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Volume 42 Issue 4
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Article Navigation >  Acta Sedimentologica Sinica  > 2024  >  42(4): 1433-1446

LIN ShuMin, ZHANG Min, LI HongBo. Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1433-1446. doi: 10.14027/j.issn.1000-0550.2023.139
Citation: LIN ShuMin, ZHANG Min, LI HongBo. Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1433-1446. doi: 10.14027/j.issn.1000-0550.2023.139
shu

Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin

doi: 10.14027/j.issn.1000-0550.2023.139
cstr: 32268.14.cjxb.62-1038.2023.139
  • 1.

    Hubei Key Laboratory of Petroleum Geochemistry and Environment (School of Resources and Environment, Yangtze University), Wuhan 430100, China

  • 2.

    Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan 430100, China

Funds:

National Natural Science Foundation of China 42072165

  • Received Date: 2023-05-17
  • Accepted Date: 2024-01-08
  • Rev Recd Date: 2023-12-21
    • Available Online: 2024-01-08
  • Publish Date: 2024-08-10

  • Objective  Coal-measure source rocks in Ordos Basin are developed, and the content of tricyclic terpane in saturated hydrocarbons is rich. However, the distribution patterns of tricyclic terpane show important differences in the study area. Therefore, it is of great significance to explore the main controlling factors for these differences.   Methods  Saturated hydrocarbons of 26 coal-measure source rock samples in the Upper Paleozoic of the Ordos Basin were analyzed in detail by gas chromatography and chromatography⁃mass spectrometry.   Results and Discussions  According to the distribution characteristics of tricyclic terpanes （TTs）, the source rock samples in the study area were divided into three types of distribution patterns. The C19TT is the main peak of the type I source rock, and the content of C19TT, C20TT and C21TT gradually decreases. At the same time, the Pr/Ph ratio is high, and the ratio distribution range is between 1.19 and 3.84, with an average value of 2.99. The content of C24 tetracyclic terpane is also rich in this type of sample. The ratio distribution range of C24 tetracyclic terpane/C26 tricyclic terpane is between 2.39 and 32.54, with an average value of 9.80. Both C28 tricyclic terpane and C29 tricyclic terpane are absent. This kind of sample is mainly humic coal and carbonaceous mudstone, revealing the characteristics of a lacustrine facies sedimentary environment. By contrast, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the content of C19TT, C20TT and C21TT gradually increases, whereas the Pr/Ph ratio is low and the ratio distribution range is mainly within the range of 0.34 and 0.90, with an average value of 0.61. The content of C24 tetracyclic terpane is not abundant, and the ratio distribution range of C24 tetracyclic terpane/C26 tricyclic terpane is mainly between 0.42 and 0.77, with an average value of 0.54. Both the C28 tricyclic terpane and C29 tricyclic terpane are abundant. In addition to coal-measure mudstone, humic coal and carbonaceous mudstone are present in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is in the middle of the type I and type II source rocks. The main peak carbon of tricyclic terpane is C23TT, whereas the relative content of C19TT, C20TT and C21TT decreases in turn with a stepwise distribution. In addition, the Pr/Ph ratio is 2.07 and the C24 tetracyclic terpane/C26 tricyclic terpane is 3.53, both of which are between the type I and type II source rocks.   Conclusions  The results reveal that with an increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows a gradually increasing trend, although the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly. To put it another way, the different distribution patterns of tricyclic terpanes in coal-measure source rocks in the study area are less affected by the thermal evolution degree of organic matter. Rather, they are mainly controlled by the sedimentary environment and parent material type.

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  • Received:  2023-05-17
  • Revised:  2023-12-21
  • Accepted:  2024-01-08
  • Published:  2024-08-10

Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin

doi: 10.14027/j.issn.1000-0550.2023.139
  • 1. Hubei Key Laboratory of Petroleum Geochemistry and Environment (School of Resources and Environment, Yangtze University), Wuhan 430100, China
  • 2. Key Laboratory of Exploration Technologies for Oil and Gas Resources (Yangtze University), Ministry of Education, Wuhan 430100, China
Funds:

National Natural Science Foundation of China 42072165

  • Received Date: 2023-05-17
  • Accepted Date: 2024-01-08
  • Rev Recd Date: 2023-12-21
  • Available Online: 2024-01-08
  • Publish Date: 2024-08-10

Abstract: 

