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滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义

李娟 陈雷 胡月 计玉冰 卢畅 董建华 谭秀成 杨莉 杨琳 陈鑫

李娟, 陈雷, 胡月, 计玉冰, 卢畅, 董建华, 谭秀成, 杨莉, 杨琳, 陈鑫. 滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义[J]. 沉积学报, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
引用本文: 李娟, 陈雷, 胡月, 计玉冰, 卢畅, 董建华, 谭秀成, 杨莉, 杨琳, 陈鑫. 滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义[J]. 沉积学报, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
LI Juan, CHEN Lei, HU Yue, JI YuBing, LU Chang, DONG JianHua, TAN XiuCheng, YANG Li, YANG Lin, CHEN Xin. Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou[J]. Acta Sedimentologica Sinica, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
Citation: LI Juan, CHEN Lei, HU Yue, JI YuBing, LU Chang, DONG JianHua, TAN XiuCheng, YANG Li, YANG Lin, CHEN Xin. Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou[J]. Acta Sedimentologica Sinica, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038

滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义

doi: 10.14027/j.issn.1000-0550.2024.038
基金项目: 

国家自然科学基金项目 41602147

中国石油—西南石油大学创新联合体科技合作项目 2020CX020000

详细信息
    作者简介:

    李娟,女,1999年出生,硕士研究生,非常规油气地质,E-mail: 3406916041@qq.com

    通讯作者:

    陈雷,男,教授,E-mail: cl211@126.com

  • 中图分类号: P618.13

Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou

Funds: 

National Natural Science Foundation of China 41602147

China University of Petroleum-Southwest Petroleum University Innovation Consortium Science and Technology Cooperation Project 2020CX020000

  • 摘要: 目的 滇黔北昭通示范区地处四川盆地边缘,在晚奥陶世—早志留世时期,相较于盆内的长宁—威远地区,其沉积水体相对更浅、更靠近物源区,该地区五峰组—龙马溪组页岩的沉积可能与盆内存在差异。因此,厘清滇黔北昭通示范区晚奥陶世—早志留世黑色页岩的硅质成因、物质来源以及源区背景,具有重要意义。 方法 以昭通示范区太阳区块为研究区,利用区内四口井样品的主量元素、微量元素和稀土元素测试数据,探讨研究区五峰组—龙马溪组页岩的源区背景、源岩属性及硅质来源。 结果 研究区五峰组—龙马溪组黑色页岩中的硅质主要来自硅质生物和陆源碎屑物质,生物成因硅自下而上先增加后减小,表现出与陆源成因硅相反的变化规律;晚奥陶世存在短暂的冰期,直到早志留世冰川发生融化,海侵事件发生,随后海平面逐渐下降,水体不断变浅,陆源输入总体表现为先减小再增加的特征,沉积速率也呈现相同的规律。部分样品稀土元素球粒陨石标准化配分模式曲线小幅度波动,反映研究区五峰组—龙马溪组页岩沉积可能存在混合物源。 结论 研究区五峰组—龙马溪组页岩原始物质可能主要来自康滇古陆和黔中隆起的花岗岩和沉积岩,沉积时受一定海底热液的影响,但原始热液组分极少;相关指标综合反映五峰组—龙马溪组页岩物源区的构造背景主要为被动大陆边缘。
  • 图  1  昭通太阳区块构造背景图与岩性柱状图(据文献[18]修改)

    Figure  1.  Structural background map and lithologic histogram in Taiyang block of Zhaotong (modified from reference [18])

    图  2  昭通太阳区块五峰组—龙马溪组页岩宏观及微观特征

    Figure  2.  Macroscopic and microscopic characteristics of the Wufeng Formation⁃Longmaxi Formation shale in Taiyang block of Zhaotong

    图  3  昭通太阳区块S6井五峰组—龙马溪组页岩地化参数垂向变化序列图

    Figure  3.  Sequence diagram depicting the vertical variation of geochemical parameters from the Wufeng Formation⁃Longmaxi Formation shale, well S6, Taiyang block of Zhaotong

    图  4  昭通太阳区块五峰组—龙马溪组页岩微量元素UCC标准化蛛网图

    Figure  4.  Trace element UCC (Upper Continental Crust) standardized spider web diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  5  昭通太阳区块五峰组—龙马溪组页岩稀土元素球粒陨石标准化配分模式图

    Figure  5.  Standardized distribution pattern diagram depicting chondrites of rare earth elements from the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  6  昭通太阳区块五峰组—龙马溪组页岩Al⁃Fe⁃Mn三角图解(底图据文献[32])

    Figure  6.  Al⁃Fe⁃Mn ternary diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong (base map is from reference [32])

    图  7  昭通太阳区块五峰组—龙马溪组页岩典型硅质生物照片

    Figure  7.  Typical siliceous biological photographs of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  8  昭通太阳区块五峰组—龙马溪组页岩及Barnett页岩Si-Al相关图(底图据文献[35])

    Figure  8.  Correlation diagram of the Wufeng Formation⁃Longmaxi Formation shale and Barnett shale Si⁃Al, Taiyang block of Zhaotong (base map is from reference [35])

    图  9  昭通太阳区块五峰组—龙马溪组页岩陆源输入箱型图

    Figure  9.  Histogram of terrestrial source input of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  10  昭通太阳区块五峰组—龙马溪组页岩TOC、陆源输入相关关系图

    Figure  10.  Correlation diagram of total organic carbon (TOC) and terrestrial source input of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  11  昭通太阳区块五峰组—龙马溪组页岩的物源区岩石属性判别交会图

    Figure  11.  Discrimination diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  12  昭通太阳区块五峰组—龙马溪组页岩物源判别图解

    Figure  12.  Provenance discrimination diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

    图  13  昭通太阳区块五峰组—龙马溪组构造背景判别图解

    Figure  13.  Tectonic setting discrimination diagrams of the Wufeng Formation⁃Longmaxi Formation, Taiyang block of Zhaotong

    表  1  昭通太阳区块五峰组—龙马溪组主量元素含量(%)统计表

    Table  1.   Principal element content (%) in the Wufeng Formation⁃Longmaxi Formation, Taiyang block of Zhaotong

    层位样号SiO2Al2O3T(Fe2O3)CaOMgOK2ONa2OTiO2P2O5MnOTOC/%
    龙马溪组Y1253.2715.478.714.272.803.931.050.680.110.040.8
    Y1349.3926.265.740.813.076.480.780.530.070.010.3
    Y1448.5113.456.638.584.173.390.820.630.110.091.0
    Y1548.2823.445.642.903.185.990.840.370.090.031.0
    Y1648.3216.285.268.542.984.350.630.700.090.051.7
    Y1754.0414.884.776.752.734.160.660.620.110.041.9
    Y1850.5915.925.267.102.824.390.690.620.110.042.1
    Y1951.7512.774.808.763.023.510.630.600.110.052.4
    Y2053.6816.384.215.772.724.460.730.550.090.032.5
    Y2154.5812.354.817.902.273.340.690.580.140.032.6
    Y2255.6512.094.378.082.323.250.650.580.140.032.7
    Y2355.2414.494.466.342.433.840.880.570.140.032.3
    Y2457.0911.544.147.462.433.120.650.550.140.042.7
    Y2556.8910.924.407.952.202.950.690.520.140.043.3
    Y2655.269.907.417.342.002.740.630.470.110.042.9
    Y2756.389.993.668.992.632.700.610.470.110.053.2
    Y2853.0611.144.467.133.483.020.810.550.140.065.4
    Y2958.8610.124.516.832.252.690.660.470.110.033.6
    Y3051.0912.114.016.892.583.161.040.620.140.048.1
    五峰组Y3147.7212.373.468.823.853.480.690.550.140.056.8
    Y3246.2419.704.734.793.335.590.620.770.110.044.2
    上地壳丰度66.0015.204.202.203.403.900.650.150.08
    下载: 导出CSV

