Shale Geochemical Characteristics and Enrichment Mechanism of Organic Matter of the Upper Devonian Shetianqiao Formation Shale in Lianyuan Sag, Central Hunan
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摘要: 上泥盆统佘田桥组是湘中地区页岩气勘探的重要层系之一.为了探讨湘中地区涟源凹陷上泥盆统佘田桥组黑色泥页岩有机质富集机理,系统采集了区内湘新地3井佘田桥组20个泥页岩岩心样品,开展有机碳含量、主量元素、微量元素以及稀土元素等地球化学特征测试,分析佘田桥组页岩的古盐度、古气候、古氧化还原条件和古生产力等古沉积环境.结果表明:佘田桥组底部层段有机碳含量较高(1.28%~2.68%),平均值为1.69%.页岩主要成分为SiO2(50.27%)、Al2O3(13.66%)、CaO(11.55%).微量元素Rb、Sr和Zr富集而Co、Mo、Sc和Hf亏损.Sr/Ba、化学蚀变指数(CIA)、Th/U、V/Sc、V/Cr、δU比值表明佘田桥组富有机质页岩段沉积环境为淡水-半咸水、干旱的贫氧-次富氧环境,贫有机质页岩段为咸水、干旱-半干旱的富氧环境.结合区域层序地层特征,综合对比佘田桥组有机碳含量(TOC)与古氧化还原条件、古生产力条件的相关性,揭示出涟源凹陷佘田桥组富有机质页岩段有机质富集的主控因素为古氧化还原环境,而贫有机质页岩段有机质主要来源于陆源供给.Abstract: The Upper Devonian Shetianqiao Formation shale is one of the most important shale gas exploration beds in Lianyuan sag of Xiangzhong depression. In order to discuss the organic matter enrichment mechanism of the Shetianqiao Formation shale, 20 shale core samples collected systematically from Well XXD-3 were chosen.The organic carbon content, major, trace and rare earth elements were analyzed to investigate the paleo-salinity, paleo-climate, paleo-redox and paleo-productivity characteristics of the Upper Devonian sedimentary environment in Lianyuan sag of Xiangzhong depression. Research illustrates that the organic carbon content is higher (1.28%-2.68%) at the bottom of Shetianqiao Formation, with an average of 1.69%. Major components of shale include SiO2(50.27%), Al2O3(13.66%), CaO (11.55%). Trace elements Rb, Sr and Zr are enriched while Co, Mo, Sc and Hf are depleted. The ratio of Sr/Ba, CIA, Th/U, V/Sc, V/Cr, and δU suggest that Shetianqiao Formation organic-rich shales should be deposited in fresh-brackish water, arid climate and suboxic to anoxic environment, while organic-poor shales were mostly deposited in salt water, arid to semi-arid climate and oxygen-enriched environment. Combining with the regional sequence stratigraphic characteristics, comprehensive comparison of the correlation among organic carbon content (TOC), paleo-redox and paleo-productivity conditions, it is revealed that the main controlling factors of organic matter enrichment of Shetianqiao Formation organic-rich shales are redox environment. The changes in organic matter content of Shetianqiao Formation organic-poor shales are predominantly controlled by terrigenous supply.