Objective  Coal-measure source rocks in Ordos Basin are developed, and the content of tricyclic terpane in saturated hydrocarbons is rich. However, the distribution patterns of tricyclic terpane show important differences in the study area. Therefore, it is of great significance to explore the main controlling factors for these differences.   Methods  Saturated hydrocarbons of 26 coal-measure source rock samples in the Upper Paleozoic of the Ordos Basin were analyzed in detail by gas chromatography and chromatography⁃mass spectrometry.   Results and Discussions  According to the distribution characteristics of tricyclic terpanes （TTs）, the source rock samples in the study area were divided into three types of distribution patterns. The C19TT is the main peak of the type I source rock, and the content of C19TT, C20TT and C21TT gradually decreases. At the same time, the Pr/Ph ratio is high, and the ratio distribution range is between 1.19 and 3.84, with an average value of 2.99. The content of C24 tetracyclic terpane is also rich in this type of sample. The ratio distribution range of C24 tetracyclic terpane/C26 tricyclic terpane is between 2.39 and 32.54, with an average value of 9.80. Both C28 tricyclic terpane and C29 tricyclic terpane are absent. This kind of sample is mainly humic coal and carbonaceous mudstone, revealing the characteristics of a lacustrine facies sedimentary environment. By contrast, the tricyclic terpanes of type II source rocks are dominated by C23TT, and the content of C19TT, C20TT and C21TT gradually increases, whereas the Pr/Ph ratio is low and the ratio distribution range is mainly within the range of 0.34 and 0.90, with an average value of 0.61. The content of C24 tetracyclic terpane is not abundant, and the ratio distribution range of C24 tetracyclic terpane/C26 tricyclic terpane is mainly between 0.42 and 0.77, with an average value of 0.54. Both the C28 tricyclic terpane and C29 tricyclic terpane are abundant. In addition to coal-measure mudstone, humic coal and carbonaceous mudstone are present in these samples, and their sedimentary environment is quite different from that of type I source rocks. The distribution of tricyclic terpanes in type III source rocks is in the middle of the type I and type II source rocks. The main peak carbon of tricyclic terpane is C23TT, whereas the relative content of C19TT, C20TT and C21TT decreases in turn with a stepwise distribution. In addition, the Pr/Ph ratio is 2.07 and the C24 tetracyclic terpane/C26 tricyclic terpane is 3.53, both of which are between the type I and type II source rocks.   Conclusions  The results reveal that with an increase of the thermal evolution degree of source rocks, the total amount of tricyclic terpanes shows a gradually increasing trend, although the distribution patterns of tricyclic terpanes in different types of source rocks have not changed significantly. To put it another way, the different distribution patterns of tricyclic terpanes in coal-measure source rocks in the study area are less affected by the thermal evolution degree of organic matter. Rather, they are mainly controlled by the sedimentary environment and parent material type.

LIN ShuMin, ZHANG Min, LI HongBo. Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1433-1446. doi: 10.14027/j.issn.1000-0550.2023.139
Citation: LIN ShuMin, ZHANG Min, LI HongBo. Origin of Tricyclic Terpane in the Upper Paleozoic Coal-measure Source Rocks in the Ordos Basin[J]. Acta Sedimentologica Sinica, 2024, 42(4): 1433-1446. doi: 10.14027/j.issn.1000-0550.2023.139
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0.   引言

  • 煤系烃源岩（煤、炭质泥岩、泥岩）是含油气盆地中一类形成于过渡环境条件的重要的烃源岩,有机前身物主要来自高等植物输入[1]。煤系烃源岩作为油气尤其是天然气的主要贡献者,其基础地球化学和分子地球化学研究引起广泛的关注[2⁃5]。傅家谟等[6]率先在富含藻质体的烛藻煤和含腐泥组的残植煤中检出三环萜烷；其后程克明[7]研究吐哈盆地时发现其泥岩和煤普遍含有三环萜烷,且利用三环/四环萜烷指纹对比图得出中、下侏罗统煤和下侏罗统煤系泥岩与台北凹陷“弧形带”原油具有相同的分布特征,表明其存在一定亲属关系。大量文献资料表明煤系烃源岩中广泛分布三环萜烷[8⁃13],三环萜类化合物具有良好的稳定性和生物降解能力,是油源对比、判断有机质来源和演化程度的重要手段。

    鄂尔多斯盆地是华北克拉通西部边缘最稳定的一个大型多旋回克拉通盆地[14]。盆内煤炭资源丰富,煤系烃源岩发育,煤生烃潜力大[15]。目前,研究鄂尔多斯盆地煤系烃源岩三环萜烷地球化学特征的文献有限,内容大多限于鄂尔多斯盆地烃源岩评价或煤系烃源岩中气体的地球化学特征研究[16⁃19]。对三环萜烷详细解剖的文章较少,而典型的煤及煤成油中富含C19三环萜烷（TT）,C23及以上碳数的三环萜烷含量极低,C19TT、C20TT、C21TT相对含量依次降低,呈阶梯状分布型态[20⁃21]。本文对鄂尔多斯盆地上古生界煤系烃源岩三环萜烷的研究发现其分布模式存在多样性,一种与典型沼泽相煤系烃源岩三环萜烷分布一致,以C19TT为主峰,C23TT含量较低,C24四环萜烷（TeT）含量丰富；饶有兴趣的是许多腐殖煤样品三环萜烷以C23TT为主峰,C19TT含量相对较低,C24四环萜烷（TeT）含量较低,对其分布模式与成因的探讨目前文献中少有报道。本次研究不仅丰富和完善了三环萜烷的成因理论,而且为生物标志化合物在油气勘探中的应用提供了新的地球化学参数。

1.   地质背景与实验

    1.1.   地质概况

  • 鄂尔多斯盆地是华北克拉通西部边缘最稳定的一个大型多旋回克拉通盆地[14]。鄂尔多斯盆地是华北地台西端的一个次级构造单元,它包括伊盟隆起、渭北隆起、伊陕斜坡、西缘逆冲带、晋西挠褶带和天环坳陷等6个二级构造单元[22]。盆地晚古生代经历了海相沉积为主的陆表海盆地、海陆过渡相为主的近海湖盆及陆相碎屑岩沉积为主的内陆坳陷湖盆的古地理演化过程[23]。鄂尔多斯盆地二叠系由下至上发育了二叠统的太原组和山西组、中二叠统的下石盒子组、上石盒子组和上二叠统石千峰组（图1）。