    表  2  昭通太阳区块五峰组—龙马溪组页岩微量元素含量(µg/g)统计表

    Table  2.   Trace element content (µg/g) in the Wufeng Formation-Longmaxi Formation, Taiyang block of Zhaotong

    层位样号ScVCoNiCuZnGaRbSrYZrNbMoCsBaHfTaTlPbThUCdCrSn
    龙马溪组Y12
    13.7011626.4075.149.3146.023.0131.0128.029.9192.016.066.712.409746.101.752.0651.620.15.780.6867.24.09
    Y13
    7.5258.47.0824.323.480.422.0112.070.827.2173.011.26.0213.307757.603.152.1112.522.06.920.3523.412.2
    Y14
    16.5012521.5052.744.879.420.4174.0227.035.3206.016.217.612.801 2496.271.771.8936.919.16.990.2464.53.81
    Y15
    9.0471.87.5926.129.656.923.5126.0111.023.9151.010.38.1817.301 0697.073.032.7925.242.38.970.2824.27.63
    Y16
    14.4019415.5060.747.8166.022.4119.0238.031.5144.015.617.615.009645.381.962.4823.223.18.130.5760.94.07
    Y17
    15.4018717.5071.448.6132.022.2142.0154.028.5148.016.315.015.709994.011.722.6828.420.79.040.8068.44.20
    Y18
    15.1019115.1067.043.9120.022.2141.0161.030.0152.014.518.915.101 0695.531.792.7733.924.09.630.7463.94.88
    Y19
    14.9022217.0075.947.1158.020.1188.0208.033.2117.014.822.613.401 0593.451.442.3122.816.611.201.0870.03.48
    Y20
    13.9017011.8051.539.668.522.9143.0143.028.6138.015.117.214.101 0094.791.822.6122.323.69.110.3256.34.72
    Y21
    14.3013922.0062.252.3192.020.1188.0216.029.7118.015.225.813.501 1093.131.302.435.315.711.200.6564.33.22
    Y22
    14.5016019.7071.850.3208.019.2177.0220.030.6118.014.926.512.801 0793.171.362.4329.615.912.200.9868.43.18
    Y23
    14.2018216.1060.047.467.121.1174.0191.025.7105.014.121.813.601 0493.261.742.3528.924.19.790.4660.23.56
    Y24
    13.6018918.7070.553.9104.019.1166.0210.029.5112.013.824.911.509993.051.272.2526.115.312.100.9464.93.16
    Y2512.6016218.9079.957.998.517.4161.0218.030.095.712.847.610.909672.731.162.3529.614.414.201.0359.02.80
    Y2611.1015316.9088.059.761.116.3144.0239.028.998.814.940.89.189992.451.072.3838.713.113.700.4651.22.58
    Y2711.7018314.4080.547.4128.016.8142.0300.030.4119.019.339.28.761 1592.911.222.0418.712.713.300.9556.02.65
    Y2813.2049117.50147.071.9492.019.3158.0197.034.9136.016.888.09.781 0893.521.273.0825.214.740.206.1968.73.01
    Y2912.0025715.00122.061.0154.016.8147.0208.032.2101.012.563.69.901 0192.691.152.8526.613.619.600.9759.72.80
    Y3013.5086917.90246.0120.0490.020.9165.0330.039.3197.015.4171.010.701 0494.961.315.0329.316.068.307.4186.22.94
    五峰组Y3117.308648.59138.0154.0330.022.1190.0234.081.2323.017.153.210.601 1197.021.321.9151.515.436.402.24103.03.99
    Y3218.9060912.40110.0116.0292.026.3161.0137.044.6283.023.132.711.501 5998.172.072.65134.021.925.602.1169.36.06
    上地壳丰度10.007012.021.017.063.018.095.0300.0170.013.00.63.3064011.600.9018.09.51.8044.0
    下载: 导出CSV

    表  3  昭通太阳区块五峰组—龙马溪组页岩稀土元素含量(µg/g)统计表

    Table  3.   Rare earth element content (µg/g) in the Wufeng Formation⁃Longmaxi Formation, Taiyang block of Zhaotong

    层位样号LaCePrNdSmEuGdTbDyHoErTmYbLu
    龙马溪组Y1251.399.011.3043.25.761.355.831.025.561.163.410.523.350.63
    Y1338.879.29.9438.05.900.825.360.935.261.173.470.553.624.00
    Y1449.7108.011.3043.26.021.626.611.136.131.243.590.543.380.54
    Y1523.244.15.3320.43.250.933.470.693.980.912.820.483.220.58
    Y1639.378.58.6032.35.121.385.310.905.221.093.190.473.070.50
    Y1746.090.59.6635.15.481.295.490.884.861.012.960.452.820.49
    Y1842.480.29.1733.95.061.255.130.894.991.093.200.493.170.65
    Y1947.497.010.3038.45.601.415.940.955.401.143.250.482.950.50
    Y2040.578.08.9032.94.931.255.010.844.821.073.180.503.220.53
    Y2145.895.510.0037.55.651.375.690.894.901.002.770.402.460.57
    Y2244.695.29.8537.36.231.445.840.945.001.012.840.422.540.40
    Y2340.379.68.3830.94.391.224.860.764.290.892.580.382.380.36
    Y2446.192.99.6536.25.441.355.560.884.760.972.810.412.520.38
    Y2543.590.99.5636.06.131.435.810.914.900.982.790.402.470.38
    Y2643.790.19.3035.44.801.305.170.884.760.982.790.402.560.43
    Y2740.882.28.4631.65.261.365.230.874.851.002.770.402.520.36
    Y2845.893.89.7337.05.601.456.051.015.571.143.290.472.950.46
    Y2941.884.28.8733.35.101.295.310.914.861.022.950.422.600.41
    Y3043.487.59.3235.55.941.475.981.066.181.283.710.553.400.54
    五峰组Y3172.7149.014.9052.89.231.3210.201.9912.102.597.511.136.911.04
    Y3253.9111.012.2043.86.381.216.851.267.591.684.990.775.180.86
    上地壳丰度33.064.07.3028.05.001.124.400.674.000.802.300.343.240.33
    下载: 导出CSV

    表  4  砂、泥岩物源区主量元素判别函数的变量及其系数[50]

    Table  4.   Variables and their coefficients of discriminant function for provenance of sandstone and mudstone[50]

    变量TiO2Al2O3T(Fe2O3)MgOCaONa2OK2O常数
    F1系数-1.7730.6070.76-1.5000.6160.509-1.224-9.090
    F2系数0.4450.070-0.25-1.1420.4381.4751.426-6.816
    注:判别公式F=a1x1+a2x2+…..+anxn+C,其中x1~xnn个判别变量,a1~an为其相应系数,C为常数。
    下载: 导出CSV

    表  5  昭通太阳区块五峰组—龙马溪组页岩样品与不同构造背景沉积盆地杂砂岩的REE特征对比[63]

    Table  5.   Comparison of rare earth element characteristics of mixed sandstones from the Wufeng Formation⁃Longmaxi Formation shale samples from Taiyang block of Zhaotong with heterogeneous sandstones in sedimentary basins with different tectonic backgrounds[63]

