天然气地质学

滨岸闭塞环境中有机质富集模式——以川西南峨边葛村剖面筇竹寺组为例

  • 李依林 , 1 ,
  • 伏美燕 1, 2 ,
  • 邓虎成 1, 2 ,
  • 刘四兵 1 ,
  • 胥旺 1 ,
  • 吴冬 1
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  • 1. 成都理工大学能源学院,四川 成都 610059
  • 2. 成都理工大学油气藏地质及开发工程国家重点实验室,四川 成都 610059

李依林(1997-),男,黑龙江大庆人,硕士研究生,主要从事碳酸盐岩成岩作用及页岩储层评价研究.E-mail:.

收稿日期: 2021-07-05

  修回日期: 2021-09-13

  网络出版日期: 2022-04-22

The enrichment model of organic matter in the coastal detention environment: Case study of the Qiongzhusi Formation in the Gecun section of Ebian in southwestern Sichuan Basin

  • Yilin LI , 1 ,
  • Meiyan FU 1, 2 ,
  • Hucheng DENG 1, 2 ,
  • Sibing LIU 1 ,
  • Wang XU 1 ,
  • Dong WU 1
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  • 1. College of Energy,Chengdu University of Technology,Chengdu 610059,China
  • 2. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation,Chengdu University of Technology,Chengdu 610059,China

Received date: 2021-07-05

  Revised date: 2021-09-13

  Online published: 2022-04-22

Supported by

The National Science and Technology Major Project of China(2017ZX05036003-007)

本文亮点

上扬子地区筇竹寺组是我国四川盆地页岩气勘探的重点层位之一。目前对于筇竹寺组细粒沉积岩的有机质富集研究集中于绵阳—长宁拉张槽内的深水陆棚中,对川西南地区滨岸浅水环境中有机质富集的研究较少。以峨边葛村剖面为例,通过开展剖面及薄片观察、X射线衍射分析、有机地球化学测试、元素地球化学分析等工作,恢复研究区的沉积古环境并探讨了有机质富集的主控因素。结果表明:峨边地区筇竹寺组发育富有机质泥页岩,有机质富集是古气候、古生产力及水体氧还原性质协调作用的结果。导致滨岸环境有机质富集的根本原因是滞留的水体及古水深的变化,筇竹寺组沉积晚期温暖的气候和古水深的增加促使生物繁盛,古生产力提高。古水深的变化先后导致水体分层和滞留水体的形成,使沉积水体还原性进一步提高。研究认为川西南地区筇竹寺组沉积晚期古水深变化的过程有利于有机质的富集,滨岸闭塞环境的富有机质泥页岩段发育于水体深度波动变化时期,在此基础上建立了滨岸环境下的有机质富集模式。

本文引用格式

李依林 , 伏美燕 , 邓虎成 , 刘四兵 , 胥旺 , 吴冬 . 滨岸闭塞环境中有机质富集模式——以川西南峨边葛村剖面筇竹寺组为例[J]. 天然气地球科学, 2022 , 33(4) : 588 -604 . DOI: 10.11764/j.issn.1672-1926.2021.10.003

Highlights

The Qiongzhusi Formation in the Upper Yangtze area is one of the key formations for shale gas exploration in the Sichuan Basin, China. At present, the research on the organic matter enrichment of the fine-grained sedimentary rocks of the Qiongzhusi Formation focuses on the deep-water shelf environment in the Mianyang-Changning elongated trough. There are few studies on the organic matter enrichment in the coastal shallow water environment in southwestern Sichuan Basin. This study takes the Gecun section of Ebian as an example. Through section and thin section observation, X-ray diffraction analysis, organic geochemical test, element geochemical analysis, etc., the ancient sedimentary environment in the area was restored and the main controlling factors of organic matter enrichment in the study area were explored. The results of the study show that organic-rich mud shale is developed in the Qiongzhusi Formation in Ebian area, and the enrichment of organic matter is the result of the coordinated effect of paleo-climate, paleo-productivity and oxygen reduction properties of the water. The key reason of the enrichment of organic matter is the change of paleo-water depth. The warmer climate and the rise of paleo-water depth during the late deposition period of Qiongzhusi Formation promoted the prosperity of organisms and the increase of ancient productivity. Paleo-water depth changes have successively led to the formation of water stratification and the formation of detaining environments, which further improved the reducibility of sedimentary water bodies. Research suggests that the late deposition period of Qiongzhusi Formation in southwestern Sichuan is conducive to the enrichment of organic matter, the organic-rich mud shale section in the riparian facies retention environment developed during the paleo-water depth rise and fall. Based on these studies, the organic matter enrichment model in the riparian environment was established.