    Figure 1.  The distribution map of coal accumulation environment and sampling wells in the Lower Permian of the Ordos Basin and the comprehensive histogram of the study area (modified from reference [24])

    鄂尔多斯盆地山西组沉积时期为陆表海背景下的海陆过渡沉积演化阶段,海陆过渡三角洲体系广泛发育,到下石盒子组时期才完全进入陆相湖盆的沉积演化阶段[24]。山西组时期盆地水体由北部向中部缓慢加深,依次发育扇缘泥炭沼泽相、岸后泥炭沼泽相、三角洲平原泥炭沼泽相和湖滨泥炭沼泽相[25]。

    • 1.2.   样品与实验

  • 研究所涉及的煤系烃源岩取自乌达、保德、柳林、澄城、韩城5个野外露头剖面,以及伊盟隆起中部的苏27井、陕北斜坡带东北部的榆20井与双1井,岩性为煤、炭质泥岩、泥岩。对26个样品进行全岩显微组分分析、镜质体反射率测定、岩石热解、有机碳测定、抽提、族组分分离和GC-MS定量分析,样品实验的GC-MS分析条件见文献[26],部分主要生标参数见表1。

    三环萜烷分布模式井号/样品名深度/m层位岩性TOC/%Ro/%Tmax/℃Pr/PhC19TT/C23TTC19-C2TT/C23-C25TT∑三环萜烷/C30HG/C30HC24TeT/ C26TT
    WD-1地表露头P1s煤51.950.664413.848.169.060.280.0232.54
    WD-2地表露头P1t煤63.030.874631.195.565.829.700.132.60
    BD-1地表露头P1s煤43.880.684322.8614.2914.740.220.026.76
    Ⅰ类BD-2地表露头P1t煤38.480.664355.293.112.680.340.035.63
    BD-3地表露头P1t炭质泥岩10.89—4342.285.143.500.220.0311.61
    苏27井SU-13 157.37~3 158.08P1s煤44.09—4462.102.803.252.320.067.07
    苏27井SU-23 185.06~3 185.38P1s炭质泥岩6.85—4503.351.421.975.080.122.39
    WD-3地表露头P1s泥岩1.82—5430.710.551.432.990.162.34
    WD-4地表露头P1t泥岩4.42—5181.130.211.164.250.160.55
    WD-5地表露头C2y泥岩1.440.875050.900.091.014.640.140.51
    WD-6地表露头C2y煤65.260.945370.900.100.943.420.160.50
    WD-7地表露头C2j泥岩3.11—5650.630.111.024.470.190.51
    WD-8地表露头C2j煤39.94—4990.340.030.754.990.210.51
    LL-1地表露头P1s煤54.541.134720.970.981.713.440.230.77
    LL-2地表露头P1t炭质泥岩10.001.235210.540.080.937.560.170.51
    Ⅱ类HC-1地表露头P1t炭质泥岩17.391.335620.320.070.849.180.160.48
    CC-1地表露头P1s炭质泥岩6.911.585210.630.231.083.630.150.46
    CC-2地表露头P1t煤46.961.594940.720.281.113.310.150.47
    CC-3地表露头C2b煤33.181.635170.650.070.814.620.160.46
    双1井 S-12 619.35~2 619.47P1s泥岩1.141.034780.450.070.712.650.170.42
    双1井 S-22 697.68~2 697.77P1s煤33.051.154780.480.180.823.020.150.54
    榆20井 Y-12 696.11~2 701.95P2sh泥岩2.201.164860.520.110.833.510.170.60
    榆20井 Y-22 733.60~2 734.84P1s煤62.661.124750.390.130.783.370.180.50
    榆20井 Y-32 744.40~2 749.41P1s炭质泥岩23.541.204740.390.080.714.790.180.62
    苏27井SU-33 155.03~3 155.97P1s泥岩2.510.824550.830.341.053.250.100.77
    Ⅲ类苏27井SU-43 184.46~3 185.06P1s煤63.140.864502.070.911.556.200.133.53

    Table 1.  Parameters of main biomarker compounds of the Upper Paleozoic coal⁃measure source rocks in the Ordos Basin

    研究所涉及煤岩的TOC介于33.05%~65.26%,显微组分富含镜质组、惰质组和腐泥组,少量壳质组；TI指数介于-69.4~-41.8；以Ⅲ型干酪根为主,符合腐殖煤的特征；Ro介于0.66%~1.63%,Tmax介于432 ℃~537 ℃。

    炭质泥岩TOC介于6.85%~23.54%,显微组分主要含镜质组和腐泥组,少量惰质组和壳质组；TI指数介于-69.4~-13.5,以Ⅲ型干酪根为主；Ro介于1.20%~1.58%,Tmax介于434 ℃~562 ℃。

