹阶),对应WF1—LM5笔石带[图2(a)],沉积于447.62±1.1 Ma 至440.77±1.2 Ma之间31-32,沉积时限约为4.55~9.15 Ma。上扬子地区N211井五峰组—龙马溪组为一套连续沉积的地层[图2(b)]。五峰组厚9.7 m(2 347.3~2 357 m),岩性为黑色硅质页岩,与下伏宝塔组瘤状灰岩平行不整合接触。五峰组顶部的观音桥段厚1 m(2 347.3~2 348.3 m),含大量赫南特动物群化石。龙马溪组厚335.3 m(2 011.98~2 347.3 m),岩性由底向上依次为硅质页岩、炭质页岩、粉砂质页岩、泥质粉砂岩,粒度逐渐变粗,与上覆石牛栏组整合接触。其中,晚赫南特期—鲁丹期沉积的龙马溪组厚度为33.86 m(2 313.44~2 347.3 m),岩性为黑色硅质页岩。鉴于五峰组—龙马溪组黑色页岩主要位于上凯迪阶—鲁丹阶[图2(b)],本文将晚凯迪期—鲁丹期沉积的黑色页岩作为研究层位,下文中出现的龙马溪组下部黑色页岩均代表晚赫南特期—鲁丹期沉积的黑色页岩。
图2 扬子板块五峰组—龙马溪组笔石带(据文献[31])(a)和上扬子地区N211井五峰组—龙马溪组柱状图(b)

Fig.2 Graptolite biozones of the Wufeng-Longmaxi formations, Yangtze Plate (from Ref.[31]) (a) and histogram of the Wufeng-Longmaxi formations in Well N211, Upper Yangtze Region (b)

2 样品、数据和方法

2.1 样品与数据

从N211井采集40个黑色页岩样品,用于分析TOC,其中,9个样品为五峰组黑色页岩(包括1个观音桥段样品),31个样品为龙马溪组下部黑色页岩[图2(b)]。利用N211井GR数据列开展轨道参数识别及高频层序划分工作,GR值由斯伦贝谢公司通过自然伽马能谱测井测得,数据点间隔为0.1524 m。

2.2  TOC与频谱分析方法

将黑色页岩样品用玛瑙研钵磨碎至<80目,酸洗、烘干后,用于TOC分析,分析仪器采用CS230碳硫分析仪,该实验在中国石油西南油气田分公司勘探开发研究院分析实验中心完成。
将五峰组和龙马溪组下部地层分为2个单元进行频谱分析。频谱分析采用基于Matlab平台的Acycle2.3.1软件包33,主要分析方法包括多窗口频谱分析(MTM)和滑动窗口频谱分析。在深时缺乏精确的天文解,只有405 ka的长偏心率周期被认为在整个地质时代是恒定的34-35。JIN等19在上奥陶统五峰组—下志留统龙马溪组识别出短偏心率周期(125 ka和95 ka)。WALTHAM36计算出早志留世(440 Ma)的精确轨道周期:斜率为33.4±3.8 ka,岁差为20.9±1.5 ka、19.9±1.3 ka、17.1±1.0 ka、17.2±1.0 ka。因此,将405∶125∶95∶33.4∶20.4∶17.15作为晚奥陶世—早志留世地球轨道参数的周期比,将频谱分析中功率峰值(高置信度)对应的沉积旋回厚度的比值与上述理论周期比值进行对比,来验证N211井五峰组和龙马溪组下部GR数据列中是否存在轨道参数的周期记录。

3 结果

3.1 GR值及TOC特征

N211井五峰组下部地层的GR值介于98.88~272.76 API之间,均值为160.26 API;五峰组顶部的观音桥段GR值介于294.59~397.58 API之间,均值为348.79 API;龙马溪组下部GR值介于92.77~273.29 API之间,均值为170.06 API。GR值由五峰组底部到五峰组顶部的观音桥段逐渐变大,观音桥段出现峰值,再由观音桥段向龙马溪组逐渐变小[图3(a)]。
图3 上扬子地区五峰组和龙马溪组下部GR值及TOC特征37

