Characteristics of organic matter-hosted pores in Lower Silurian Longmaxi shale with different maturities, Sichuan Basin
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摘要: 有机质孔隙是页岩气的主要储集空间,也是页岩天然渗流通道的重要组成部分。选取四川盆地不同热成熟度的下志留统龙马溪组页岩,采用场发射扫描电镜开展页岩不同显微组分有机质孔隙形成演化研究。笔石是龙马溪组页岩主要的结构有机质,含量较少,自身不发育有机质孔隙。成岩过程中由脂类原位聚合或外部地质聚合物交代使得笔石化石具有一定的生烃潜力,导致局部存在有机质孔隙。固体沥青是龙马溪组页岩最主要的显微组分,也是有机质孔隙发育的主要载体。考虑细粒沉积物成岩和有机质生烃演化,结合固体沥青赋存形态,可将固体沥青区分为前油沥青和后油沥青,且后油沥青含量占绝对优势。固体沥青有机质孔隙演化与热成熟度密切相关。总体上,随着热成熟度增加,固体沥青有机质孔隙越来越发育。成熟-高成熟早期(GRo < 2.3%),固体沥青孔隙不太发育,可能受到有机质生成的烃类物质掩盖。高成熟晚期-过成熟早期(2.3% < GRo < 4.5%)是固体沥青有机质孔隙大量发育的主要时期,含海绵状孔隙和气泡状孔隙2种类型,富有机质页岩有机质孔隙度对总孔隙度的贡献达50%以上。过成熟晚期(GRo > 4.5%),有机质炭化对页岩孔隙产生强烈破坏作用,导致页岩气勘探风险加剧。Abstract: Organic matter-hosted pores provide reservoir space and migration pathways for shale gas. The evolution of organic matter-hosted pores (OM pores) of different macerals from Longmaxi shale with a wide variety of thermal maturities has been investigated using field emission scanning electron microscopy (FE-SEM). The Longmaxi shale contains rare graptolites which are the main component of structured organic matter. No OM pores occur in graptolite fragments, irrespective of thermal maturity. OM pores locally developed in graptolite fragments are formed from hydrocarbon generation of organic matter which was replaced by macromolecular material from surrounding sediment or in situ polymerized by lipids from the organism itself. Solid bitumen is not only the major organic component in the Longmaxi shale, but also the main host of OM pore development. The diagenesis of fine-grained sediments and thermal evolution of organic matter have been combined with the morphology of solid bitumen to distinguish pre-oil solid bitumen and post-oil solid bitumen. The post-oil solid bitumen is dominant. The evolution of OM pores within solid bitumen is closely related to thermal maturity. Generally, OM pores within solid bitumen become greater as thermal maturity increases. During the mature to early postmature (GRo < 2.3%) stages, OM pores within solid bitumen are not well developed probably due to the masking by oil and bitumen generated from organic matter. OM pores within solid bitumen are well developed during the late postmature to early overmature (2.3% < GRo < 4.5%) stages, with two main types being spongy and bubble-shaped. For organic-rich Longmaxi shale, the contribution of OM porosity to total porosity is more than 50%. During the late overmature (GRo>4.5%) stage, organic matter carbonization will cause intense damage to shale pores so that the exploration risk of shale gas increases.
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Key words:
- organic matter-hosted pore /
- thermal maturity /
- maceral /
- solid bitumen /
- shale /
- Longmaxi Formation /
- Lower Silurian /
- Sichuan Basin
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表 1 四川盆地龙马溪组页岩样品基本参数
Table 1. Basic parameters of Longmaxi shale, Sichuan Basin
样品编号 深度/m 有机碳含量/% 矿物组成/% 黏土 石英 长石 方解石 白云石 黄铁矿 其他 CMB 露头 2.12 36.6 53.0 4.4 0.9 0.6 3.2 1.3 YC1 1 099.0 1.11 55.4 31.2 5.7 1.4 0.8 1.0 4.5 CHP 露头 4.05 26.8 63.8 3.9 1.1 0.2 2.8 1.4 DBY 露头 4.87 39.1 53.3 1.3 0.9 0.1 1.6 3.7 GD1 1 247.1 1.17 38.7 40.4 11.5 4.5 1.2 2.1 1.6 WY35 3 711.9 3.43 45.3 29.7 8.0 3.5 5.8 6.2 1.5 N208 1 269.3 1.21 45.0 34.1 10.1 5.7 2.0 1.9 1.2 YY1 3 813.6 5.04 38.5 45.6 3.9 2.3 3.2 5.2 1.3 PY1 2 153.3 2.47 48.4 42.4 4.9 0.3 0.8 1.7 1.5 LY1 2 829.3 4.58 25.2 59.6 6.7 2.7 1.7 2.3 1.8 JY41 2 609.3 3.74 48.6 38.0 6.4 1.0 1.8 2.6 1.6 YZ1 4 503.1 4.81 35.9 50.9 7.0 0.3 1.5 1.4 3.0 MY1 3 093.2 3.66 4.8 9.2 0.7 34.6 49.0 1.3 0.4 表 2 四川盆地涪陵页岩气田焦页2井龙马溪组页岩有机显微组分组成特征
Table 2. Maceral groups of Longmaxi shale from well Jiaoye 2, Fuling Shale Gas Field, Sichuan Basin
深度/m 层位 有机碳含量/% 全岩有机显微组分含量/% 固体沥青 动物碎屑 类脂组 合计 2 542.68 下志留统 2.95 5 0.2 0.1 5.3 2 567.62 下志留统 3.82 7 0.2 0.1 7.3 2 568.50 下志留统 7.13 15 0.4 0.1 15.5 2 572.30 上奥陶统 5.27 10 0.2 0.1 10.3 表 3 四川盆地龙马溪组页岩固体沥青扫描电镜有机质孔隙度与计算有机质孔隙度数据
Table 3. Calculated organic matter porosity based on SEM of solid bitumen in Longmaxi shale in Sichuan Basin
样品编号 w(TOC)/% SEM有机质孔隙度/% 含孔有机质含量/% 有机质孔隙度1/% 有机质孔隙度2/% YC1 1.11 0.38 2.81 0.01 0.02 CHP 4.05 1.20 9.92 0.12 0.18 DBY 4.87 28.89 11.82 3.42 5.25 GD1 1.17 29.35 2.96 0.87 1.34 WY35 3.43 27.95 8.46 2.37 3.64 N208 1.21 21.20 3.06 0.65 0.99 YY1 5.04 16.23 12.21 1.98 3.05 PY1 2.47 17.72 6.16 1.09 1.68 LY1 4.58 20.04 11.16 2.24 3.44 JY41 3.74 31.34 9.19 2.88 4.43 YZ1 4.81 27.49 11.69 3.21 4.94 MY1 3.66 15.40 9.01 1.39 2.13 注:含孔有机质含量=w(TOC)×0.95/[w(TOC)/1.25+(100-w(TOC))/2.7];有机质孔隙度1=SEM有机质孔隙度×含孔有机质含量/100,有机质孔隙度2=有机质孔隙度1/0.65。 -
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