0 引言

在现今经济发展的过程中,我国能源面临着巨大的挑战,其中最突出的问题是油气资源对外依存度过高,对我国能源的安全构成了严重威胁1-3。而我国具有丰富的非常规油气资源,以页岩气为代表的非常规油气资源在未来将成为我国增储的主要组成部分4-8。作为生烃的基础,有机质富集问题一直是页岩气勘探研究的重点。前人9-15对海相及湖相泥页岩有机质富集的主控因素已经进行了大量的研究并取得了丰硕成果,归纳总结并建立了古生产力、缺氧保存条件及两者混合作用的3种有机质富集模式16-28。但目前建立的页岩有机质富集模式更多集中于具有较高古生产力,水体还原性强的湖盆相或深水陆棚相沉积。在通常情况下,近岸环境由于水体较浅,还原性较差而不利于有机质的保存,所以对于近岸环境下的有机质富集模式研究相对较少。近年来越来越多的研究人员发现在滨岸环境下发育的富有机质页岩,但未能明确其富集机理29-30。丁江辉等26在四川盆地龙潭组海陆过渡相页岩的研究中提出了滨岸环境下富有机质页岩的富集模式,但这种模式强调潮坪环境下的高等植物生长和较快的沉积速率,不具有普适性。所以针对滨岸相富有机质机理需要进一步的探讨。此外,前人的研究更强调古生产力和保存条件对于有机质富集的控制,通常对海相沉积中古水深变化对有机质富集的影响缺乏深入的研究,针对有机质富集控制因素的研究仍不够全面。
四川盆地筇竹寺组是近年来页岩气勘探的重点层位之一,目前在威远—长宁地区及井研—犍为探区都获得了油气显示,展现出良好的勘探前景31-35。前人已对筇竹寺组及牛蹄塘组有机质特征开展了大量工作,但目前对于筇竹寺组的有机质富集机理研究仍存在不足,现有研究主要探讨了拉张槽内及四川盆地东南深水陆棚区内有机质富集主控因素并建立了缺氧保存的富集模式。然而前人36-40对其他地区筇竹寺组富有机质泥页岩的研究较少,针对川西南地区筇竹寺组广泛发育的滨岸相细粒沉积岩中有机质富集的主控因素没有明确认识。本文通过研究川西南峨边葛村剖面的岩石学、地球化学特征,结合矿物组分特征对葛村剖面沉积古环境进行恢复,综合总有机碳含量(TOC)分析有机质富集的主控因素及古水深的变化在沉积过程中的影响,建立川西南地区筇竹寺组滨岸环境中的有机质富集模式。旨在为四川盆地筇竹寺组页岩气下一步的勘探提供依据,进一步丰富泥页岩中有机质富集模式的理论研究。

1 地质背景

四川盆地下寒武统筇竹寺组是我国页岩气勘探潜力较大的层位之一,该层位在四川盆地及周缘沉积页岩厚度较大,分布范围广,具有形成规模页岩气储层的条件441-43。受古地理条件及构造运动等多种因素影响,四川盆地及周缘沉积了一套地层展布不均,相变较快的细粒沉积岩。除筇竹寺组外,同时期还沉积有牛蹄塘组、水井沱组、九老洞组等(图1)。整体上看,上扬子地台沉积环境自西向东由古陆—滨岸向陆棚边缘斜坡过渡,随着水深增加砂质含量逐渐减小。但盆地中西部发育了南北向的绵阳—长宁拉张槽,拉张槽内沉积了一套深水陆棚相泥页岩,拉张槽边缘逐渐向浅水陆棚相过渡。所以在四川盆地北部及西部主要为滨岸相沉积,盆地中拉张槽范围内为深水陆棚相,盆地内其他地区及东南部主要为浅水陆棚相沉积,向东部及南部逐渐变化为深水陆棚相(图2)。西部滨岸—浅水陆棚相受陆源物质影响,沉积的泥页岩中含有大量粉砂质及砂质(图3)。筇竹寺组是早寒武世一套海相泥砂质沉积,以绿色、灰色、黄色及黑色泥页岩为主夹薄层砂岩。该时期可划分为2个沉积旋回,海平面变化表现为早期快速上升,中期波动升降,晚期持续下降的变化特征324244-47。峨边葛村剖面位于四川盆地西南缘,靠近西部的泸定古陆,属于滨岸—浅水陆棚相。有学者认为该地区以滨岸环境为主,也有学者认为峨边筇竹寺组主要发育浅水陆棚相3436-3845
图1 四川盆地下寒武统地层发育特征32

Fig.1 Stratigraphic development characteristics of the Lower Cambrian in Sichuan Basin32