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表 1 涟源凹陷湘新地3井佘田桥组页岩主量元素组成(%)
Table 1. Major elements of the shales from Shetianqiao Formation in Well XXD-3 of Lianyuan sag (%)
样品编号 段 TOC SiO2 Al2O3 Fe2O3 FeO CaO MgO K2O Na2O TiO2 P2O5 MnO CIA Al/ (Al+Fe+Mn) XXD3-44 富有机质页岩段 1.58 60.33 6.78 0.701 1.25 12.68 1.47 1.64 0.354 0.278 0.057 0.023 31.60 0.71 XXD3-46 1.28 54.12 11.25 0.00 2.96 11.87 2.10 2.69 0.567 0.443 0.056 0.027 42.65 0.72 XXD3-48 2.68 61.90 13.66 1.36 1.72 3.66 2.36 3.38 0.522 0.523 0.079 0.017 64.37 0.76 XXD3-58 2.10 63.22 12.81 1.70 1.71 5.31 1.19 2.57 0.796 0.415 0.155 0.016 59.62 0.73 XXD3-64 1.44 59.19 12.20 1.69 2.27 7.70 1.42 2.54 0.717 0.423 0.119 0.024 52.68 0.69 XXD3-73 1.30 61.59 12.27 1.18 2.99 5.67 2.04 2.70 0.620 0.463 0.070 0.027 57.71 0.67 XXD3-91 2.01 49.55 13.52 1.16 3.10 11.61 1.48 3.00 0.706 0.496 0.079 0.052 46.89 0.69 XXD3-106 1.12 53.43 14.92 1.41 3.14 8.90 1.54 3.18 0.809 0.606 0.071 0.047 53.65 0.70 XXD3-110 1.53 50.19 14.08 1.13 3.71 10.79 1.67 3.19 0.749 0.531 0.079 0.068 48.87 0.67 XXD3-112 1.90 51.66 15.79 0.812 3.53 8.77 1.70 3.72 0.659 0.551 0.076 0.059 54.56 0.71 XXD3-119 贫有机质页岩段 0.511 58.92 19.28 1.07 4.62 1.80 1.84 4.09 1.02 0.669 0.112 0.036 73.62 0.70 XXD3-125 0.748 53.31 17.72 2.18 3.80 5.52 2.05 3.60 1.07 0.586 0.141 0.035 63.49 0.68 XXD3-127 0.809 43.57 9.41 0.888 2.54 19.20 1.78 2.12 0.503 0.436 0.067 0.055 30.13 0.65 XXD3-136 0.850 21.99 4.75 0.370 1.06 37.10 0.960 0.871 0.244 0.220 0.096 0.036 11.06 0.69 XXD3-137 0.503 53.64 19.34 2.31 1.92 5.76 1.36 3.20 1.15 0.646 0.148 0.026 65.67 0.77 XXD3-150 0.411 45.08 17.51 1.06 6.41 10.08 1.76 2.49 0.589 0.634 0.159 0.150 57.09 0.61 XXD3-152 0.340 43.42 17.78 1.42 6.28 10.65 1.54 2.28 0.532 0.634 0.144 0.131 56.91 0.61 XXD3-155 0.264 49.72 19.30 1.08 5.38 7.30 1.36 2.52 0.540 0.685 0.127 0.075 65.07 0.67 XXD3-146 0.261 37.41 12.07 0.658 3.68 20.90 1.51 1.71 0.291 0.504 0.110 0.080 34.51 0.65 XXD3-157 0.248 33.26 8.80 0.412 3.88 25.71 1.62 1.22 0.238 0.438 0.134 0.090 24.47 0.58 注:CIA=100×Al2O3/(Al2O3+CaO+Na2O+K2O). 表 2 涟源凹陷佘田桥组页岩微量元素分析及计算结果(10-6)
Table 2. Analytical and calculated results of trace elements of Shetianqiao Formation shale in Lianyuan sag (10-6)
样品编号 段 微量元素质量分数 V/Cr Ni/Co Th/U V/Sc V/(V+Ni) δU Sr/Ba Baxs P/Ti Cr Ni Co Rb Mo Sr Ba V Sc Zr Hf U Th XXD3-44 富有机质页岩段 43.6 36.7 6.67 72.1 12.7 214 224 0.96 6.55 62.7 1.59 3.95 7.04 2.34 5.50 1.78 15.57 0.74 1.25 0.96 43.3 0.15 XXD3-46 61.9 37.9 7.93 123.4 5.62 325 345 0.94 8.11 104 2.55 3.52 8.76 3.25 4.78 2.49 24.78 0.84 1.09 0.94 57.05 0.09 XXD3-48 71.6 57.9 11.0 151 17.0 72.4 352 0.21 