    泥岩TOC介于1.14%~4.42%,显微组分主要为镜质组和腐泥组,几乎不含惰质组和壳质组；TI指数介于-8.8~21.4；以Ⅱ1型和Ⅲ型干酪根为主。Ro介于0.82%~1.16%,Tmax介于455 ℃~565 ℃。

2.   结果与讨论

    2.1.   三环萜烷分布模式

  • 鄂尔多斯盆地上古生界26个煤系烃源岩样品,利用三环萜烷（TT）的地球化学特征,将鄂尔多斯盆地石炭系—二叠系的煤系烃源岩样品划分为三种类型（图2）。

    Figure 2.  Tricyclic terpane distribution of the Upper Paleozoic coal⁃measure source rock samples in the Ordos Basin (m/z 191)

    Ⅰ类烃源岩以煤岩样品BD-2和炭质泥岩样品苏27井3 185.06~3 185.38 m（SU-2）为例,以C19TT为主峰,C19TT、C20TT、C21TT相对含量依次降低,呈阶梯状分布,C24四环萜烷（TeT）含量丰富,C28TT-C29TT缺失。

    Ⅱ类烃源岩以煤岩样品WD-8、炭质泥岩样品HC-1、泥岩样品榆20井2 696.11~2 701.95 m（Y-1）为例,主峰为C23TT,C19TT、C20TT、C21TT相对含量依次增加,呈正态分布,C24TeT含量不丰富,但存在较丰富的C28TT-C29TT。

    Ⅲ类烃源岩为煤岩苏27井3 184.46~3 185.06 m（SU-4）,以C23TT为主峰,C19TT、C20TT、C21TT相对含量依次降低,呈阶梯状分布,C24TeT含量丰富。

    综上所述,在研究的煤系烃源岩中三环萜烷存在三种分布模式,Ⅰ类煤岩和炭质泥岩均表现出湖沼相分布特征,该分布模式与典型煤系烃源岩三环萜烷分布模式一致[20⁃21]。但还存在位于WD-6、WD-8、LL-1、CC-2、CC-3和双1井2 697.68~2 697.77 m（S-2）、榆20井2 733.60~2 739.84 m（Y-2）部分腐殖煤样品,其三环萜烷分布有明显的特殊性,表现出以C23TT为主峰,此分布模式一般出现在湖相或海相中烃源岩中,在腐殖煤中出现此种三环萜烷的分布模式文献少有报道,其成因有待进一步的探讨。Ⅲ类炭质泥岩的分布模式介于Ⅰ类和Ⅱ类之间,呈过渡形式的分布。

    • 2.2.   三环萜烷组成特征与成熟度关系

  • 因本文样品镜质体反射率Ro数据有限,且Ro与成熟度指标Tmax有较好的正相关关系（图3a）,因此用Tmax可以代替Ro表征样品的成熟度。

    Figure 3.  Relationship between the Upper Paleozoic coal⁃measure source rocks in the Ordos Basin

    随着成熟度的增高,三环萜烷会优先从干酪根或沥青质中释放,且其热稳定性强于藿烷,导致三环萜烷的相对丰度高于藿烷[27⁃28]。本次研究分析的26件煤系烃源岩样品随着热演化程度的增大（图3b）,Ⅰ类煤岩和炭质泥岩∑三环萜烷/C30H比值主体介于0.22~0.34,平均值为0.27；Ⅱ类煤系烃源岩∑三环萜烷/C30H比值主体介于2.65~7.56,平均值为3.99；Ⅲ类炭质泥岩∑三环萜烷/C30H比值介于Ⅰ类和Ⅱ类分布模式之间,其值为6.20（表1）。本文所研究样品的镜质体反射率Ro范围跨度较大,介于0.66%~1.63%,整体表现出随着成熟度的增大,三环萜烷总量有明显的增加。

    Ⅰ类煤岩和炭质泥岩的C19-C21TT/C23-C25TT介于1.97~14.74,平均值为5.86、C19TT/C23TT介于1.42~14.29,平均值为5.78；Ⅱ类煤系烃源岩的C19-C21TT/C23-C25TT主体介于0.71~1.08,平均值为0.94、C19TT/C23TT主体介于0.03~0.34,平均值为0.16；Ⅲ类炭质泥岩的C19-C21TT/C23-C25TT为1.55、C19TT/C23TT为0.91均介于Ⅰ类和Ⅱ类之间（表1）。其中,I类煤岩和炭质泥岩的Ro介于0.66%~0.87%,Tmax介于432 ℃~463 ℃小范围地变化,但C19-C21TT/C23-C25TT和C19TT/C23TT含量有明显的增大幅度。相反,在Ⅱ类煤系烃源岩中,其成熟度跨度较大,Ro介于0.82%~1.63%,Tmax介于455 ℃~565 ℃,但其C19-C21TT/C23-C25TT和C19TT/C23TT随成熟度的增加含量未有明显变化,说明成熟度对三环萜烷的分布模式影响较小。陈哲龙等[29]对准噶尔盆地玛湖凹陷二叠系烃源岩的加水热模拟实验分析表明,当成熟度处于成熟—高成熟阶段,随着成熟度的增大不同碳数的三环萜烷含量以及三环萜烷总量均明显改变,但成熟度对C19-C23TT的分布样式影响不大,此结论与本文研究不谋而合。而在腐殖煤样品中有两种分布模式,一种以C19TT为主峰,C24TeT含量丰富；另一种以C23TT为主峰,C24TeT含量不丰富。分布模式存在差异的原因可能与沉积环境和母质类型有关[30⁃31]（图3c,d）。