    构造背景La/(µg/g)Ce/(µg/g)ΣREE/(µg/g)LREE/HREELa/YbSc/Cr
    大洋岛弧8.7±2.519±3.758±103.8±0.94.2±1.30.57±0.16
    大陆岛弧24.4±2.359±8.2146±207.7±1.711.0±3.60.32±0.06
    活动大陆边缘33.0±4.5781869.112.50.30±0.02
    被动大陆边缘33.5±5.8852108.515.90.16±0.02
    研究区平均值44.7990.68225.989.1114.730.24
    下载: 导出CSV
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出版历程
  • 收稿日期:  2023-07-28
  • 修回日期:  2024-03-05
  • 录用日期:  2024-04-03
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  • 刊出日期:  2024-06-10

目录

    滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义

    doi: 10.14027/j.issn.1000-0550.2024.038
      基金项目:

      国家自然科学基金项目 41602147

      中国石油—西南石油大学创新联合体科技合作项目 2020CX020000

      作者简介:

      李娟,女,1999年出生,硕士研究生,非常规油气地质,E-mail: 3406916041@qq.com

      通讯作者: 陈雷,男,教授,E-mail: cl211@126.com
    • 中图分类号: P618.13

    摘要: 目的 滇黔北昭通示范区地处四川盆地边缘,在晚奥陶世—早志留世时期,相较于盆内的长宁—威远地区,其沉积水体相对更浅、更靠近物源区,该地区五峰组—龙马溪组页岩的沉积可能与盆内存在差异。因此,厘清滇黔北昭通示范区晚奥陶世—早志留世黑色页岩的硅质成因、物质来源以及源区背景,具有重要意义。 方法 以昭通示范区太阳区块为研究区,利用区内四口井样品的主量元素、微量元素和稀土元素测试数据,探讨研究区五峰组—龙马溪组页岩的源区背景、源岩属性及硅质来源。 结果 研究区五峰组—龙马溪组黑色页岩中的硅质主要来自硅质生物和陆源碎屑物质,生物成因硅自下而上先增加后减小,表现出与陆源成因硅相反的变化规律;晚奥陶世存在短暂的冰期,直到早志留世冰川发生融化,海侵事件发生,随后海平面逐渐下降,水体不断变浅,陆源输入总体表现为先减小再增加的特征,沉积速率也呈现相同的规律。部分样品稀土元素球粒陨石标准化配分模式曲线小幅度波动,反映研究区五峰组—龙马溪组页岩沉积可能存在混合物源。 结论 研究区五峰组—龙马溪组页岩原始物质可能主要来自康滇古陆和黔中隆起的花岗岩和沉积岩,沉积时受一定海底热液的影响,但原始热液组分极少;相关指标综合反映五峰组—龙马溪组页岩物源区的构造背景主要为被动大陆边缘。

    English Abstract

    李娟, 陈雷, 胡月, 计玉冰, 卢畅, 董建华, 谭秀成, 杨莉, 杨琳, 陈鑫. 滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义[J]. 沉积学报, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
    引用本文: 李娟, 陈雷, 胡月, 计玉冰, 卢畅, 董建华, 谭秀成, 杨莉, 杨琳, 陈鑫. 滇黔北昭通地区晚奥陶世—早志留世黑色页岩地球化学特征及其地质意义[J]. 沉积学报, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
    LI Juan, CHEN Lei, HU Yue, JI YuBing, LU Chang, DONG JianHua, TAN XiuCheng, YANG Li, YANG Lin, CHEN Xin. Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou[J]. Acta Sedimentologica Sinica, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
    Citation: LI Juan, CHEN Lei, HU Yue, JI YuBing, LU Chang, DONG JianHua, TAN XiuCheng, YANG Li, YANG Lin, CHEN Xin. Element Geochemical Characteristics and Their Geological Significance of Late Ordovician-Early Silurian Black Shale in Zhaotong Area of Northern Yunnan and Guizhou[J]. Acta Sedimentologica Sinica, 2024, 42(3): 738-756. doi: 10.14027/j.issn.1000-0550.2024.038
      • 暗色富有机质页岩是目前油气勘探开发领域中的研究热点[13]。自2009年以来,我国先后在四川盆地及其周缘多个区块实现页岩气勘探重大突破,并建立了长宁—威远、涪陵和昭通国家级页岩气示范区[46]。富有机质页岩的沉积受古气候、古海洋等多种地质要素影响[1]。五峰组—龙马溪组沉积时期,上扬子地块构造运动、火山活动以及冰川作用频发,区域上控制页岩沉积,不同区域页岩中的矿物结构、有机质含量等受原岩性质、风化剥蚀作用以及后期成岩作用的多重影响而有所不同[7],间接影响页岩的含气量。因此,阐明富有机质页岩的原岩性质、构造背景等,是研究其富集机理及工业效益的关键一步[2]

        昭通国家级页岩气示范区是我国目前重点开发的区块之一,位于四川盆地南缘及滇黔北坳陷的交界地带,具有构造活跃、改造作用强、地形复杂的特点,勘探开发难度较大[89]。前人重点对该地区五峰组—龙马溪组页岩气的富集机理及有机质富集机制进行了研究[811]。如梁兴等[9]、杜建平等[10]对昭通太阳背斜区块整体的沉积环境、保存条件、储层特征及甜点展布进行研究,提出“三元控藏”规律;张廷山等[11]提出了“水下地貌是基础、有机质保存是关键、优势岩相控储控气、纳米孔隙系统发育、顶底板封盖”是昭通示范区内五峰组—龙马溪组页岩气富集关键要素;李琪琪等[1]采用地球化学手段对黔中隆起北缘的沉积环境及物源区构造背景进行研究,认为该区五峰组—龙马溪组页岩沉积环境主要为贫氧—厌氧环境,源岩总体以花岗岩为主,源区构造背景主要为主动大陆边缘;王鹏万等[1213]、王跃等[14]对昭通示范区页岩的沉积环境及有机质富集机制进行了研究。总体而言,学者们对于昭通示范区富有机质页岩的有机质富集机制和页岩气富集机理已经取得了一系列认识,但对其源区属性及背景等方面的研究尚少。基于此,以昭通示范区太阳区块为研究区,利用区内四口井的分析测试数据探讨研究区五峰组—龙马溪组页岩的源区背景、源岩属性及硅质来源,为后续页岩气勘探开发研究奠定一定的基础。

      • 昭通页岩气示范区位于四川盆地南缘台坳与滇东—黔中隆起的交界带,地势整体呈北低南高的特征[1516]。示范区北部处于川南低陡褶皱带之内,由川南低陡褶皱带—滇黔北坳陷—黔中隆起构造形变由弱到强[16],地形较为复杂,示范区内由西向东多个南西—北东走向的复背斜和向斜带相间发育,形成典型的隔槽式褶皱[17]。研究区位于昭通页岩气示范区东北部,受燕山期、喜山期等多期构造活动影响,形成叙永向斜、太阳背斜等多个褶皱,走向大致呈东西向,并且断层发育,多为逆断层,构造复杂(图1[10,1517]

        图  1  昭通太阳区块构造背景图与岩性柱状图(据文献[18]修改)

        Figure 1.  Structural background map and lithologic histogram in Taiyang block of Zhaotong (modified from reference [18])

