Lithofacies Characteristics and Its Effect on Pore Structure of the Marine Shale in the Low Silurian Longmaxi Formation, Southeastern Chongqing
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摘要: 目前对优势页岩岩相的划分尚缺乏明确的标准.另外，合理连接不同孔隙测试方法的结果，实现页岩的全孔径孔隙结构定量化表征，成为现阶段亟待解决的关键问题.基于页岩的有机质丰度和矿物组分建立了岩相划分方案，查明了渝东南下志留统龙马溪组页岩发育硅质页岩、混合质页岩和粘土质页岩3类，根据有机质丰度将每类页岩细分为富、含和贫有机质等共计9种岩相，在此基础上开展了低压N2吸附和高压压汞实验.研究区龙马溪组中富有机质页岩孔径呈现多峰分布特征，主要孔径峰值位于2~3nm、70~90nm和200~300nm，页岩的孔体积主要来源于中孔（2~0nm）和宏孔（>0nm），比表面积主要来自中孔和微孔（ < 2nm）；对于孔体积贡献，微孔最高可占12%，中孔占3%，宏孔占2%.对于孔面积贡献，微孔最高占47%，中孔占7%，宏孔占11%.随着粘土矿物含量升高，在200~400nm范围内孔体积显著升高.中孔和宏孔贡献了超过90%的孔体积，微孔和中孔贡献了超过90%的比表面积.富有机质硅质页岩微孔比例高，对比表面积贡献高，孔体积和比表面积最大，有利于页岩气富集，是最有利的页岩岩相.Abstract: The favorable facies classification for shale has not been established so far, and it is a key issue in shale gas evaluation to characterize the full scale pore size distribution by reasonable relating of different results. In this study, lithofacies classification is established based on TOC and XRD experiments on cores. There are 9 types of lithofacies, namely the Organic-rich siliceous shale (ORS), Organic-rich mixed shale (ORM), Organic-rich argillaceous shale (ORA), Organic-fair siliceous shale (OMS), Organic-fair mixed shale (OMM), Organic-fair argillaceous shale (OMA), Organic-poor siliceous shale (OPS), Organic-poor mixed shale (OPM), and Organic-poor argillaceous shale (OPA) develop in southeastern Chongqing. Low pressure nitrogen adsorption and high pressure mercury intrusion experiments are conducted to quantitatively characterize the full scale pore size distribution. It is found that spectrum of pore size distribution for ORS have multiple peaks at 2-3nm, 70-90nm and 200-300nm, and the peak shifts to the small pore size when the TOC value increases. Mesopores and macropores have the largest proportion in pore volumes, whereas mesopores and micropores take the largest part of the pore surface areas. The pore volume percentage of micropore, mesopore and marcropore is 12%, 53%, and 52% respectively, and the pore surface area percentage of micro pore, mesopore and marcropore is 47%, 57%, and 11% respectively. The volume of 200-400nm pore obviously increases when clay mineral content increases. The contribution to the surface area is over 90% for micropores and mesopores. The contribution to the pore volume is over 90% for mesopores and marcropores. The organic-rich siliceous shale, which has the largest pore volume and surface area, is the most favourable lithofacies for shale gas enrichment.
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Key words:
- southeastern Chongqing /
- Longmaxi Formation /
- shale lithofacies /
- siliceous shale /
- pore structure /
- petroleum geology
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图 1 渝东南地区构造位置(a)及井位分布(b)
Fig. 1. Tectonic characteristics (a) and wells distribution (b) of southeastern Chongqing
图 2 海相页岩岩相划分方案
Clay.粘土矿物；Ca.碳酸盐矿物，主要包括方解石和白云石；Si.石英；Ⅰ.粘土质页岩；Ⅱ.钙质页岩；Ⅲ.混合质页岩；Ⅳ.硅质页岩
Fig. 2. The lithofacies classification of marine shale
图 3 渝东南下志留统龙马溪组页岩矿物组成
Fig. 3. The mineral composition of the Longmaxi Formation shale in southeastern Chongqing
图 4 基于高压压汞法表征渝东南地区龙马溪组页岩孔径分布
Fig. 4. Pore size distribution calculated using mercury intrusion data for Longmaxi Formation shale in southeastern Chongqing
图 5 基于低压N2法表征渝东南地区龙马溪组页岩孔径变化率分布
Fig. 5. Incremental pore volume calculated using N2 adsorption data for Longmaxi Formation shale in southeastern Chongqing
图 6 基于低压N2法表征渝东南地区龙马溪组页岩孔径分布
Fig. 6. Pore size distribution calculated using N2 adsorption data for Longmaxi Formation shale in southeastern Chongqing
图 7 渝东南地区龙马溪组页岩全尺度孔径分布特征
Fig. 7. Full scale pore size distribution characteristics for Longmaxi Formation shale in southeastern Chongqing
图 8 渝东南下志留统龙马溪组不同岩相页岩孔隙比例
Fig. 8. Proportion of pore size of the Longmaxi Formation shale in southeastern Chongqing
表 1 渝东南下志留统龙马溪组页岩岩相特征
Table 1. Lithofacies characteristics of the Longmaxi Formation shale in southeastern Chongqing
井号 埋深(m) TOC (%) 矿物组分(%) 岩相类型 岩相代号 石英 长石 碳酸盐岩 粘土矿物 黄铁矿 CY-1 2 410.1 3.7 54.2 22.5 9.5 8.6 3.9 富有机质硅质页岩 ORS CY-2 718.6 2.3 36.5 14.4 15.9 27.6 4.2 富有机质混合质页岩 ORM CY-2 751.2 2.6 35.6 11.3 7.6 41.2 3.8 富有机质粘土质页岩 ORA CY-4 782.0 1.4 44.5 17.0 9.2 25.1 3.3 含有机质硅质页岩 OMS CY-4 756.6 1.3 36.7 17.6 15.1 23.9 4.0 含有机质混合质页岩 OMM CY-3 775.0 1.3 33.2 14.5 6.9 41.0 3.1 含有机质粘土质页岩 OMA CY-6 818.0 0.7 44.9 18.6 8.1 25.7 1.7 贫有机质硅质页岩 OPS CY-5 761.0 0.6 38.9 21.3 17.5 19.6 1.8 贫有机质混合质页岩 OPM CY-6 763.0 0.6 36.4 10.8 3.8 45.4 2.1 贫有机质粘土质页岩 OPA 
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表 2 渝东南下志留统龙马溪组不同岩相页岩孔隙比例
Table 2. Proportion of pore size of the Longmaxi Formation shale in southeastern Chongqing
井号 埋深(m) TOC (%) 岩相代号 孔体积比例(%) 孔体积(mL/g) 孔比表面积比例(%) 孔比表面积(m2/g) 微孔 中孔 宏孔 微孔 中孔 宏孔 CY-1 2 410.1 3.7 ORS 12 36 52 0.031 47 48 5 18.75 CY-2 718.6 2.3 ORM 8 42 50 0.042 44 49 7 25.64 CY-2 751.2 2.6 ORA 6 45 49 0.024 46 43 11 18.35 CY-4 782.0 1.4 OMS 6 44 52 0.035 41 52 7 9.86 CY-4 756.6 1.3 OMM 7 53 40 0.028 42 52 6 6.77 CY-3 775.0 1.3 OMA 4 53 43 0.022 40 51 9 9.11 CY-6 818.0 0.7 OPS 3 52 45 0.009 39 57 4 5.70 CY-5 761.0 0.6 OPM 5 58 37 0.014 44 53 3 6.93 CY-6 763.0 0.6 OPA 8 40 54 0.018 40 55 5 10.58
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