    • 2.3.   三环萜烷组成特征与沉积环境关系

  • 从沉积学角度来看,研究区广泛发育泥炭沼泽相、泥炭坪相体系,沉积水体表现为淡—微咸水沉积环境。类异戊二烯烷烃中姥鲛烷和植烷的比值（Pr/Ph）、Pr/nC17和Ph/nC18、伽马蜡烷指数（G/C30H）以及含硫芳烃化合物相对丰度,常用来表示烃源岩沉积时的氧化还原性和咸度[32⁃35]。Pr/nC17和Ph/nC18关系图显示不同分布模式煤系烃源岩所处沉积环境的氧化还原性存在差异（图4a）：Ⅰ类煤岩和炭质泥岩大体处于氧化型沉积环境；Ⅱ类煤系烃源岩大体处于还原—强还原型沉积环境中；Ⅲ类炭质泥岩为氧化型沉积环境；因此,有机质沉积时水体的氧化还原性是影响不同分布模式煤系烃源岩三环萜烷相对丰度的因素。研究区中腐殖煤存在两种迥然不同的分布模式,主要现在C19TT和C23TT的相对丰度上,故此选择C19TT/C23TT的比值分别与Pr/Ph比值、G/C30H比值和DBT/PHEN比值的相关关系来研究不同环境参数与单个三环萜烷比值的内在联系。鄂尔多斯盆地上古生界不同岩性煤系烃源岩样品姥植比变化幅度较大,介于0.32~5.29,暗示其沉积环境存在明显的差异性,具体表现为Ⅰ类煤岩和炭质泥岩Pr/Ph值相对较高,主体大于2.0,平均值为2.99；Ⅱ类煤系烃源岩Pr/Ph值相对较低,主体小于1.0,平均值仅为0.61；Ⅲ类炭质泥岩Pr/Ph值介于Ⅰ类和Ⅱ类煤系烃源岩之间,Pr/Ph值为2.07。研究区煤系烃源岩Pr/Ph这种变化趋势导致了样品中三环萜烷的分布模式发生了规律性变化。具体而言,三环萜烷单个化合物的相对比值如C19TT/C23TT比值随着Pr/Ph值呈现出正相关关系,因此,Pr/Ph和C19TT/C23TT比值可以很好地将研究区煤系烃源岩分成三种类型（表1、图4b）。

    Figure 4.  Relationship between the Upper Paleozoic coal⁃measure source rocks in the Ordos Basin

    伽马蜡烷（G）能够较好地反映沉积环境的盐度和水体分层程度[36]。三类烃源岩的G/C30H比值介于0.02~0.23,平均值为0.14,整体上研究区介于淡—微咸水沉积环境。Ⅰ类煤岩和炭质泥岩的G/C30H比值介于0.02~0.13,平均值为0.06,相对较低以淡水为主；Ⅱ类煤系烃源岩的G/C30H比值介于0.10~0.23,平均值为0.17,相对较高以微咸水为主（表1）。三类煤系烃源岩三环萜烷的相对丰度与水体的咸化程度有较好的负相关性,水体咸化程度的高低影响C19TT/C23TT比值变化；即随着水体咸度的增加,C19TT/C23TT比值逐渐减小（图4c）。

    三芴系列化合物中的二苯并噻吩（DBT）结合其他指标如菲（PHEN）、姥植比（Pr/Ph）等可进一步判识成煤环境[37]。Ⅰ类煤岩和炭质泥岩的DBT/PHEN比值分布集中,主体介于0.001 7~0.068 8,平均值为0.023 1,表示样品中含硫化合物的含量相对较低；相反Ⅱ类煤系烃源岩的DBT/PHEN比值分布跨度较大,介于0.001 6~0.667 8,平均值为0.118 4,样品中含硫化合物的含量相对较高,暗示成烃母质在相对还原的环境中生成。由此可知,Ⅱ类煤系烃源岩DBT/PHEN比值平均值是Ⅰ类煤岩和炭质泥岩DBT/PHEN比值平均值的5倍,表明Ⅱ类煤系烃源岩还原性强于Ⅰ类煤岩和炭质泥岩（表1、图4d）。