        晚奥陶世—早志留世时期,由于区域构造活动、古气候条件变化,发生海侵事件,海平面快速上升,昭通示范区受到北部川中古隆起以及南部黔中隆起的阻隔,水体变深,整体处于深水陆棚环境,由于黔中隆起的不断抬升,海平面不断下降,沉积环境逐渐转变为半深水—浅水陆棚环境[1819],整体沉积一套暗色富有机质页岩。研究区五峰组—龙马溪组的上覆地层因多期构造活动抬升遭受剥蚀而不断减薄[20],页岩气埋深整体较浅,主要为一套黑色页岩沉积,厚度为50~200 m,并见有斑脱岩夹层。龙马溪组依据岩性、颜色等可划分为两段,龙一段又划分为两个亚段,其中底部的龙一1亚段依据古生物等特征划分为4个小层(图1),本文研究主要层段为五峰组—龙一1亚段[1718,21]

      • 以昭通太阳区块五峰组—龙马溪组海相页岩为主要对象,岩性主要为灰黑色—黑色页岩,偶见粉砂质页岩。五峰组顶部可见赫南特贝发育(图2a),龙马溪组黑色页岩偶见钙质夹层(图2b),构造裂缝发育,富含笔石化石(图2c),矿物组分主要包括石英、长石、碳酸盐矿物以及黏土矿物,黏土矿物以绿泥石、伊利石为主。微观视角下大量水平层理发育,暗色黏土纹层与浅色石英纹层或碳酸盐纹层呈互层状发育(图2d),碎屑矿物多呈次棱角状—棱角状,较多微裂缝顺层发育,部分被方解石充填(图2e,f),片状伊利石充填于微晶石英聚合体之间(图2g),大量草莓状黄铁矿发育,晶间多被有机质充填,并发育大量有机质孔(图2h)。此次分析测试的样品来自滇黔北昭通太阳区块S1、S3、S6、S7四口井五峰组—龙马溪组岩心(图1),共采集85块岩心样品进行地球化学测试分析,采样间隔为1~2 m。其中,五峰组样品12块,龙马溪组样品73块。

        图  2  昭通太阳区块五峰组—龙马溪组页岩宏观及微观特征

        Figure 2.  Macroscopic and microscopic characteristics of the Wufeng Formation⁃Longmaxi Formation shale in Taiyang block of Zhaotong

      • 根据国标GB/T19145—2003《沉积岩中总有机碳的测定》,利用LECO CS-230碳硫分析仪对样品进行总有机碳(Total Organic Carbon,TOC)含量测试,分析精度优于2%。根据国标GB/T 21114—2019《耐火材料X射线荧光光谱化学分析熔铸玻璃片法》,采用熔铸玻璃片方法制样,X射线荧光光谱(X-Ray Fluorescene Spetrometry,XRF)对研究区样品进行主量元素测定,分析误差小于5%。根据国标GB/T 14506.30—2010《硅酸盐岩石化学分析方法第30部分:44个元素量测定》,采用高温高压消解法测定样品微量元素及稀土元素含量,首先将0.5 g左右的样品粉末置于坩埚中,放入高温炉中煅烧去除有机质后再称取约0.5 mg烧失后的样品放入溶样瓶中,记录每个样品的称量结果后,滴入7.5 mL比例为2∶2∶1的HNO3、HF和HClO4溶解样品,混合均匀后放置于100 ℃电热板上蒸干,后加入1∶1纯化的HNO3和HF,放入高压釜中190 ℃保温48 h。冷却后再放置于140 ℃电热板上将溶液蒸至湿盐状,然后缓慢加入4 mL的HNO3,再次放入高压釜中170 ℃恒温4 h。冷却后,用2%的HNO3将溶解样稀释2 000倍。溶解稀释后的样品在安捷伦电感耦合等离子体质谱仪(Agilent 7800)上进行测试分析,微量元素及稀土元素含量分析误差小于5%。以上测试均在北京通标标准技术服务有限公司的矿产实验室完成。

      • 昭通太阳区块五峰组页岩TOC含量介于4.2%~6.8%,平均值为5.5%(表1图3),整体含量较高。龙马溪组TOC含量介于0.1%~8.1%,平均值为14.9%,TOC值在龙一11小层达最大值,自下而上逐渐减小。五峰组—龙马溪组页岩主要包括SiO2、Al2O3、CaO、K2O等主量元素,其中龙马溪组页岩SiO2含量介于34.50%~61.40%、平均值为53.61%,五峰组页岩SiO2含量较龙马溪组稍低。龙马溪组页岩Al2O3含量介于9.90%~26.26%,平均值为14.56%,较五峰组Al2O3含量稍低,自下而上Al2O3含量整体呈增大的趋势,说明陆源输入逐渐增加[2223]。五峰组、龙马溪组页岩CaO含量平均值分别为6.80%、6.30%,而MgO含量平均值为3.59%、2.74%,普遍低于CaO含量,表明页岩中方解石含量大于白云石含量。相比于大陆上地壳(Upper Continental Crust,UCC)的元素丰度[24],五峰组—龙马溪组页岩的SiO2、Al2O3、Na2O、TiO2、P2O5表现为弱亏损的特征,CaO、MgO明显富集。

        表 1  昭通太阳区块五峰组—龙马溪组主量元素含量(%)统计表

        Table 1.  Principal element content (%) in the Wufeng Formation⁃Longmaxi Formation, Taiyang block of Zhaotong

        层位样号SiO2Al2O3T(Fe2O3)CaOMgOK2ONa2OTiO2P2O5MnOTOC/%
        龙马溪组Y1253.2715.478.714.272.803.931.050.680.110.040.8
        Y1349.3926.265.740.813.076.480.780.530.070.010.3
        Y1448.5113.456.638.584.173.390.820.630.110.091.0
        Y1548.2823.445.642.903.185.990.840.370.090.031.0
        Y1648.3216.285.268.542.984.350.630.700.090.051.7
        Y1754.0414.884.776.752.734.160.660.620.110.041.9
        Y1850.5915.925.267.102.824.390.690.620.110.042.1
        Y1951.7512.774.808.763.023.510.630.600.110.052.4
        Y2053.6816.384.215.772.724.460.730.550.090.032.5
        Y2154.5812.354.817.902.273.340.690.580.140.032.6
        Y2255.6512.094.378.082.323.250.650.580.140.032.7
        Y2355.2414.494.466.342.433.840.880.570.140.032.3
        Y2457.0911.544.147.462.433.120.650.550.140.042.7
        Y2556.8910.924.407.952.202.950.690.520.140.043.3
        Y2655.269.907.417.342.002.740.630.470.110.042.9
        Y2756.389.993.668.992.632.700.610.470.110.053.2
        Y2853.0611.144.467.133.483.020.810.550.140.065.4
        Y2958.8610.124.516.832.252.690.660.470.110.033.6
        Y3051.0912.114.016.892.583.161.040.620.140.048.1
        五峰组Y3147.7212.373.468.823.853.480.690.550.140.056.8
        Y3246.2419.704.734.793.335.590.620.770.110.044.2
        上地壳丰度66.0015.204.202.203.403.900.650.150.08

        图  3  昭通太阳区块S6井五峰组—龙马溪组页岩地化参数垂向变化序列图

        Figure 3.  Sequence diagram depicting the vertical variation of geochemical parameters from the Wufeng Formation⁃Longmaxi Formation shale, well S6, Taiyang block of Zhaotong