Fig.3 Characteristics of GR value and TOC in the Wufeng Formation and lower Longmaxi Formation, Upper Yangtze Region37

五峰组和龙马溪组下部黑色页岩具有较高的TOC值[图3(a)]。N211井五峰组TOC值介于1.01%~8.35 %之间,由底向上逐渐变大。其中,五峰组下部地层的TOC值介于1.01%~5.75 %之间,均值为3.66%,五峰组顶部的观音桥段TOC值为8.35%,为五峰组和龙马溪组下部黑色页岩TOC的最大值,观音桥段富含底栖生物——赫南特贝36,说明观音桥段沉积期底层海水可能富氧,但观音桥段沉积物中的孔隙水是缺氧的。龙马溪组下部TOC值介于1.28%~6.04%之间,均值为3.15%,由底向上逐渐变小[图3(a)]。
上扬子地区华蓥三百梯剖面、L210井和JYT1井五峰组—龙马溪组黑色页岩的GR值或TOC值显示出同样的纵向变化特征,均在观音桥段出现峰值37图3(b)]。
GR值与TOC值在纵向上显示出一致的变化特征[图3(a)]。前人38研究表明,五峰组—龙马溪组黑色页岩中GR值与TOC值具有很好的正相关性。五峰组—龙马溪组黑色页岩中富含笔石生物化石,笔石生物的丰度由龙马溪组底部向上逐渐变小39,与N211井龙马溪组GR值和TOC值的变化特征一致。因此,GR值的纵向变化可以代表TOC值的纵向变化。

3.2 频谱分析

GR曲线可能包含有与地质因素无关的干扰信号,为了更好地识别GR曲线中存在的轨道参数周期,在进行频谱分析之前,采用Acycle2.3.1软件包的“Detrending”模块,分别去除五峰组和龙马溪组下部GR曲线低频部分的形变影响(图4)。
图4 上扬子地区N211井龙马溪组下部和五峰组GR数据列、GR(去趋势)数据列、GR长偏心率滤波曲线及四级层序划分

Fig.4 GR data series, GR(detrended) data series, long eccentricity filter curves of GR and fourth-order sequence division of lower Longmaxi Formation and Wufeng Formation in the Well N211, Upper Yangtze Region

轨道参数周期对应的沉积旋回厚度必须大于数据点间隔的4倍才有可能被识别出来40。N211井五峰组WF2—WF4笔石带发育齐全41,根据地层的同位素年龄,其沉积时限介于3.19~3.79 Ma之间[图2(a)]。根据沉积时限,可以计算出N211井五峰组沉积速率介于2.56~3.04 m/Ma之间,这只是一个大概的沉积速率,因为地层同位素年龄一般存在0.7~1.5 Ma的误差32。将该沉积速率与不同轨道参数周期相乘得到对应的沉积旋回厚度,然后,与数据点间隔(0.154 2 m)的4倍比较,判断五峰组GR数据可能仅能识别出长偏心率周期。去趋势后的五峰组GR曲线的MTM频谱图显示功率峰值对应的沉积旋回厚度为1.60 m,置信度大于90 %的沉积旋回厚度介于1.03~2.29 m之间[图5(a)],假设此沉积旋回对应405 ka的长偏心率周期,页岩沉积速率则为2.6~5.7 m/Ma,与同位素年龄计算的沉积速率(2.56~3.04 m/Ma)较为相似,因此,可以判断1.03~2.29 m是N211井五峰组沉积期长偏心率周期对应的沉积旋回。
图5 上扬子地区N211井五峰组和龙马溪组下部GR数据列MTM频谱与滑动窗口频谱

Fig.5 Multi-taper-method spectra and fast-Fourier transform sliding-window spectra produced from the GR data series of the Wufeng Formation and lower Longmaxi Formation in Well N211, Upper Yangtze Region