    为了更好地研究三环萜烷系列相对丰度的变化,选用三环萜烷参数C19-C21TT/C23-C25TT研究沉积环境对其分布模式的影响。三类煤系烃源岩烃样品三环萜烷的相对丰度与沉积环境的氧化还原性有较好的正相关性（图5a）：Ⅰ类煤岩和炭质泥岩Pr/Ph比值主体大于2.0,C19-C21TT/C23-C25TT比值主体介于1.97~5.82,平均值为3.44,低碳数三环萜烷含量异常丰富,该比值较高；Ⅱ类煤系烃源岩Pr/Ph比值分布集中主体小于1.0,C19-C21TT/C23-C25TT比值主体介于0.71~1.08,平均值为0.89,高碳数的三环萜烷化合物含量大于低碳数的三环萜烷；Ⅲ类炭质泥岩Pr/Ph值为2.07,C19-C21TT/C23-C25TT为1.55均介于Ⅰ类和Ⅱ类煤系烃源岩之间（表1）。与图4b结果相同,Pr/Ph值的含量同样影响着C19-C21TT/C23-C25TT参数的变化,表明沉积环境的氧化还原性不仅对三环萜烷的单个化合物（C19TT、C23TT）分布模式产生影响,而且对低碳数（C19-C21TT）和高碳数（C23-C25TT）的三环萜烷分布模式同样产生影响。C19-C21TT/C23-C25TT比值与指示水体盐度的参数G/C30H和表征成煤环境的参数DBT/PHEN也有较好的负相关性,其变化趋势与C19TT/C23TT比值一致（图5b,c）。且烷基二苯并噻吩与烷基二苯并呋喃的比值（MDBTs/MDBFs）也能推断环境类型[37⁃39]。Ⅰ类煤岩和炭质泥岩的甲基二苯并噻吩含量较少,MDBTs/MDBFs比值低于1.0,介于0.008~0.794；Ⅱ类煤系烃源岩的甲基二苯并噻吩含量丰富,MDBTs/MDBFs比值高于1.0,主体介于1.189~8.515。且C19-C21TT/C23-C25TT与MDBTs/MDBFs特征和C19-C21TT/C23-C25TT与DBT/PHEN特征类似（表1、图5d）。

    Figure 5.  Relationship between the Upper Paleozoic coal⁃measure source rocks in the Ordos Basin

    综上所述,从三环萜烷单化合物比值和三环萜烷系列化合物的比值可以看出,这些比值与沉积环境参数,诸如姥植比（Pr/Ph）、伽马蜡烷指数（G/C30H）和含硫芳烃化合物（DBT/PHEN、MDBTs/MDBFs）均有较好的相关关系,暗示三环萜烷分布模式明显受到沉积环境的影响：在偏氧化的沉积环境中,C19TT/C23TT值和C19-C21TT/C23-C25TT值相对含量高；在偏还原的沉积环境中,C19TT/C23TT值和C19-C21TT/C23-C25TT值相对含量较低。沉积学的观点也支持此种现象,Ⅰ类煤岩和炭质泥岩主要发育于泥炭沼泽,为沼泽相；Ⅱ类煤系烃源岩主要分布在泥炭坪和泥炭沼泽,主要为海相和沼泽相[24],但部分样品虽然为沼泽相却表现出Ⅱ类分布模式,结合姥鲛烷与植烷、三环萜烷、伽马蜡烷、二苯并噻吩和菲等分子地球化学特征,可能与煤系烃源岩遭受海水作用有关[40⁃41]。

    • 2.4.   三环萜烷组成特征与母质来源关系

  • 在沉积环境中可能存在海水的作用导致煤系烃源岩三环萜烷的三种分布模式,下面将从母质来源研究其分布模式的特征。甾烷化合物可以为有机质来源提供一定的资料,如C27规则甾烷丰度较高表明有机质主要来源于低等水生生物,而C29规则甾烷生源组成比较复杂,既可能来自高等植物,也可能来自低等浮游生物（如蓝绿藻等）[42⁃44]。三环萜烷单化合物比值（C19TT/C23TT）和三环萜烷系列化合物的比值（C19-C21TT/C23-C25TT）均与∑C27规则甾烷/∑C29规则甾烷有较明显的负相关性（图6a,b）。Ⅰ类煤岩和炭质泥岩的∑C27规则甾烷/∑C29规则甾烷值均小于1.0,介于0.10~0.71；Ⅱ类煤系烃源岩的∑C27规则甾烷/∑C29规则甾烷值介于0.75~1.32；Ⅲ类炭质泥岩的∑C27规则甾烷/∑C29规则甾烷值为0.48（表1）。Ⅱ类煤系烃源岩的∑C27规则甾烷/∑C29规则甾烷值高于Ⅰ类,表明Ⅰ类煤岩和炭质泥岩陆源有机质的输入多于Ⅱ类煤系烃源岩,而Ⅱ类煤系烃源岩主要是微生物对高等植物的改造作用形成的细菌或/和藻类成烃。

    Figure 6.  Relationship between the Upper Paleozoic coal⁃measure source rocks in the Ordos Basin

    C24四环萜烷（TeT）通常被认为来源于陆源高等植物输入,且C24TeT/C26TT是判断有机质母质来源的一个常用生源指标[45⁃46]。同样三环萜烷单化合物比值（C19TT/C23TT）和三环萜烷系列化合物的比值（C19-C21TT/C23-C25TT）相关图均与C24TeT/C26TT有良好的正相关性。Ⅰ类煤岩和炭质泥岩的C24TeT/C26TT值均大于1.0,介于2.39~32.54；Ⅱ类煤系烃源岩的C24TeT/C26TT值主体分布小于1.0,介于0.42~0.77（表1）。Ⅰ类煤岩和炭质泥岩的C24TeT/C26TT值明显高于Ⅱ类分煤系烃源岩,表明Ⅰ类煤岩和炭质泥岩陆源有机质输入较Ⅱ类煤系烃源岩多,Ⅱ类煤系烃源岩主要为高等植物经微生物改造后的低等水生生物成烃,唯独WD-3样品的C24TeT/C26TT值与Ⅱ类其他样品有区别,其C24TeT/C26TT值为2.34,大于1.0,表明微生物对高等植物的改造作用不完全,导致其母质来源仍表现为高等植物的输入（图6c,d）。