      • 研究区五峰组—龙马溪组页岩样品微量元素分析结果见表2。研究区五峰组页岩较UCC元素丰度Mo、U表现出强烈富集的特征[24],富集系数分别达71.58、17.22,Be、V、Ni、Cu、Zn、Cs、Pb表现为中等富集,Li、Sc、Ga、Rb、Zr、Nb、Ba、Th、Cr表现为弱富集特征,而Co、Sr、Hf表现弱亏损的特征(表2);龙马溪组页岩Mo元素与五峰组特征相似,富集系数达48.22,V、Ni、Cu、Zn、Cs、Th、U中等富集,Li、Be、Sc、Co、Ga、Ba、Rb、Zr、Nb、Pb、Cr弱富集或不富集,而Sr、Hf呈现亏损的特征(图4)。

        表 2  昭通太阳区块五峰组—龙马溪组页岩微量元素含量(µg/g)统计表

        Table 2.  Trace element content (µg/g) in the Wufeng Formation-Longmaxi Formation, Taiyang block of Zhaotong

        层位样号ScVCoNiCuZnGaRbSrYZrNbMoCsBaHfTaTlPbThUCdCrSn
        龙马溪组Y12
        13.7011626.4075.149.3146.023.0131.0128.029.9192.016.066.712.409746.101.752.0651.620.15.780.6867.24.09
        Y13
        7.5258.47.0824.323.480.422.0112.070.827.2173.011.26.0213.307757.603.152.1112.522.06.920.3523.412.2
        Y14
        16.5012521.5052.744.879.420.4174.0227.035.3206.016.217.612.801 2496.271.771.8936.919.16.990.2464.53.81
        Y15
        9.0471.87.5926.129.656.923.5126.0111.023.9151.010.38.1817.301 0697.073.032.7925.242.38.970.2824.27.63
        Y16
        14.4019415.5060.747.8166.022.4119.0238.031.5144.015.617.615.009645.381.962.4823.223.18.130.5760.94.07
        Y17
        15.4018717.5071.448.6132.022.2142.0154.028.5148.016.315.015.709994.011.722.6828.420.79.040.8068.44.20
        Y18
        15.1019115.1067.043.9120.022.2141.0161.030.0152.014.518.915.101 0695.531.792.7733.924.09.630.7463.94.88
        Y19
        14.9022217.0075.947.1158.020.1188.0208.033.2117.014.822.613.401 0593.451.442.3122.816.611.201.0870.03.48
        Y20
        13.9017011.8051.539.668.522.9143.0143.028.6138.015.117.214.101 0094.791.822.6122.323.69.110.3256.34.72
        Y21
        14.3013922.0062.252.3192.020.1188.0216.029.7118.015.225.813.501 1093.131.302.435.315.711.200.6564.33.22
        Y22
        14.5016019.7071.850.3208.019.2177.0220.030.6118.014.926.512.801 0793.171.362.4329.615.912.200.9868.43.18
        Y23
        14.2018216.1060.047.467.121.1174.0191.025.7105.014.121.813.601 0493.261.742.3528.924.19.790.4660.23.56
        Y24
        13.6018918.7070.553.9104.019.1166.0210.029.5112.013.824.911.509993.051.272.2526.115.312.100.9464.93.16
        Y2512.6016218.9079.957.998.517.4161.0218.030.095.712.847.610.909672.731.162.3529.614.414.201.0359.02.80
        Y2611.1015316.9088.059.761.116.3144.0239.028.998.814.940.89.189992.451.072.3838.713.113.700.4651.22.58
        Y2711.7018314.4080.547.4128.016.8142.0300.030.4119.019.339.28.761 1592.911.222.0418.712.713.300.9556.02.65
        Y2813.2049117.50147.071.9492.019.3158.0197.034.9136.016.888.09.781 0893.521.273.0825.214.740.206.1968.73.01
        Y2912.0025715.00122.061.0154.016.8147.0208.032.2101.012.563.69.901 0192.691.152.8526.613.619.600.9759.72.80
        Y3013.5086917.90246.0120.0490.020.9165.0330.039.3197.015.4171.010.701 0494.961.315.0329.316.068.307.4186.22.94
        五峰组Y3117.308648.59138.0154.0330.022.1190.0234.081.2323.017.153.210.601 1197.021.321.9151.515.436.402.24103.03.99
        Y3218.9060912.40110.0116.0292.026.3161.0137.044.6283.023.132.711.501 5998.172.072.65134.021.925.602.1169.36.06
        上地壳丰度10.007012.021.017.063.018.095.0300.0170.013.00.63.3064011.600.9018.09.51.8044.0

        图  4  昭通太阳区块五峰组—龙马溪组页岩微量元素UCC标准化蛛网图

        Figure 4.  Trace element UCC (Upper Continental Crust) standardized spider web diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

      • 研究区五峰组—龙马溪组黑色页岩稀土元素(Rare Earth Element,REE)在不同层段呈不同规律(表3)。五峰组—龙马溪组页岩∑REE介于113.36~512.37 µg/g,平均值为224.28 µg/g,相较于大陆上地壳稀土元素(Rare Earth Element,REE)总量表现为明显富集的特征,其中五峰组∑REE较龙马溪组更高(表3)。研究区页岩LREE/HREE值介于6.02~10.48,平均值为9.14,反映研究区更富集轻稀土元素,且LREE/HREE值普遍小于大陆上地壳值,其中五峰组页岩的LREE/HREE平均值为7.37,龙马溪组页岩的平均值为9.25,二者稀土元素分异程度较为相近,表明研究区五峰组与龙马溪组页岩的物源大致相同[2]。通常LaN/YbN(N表示球粒陨石标准化)值会受陆源碎屑输入的影响,随着陆源输入增多LaN/YbN值不断增大[2528]。研究区五峰组—龙马溪组页岩LaN/YbN值介于4.75~12.26,平均为9.44,反映其受物源影响较大[22]。δEu值介于0.42~0.85,平均为0.71,表现为负异常特征,其中五峰组δEu平均为0.49,较龙马溪组亏损严重。δCe值介于0.90~1.05,平均为0.97,以弱负异常为主。据稀土元素球粒陨石标准化配分模式图来看,五峰组与龙马溪组均呈较为平缓的右倾特征,与大陆上地壳相似(图5)。

        表 3  昭通太阳区块五峰组—龙马溪组页岩稀土元素含量(µg/g)统计表

        Table 3.  Rare earth element content (µg/g) in the Wufeng Formation⁃Longmaxi Formation, Taiyang block of Zhaotong

        层位样号LaCePrNdSmEuGdTbDyHoErTmYbLu
        龙马溪组Y1251.399.011.3043.25.761.355.831.025.561.163.410.523.350.63
        Y1338.879.29.9438.05.900.825.360.935.261.173.470.553.624.00
        Y1449.7108.011.3043.26.021.626.611.136.131.243.590.543.380.54
        Y1523.244.15.3320.43.250.933.470.693.980.912.820.483.220.58
        Y1639.378.58.6032.35.121.385.310.905.221.093.190.473.070.50
        Y1746.090.59.6635.15.481.295.490.884.861.012.960.452.820.49
        Y1842.480.29.1733.95.061.255.130.894.991.093.200.493.170.65
        Y1947.497.010.3038.45.601.415.940.955.401.143.250.482.950.50
        Y2040.578.08.9032.94.931.255.010.844.821.073.180.503.220.53
        Y2145.895.510.0037.55.651.375.690.894.901.002.770.402.460.57
        Y2244.695.29.8537.36.231.445.840.945.001.012.840.422.540.40
        Y2340.379.68.3830.94.391.224.860.764.290.892.580.382.380.36
        Y2446.192.99.6536.25.441.355.560.884.760.972.810.412.520.38
        Y2543.590.99.5636.06.131.435.810.914.900.982.790.402.470.38
        Y2643.790.19.3035.44.801.305.170.884.760.982.790.402.560.43
        Y2740.882.28.4631.65.261.365.230.874.851.002.770.402.520.36
        Y2845.893.89.7337.05.601.456.051.015.571.143.290.472.950.46
        Y2941.884.28.8733.35.101.295.310.914.861.022.950.422.600.41
        Y3043.487.59.3235.55.941.475.981.066.181.283.710.553.400.54
        五峰组Y3172.7149.014.9052.89.231.3210.201.9912.102.597.511.136.911.04
        Y3253.9111.012.2043.86.381.216.851.267.591.684.990.775.180.86
        上地壳丰度33.064.07.3028.05.001.124.400.674.000.802.300.343.240.33

