摘要:通过有机地化分析、全岩X衍射矿物分析、甲烷等温吸附及低压氮气吸附实验，本文对桂中坳陷环江凹陷上古生界页岩样品的孔隙结构及分形特征进行了研究。结果表明：研究区页岩总有机碳含量（TOC）平均为2.40%，热成熟度（Ro）平均为2.65%，处于过成熟演化阶段。页岩主要的矿物组成为石英和黏土。页岩的比表面积平均为5.86 m2/g，孔容平均为0.014 9 mL/g，平均孔径为11.2 nm。页岩中发育大量的中孔，主要呈两端开口的圆筒形孔或四边开放的平行板状孔。页岩中TOC含量和石英含量越多，微-中孔越发育、比表面积和孔容越大，而平均孔径则变小。通过Frenkel-Halsey-Hill（FHH）模型和氮气吸附实验数据计算得到孔隙表面分形维数D1（平均为2.428 4）和孔隙结构分形维数D2（平均为2.622 2），对应的相对压力（P/P0）分别是0～0.45和0.45～0.99。分形维数D1、D2随着比表面积、孔容的增加而增加，而平均孔径随着前者的增加而减小。分形维数D1、D2、TOC含量、石英含量和甲烷吸附量之间呈现较好的正相关性，但随着黏土矿物含量的增多而减小。分形维数D1与Langmuir压力存在弱负相关性，分形维数D2随Langmuir压力增大有变大的趋势。桂中坳陷西北部页岩分形维数越大，孔隙结构越复杂，其对天然气的吸附和存储能力越强。
关键词: 桂中坳陷/
环江凹陷/
页岩/
孔隙结构/
分形维数/
低压氮气吸附
Abstract:Pore structure and fractal characteristics are of great significance for gas adsorption and transport in shales. In order to study pore structure and fractal characteristics of shale, samples from the Upper Paleozoic in Huanjiang sag, Guizhong depression were investigated by organic geochemical analysis, X-ray diffraction analysis, methane adsorption experiments and low-pressure nitrogen adsorption experiments. The results show that the average content of total organic carbon（TOC）is 2.40%, vitrinite reflectance（Ro）is 2.65% on average, indicating the shale are at over-mature evolution stage. Mineral compositions of the samples are dominated by quartz and clay minerals. The specific surface area of shale averages 5.86 m2/g, the total pore volume averages 0.014 9 mL/g, the average pore diameter is 11.2 nm. There are a large number of mesopores in shales. The pore system is dominated by cylindrical pores and plate-like pores. The contents of TOC and quartz minerals increase with the increasing micro-mesopores in shale, the larger the specific surface area and the total pore volume, the smaller the average pore diameter. Surface fractal dimension?D1（average of 2.428 4）and pore structure fractal dimension?D2（average of 2.622 2）are calculated by Frenkel-Halsey-Hill（FHH）model and nitrogen adsorption-desorption curves（at relative pressure of 0～0.45 and 0.45～0.99, respectively）. With increasing the specific surface area and the total pore volume and decreasing the average pore diameter, fractal dimension?D1 and?D2 display an increasing trend. Meanwhile, fractal dimensions?D1 and?D2 have a positive linear correlation with TOC, quartz minerals content and methane adsorption, but have an infirm negative correlation with clay minerals content. Additionally, it is found that fractal dimension?D1 decrease with the increasing Langmuir pressure but fractal dimension?D2 increase with the increasing Langmuir pressure. In general, the higher fractal dimension of the shale in Guizhong depression, the more complicated pore structure, which made a great contribute to the gas adsorption and accumulation capacity in shales.
Key words:Guizhong depression/
Huanjiang sag/
Shale/
Pore structure/
Fractal dimension/
Low-pressure nitrogen adsorption
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