9.35 132 3.54 6.52 9.88 3.30 5.26 1.52 25.24 0.80 1.33 0.21 12.05 0.11 XXD3-58 68.6 58.2 13.0 135.7 4.76 166 373 0.45 9.41 92.9 2.39 3.59 9.30 2.73 4.48 2.59 19.87 0.76 1.07 0.45 103.25 0.27 XXD3-64 69.0 35.4 10.7 132.7 1.80 273 388 0.70 8.39 95.2 2.43 3.28 8.62 2.38 3.31 2.63 19.55 0.82 1.07 0.70 113.05 0.20 XXD3-73 68.2 50.9 12.5 144.3 9.32 186 371 0.50 9.48 109 2.76 4.20 9.40 3.20 4.07 2.24 23.00 0.81 1.15 0.50 70.05 0.11 XXD3-91 65.9 37.4 11.6 157 7.22 492 438 1.12 9.90 130 3.47 4.26 8.64 1.70 3.22 2.03 11.31 0.75 1.19 1.12 115.6 0.12 XXD3-106 81.4 36.2 12.2 170 2.19 403 452 0.89 10.5 151 4.09 3.54 11.8 1.84 2.97 3.33 14.29 0.81 0.95 0.89 58.1 0.09 XXD3-110 78.3 47.9 11.9 155 4.46 494 508 0.97 11.4 140 4.12 4.75 12.0 3.82 4.03 2.53 26.23 0.86 1.09 0.97 162.85 0.11 XXD3-112 86.7 63.3 11.3 208.3 2.87 392 452 0.87 11.7 136 3.98 5.14 11.7 4.39 5.60 2.28 32.56 0.86 1.14 0.87 93.85 0.10 XXD3-119 贫有机质页岩段 94.8 41.6 14.7 177 0.37 69.4 252 0.28 8.31 184 5.60 2.56 9.15 1.46 2.83 3.57 16.61 0.77 0.91 0.28 -182.85 0.12 XXD3-125 86.5 33.0 13.6 165.6 0.32 244 397 0.61 10.7 162 4.81 2.52 13.3 1.45 2.43 5.28 11.68 0.79 0.72 0.61 16.1 0.18 XXD3-127 49.2 24.6 10.0 115 1.02 808 283 2.86 8.53 116 3.31 1.92 10.5 1.38 2.46 5.47 7.95 0.73 0.71 2.86 -0.4 0.11 XXD3-136 22.1 17.4 3.50 45.5 0.99 2 540 75.8 33.51 4.36 46.7 1.51 1.95 5.88 3.72 4.97 3.02 18.85 0.83 1.00 33.51 -67.2 0.32 XXD3-137 94.0 30.4 9.96 197 0.32 804 308 2.61 11.7 130 3.86 2.40 14.3 1.32 3.05 5.96 10.60 0.80 0.67 2.61 -111.9 0.17 XXD3-146 57.8 22.4 9.24 108.6 0.33 927 195 4.75 10.5 117 3.30 1.97 12.6 1.39 2.42 6.40 7.68 0.78 0.64 4.75 -217.1 0.18 XXD3-150 77.7 28.9 15.9 154.8 0.48 524 219 2.39 11.7 110 2.32 1.32 9.66 1.40 1.82 7.32 9.32 0.79 0.58 2.39 -193.1 0.17 XXD3-152 83.4 31.2 20.2 154 0.44 650 206 3.16 12.5 118 3.42 2.04 13.0 1.29 1.54 6.37 8.64 0.78 0.64 3.16 -239.25 0.13 XXD3-155 93.8 30.0 15.5 159.2 0.40 327 201 1.63 10.4 145 4.23 2.17 11.1 1.29 1.94 5.12 11.63 0.80 0.74 1.63 -126.6 0.16 XXD3-157 45.0 19.0 11.6 74.6 0.34 771 226 3.41 8.62 106 2.95 1.68 10.1 1.23 1.64 6.01 6.43 0.74 0.67 3.41 -58.7 0.22 注:δU=U/[0.5×(Th/3+U)]; Baxs=Ba样品-Ti样品(Ba/Ti)PAAS, PAAS为太古宙澳大利亚页岩(PAAS数据引自Taylor and McLennan, 1985); 负值(-)表示样品组该元素含量主要由陆源物质贡献. 表 3 涟源凹陷佘田桥组贫有机质页岩段稀土元素分析结果(10-6)
Table 3. REE analytical data of the upper member of Shetianqiao Formation shale in Lianyuan sag (10-6)