图2 四川盆地及周缘峨边筇竹寺组沉积环境及峨边葛村剖面位置(修改自文献[3438-40])

Fig.2 Sedimentary environment of Qiongzhusi Formation in Sichuan Basin and its periphery and position of Gecun section of Ebian (modified from Refs.[3438-40])

图3 川西南地区峨边葛村—乐山范店—金页1井筇竹寺组岩性对比(修改自文献[34])

Fig.3 Lithology comparison of Qiongzhusi Formation in Gecun section of Ebian-Fandian section of Leshan-Well Jinye 1 in southwestern Sichuan(modified from Ref.[34])

由于该地区筇竹寺组碎屑含量普遍较高,粒度较大,发育交错层理,所以综合判断研究区筇竹寺组主要为滨岸相沉积36。峨边地区在筇竹寺组沉积期发育一套砂泥交互的细粒沉积岩,下段以砂质为主,上段以泥质为主3247-48

2 样品采集与实验方法

样品取自川西南峨边葛村剖面筇竹寺组,共取样117块,挑选部分样品(自27.5~262 m之间挑选31块样品,样品S1—S31位置标注于图4岩性剖面上)进行了薄片观察、X射线衍射(XRD)分析及主微量元素测定,对所有样品进行总有机碳含量(TOC)测定,所有实验均完成于成都理工大学油气藏地质及开发工程国家重点实验室和四川省煤田地质局科源工程技术测试中心。主微量元素使用来自Perkin-Elmer公司的Optima 5300V电感耦合等离子体原子发射光谱仪(ICP-AES)和电感耦合等离子体质谱(ICP-MS)进行测试(依据行业标准《GB/T 21114—2007》),误差小于5%;TOC使用美国Leco公司生产的LecoCS230碳硫分析仪测试(依据行业标准《GB/T 19145—2003》),误差小于5%;XRD测试使用日本理学株式会社的DMAX-3C型X射线衍射仪(40kV,20 mA),测试行业依据《SY/T5163—2018》。使用JADA5.0软件进行半定量相分析。当矿物含量大于40%时,相对偏差小于10%。
图4 峨边葛村剖面岩性及取样位置

Fig.4 Lithology and sampling position of the Gecun section of Ebian

3 实验结果

3.1 岩石学特征

3.1.1 剖面及薄片观察

研究区内岩性以粉砂岩和泥岩为主,主要成分为陆源碎屑及黏土矿物。在剖面上岩性表现为砂岩—泥岩的交替变化,峨边葛村剖面筇竹寺组下段砂质含量高,岩性为砂岩夹小段泥岩及粉砂质泥岩;上段泥质含量高,泥页岩厚度大,连续厚度最大超过40 m(图4)。泥页岩多为黑色,砂岩为灰色和灰绿色[图5(a)—图5(d)]。在镜下细粒沉积岩表现出海陆混积的特征,既有大量的碎屑颗粒,又有泥质及白云质成分[图5(e)—图5(j)],矿物粒径大多为粉砂级—泥级(依据行业标准《SY/T 5368—2016》),分选性中等,形态为次圆—次棱角状。细粒沉积岩在镜下可见砂泥交互形成的纹层[图5(k),图5(l)]。在地层顶部泥岩中还可见砂质透镜体发育[图5(m),图5(n)]。有机质在镜下呈分散状或条带状[图5(o),图5(p)],主要分布于泥页岩中。
图5 研究区细粒沉积岩镜下特征及宏观特征

(a)黑色泥岩,峨边葛村剖面;(b)灰绿色粉砂岩,峨边老鸦村剖面;(c)炭质页岩样品,56 m;(d)灰绿色粉砂质泥岩样品,34 m;(e)砂质泥岩,238.5 m,单偏光;(f)砂质泥岩,238.5 m,正交光;(g)含泥粉砂岩,126 m,单偏光;(h)含泥粉砂岩,126 m,正交光;(i)黑色泥页岩,65 m,单偏光;(j)黑色泥页岩,65 m,正交光;(k)纹层,77.5 m,单偏光;(l)纹层,77.5 m,正交光;(m)砂质透镜体,30.2 m,单偏光;(n)砂质透镜体,30.2 m,正交光;(o)条带状有机质,24 m,单偏光;(p)条带状有机质,24 m,正交光

Fig.5 Microscopic and macroscopic characteristics of fine-grained sedimentary rocks in the study area

3.1.2 XRD分析结果

根据XRD数据,研究区细粒沉积岩矿物种类有石英、长石、黏土矿物及白云石,含有少量黄铁矿及菱铁矿。其中,石英、长石和黏土矿物为主要成分,平均含量分别为33%、10%、37%,此外在泥页岩段中部及顶部砂岩段中还发育大量的白云石,其含量最高超过50%。纵向上看陆源碎屑及黏土矿物含量长期波动变化,在上段黏土矿物含量高,陆源碎屑含量低,而下段陆源碎屑含量总体偏高(图6)。
图6 研究区样品矿物组分特征

