Geochemical Characteristics and Implications of Shale Gas in Jiaoshiba, Eastern Sichuan, China
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摘要: 焦石坝地区五峰组-龙马溪组页岩富集有大量的天然气，但针对页岩气地球化学特征研究还较薄弱，其蕴含的地质意义不甚明确.通过页岩气组分及其碳同位素特征和页岩干酪根的碳同位素特征分析，讨论了页岩气的来源、成因类型和完全倒转的碳同位素分布特征.五峰组-龙马溪组页岩具有很好的生烃能力、有机质丰度与含气量的关系有明显的正相关性、甲烷与干酪根相似的碳同位素特征、地层的超压特征等，综合表明研究区天然气应为页岩自生自储的页岩气；页岩气的甲烷含量均超过98%，其碳同位素平均为-29.93‰，反映了成熟度已经达到过成熟干气阶段；相对稳定的ln（C1/C2）和快速增大的ln（C2/C3）揭示其成因主要为二次裂解气；页岩气碳同位素完全倒转的分布特征主要受到在相对封闭环境中的原油裂解生气作用的影响，其完全倒转的碳同位素分布特征也反映了研究区良好的页岩气保存、富集条件.Abstract: The Wufeng-Longmaxi shales in Jiaoshiba is rich in natural gas. However, the research on the geochemical characteristic of the shale gas is weak, and its geological significance is still unclear. This article discusses the origin type and the "full reversal" carbon isotopic composition of shale gas from Wufeng Formation-Longmaxi Formation based on the analysis of the shale gas component and its isotope. The high hydrocarbon generation potential of Wufeng-Longmaxi shales, the positive correlation of the abundance of organic carbon and the content of gas, the similar carbon isotope distribution between methane and kerogen and the overpressure in Wufeng Formation-Longmaxi Formation suggest the shale gas should be generated from and preserved in the Wufeng Formation-Longmaxi Formation. The content of methane and its isotope indicate the stage of thermal maturity for shale gas has reached the overmature phase and the shale gas is mainly dry gas. The shale gas is mainly formed from the cracking of oil generated early based on the ratio of ln(C1/C2) and ln(C2/C3). The "full reversal" carbon isotope distribution of shale gas resulted from the oil cracking in a relatively closed system and it also reflects a favorable preservation and accumulation conditions for shale gas.
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Key words:
- carbon isotope /
- shale gas /
- Wufeng Formation /
- Longmaxi Formation /
- Jiaoshiba /
- petroleum geology
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图 1 焦石坝地区区域构造与地层简图
a.构造分区；b.地理位置；c.剖面图；d.地层柱状图；e.四川盆地剖面图
Fig. 1. The regional tectonic and stratigraphic of Jiaoshiba
图 2 焦页1井(a)、焦页2井(b)五峰-龙马溪组有机碳分布特征
Fig. 2. The variations of TOC with depth for Wufeng Formation-Longmaxi Formation in wells JY1 (a) and JY2 (b)
图 3 焦石坝地区天然气ln (C1/C2)与ln (C2/C3)关系
Fig. 3. The relation of ln (C1/C2) and ln (C2/C3) on shale gas in Jiaoshiba
图 4 焦石坝地区天然气成因的“Bernard”分类图版
Fig. 4. Modified Bernard diagram characterizing shale gas in Jiaoshiba
图 5 焦石坝地区页岩气δ13C1与C2+关系
Fig. 5. The relation of δ13C1 and C2+ on shale gas in Jiaoshiba
图 6 焦石坝地区页岩气δ13C1与δ13C2关系
Fig. 6. The relation of δ13C1 and δ13C2 on shale gas in Jiaoshiba
图 7 焦石坝地区天然气碳同位素分布特征
Fig. 7. The carbon isotope characteristics of shale gas from Jiaoshiba
图 8 焦石坝地区五峰组-龙马溪组页岩有机碳含量与含气量关系
Fig. 8. The relation of TOC and the content of gas on Wufeng-Longmaxi shales in Jiaoshiba
表 1 焦页1井龙马溪组干酪根镜检测定
Table 1. Microcomponents of kerogen for Longmaxi Formation in well JY1
井深(m) 地层 岩性 显微组分(%) 类型指数 类型 腐泥无定形体 藻类体 动物碎屑 2 339.33 龙马溪组 灰黑色页岩 40.27 52.57 7.16 92.8 Ⅰ 2 349.23 龙马溪组 灰黑色页岩 71.21 28.79 / 100.0 Ⅰ 
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表 2 焦石坝地区龙马溪组干酪根碳同位素分布特征
Table 2. The carbon isotopic composition of kerogen for Longmaxi Formation in Jiaoshiba
编号 地层 岩性 TOC (%) δ13C干酪根(‰) JSB-1 龙马溪组 灰黑色页岩 1.49 -29.20 JSB-2 龙马溪组 黑色碳质页岩 2.09 -29.30 JSB-3 龙马溪组 黑灰色含碳质泥岩 0.49 -29.70 JSB-4 龙马溪组 灰黑色页岩 0.45 -29.24 JSB-5 龙马溪组 灰黑色页岩 2.41 -29.24 JSB-6 龙马溪组 灰黑色页岩 1.67 -28.61 JSB-7 龙马溪组 黑色碳质页岩 1.46 -29.06 JSB-8 龙马溪组 黑色碳质页岩 2.02 -29.11 JSB-9 龙马溪组 黑色碳质页岩 3.40 -29.94 JSB-10 龙马溪组 黑灰色含碳质泥岩 5.65 -29.83 JSB-11 龙马溪组 黑灰色含碳质泥岩 3.85 -29.83
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表 3 焦石坝地区天然气组成特征
Table 3. The characteristics of shale gas in Jiaoshiba
井号 气体组分(%) 稳定碳同位素δ13C (‰) CH4 C2H6 C3H8 CO2 N2 CH4 C2H6 C3H8 焦页1HF① 98.35 0.63 0.02 0.20 0.80 -28.36 -34.18 -36.72 焦页1HF② 98.31 0.60 0.02 0.32 0.75 -30.51 -34.10 焦页1HF③ 98.41 0.52 0.02 0.27 0.78 -30.33 -34.34 -37.56 焦页7-2HF① 97.90 0.94 0.02 0.18 0.96 -29.03 -34.47 -37.05 焦页7-2HF② 98.18 0.50 0.02 0.36 0.94 -30.71 -34.37 焦页7-2HF③ 98.32 0.51 0.33 0.84 -30.51 -34.47 焦页8-2HF① 98.37 0.54 0.02 0.25 0.82 -29.07 -34.34 -37.14 焦页8-2HF② 98.33 0.65 0.02 0.27 0.73 -30.12 -34.31 焦页8-2HF③ 98.65 0.43 0.18 0.74 -30.41 -34.33 -36.09 焦页12-2HF② 98.15 0.67 0.02 0.46 0.70 -30.20 -34.60 ①.采样日期2013年10月25日；②.采样日期2014年4月10日；③.采样日期2014年8月6日.
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