样品编号 段 稀土元素质量分数 ∑REE ∑LREE ∑HREE ∑LREE/ ∑HREE LaN/ YbN δEu δCe La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y XXD3-44 富有机质页岩段 53.7 36.8 27.1 20.4 11.7 4.85 8.13 7.04 6.13 5.76 5.49 5.51 5.78 5.17 15.3 203.56 154.55 49.01 3.15 0.90 2.18 0.21 XXD3-48 93.7 69.9 48.7 36 17.6 5.77 11.9 9.68 8.62 8.3 8.31 8.77 9.44 8.79 19.3 345.48 271.67 73.81 3.68 0.96 1.75 0.23 XXD3-58 72 55.2 45.4 36.5 23.3 10.6 15.2 13 11.1 10 9.22 8.77 9.36 8.27 25.1 327.92 243 84.92 2.86 0.75 2.47 0.21 XXD3-64 75.7 54.3 40.6 30.4 15.9 8.2 10.7 8.8 7.64 7.37 7.49 7.77 8.63 7.75 19 291.25 225.1 66.15 3.40 0.85 2.76 0.21 XXD3-73 77.8 61.8 44.9 34.8 20 9.7 13.2 11.4 9.77 8.99 8.63 8.52 9.2 8.27 22.4 326.98 249 77.98 3.19 0.82 2.62 0.23 XXD3-91 69.3 55.3 41 32.1 19.4 9.93 12.7 10.9 9.33 8.53 8.35 8.52 9.48 8.53 21.0 303.37 227.03 76.34 2.97 0.71 2.78 0.23 XXD3-106 95.8 81.4 56 42.7 22.4 11.4 14 11.6 9.97 9.45 9.41 9.52 10.5 9.3 22.4 393.45 309.7 83.75 3.70 0.88 2.83 0.24 XXD3-110 68.8 53.6 40.4 30.9 17.9 9.7 11.8 10.2 9.15 8.76 8.78 9.52 10.2 9.3 22.5 299.01 221.3 77.71 2.85 0.65 2.93 0.22 XXD3-112 91.8 74.4 54.4 43.2 26.1 14 16.8 14.8 12.9 11.9 11.5 11.5 12.7 11.1 29.9 407.1 303.9 103.2 2.94 0.70 2.94 0.23 XXD3-119 XXD3-125 贫有机质页岩段 116 95.2 96.6 77.8 69 55.3 53.8 42.2 30.7 21.9 13.3 8.78 19.5 14.0 16.4 11.6 14.3 10.2 13.1 9.68 12.5 9.73 12.5 9.77 13.7 10.8 12.1 9.56 30.1 23.3 493.5 396.29 379.4 301.18 114.1 85.34 3.33 3.53 0.82 0.85 2.39 2.20 0.24 0.23 XXD3-127 67.7 52.9 39.0 30.2 18.3 9.93 12.0 10.2 8.74 7.83 7.41 7.52 8.03 7.24 18.8 294.52 218.03 68.97 3.16 0.82 2.94 0.22 XXD3-136 37.3 28.5 20.6 16.2 9.91 7.39 7.04 6.16 5.26 4.72 4.47 4.26 4.46 3.88 12.8 164.41 119.9 40.25 2.98 0.81 3.88 0.22 XXD3-137 110 91.0 60.1 44.7 22.1 8.31 14.0 10.2 8.08 7.49 7.53 7.77 8.84 8.01 16.2 415.9 336.21 71.92 4.67 1.21 2.07 0.24 XXD3-146 76.2 61.3 40.7 30.6 17.5 9.35 11.9 9.51 8.21 7.49 7.41 7.52 8.80 8.01 18.5 312.02 235.65 68.85 3.42 0.84 2.84 0.24 XXD3-150 104 88.9 58.6 45.2 26.5 15.1 17.6 14.6 12.2 10.7 10.0 10.0 10.5 9.04 25.3 442.94 338.3 94.64 3.57 0.96 3.07 0.25 XXD3-152 101 86.2 56.1 42.7 24.5 13.7 16.2 13.2 11.2 9.91 9.33 9.02 9.80 8.79 22.6 420.67 324.2 87.45 3.71 1.00 3.02 0.25 XXD3-155 107 90.3 59.4 45.1 26.3 14.1 17.3 14.1 11.8 10.6 10.2 10.0 10.8 9.30 24.5 446.3 342.2 94.1 3.64 0.96 2.90 0.25 XXD3-157 64.8 53.3 37.7 29.5 18.4 10.6 12.7 11.1 9.54 8.53 7.84 7.52 8.03 6.98 20.6 294.06 214.3 72.24 2.97 0.78 3.04 0.23 注:δCe=CeN/(LaN×PrN)1/2; δEu=EuN/(SmN×GdN)1/2(N代表北美页岩标准化数据; Haskin and Haskin, 1966). 表 4 氧化还原环境的元素判别指标
Table 4. Element discrimination parameters in redox condition
古氧化还原环境 V/Cr Ni/Co Th/U V/(V+Ni) V/Sc δU 缺氧、极贫氧环境 > 4.25 > 7.0 < 2.0 > 0.84 > 30 > 1.0 贫氧、次富氧环境 2.0~4.25 5.0~7.0 2.0~8.0 0.60~0.84 9~30 < 1.0 富氧环境 < 2.0 < 5.0 < 0.60 < 9 -
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