Fig.6 Mineral composition characteristics of samples in the study area

3.2  TOC含量

葛村筇竹寺剖面TOC含量分布于0.07%~2%之间,平均值为0.47%,TOC平均含量较低。但由于TOC纵向上变化幅度较大,不同深度下对应不同岩性中TOC差异较大,部分层段的TOC可以达到烃源岩标准。在上段富有机质泥页岩TOC较高。其中在46.2~65 m的泥页岩段中平均TOC含量为1.48%,最大可达2%,远高于下伏地层中砂质泥岩TOC,富有机质泥页岩段中TOC含量自下而上呈缓慢升高后快速降低的趋势。总体上看TOC变化在剖面上表现为上段高于下段,泥页岩高于砂岩。

3.3 主微量元素特征

本文研究重点对Al2O3、CaO、MgO、Na2O、K2O、P2O5、MnO共7种主量元素(氧化物)以及Mo、V、Cr、Ni、Cu、Co共6种微量元素进行了分析,实验结果表明不同时期下沉积的地层元素组成有明显的差异。
主量元素中Al2O3含量在纵向上大幅度变化,分布在4.5%~17.4%之间,平均值为14.5%。CaO含量除了在76 m处出现15.4%的异常高值外,大多数样品CaO含量都在0%~6.9%之间,平均值为2.6%。CaO与Al2O3都富集于泥页岩中,但两者含量呈现出密切的负相关关系。MgO含量分布特征与CaO类似,但变化幅度小,其含量大多在2.9%~6.9%之间变化,同样在76 m处出现异常高值,MgO含量平均值为5.0%。Na2O含量在0.12%~0.72%之间,平均值为0.29%,在下段含量显著高于上段。K2O、P2O5与MnO含量分布较稳定,仅在个别数据点出现大幅变化。K2O含量大多分布在4.7%~8.0%之间,平均值为5.6%。P2O5含量大多分布于0.20%~1.24%之间,平均值为0.43%。MnO含量大多在0.1%以下,平均值为0.07%。
本文研究的几种微量元素含量在纵向上展现出相似的规律,即上段含量高于下段,自下而上含量高低波动,在上段表现出先缓慢升高后急剧下降的趋势(表1)。多种微量元素在泥页岩中的含量都明显高于在砂岩中的含量。其中Mo元素含量在(21.1~217.1)×10-6之间,平均值为118.9×10-6。Cr元素含量在(32.1~62.5)×10-6之间,平均值为35.9×10-6。Ni元素含量在(6.6~62.5)×10-6之间,平均值为35.9×10-6。Cu元素含量在(3.7~81.6)×10-6之间,平均值为27.7×10-6。Co含量在(3.1~26.7)×10-6之间,平均值为14.08×10-6
表1 研究区样品主要元素分析结果