    • 2.5.   讨论

  • 单一的因素难以解释鄂尔多斯盆地上古生界煤系烃源岩三环萜烷分布与形成,须将煤系烃源岩形成的时空差异、主要沉积相、沉积环境、生源母质等因素的研究综合起来,才能阐明具有差异性分布模式的形成及变化的主控因素。

    晚古生代进入克拉通盆地的发育时期,由于海水的快速入侵及缓慢退去,在空间上形成了各种成煤环境,而泥炭沼泽与泥炭坪是鄂尔多斯盆地的两大基本成煤环境[24,47]。陆相淡水成煤环境——泥炭沼泽,成煤植物主要为陆地森林、淡水水生草本,煤的硫分含量较低；滨海咸水成煤环境——泥炭坪,成煤植物主要为潮汐适盐植物,煤的硫分含量较高[48]。Ⅰ类烃源岩主要分布在泥炭沼泽,沉积环境为偏氧化型,淡水为主,硫分含量较低；生源母质主要为陆源高等植物。Ⅱ类烃源岩主要分布于泥炭坪（潟湖泥炭坪和潮坪泥炭坪）和泥炭沼泽,沉积环境为弱还原—强还原型,微咸水,硫分含量较高；生源母质主要为低等水生生物和陆源高等植物混源。Ⅲ类烃源岩主要分布在泥炭坪（潮控三角洲泥炭坪）,沉积环境为氧化型,淡水—微咸水,硫分含量较低,处于Ⅰ类烃源岩和Ⅱ类烃源岩之间（表2）。而泥炭坪与一般泥炭沼泽有很大差别,它属于海相沉积,泥炭坪的水深很浅,且成煤植物在周期性潮汐作用下与正常陆生植物存在差异,导致煤层的硫分高[49]。由于泥炭坪主要受潮汐流的影响,海水规律性地侵入及退出,使得微生物活跃,并强烈降解部分高等植物遗体。虽碱性沼泽水体大量溶解并流失植物木质纤维经降解后形成的腐殖质,但仍有部分残留形成煤基质。另有化学性质稳定,不易被降解的类脂化合物在此强还原条件下保存,由于水动力条件的加强使多层型树皮体分离形成碎片状树皮体,并在海水和沼泽的反复交换下富集成煤[50]。

    三环萜烷分布模式主要特征沉积相沉积环境生源母质三环萜烷分布特征
    Ⅰ类泥炭沼泽Pr/Ph介于1.19~3.84,平均值为2.99；G/C30H介于0.02~0.13,平均值为0.06；总硫芴/总氧芴介于0.01~0.94,平均值为0.31∑C27规则甾烷/∑C29规则甾烷 介于0.10~0.71,平均值为0.43；C24四环萜烷/C26TT 介于2.39~32.54,平均值为9.80C19TT/C23TT介于1.42~14.29,平均值为5.78；C19~C21TT/C23~C25TT 介于1.97~14.74,平均值为5.86； ∑三环萜烷/C30H介于0.22~0.34, 平均值为0.27
    Ⅱ类泥炭坪、泥炭沼泽Pr/Ph主体介于0.34~0.90,平均值为0.61；G/C30H介于0.10~0.23,平均值为0.17；总硫芴/总氧芴主体 介于1.22~9.43,平均值为3.58∑C27规则甾烷/∑C29规则甾烷介于 0.75~1.32,平均值为1.08；C24四环萜烷/C26TT主体 介于0.42~0.77,平均值为0.54C19TT/C23TT主体介于0.03~0.34,平均值为0.16；C19~C21TT/C23~C25TT主体介于0.71~1.08,平均值为0.94；∑三环萜烷/C30H介于2.65~7.56,平均值为3.99
    Ⅲ类泥炭坪Pr/Ph比值为2.07；G/C30H比值为0.13；总硫芴/总氧芴比值为0.28∑C27规则甾烷/∑C29规则甾烷比值为0.48；C24四环萜烷/C26TT比值为3.53C19TT/C23TT比值为0.91； C19-C21TT/C23-C25TT比值为1.55； ∑三环萜烷/C30H 比值为6.20

    Table 2.  Geological⁃geochemical parameters of different types of tricyclic terpane distribution patterns in the Upper Paleozoic coal⁃measure source rocks in the Ordos Basin