去趋势后的龙马溪组下部GR曲线的MTM频谱图显示功率峰值对应的沉积旋回厚度分别为5.64 m、4.83 m、3.26 m、1.47 m和1.16 m[图5(b)],它们的置信度均超过90 %,其中,4.83~5.64 m与1.47 m、1.16 m的比值(4.83~5.64∶1.47∶1.16=4.17~4.87∶1.27∶1)与长、短偏心率周期之比(405∶125∶95=4.27∶1.32∶1)非常接近,因此,判断5.64~4.83 m、1.47 m和1.16 m分别为长偏心率周期、125 ka的短偏心率周期、95 ka的短偏心率周期对应的沉积旋回厚度。
为了识别五峰组和龙马溪组下部GR数据列的主频率(或主沉积旋回)随深度的变化,观察轨道参数周期记录在深度上是否具有稳定性,对N211井五峰组和龙马溪组下部GR曲线进行滑动窗口频谱分析。五峰组滑动窗口频谱分析中的窗口步长为0.152 4 m,滑动窗口长度为2 m。龙马溪组下部滑动窗口频谱分析中的窗口步长为0.152 4 m,滑动窗口长度为8 m。滑动窗口频谱中高功率对应的频率与MTM频谱中功率峰值对应的频率基本一致[图5(c),图5(d)],表明五峰组GR数据列各深度段保存了长偏心率周期信号,龙马溪组下部GR数据列各深度段保存了长偏心率和短偏心率周期信号。

3.3 滤波

基于频谱分析的结果,将代表龙马溪组下部长偏心率周期(E:405 ka)的沉积旋回(4.83 m)通过高斯带通滤波提取出来,其滤波频率为0.21±0.04 周/m,滤波结果显示N211井龙马溪组下部记录了约7个长偏心率周期[图4(a)]。将代表五峰组长偏心率周期(E:405 ka)的沉积旋回(1.60 m)通过高斯带通滤波提取出来,其滤波频率为0.63±0.12 周/m,滤波结果显示N211井五峰组记录了约6个长偏心率周期[图4(b)]。

4 讨论

4.1 GR和TOC的海平面变化意义

GR值和TOC值由五峰组下部到观音桥段逐渐变大,在观音桥段出现峰值,由观音桥段向龙马溪组逐渐变小[图6(a)]。但是,五峰组—龙马溪组的古生物证据表明,上扬子地区海平面由早—中五峰期到观音桥段沉积期逐渐下降,在观音桥段沉积期最低,由观音桥段沉积期向龙马溪组下部黑色页岩沉积期逐渐升高42-43图6(a)],其中,观音桥段沉积期上扬子地区的平均水深为20~30 m44,发育底栖型生物——赫南特贝45,表明该时期底层海水富氧。五峰组—龙马溪组黑色页岩沉积期上扬子地区的海平面变化与全球海平面变化一致[图6(a), 图6(b)],此时期上扬子地区海平面变化可能与晚奥陶世—早志留世冰川的发育和消融有关42-4346
图6 上扬子地区五峰组和龙马溪组下部GR值、TOC与海洋生产力、上升流强度、海平面的关系(a)与晚奥陶世—早志留世全球海平面变化(b)

注:生产力指标据文献[57];上升流强度据文献[10];上扬子地区海平面据文献[42-43];全球海平面变化据文献[46

Fig.6 Relationship between GR value and TOC of Wufeng Formation and lower Longmaxi Formation and marine productivity, upwelling intensity and sea level in the Upper Yangtze Region (a) and global sea level change of Late Ordovician-Early Silurian (b)