Table 1 Analysis results of main elements of samples in the study area

样品编号 深度/m 主量元素含量/% 微量元素含量/10-6
Al2O3 CaO MgO Na2O K2O P2O5 MnO Mo V Cr Ni Cu Co
S1 262.0 16.062 0.254 2.925 0.245 5.714 0.260 0.083 1.06 96.58 91.62 32.17 28.64 19.92
S2 258.0 16.121 0.380 4.380 0.651 4.921 0.276 0.040 0.65 95.24 48.35 35.61 38.66 10.86
S3 239.0 15.709 0.393 4.245 0.689 4.865 0.283 0.040 0.62 93.82 47.43 35.29 38.21 12.02
S4 237.5 15.071 1.278 5.444 0.660 4.782 0.228 0.064 1.96 106.29 53.53 46.25 27.05 20.83
S5 237 15.774 1.012 5.500 0.584 4.886 0.232 0.056 1.49 123.55 61.28 46.71 22.76 21.34
S6 223 15.194 1.091 5.359 0.693 4.759 0.244 0.050 1.28 110.57 49.41 42.93 18.34 19.11
S7 221 14.950 1.208 5.389 0.722 4.699 0.249 0.050 1.11 103.00 51.21 39.67 18.44 16.55
S8 183 12.456 3.458 5.744 0.352 4.770 0.275 0.078 0.51 72.77 36.41 21.53 5.68 9.97
S9 177 17.417 0.619 3.334 0.323 7.506 0.482 0.087 0.42 69.56 32.59 19.16 4.89 10.52
S10 161 14.958 0.704 4.368 0.182 5.942 0.414 0.069 0.54 86.98 48.25 28.83 5.25 10.39
S11 160 14.685 2.839 5.724 0.323 5.434 0.288 0.065 0.51 87.72 39.92 27.12 4.51 9.04
S12 157 13.910 3.130 5.721 0.353 5.330 0.283 0.071 0.45 81.42 41.53 25.79 20.26 8.61
S13 100 14.195 0.711 3.428 0.188 6.247 0.570 0.114 0.51 73.00 41.87 24.09 3.66 7.32
S14 89.0 15.275 0.542 2.495 0.193 8.054 0.439 0.028 0.44 100.51 54.65 24.99 7.30 10.57
S15 86.0 14.996 1.832 3.615 0.179 7.597 0.299 0.055 1.06 91.55 54.83 30.79 11.81 15.93
S16 81.0 10.494 4.671 4.813 0.167 6.053 0.294 0.102 0.55 47.20 40.22 12.21 4.79 5.37
S17 77.0 12.010 5.412 6.880 0.167 5.198 2.488 0.066 0.77 84.41 57.54 25.50 14.38 22.54
S18 76.0 4.515 15.460 14.316 0.122 1.608 0.443 0.186 1.19 21.06 32.14 6.58 6.73 3.12
S19 75.8 12.491 3.545 4.385 0.171 6.703 0.481 0.084 1.31 88.25 65.92 20.26 8.94 9.14
S20 74.0 11.427 4.994 5.518 0.166 5.456 1.235 0.085 2.86 72.00 55.50 19.92 10.96 9.57
S21 69.0 15.987 1.748 4.138 0.169 6.031 0.354 0.052 1.26 149.63 96.36 45.87 22.92 14.38
S22 66.0 11.954 4.761 5.173 0.162 5.316 0.415 0.099 2.41 83.46 68.08 27.32 18.28 9.22
S23 57.5 14.239 4.293 4.704 0.167 5.548 0.922 0.086 2.65 170.77 80.16 43.88 29.27 10.83
S24 56.0 16.133 1.641 3.914 0.162 6.136 0.283 0.053 9.08 202.39 60.66 58.62 62.98 22.55
S25 46.0 16.582 1.666 4.370 0.166 5.729 0.238 0.057 2.32 217.11 69.29 55.30 78.29 20.65
S26 45.0 16.762 2.140 4.865 0.162 5.390 0.222 0.063 2.88 205.77 69.69 53.93 66.47 19.41
S27 43.8 16.663 2.118 4.851 0.163 5.394 0.228 0.063 1.30 203.61 62.76 51.91 74.63 19.29
S28 41.5 16.510 2.406 5.059 0.160 5.209 0.220 0.070 1.48 193.49 60.73 50.60 81.64 19.13
S29 32.0 14.526 3.985 5.523 0.166 5.665 0.199 0.061 1.62 167.36 68.07 46.56 46.60 20.24
S30 30.0 16.454 1.681 4.554 0.171 6.301 0.237 0.043 1.25 184.75 58.86 51.47 39.67 23.69
S31 27.5 16.439 1.984 5.126 0.164 5.711 0.266 0.045 1.29 201.36 63.75 62.45 36.33 26.73

3.4 数据处理

根据本文研究需要,计算了Cu/Al、V/(V+Ni)、V/Cr、Mn×Co的值,并利用式(1)计算化学蚀变指数CIA
C I A =   A l 2 O 3 / ( A l 2 O 3 + C a O * + N a 2 O + K 2 O )   × 100  
使用样品中各元素的摩尔含量计算CIA,式中所有氧化物单位为mol。CaO*指硅酸盐中矿物中的CaO,采用式(2)计算38
C a O * = C a O - P 2 O 5 × 10 / 3       
计算后取Na2O与CaO*结果中的最小值作为CaO*,并代入式(1)中求得结果。处理后数据如图7所示。另外,引入了筇竹寺组沉积时全球海平面变化特征作为参考49,对比岩性旋回判断沉积期古水深的特征。
图7 研究区样品地球化学参数计算结果及全球海平面变化在垂向上的分布(海平面变化数据引自文献[49])

Fig.7 Calculation results of geochemical parameters of samples in the study area and vertical distribution of global sea level change (sea level changes data is modified from Ref.[49])