        图  5  昭通太阳区块五峰组—龙马溪组页岩稀土元素球粒陨石标准化配分模式图

        Figure 5.  Standardized distribution pattern diagram depicting chondrites of rare earth elements from the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

      • 硅质页岩中SiO2的成因主要包括陆源、生物、热液三种[29]。根据前人研究,陆源输入通常与Al元素的含量相关,而热液的参与则影响Fe、Mn元素的富集[3031],因此常用Al-Fe-Mn三角图和Al/(Al+Fe+Mn)比值来判断硅质页岩的成因。Al/(Al+Fe+Mn)比值通常反映热液对沉积物的贡献量,热液物质的贡献量越多,Fe、Mn元素越富集,比值就越小。通常纯热液沉积物的Al/(Al+Fe+Mn)比值近似0.01,受热液影响的沉积物Al/(Al+Fe+Mn)比值通常小于0.35,当比值大于0.4时,表明主要受陆源输入的影响,而纯生物作用形成的硅质岩该比值接近0.6[3233]。例如东太平洋洋中脊纯热液沉积物的Al/(Al+Fe+Mn)比值低至0.01,半远洋环境下纯生物成因的Kamiaso硅质岩的比值为0.60[32]。研究区五峰组—龙马溪组Al/(Al+Fe+Mn)比值介于0.42~0.78,表明该套页岩硅质成分受热液影响较小,生物成因和陆源输入可能是该套页岩SiO2形成的主要因素。据样品测试点在Al-Fe-Mn三角图上的投点显示,研究区样品主要落在生物成因区内,仅有一个样品点落在生物成因区外,靠近热液成因区,进一步表明研究区五峰组—龙马溪组页岩硅质成因以生物和陆源输入为主(图6),但也不能排除热液的影响;另外研究区五峰组—龙马溪组页岩镜下可见海绵骨针、腕足类化石以及放射虫等硅质生物化石(图7)。

        图  6  昭通太阳区块五峰组—龙马溪组页岩Al⁃Fe⁃Mn三角图解(底图据文献[32])

        Figure 6.  Al⁃Fe⁃Mn ternary diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong (base map is from reference [32])

        图  7  昭通太阳区块五峰组—龙马溪组页岩典型硅质生物照片

        Figure 7.  Typical siliceous biological photographs of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

        Si/Al比值也可用来判断硅质成因。研究区五峰组—龙马溪组页岩Si/Al比值较高,Holdaway et al.[34]提出过量硅是指高于正常碎屑沉积环境下的SiO2含量,通常反映生物成因部分的硅质,其计算公式为:Si过量=Si样品-[(Si/Al)背景×Al样品],(Si/Al)背景采用平均页岩比值3.11。研究区五峰组—龙马溪组页岩Si过量介于0.29%~23.73%,平均值为8.52%。Si-Al相关图显示,五峰组—龙马溪组页岩与Barnett页岩类似,在伊利石Si/Al线上为过量硅部分,反映硅质的生物成因部分[29,35];而部分样品点落入伊利石Si/Al线下区域(图8),反映五峰组—龙马溪组页岩硅质的陆源碎屑来源。通过硅质总量减去过量硅得到陆源硅的含量,五峰组—龙马溪组页岩陆源硅含量介于6.94%~43.23%,平均值为20.11%,自下而上陆源硅含量逐渐增多,生物硅含量逐渐减少(图3),表明在五峰组—龙马溪组沉积时期陆源输入不断增多。焦石坝地区五峰组—龙马溪组下段页岩样品过量硅含量介于19.79%~55.31%,平均值为33.4%[3031],川南长宁地区五峰组—龙马溪组页岩的生物硅含量多介于0.24%~61.23%,五峰组—龙马溪组底部平均值可达47.52%[3637]。而研究区陆源硅总体偏低,推测原因为昭通示范区靠近黔中古陆、康滇古陆,导致陆源硅输入较多,生物硅相对较少。

        图  8  昭通太阳区块五峰组—龙马溪组页岩及Barnett页岩Si-Al相关图(底图据文献[35])

        Figure 8.  Correlation diagram of the Wufeng Formation⁃Longmaxi Formation shale and Barnett shale Si⁃Al, Taiyang block of Zhaotong (base map is from reference [35])

      • 陆源碎屑物质的输入对有机质的富集起到关键性作用[38]。通常陆源碎屑的输入会带来丰富的营养物质,使得水体中的藻类大量繁殖,由此促进有机质的富集;当陆源碎屑输入过多时,会对有机质产生稀释作用,从而抑制有机质的富集[3839]。Al、Ti等元素在沉积过程中不易受风化作用和成岩作用的影响,常被用来指示陆源物质输入[39]。Murry et al.[40]提出利用公式:陆源物质质量分数(%)=ω(Ti样品)/ω(TiPAAS)×100来计算陆源输入量。经计算,研究区五峰组—龙马溪组页岩陆源输入量介于10.47%~53.49%,平均值为35.0%,自下而上表现为先减小再增加的特征(图9)。陆源输入的变化与沉积时的古环境密切相关[12]。在晚奥陶世末期存在短暂的冰期,导致全球海平面下降,陆源输入增多;早志留世早期冰川发生大规模融化,海平面快速上升,发生海侵,陆源输入迅速减少,随着早志留世中后期海退的发生,水体逐渐变浅,陆源碎屑输入逐渐增多[3739],这与U/Th、V/Cr所指示的沉积环境变化(由缺氧环境转变为富氧环境)相符(图3),随着陆源碎屑物质不断沉积,海水中的沉积物趋于稳定[41]

        图  9  昭通太阳区块五峰组—龙马溪组页岩陆源输入箱型图

        Figure 9.  Histogram of terrestrial source input of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

        研究区五峰组—龙马溪组页岩TOC含量在龙一1亚段底部1、2小层达最大,向上逐渐减小。据陆源输入与TOC交会图显示,龙一1亚段底部1、2小层的TOC与陆源输入呈较好的正相关关系,R2分别为0.40、0.57(图10),表明在1、2小层沉积时期陆源碎屑的输入带来了丰富的营养物质,有利于生物繁盛,进而促进有机质的富集,在五峰组、龙一1亚段3、4小层及龙一2亚段陆源碎屑的输入较多,对有机质产生一定的稀释作用,因此在一定程度上抑制了有机质的富集。

        图  10  昭通太阳区块五峰组—龙马溪组页岩TOC、陆源输入相关关系图

        Figure 10.  Correlation diagram of total organic carbon (TOC) and terrestrial source input of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