前人通过古生物学、矿物学、地球化学等证据,认为上扬子地区在五峰组—龙马溪组黑色页岩沉积期发育上升流1047-48。冰期时大洋东侧上升流区的上升流强度变大,海洋初级生产力会因地球冰期强劲的上升流而激增49-56,可能会导致页岩的GR值和TOC值较高。五峰组—龙马溪组黑色页岩沉积期经历了晚奥陶世—早志留世冰川逐渐发育到逐渐消融的过程,观音桥段沉积期是冰川发育规模最大的时期,上升流因此产生的强弱变化导致上扬子地区海洋生产力由低到高再变低[图6(a)]。因此,在五峰组沉积期,冰川逐渐发育,海平面降低,但是冰川逐渐发育导致上扬子地区上升流强度变大,导致页岩中GR值和TOC值升高;反之,龙马溪组下部黑色页岩沉积期,冰川逐渐消融,海平面升高,但是冰川逐渐消融导致上升流强度变小,致使页岩中GR值和TOC值降低。同时,观音桥段沉积期冰川规模最大,上扬子地区海平面最低,GR值和TOC值出现峰值。上述分析表明,上扬子地区五峰组—龙马溪组黑色页岩GR值和TOC值降低(升高)对应该时期百万年尺度的海平面上升(下降)。上扬子地区海平面变化对页岩中有机质的富集并没有影响,上扬子地区海平面变化以及页岩中GR值、TOC值的纵向变化都是晚奥陶世—早志留世冰川发育和消融的结果。

4.2 高频层序划分

高频层序(四级至六级层序)是冰川型海平面变化的产物,与地球轨道参数的周期变化有关,其中,四级层序与地球长偏心率周期有关658。通过识别五峰组和龙马溪组下部黑色页岩中的轨道参数信息,并探讨该套页岩GR值和TOC值的海平面变化意义,可以在五峰组和龙马溪组下部黑色页岩中划分出四级层序。
晚奥陶世—早志留世冰期上升流导致的海洋生产力变化,造成海平面上升(下降)期间页岩GR值和TOC值降低(升高)。在此时期,地球长偏心率变化导致冰川规模与海平面出现周期性改变,偏心率尺度的海平面上升(下降)会同样伴随页岩GR值和TOC值降低(升高)。因此,五峰组和龙马溪组下部GR长偏心率滤波曲线中GR值升高(降低)代表了海平面下降(上升)。根据水进—水退(T—R)旋回理论59-62,结合GR长偏心率滤波曲线,五峰组和龙马溪组下部可以划分为12个完整的四级层序和底部1个不完整的四级层序[图4(a),图4(b)]。GR长偏心率滤波曲线的高值为层序边界,低值为最大海泛面,层序边界到最大海泛面之间为海进旋回(T旋回),表现为GR值的逐渐变小,最大海泛面到层序边界之间为海退旋回(R旋回),表现为GR值的逐渐变大[图4(a),图4(b)]。五峰组底部的四级层序仅包含一个R旋回[图4(b)]。
高频层序划分结果表明,即使层序边界附近的黑色页岩沉积在海平面相对较低时期,但是,由于此时期为地球的相对冷期,较强的上升流会导致该处黑色页岩的有机质富集程度相对较高。

5 结论

(1)上扬子地区N211井五峰组和龙马溪组下部黑色页岩GR值和TOC值由五峰组底部到五峰组顶部的观音桥段逐渐变大,观音桥段出现峰值,再由观音桥段向龙马溪组上部逐渐变小。
(2)N211井五峰组GR曲线可以识别出长偏心率周期,对应的沉积旋回厚度为1.03~2.29 m,五峰组记录了约6个长偏心率旋回。龙马溪组下部GR曲线可以识别出长偏心率周期和短偏心率周期,长偏心率周期对应的沉积旋回厚度为4.83~5.64 m,125 ka的短偏心率周期对应的沉积旋回厚度为1.47 m,95 ka的短偏心率周期对应的沉积旋回厚度为1.16 m,龙马溪组下部记录了约7个长偏心率旋回。
(3)上扬子地区五峰组—龙马溪组黑色页岩GR值和TOC值降低(升高)对应该时期海平面的上升(下降)。五峰组和龙马溪组下部可以划分为12个四级层序和底部1个不完整的四级层序。层序边界附近的黑色页岩有机质富集程度相对较高。
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