4 讨论

4.1 沉积古环境演化

4.1.1 古水深

根据HAQ等49对海平面变化恢复的结果,筇竹寺组可以划分为2期海侵—海退。全球海平面变化与研究区岩性特征之间表现出密切的联系,低水位时期发育粉砂质泥岩及砂岩;高水位时期主要发育泥页岩,在筇竹寺组沉积后期海平面的大幅度上升导致在上部沉积了一套较厚的泥页岩。根据岩性及矿物组分含量的变化判断古水深变化与海平面变化之间存在一定的相关性。同时由于Fe的化合物易在滨岸环境下富集,Mn可长期迁移至远滨或湖盆中,据此引入Fe/Mn值判断古水深的变化,比值越高说明水深越浅,反之说明水体较深50-52。将计算得出的结果与海平面变化对比认为筇竹寺组沉积时期古水深变化与海平面变化相近但不完全一致,然而都表现出了2次明显的升降,Fe/Mn值与海平面变化特征都反映了在筇竹寺组上段沉积时期水体深度较大。

4.1.2 古气候

由于古气候的变化会影响化学风化作用和陆源物质输入,进而改变岩石的组分,所以采用与岩石化学成分相关性较强的化学蚀变指数(CIA)可以反映沉积物沉积时期的古气候变化53-55。通常认为CIA值在50~65之间为伴随弱风化作用的寒冷干旱型气候,当65<CIA<85时反映中等化学风化背景下的暖湿型气候,当CIA值介于85~100之间时反映炎热潮湿的气候,此时沉积物所受到的风化作用较强26。研究区内CIA值介于55~75之间,大部分样品CIA>65,部分样品CIA值在65上下频繁变化,说明筇竹寺组沉积时期处于一个温暖湿润气候之下。在早期气候相对温暖湿润,之后气候向寒冷干旱过渡,后期先突变为寒冷干旱的环境(图7中80 m处),其后逐渐演化为相比早期更加温暖潮湿的环境。

4.1.3 古氧化还原条件

古氧化还原条件对有机质富集有着重要的影响,在还原性水体中有机质相对更容易保存和富集1056-59。古海洋中的部分元素对氧化还原条件的敏感性较强,氧化还原条件控制着它们的迁移和富集。本文研究利用V/Cr值与V/(V+Ni)值对水体氧化还原条件的敏感性来判断沉积水体性质60-63,通常认为V/Cr<2代表沉积水体富氧,2<V/Cr<4.25代表沉积水体贫氧,V/Cr>4.25代表沉积水体缺氧。V/(V+Ni)<0.6为富氧水体,0.6<V/(V+Ni)<0.8时为贫氧水体,V/(V+Ni)>0.8时为缺氧水体。根据本文研究结果,样品中V/(V+Ni)值与V/Cr值变化趋势虽然在局部上有差异,但2种指标都反映了筇竹寺组沉积水体前期还原性较弱,后期还原性急剧上升,迅速转变为缺氧环境。

4.1.4 古生产力

现有的研究表明,上扬子地区筇竹寺组可能受到热液的作用4664-67。因此在对热液具体作用范围和强度没有明确认识的情况下,不宜选用生物Ba作为古生产力的判断指标。然而P虽然是生物富集的重要营养物质,但其易受氧化还原条件的影响而产生误差68-69,所以不使用这2种常用元素的结果量化古生产力。Cu与古生产力之间的关系长期以来已经得到了众多学者的认可70-73,而Mo与TOC的相关性也让Mo越来越多地被用于评价古生产力中3174-77。所以本文研究选取了Mo、Cu/Al来作为衡量古生产力的指标。Mo与Cu/Al在纵向上的变化特征不完全相同,但都反映了筇竹寺组沉积前中期古生产力稳定地保持较低水平,后期逐渐升高后骤降的特征。

4.2 有机质富集主控因素

4.2.1 沉积古环境对有机质富集的影响

图8所示,通过对比沉积古环境指标与TOC之间的关系,认为古生产力与TOC的相关性最好,Mo含量与TOC表现出密切的相关性,古气候和古氧化还原条件与有机质富集的相关性很差,相关性分析结果表明有机质富集只受控于古生产力的作用。但是在剖面上纵向对比TOC与古环境指标的变化趋势,发现在筇竹寺组上段TOC较高的地层,即剖面上45~75 m发育的泥页岩中(图7),沉积水体长期处于贫氧—缺氧的状态,有利于有机质的保存。在剖面上66 m处及74 m处,Mo含量达到了富有机质泥页岩中Mo含量的标准,但TOC含量远低于上部60 m页岩段TOC图7),由此说明水体还原性对有机质富集的影响。泥页岩沉积时期处于远滨环境下,温暖潮湿的古气候促进了古生物的发育,长期处于贫氧—缺氧的水体满足了有机质的保存条件,远滨环境下陆源碎屑的输入既能够为古生物提供营养物质,又不会因陆源物质过多而导致沉积时稀释有机质。综合对比古水深变化、气候、保存条件及古生产力特征发现,在筇竹寺组沉积后期沉积古环境全面向利于有机质富集的环境特征演化,这种有机质富集模式更近似于古生产力和保存条件的混合作用。
图8 研究区样品TOC与古生产力、古气候、氧化还原性质指标的相关性