      • 稀土元素在水体中主要赋存在细粒碎屑物质或悬浮物中,这些载体在水体中停留时间的长短决定了REE的分异程度,因此REE分异程度是研究沉积速率的重要手段之一。在沉积过程中轻稀土元素通常会被吸附在黏土矿物或者有机质中优先沉积,而重稀土元素常常在水体中形成稳定络合物,进而导致轻重稀土元素强烈分异[42]。La和Yb分别指示轻稀土元素和重稀土元素。因此,可以利用LaN/YbN值和稀土元素标准化配分模式曲线的斜率来体现沉积速率相对大小[4243]。当沉积物在水体中的停留时间越长,沉积速率越慢时,稀土元素的分解程度越彻底,有机质等物质发生反应的程度越高,REE分异程度越强,稀土元素配分模式曲线斜率越大,LaN/YbN值明显大于1或小于1,即偏离1的程度越大;反之,沉积物与水体中的物质交换少,REE分异程度弱,曲线越平缓,分异程度越小,LaN/YbN值越接近1 [43]。研究区五峰组—龙马溪组LaN/YbN值介于3.72~14.42,平均为9.19,自下而上LaN/YbN值呈由低—高—低—高的变化趋势,反映五峰组—龙马溪组页岩的沉积速率降低—增加—降低的变化过程,与陆源输入呈大致相同的变化趋势(图3)。沉积速率的变化与前文所述的沉积环境变化相符合。五峰组沉积时期陆源输入较多,沉积速率较快;龙马溪组沉积早期陆源输入减少,沉积物在水体中长时间滞留,沉积速率减慢,早志留世中后期陆源碎屑输入不断增多,沉积速率增大,随着沉积物稳定沉降[3839],陆源输入减小,沉积速率也再次减小。研究区五峰组—龙马溪组稀土元素标准化配分模式曲线较为平缓(图5),整体沉积速率较快,表明其沉积时期距离物源区较近。

      • 由于各元素稳定性的差异,部分稳定性较强微量及稀土元素能够较好地保留源区的地球化学信息,因此常被用来表征物源类型[1,44]。通常物质来源稳定的沉积物稀土元素配分模式较为相似[4546]。例如何佳伟等[2]研究发现盐津牛寨剖面龙马溪组各页岩样品之间的配分模式差异不大,配分模式曲线间近于平行,源岩主要为沉积岩及花岗岩,来源基本一致。对研究区五峰组—龙马溪组页岩样品进行球粒陨石标准化[47],据稀土元素配分模式图显示,五峰组和龙马溪组的稀土元素配分模式基本一致,呈较为平缓的右倾特征,五峰组的配分模式曲线整体波动幅度较小,龙马溪组大多数样品配分模式曲线基本平行,个别样品出现异常,稀土元素含量整体偏高(图5),说明可能存在混合物源;此外,配分模式图中还显示研究区五峰组—龙马溪组页岩存在Eu负异常和Ce弱负异常,与上地壳的稀土元素配分模式一致,反映研究区五峰组—龙马溪组的源岩来自上地壳[1,44,46]

        通常情况下花岗岩的δEu值多小于0.90,玄武岩、中性斜长石等δEu值大于0.90[4849]。研究区五峰组—龙马溪组δEu介于0.42~0.85,均小于0.9,指示其源岩主要为花岗岩。Roser et al.[50]最早提出了依据主量元素含量计算的砂、泥岩物源区判别函数(表4),判别函数图(F1-F2)显示,研究区五峰组—龙马溪组页岩样品主要落在石英质沉积岩源区、酸性火山岩源区,少部分样品落在中性火山岩源区(图11a)。微量元素Hf-La/Th图解中[51],样品点主要落在长英质物源以及与基性岩混合物源区域,部分样品落在被动大陆边缘附近(图11b)。随着岩浆不断演化,La、Th等元素不断富集,Sc、Co等元素逐渐亏损,因此酸性火成岩通常显示有更高的La/Sc值和更低的Co/Th值[49,5253]。研究区样品La/Sc比值介于2.57~9.74,Co/Th值介于0.10~1.93,在La/Sc-Co/Th图解中主要分布在长英质火成岩附近[53],少数样品点散落在长英质火成岩和花岗岩中间区域(图11c),判断其原始物质可能来自长英质火成岩和花岗岩。ΣREE-La/Yb图解常能够有效判别源岩属性[46,54],该图解显示研究区五峰组—龙马溪组样品主要落在沉积岩和花岗岩交汇区域,少数落在碱性玄武岩、花岗岩交汇区(图11d),反映五峰组—龙马溪组页岩的源岩主要为沉积岩类和花岗岩类。综合上述判断研究区五峰组—龙马溪组可能存在混合物源,其源岩主要来自花岗岩和沉积岩。

        表 4  砂、泥岩物源区主量元素判别函数的变量及其系数[50]

        Table 4.  Variables and their coefficients of discriminant function for provenance of sandstone and mudstone[50]

        变量TiO2Al2O3T(Fe2O3)MgOCaONa2OK2O常数
        F1系数-1.7730.6070.76-1.5000.6160.509-1.224-9.090
        F2系数0.4450.070-0.25-1.1420.4381.4751.426-6.816
        注:判别公式F=a1x1+a2x2+…..+anxn+C,其中x1~xnn个判别变量,a1~an为其相应系数,C为常数。

        图  11  昭通太阳区块五峰组—龙马溪组页岩的物源区岩石属性判别交会图

        Figure 11.  Discrimination diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

        海底热液对黑色页岩沉积具有重要影响[48,55]。前人常用Ba/Sr、Co/Zn值来识别海底热液活动。通常正常海相沉积物的Ba/Sr值小于1,受海底热液影响的沉积物Ba/Sr值大于1[56]。研究区五峰组—龙马溪组页岩的Ba/Sr值介于1.16~17.50,普遍大于1,表明其沉积时受到海底热液影响。热液成因的沉积物中Co/Zn值约为0.15,铁锰结核等正常沉积形成的岩石中Co/Zn值一般在2.5左右[27]。研究区五峰组—龙马溪组页岩的Co/Zn值介于0.01~0.40,平均为0.12,接近0.15,同样反映存在热液活动。Zn⁃Ni⁃Co三角图也是判断热液影响的方法之一[5758],该图解显示样品主要落在受热液影响的沉积区及其附近,进一步反映热液活动的存在(图12a)。通常可以利用ΣREE/Fe值来反映受热液影响的沉积物距离热液活动中心的远近,该比值越小,距离热液活动中心就越近,反之则越远[5960]。如东太平洋洋隆附近沉积物的ΣREE/Fe值为6.2×10-4,而距离洋隆802 km之外的沉积区该比值约为28.8×10-4[59]。研究区五峰组页岩的ΣREE/Fe值介于57.32×10-4~141.91×10-4,平均为96.77×10-4,龙马溪组页岩的ΣREE/Fe值介于14.90×10-4~563.30×10-4,平均为73.85×10-4,说明研究区五峰组—龙马溪组页岩沉积时离热液活动中心较远。Alexander et al.[61]提出Eu/Sm-Sm/Yb二元图解用于判断海水与海底热液的贡献量。研究区五峰组—龙马溪组页岩接近于水成铁锰质地壳与海水(图12b),表明在原始溶液中海底热液的占比尚不足0.1%。此外海底热液常表现出强烈的Eu正异常,学者们利用Eu正异常程度判别高温热液的贡献量[62],当样品的δEu值大于1,并且数值越大,说明样品正异常程度越高,高温热液的贡献量也就越大。研究区样品均表现为Eu负异常,进一步说明原始溶液中海底热液组分较少,对页岩沉积影响极小,推测其为远离热液活动中心的远端沉积[27]。前文的Al-Fe-Mn三角图解(图6)主要是针对硅质成因进行判别,该图解没有样品点落入热水成因区中,并不表示无热液影响。通过以上研究证实研究区五峰组—龙马溪组页岩的硅质来源受到了热液的影响。但由于热液组分含量极低,从而对于Fe、Mn元素的响应不大,导致无样品点落入热水成因区,热液对于硅质成分的贡献极小,因此这与上文黑色页岩受热液影响的结论并不相悖[29]