    为了更好地研究鄂尔多斯盆地上古生界煤系形成的时空及岩性差异,选择乌达剖面进一步探讨其沉积环境及其生源母质对三环萜烷分布模式的影响。乌达剖面石炭系煤岩和泥岩样品三环萜烷分布模式均为Ⅱ型,且均分布于泥炭坪相。虽然石炭系样品岩性不同,但沉积环境及其生源母质对三环萜烷分布模式的影响有相似性,具体表现在乌达剖面石炭系样品的C19TT/C23TT比值介于0.03~0.11,平均值为0.08；C19-C21TT/C23-C25TT比值介于0.75~1.02,平均值为0.93；∑三环萜烷/C30H比值介于3.42~4.99,平均值为4.38。根据表征沉积环境的参数诸如Pr/Ph（比值均小于1.0）、G/C30H（比值介于0.14~0.21）、总硫芴/总氧芴（比值均大于1.0,最高达47.22）以及生源母质相关参数诸如∑C27规则甾烷/∑C29规则甾烷（比值介于0.87~0.99）和C24TeT/C26TT（比值均小于1.0）（表3）揭示研究区石炭系沉积环境为还原—强还原型、微咸水、硫分含量较高,而生烃母质主要为低等水生生物,与泥炭坪相特征符合。早二叠世早期,由于地壳沉降,来自盆地东西部的海侵进一步向北部及中央隆起侵入,形成各种海相环境的泥炭坪。到早二叠世山西期,在大规模海退的背景下,进入陆相成煤阶段,以潮控三角洲泥炭坪、潟湖泥炭坪等为主体的成煤环境逐步被三角洲平原和岸后泥炭沼泽取代[24,51]。乌达剖面太原组腐殖煤和泥岩样品,虽均位于潮控三角洲泥炭坪相,但其三环萜烷分布模式迥然不同,腐殖煤表现为Ⅰ型,泥岩表现为Ⅱ型。出现此种现象的原因是腐殖煤和泥岩在沉积时沉积环境的氧化还原性、水体咸度、富硫物质含量以及生烃母质的变化造成三环萜烷分布的差异性。而山西组腐殖煤和泥岩样品均位于岸后泥炭沼泽,同理因为沉积环境的改变导致其三环萜烷分布迥然不同,其沉积环境由弱还原—弱氧化型逐渐变为偏氧化型,淡水为主,氧芴含量增加,生源母质以低等水生生物和高等植物混源为主。乌达剖面石炭系—二叠系,沉积相由泥炭坪相向泥炭沼泽相转化,在地史时期当地壳抬升或者海退时,泥炭坪缓慢上升,水介质或逐渐淡化或高盐度化,随着陆源物质的堆积,潟湖逐渐演变为陆生植物繁衍的泥炭沼泽[50]。沉积相的转化较好地解释了沉积环境及生源母质的变化,同时也说明了这是影响三环萜烷分布模式的重要因素。

    三环萜烷分布模式样品层位沉积相岩性C19TT/C23TTC19-C21TT/C23-C25TT∑三环萜烷/C30HPr/PhG/C30H总硫芴/总氧芴∑C27规则甾烷/∑C29规则甾烷C24TeT/ C26TT
    Ⅰ类WD-1P1s岸后泥炭沼泽煤8.169.060.283.840.020.010.1032.54
    Ⅱ类WD-3P1s岸后泥炭沼泽泥岩0.551.432.990.710.160.040.862.34
    Ⅰ类WD-2P1t潮控三角洲泥炭坪煤5.565.829.71.190.130.940.402.60
    Ⅱ类WD-4P1t潮控三角洲泥炭坪泥岩0.211.164.251.130.161.431.090.55
    Ⅱ类WD-5C2y潟湖泥炭坪泥岩0.091.014.640.90.141.390.870.51
    Ⅱ类WD-6C2y潟湖泥炭坪煤0.100.943.420.90.161.240.990.50
    Ⅱ类WD-7C2 j潮坪泥炭坪泥岩0.111.024.470.630.1947.220.990.51
    Ⅱ类WD-8C2 j潮坪泥炭坪煤0.030.754.990.340.218.800.990.51

    Table 3.  Molecular compound parameters of coal⁃measure source rocks with different ages and lithology in the Wuda section

    综上所述,结合煤系烃源岩时空差异及主要沉积相、沉积环境、生源母质等因素可知,鄂尔多斯盆地上古生界煤系烃源岩三环萜烷分布模式差异性是由于研究区沉积环境及沉积相的改变所造成的,又受到沉积环境对母质类型改造的影响。

3.   结论

  • （1） 厘定了鄂尔多斯上古生界煤系烃源岩三环萜烷的分布特征,提出了三环萜烷存在三种分布模式,且不同的三环萜烷分布模式差异性明显。

    （2） 系统剖析了成熟度与三环萜烷分布模式的相关关系,研究发现尽管三环萜烷总量受成熟度的影响,但是研究区煤系烃源岩中三环萜烷的分布模式基本不受成熟度的影响。

    （3） Ⅰ类煤岩和炭质泥岩为偏氧化型的沉积环境,表现在其Pr/Ph值较高、G/C30H比值较低、C24四环萜烷含量异常丰富、C29规则甾烷相对含量较高,明显与陆源高等植物输入有关；Ⅱ类煤系烃源岩三环萜烷分布特征正好与Ⅰ类煤岩和炭质泥岩相反；Ⅲ类炭质泥岩三环萜烷分布特征处于Ⅰ类和Ⅱ类煤系烃源岩之间。

    （4） 导致Ⅱ类煤系烃源岩的三环萜烷分布模式形成的主要原因是,这类煤系烃源岩在沉积后受到海水的作用使沉积环境变得较为还原环境,成烃母质主要为低等水生生物和高等植物经微生物改造后的菌藻类成烃,这与Ⅰ类煤系烃源岩以原始陆源高等植物为生烃母质形成的三环萜烷迥然不同。

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