Fig.8 Correlation between TOC and palaeoproductivity, palaeoclimate and redox properties of samples in the study area

4.2.2 古水深变化对有机质富集的影响

古水深的变化在滨岸环境下有机质富集过程中扮演着重要的角色。一方面,古水深的变化改变了古生物繁殖的环境,导致古生产力的变化;另一方面,古水深的变化会改变底层沉积水体的还原性,影响有机质的保存条件。此外,古水深的变化较大程度地影响了陆源碎屑及水体滞留程度与有机质富集的关系,使不同时期沉积的有机质表现为不同的富集模式。
有机质的产生主要源于沉积环境中死亡的古生物,同时又受外来物质的影响。研究区筇竹寺组沉积于滨岸环境,在这种环境下水体滞留程度较高,易受到陆源碎屑的作用。据前人研究,由于深部海洋流体缺乏Mn与Co,大陆边缘上升流环境中的沉积物同样含Mn、Co较少,因此本文研究通过Mn×Co的值来衡量水体滞留程度/上升洋流的影响,通常认为Mn×Co<0.4×10-8则受上升洋流的影响,Mn×Co>0.4×10-8则水体处于滞留环境3178-79。由于研究区石英为陆源输入,所以根据石英的含量可以很好地判断陆源碎屑的输入量。对比TOC与陆源碎屑和滞留程度的关系发现,虽然TOC与2种指标的相关性很低,但在散点图上的分布是有一定规律性的。随着水体滞留程度的增强以及陆源碎屑输入量的增加,TOC值在图中分别按照2个特定的斜率方向变化[图9(a),图9(b)]。所以判断TOC的分布与水体滞留程度和陆源碎屑的输入是相关的。
图9 研究区样品TOC与陆源碎屑及水体滞留程度指标在不同水深下的相关性差异

Fig.9 Correlation difference between TOC and retention degree of terrigenous debris and water under different water depths in the study area

观察TOC在纵向上的变化可知,富有机质泥页岩发育于筇竹寺组后期黏土矿物含量升高,泥页岩厚度较大的层段。该层段沉积时期古水深出现明显的升降变化。筛选出筇竹寺组上段富有机质泥页岩样品作相关性分析,发现该段地层的陆源碎屑输入量及滞留程度都与TOC分布呈一定的正相关性,相关系数R 2值分别为0.518和0.543 3[图9(c),图9(d)],说明此时有机质的富集与陆源碎屑输入量及水体滞留程度呈正相关。在全球海平面变化、气候及构造运动的影响下,海水到达一定深度后,少量陆源碎屑物质伴随着有机质进入研究区,同时陆源碎屑中的营养物质也有利于古生产力的提高,相对滞留的缺氧水体也为有机质提供了优越的保存条件。当古水体深度减小,海水较浅时,沉积环境变为近滨。此时由于古生物的发育环境被破坏,陆源营养物质的输入对古生产力基本无影响,过量的陆源碎屑反而在沉积时稀释了有机质,陆源碎屑输入与有机质富集呈现出微弱的负相关性。同时由于古生产力降低,在有机质供给不足的情况下水体滞留程度对有机质的影响也远小于高水位时期,此时陆源碎屑输入量和滞留程度与TOC之间形成了另一种相关性模式。
综上所述,在古水深不同的情况下有机质富集过程中古生产力、保存条件和有机质的沉积都会出现一定的差异。因此古水深对有机质富集过程有重要的影响。