        图  12  昭通太阳区块五峰组—龙马溪组页岩物源判别图解

        Figure 12.  Provenance discrimination diagram of the Wufeng Formation⁃Longmaxi Formation shale, Taiyang block of Zhaotong

      • 伴随着沉积作用及构造活动的发生,不同地质背景下的陆源碎屑岩中地球化学特征会存在差异,部分微量元素和稀土元素在沉积过程中比较稳定,其元素特征和比值范围能够较好地保留物源区的相关信息,常被用来研究物源区母岩属性及构造背景[46,4849,63]。Bhatia et al.[63]研究发现不同构造背景下的杂砂岩的REE分布存在一定差异,并总结出规律作为不同构造背景的识别依据(表5),通过对研究区La、Ce、∑REE及La/Yb平均值等数值进行计算,再与前人研究总结出的各背景下的范围值进行比较,依据对比结果综合判断出最为接近的构造背景。此外利用主量元素判别不同构造背景已得到广泛应用,常用的主量元素指标有SiO2、K2O、Na2O和Al2O3[24,63]

        表 5  昭通太阳区块五峰组—龙马溪组页岩样品与不同构造背景沉积盆地杂砂岩的REE特征对比[63]

        Table 5.  Comparison of rare earth element characteristics of mixed sandstones from the Wufeng Formation⁃Longmaxi Formation shale samples from Taiyang block of Zhaotong with heterogeneous sandstones in sedimentary basins with different tectonic backgrounds[63]

        构造背景La/(µg/g)Ce/(µg/g)ΣREE/(µg/g)LREE/HREELa/YbSc/Cr
        大洋岛弧8.7±2.519±3.758±103.8±0.94.2±1.30.57±0.16
        大陆岛弧24.4±2.359±8.2146±207.7±1.711.0±3.60.32±0.06
        活动大陆边缘33.0±4.5781869.112.50.30±0.02
        被动大陆边缘33.5±5.8852108.515.90.16±0.02
        研究区平均值44.7990.68225.989.1114.730.24

        利用Sc/Cr-La/Y图解对研究区五峰组—龙马溪组样品数据进行投点,结果显示研究区样品投点较为集中,大部分样品落在被动大陆边缘内,少数样品落在活动大陆边缘和大陆岛弧内及其附近(图13a)。利用前人建立的SiO2/Al2O3-K2O/Na2O双变量判别图版进行投点[63],,所有样品集中落在被动大陆边缘范围内(图13b),说明研究区构造背景为被动大陆边缘。通常处于被动大陆边缘的沉积物稀土元素具有轻稀土富集,Eu负异常的特征,而活动大陆边缘的沉积物多以重稀土元素富集、无Eu亏损为特征,且多为分异程度较低的火成岩[43,48,64]。研究区样品的LREE/HREE平均值为9.11,δEu平均值为0.69,指示构造背景为被动大陆边缘。将研究区样品稀土元素含量与各背景的范围值对比[63],结果表明研究区La、Ce、∑REE及La/Yb平均值分别为44.79 µg/g、90.68 µg/g、225.98 µg/g、14.73(表5),均与被动大陆边缘接近。综合以上研究结果,判断研究区五峰组—龙马溪组页岩构造背景主要为被动大陆边缘。

        图  13  昭通太阳区块五峰组—龙马溪组构造背景判别图解

        Figure 13.  Tectonic setting discrimination diagrams of the Wufeng Formation⁃Longmaxi Formation, Taiyang block of Zhaotong

        研究区主要位于上扬子地块东南部,而扬子地块是前寒武纪华南板块的重要组成部分,在早古生代与华北板块、塔里木板块发生分离[22,29]。寒武纪末期,华夏板块与扬子地块受广西造山运动的影响开始发生汇聚,扬子板块东南部不断抬升,扬子地区进入被动大陆边缘演化阶段。扬子地区奥陶世先以碳酸盐台地沉积为主,自晚奥陶世起,以碳酸盐岩—碎屑岩混合沉积为主。在奥陶纪—志留纪之交发生都匀运动,华南板块向北持续俯冲,与华北板块、滇缅板块等发生碰撞,使得扬子板块内部发生挤压变形,川中隆起、黔中—滇东隆起、雪峰隆起等开始形成,由此控制了晚古生代的构造演化,扬子西缘整体仍在持续抬升,因此仍处于被动大陆边缘环境,与上述研究结论一致[22,64]。奥陶纪末期冈瓦纳大陆南极冰盖开始扩张,扬子地台发生海退,早志留世早期,随着华夏板块的不断扩张以及华南板块大部分地区的持续隆升,康滇古陆、黔中隆起以及雪峰隆起等古陆及水下高地开始形成,并且该时期伴随着冰川的大规模融化,由于各个古隆起的阻隔,上扬子地区处于半封闭的沉积环境,广泛沉积一套黑色页岩,即龙马溪组页岩[3,8,22]

        研究区西侧的康滇古陆位于扬子地台西南缘。康滇古陆自显生宙以来长期暴露地表[65],前寒武纪以前,康滇古陆逐渐开始形成岛弧褶皱带,晋宁运动导致岛弧褶皱发生转变,转变成早震旦世的安第斯型山弧,并与扬子大陆统一[6667]。晚震旦世康滇构造带基本处于稳定状态,中奥陶世—二叠纪,地幔物质上涌,上地壳发生上隆,康滇古陆区域也大面积隆升。据前人研究发现,康滇古陆部分区域(如泸定—石棉一线)主要出露新元古代地台盖层,岩性主要为一套中—酸性火成岩—火山碎屑岩,在其内部发现有大量同源花岗岩侵入体[2,29,67],与前文研究得出的花岗岩源岩属性结论一致。结合五峰组—龙马溪组页岩沉积时构造背景,在晚奥陶世—早志留世黔中隆起紧邻研究区开始发育,与龙马溪组开始沉积时间基本同步[2,68],据此综合推断研究区五峰组—龙马溪组页岩的源岩可能来自康滇古陆新元古地台盖层和黔中隆起。

      • (1) 研究区五峰组—龙马溪组页岩硅质主要来源于硅质生物和陆源输入,受热液影响较小;在晚奥陶世—早志留世时期研究区构造活跃,且靠近黔中古陆、康滇古陆,因此研究区页岩生物成因硅相较于焦石坝等地区整体偏低,陆源硅相对偏高,自下而上生物成因硅先增加后减小,陆源硅呈相反趋势。

        (2) 研究区五峰组—龙马溪组页岩沉积时期陆源输入具有明显的变化,其对TOC的富集具有双重作用。龙一1亚段1、2小层时期,陆源输入带来的营养物质促进了TOC的富集,而五峰组、龙一1亚段3、4小层时期则由于陆源输入的大量增加,反而起到了稀释作用,不利于TOC的富集。LaN/YbN值反映五峰组—龙马溪组页岩的沉积速率降低—增加的变化过程,整体与陆源输入变化趋势相同。

        (3) 物源分析表明五峰组—龙马溪组页岩可能存在混合物源,源岩可能来自康滇古陆和黔中隆起,沉积时受海底热液影响较小,构造背景主要为被动大陆边缘。

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