4.3 有机质富集模式

前人提出的古生产力模式、保存模式以及混合模式实际上都需要有机质的生产、保存和沉积条件的协调。之所以区分不同的模式仅仅是因为古生产力,保存条件或者其他因素在长期沉积过程中也会发生差异性变化,所以研究区有机质富集属于多因素的混合作用模式。而本文着重提出有机质富集受古水深变化的影响是因为虽然在有机质富集过程中各种因素长期差异性变化且相互影响,但古水体深度的变化在这个过程中起到主导的作用。
峨边地区筇竹寺组沉积期整体上处于一个不利于有机质富集的环境下。不稳定的气候、缺乏上升洋流带来的营养物质、陆源碎屑的大量输入等因素都阻碍了有机质的富集。但是如本文4.2节所提及的,陆源碎屑和滞留程度与TOC随水深的变化可以形成2种相关性模式,古水深的变化能够改变有机质富集的控制因素以它他们之间的相互作用。筇竹寺组下段沉积时期,水体深度处于一个相对较低的水平,除早期和末期的气候温暖湿润外其他时期古气候都处于寒冷干旱—温暖湿润之间,缺少一个利于古生物繁殖的环境,导致在有陆源营养物质供给的条件下古生产力依然较低;同时浅水环境中上层富氧水体可能将氧气带入底部沉积水体中,底部沉积水体的还原性较差,有机质保存条件恶劣;过量的陆源碎屑输入在沉积时一定程度上稀释了有机质,整体环境不利于有机质富集。在这种环境下即使有少量古生物繁殖也很难使TOC值明显升高(图7中237 m处样品特征)。筇竹寺组沉积后期远滨环境下古生产力随着古气候变得温暖湿润和陆源营养物质输入量的增加而提高。水体深度增加而产生的水体分层导致底部水体还原性增强,为有机质保存提供一个良好的环境[图7中75 m处样品特征,图10(a)]。由于此时陆源碎屑输入量较少,在沉积过程中陆源物质对有机质的稀释作用降低,此时水体滞留程度和陆源碎屑输入量与有机质含量都呈现出正相关的关系[图9(c),图9(d)]。在之后的海退过程中,由于温暖潮湿的气候以及陆源营养物质的输入,古生产力在短期内没有下降。而海退过程中由于水深降低,闭塞环境中水体流动性变差使底部水体滞留程度越来越高。虽然水深的降低导致水体分层不明显,但水体的滞留程度增加以及古生物呼吸对于水中氧气的消耗使水体还原性大幅上升[见图7中56 m处样品特征,图10(b)]。在这段沉积时期仍然满足有机质富集条件,TOC保持在较高水平。之后随着水深降低演化为近滨环境,筇竹寺组沉积末期古生物繁殖被破坏,古生产力迅速降低。虽然水体具有更强的还原性,但缺少有机质来源,无法形成有机质富集,TOC随之骤降(见图7中40 m处样品特征)。这种滨岸环境下的有机质富集模式具有以下几个特征:富有机质泥页岩矿物组分呈现出混积特征,既有大量的黏土矿物,又同时含有一定的陆源碎屑和碳酸盐矿物组分;虽然处于滨岸环境下,但富有机质泥页岩发育于水体较深的时期,只有古水深到达一定深度后才创造出利于有机质富集的环境,这种环境下黏土矿物含量较高,在筇竹寺组中表现为上段沉积的厚层泥页岩,有机质富集段存在于厚层泥页岩中;由于峨边地区筇竹寺组后期水体深度的大幅度变化,很可能会出现岩性突变,这种富有机质泥页岩上易沉积灰绿色砂质泥岩或粉砂岩。根据以上特征可进一步预测滨岸环境下有机质富集区带。
图10 峨边地区筇竹寺组上段泥页岩有机质富集模式

(a)古水深升高过程中有机质富集特征;(b)古水深降低过程中有机质富集特征

Fig.10 The enrichment pattern of organic matter in the shale of the Upper Qiongzhusi Formation in Ebian area

5 结论

(1)川西南峨边葛村剖面筇竹寺组沉积于滨岸浅水的环境中,发育了一套细粒沉积岩。与全球海平面变化相对应,剖面上可识别出2期海侵—海退,上段沉积了一套远滨相泥页岩,下段主要为近滨相粉砂岩及砂质泥岩。有机质富集于上段远滨相泥页岩中,该段页岩厚度超过30 m,TOC平均值为1.48%,最高可达2%。对比威远地区筇竹寺组页岩条件认为滨岸环境下富有机质页岩同样具有一定的勘探潜力。
(2)研究区处于滨岸浅水环境下,缺乏洋流输入的营养物质以及适宜生物发育的水体条件,整体上处于一个气候温暖湿润、古生产力相对较低、贫氧—滞留的古环境中。陆源物质的过量输入和水体环境的不稳定变化都阻碍了有机质的富集,仅在筇竹寺组沉积后期满足富集条件。
(3)研究区有机质富集是由于古生产力和氧化还原条件的共同作用,同时受古水深的影响。陆源物质、水体滞留程度与TOC的关系和古生产力与古气候的关系在水体深度不同的情况下表现为不同的相关性。在海水达到一定深度时古生产力提高,古水深波动变化致使沉积水体长期缺氧,为有机质富集创造了优越的环境,在此基础上建立了滨岸环境下的有机质富集模式。
(4)川西南地区及川北地区近滨环境下发育的筇竹寺组中有机质富集层段应集中于筇竹寺组上段发育的厚层泥页岩内,对于其他滨岸环境下有机质富集层段的预测重点应该在古水深较大的滞留环境中沉积的泥页岩中。

感谢三位匿名审稿专家的意见和认真指导!同时也感谢四川省煤田地质局科源测试中心提供的实验条件!

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