过成熟页岩孔隙结构变化的石英管热模拟研究

Quartz-tube thermal simulation study on the pore structure transformation in over-matured shales

对上扬子区寒武系牛蹄塘组和志留系龙马溪组两套过成熟页岩开展了系列温度点石英管热模拟实验,在应用氦孔隙度测试法、高压压汞实验和氮气吸附法等测试技术分析模拟样品孔隙结构参数的基础上,研究了页岩孔隙结构随温度的变化特征。结果表明:(1)两组页岩孔隙度、成熟度随热模拟温度的升高有增加的趋势,热模拟后牛蹄塘组页岩孔隙度变化范围介于4.2%?12.2%之间,成熟度介于3.04%?3.46%之间,龙马溪组页岩孔隙度介于5.8%?11.1%之间,成熟度介于2.87%?3.38%之间。页岩孔容的增加主要源于介孔、矿物微裂缝以及基质微裂缝的显著增多;(2)牛蹄塘组页岩热模拟后孔体积和比表面积变化范围分别为0.0031?0.031 cm^3/g和0.47?2.93 m^2/g,而龙马溪组页岩的变化范围分别为0.015?0.054 cm^3/g和3.62?13.93 m^2/g;两组页岩原样的比表面积均来自孔径小于10 nm的纳米孔的贡献,而热模拟后的页岩比表面积则主要来自大于10 nm的孔隙贡献。(3)热模拟后的牛蹄塘组页岩和龙马溪组页岩比表面积与有机碳(TOC)含量减少量成一定的正相关性,与脆性矿物变化量和黏土矿物含量之间的相关性较小,显示比表面积的变化主要与有机质热演化导致的有机纳米孔的增加有关,而黏土矿物在过成熟阶段趋于稳定,对比表面积变化影响较小。

The Niutitang Formation and Longmaxi Formation shales of the upper-Yangtze region were pyrolized by using the quartz-tube thermal simulation method with an aim to characterize the pore structure changes in over-matured shales. The pore structure parameters of the pyrolyzed shale samples were measured by using the helium porosity method, nitrogen adsorption and mercury intrusion experiments. The results show that the porosity and maturity of the Longmaxi Formation and Niutitang Formation shales tend to increase with increasing thermal temperature. The total helium porosity of the pyrolyzed Niutitang Formation shales varies from 4.2% to 12.2% and their maturities are within the range of 3.04%-3.46%, while the porosity and maturity of the pyrolyzed Longmaxi Formation shales are within the range of 5.8% to 11.1% and 2.87%-3.38%, respectively. The increase in the pore volume of pyrolyzed shales is mainly contributed to the mesoporous and micro-fractures in brittle minerals as well as in the matrix. The total pore volume of the pyrolyzed Niutitang Formation and Longmaxi Formation shales range from 0.0031 cm3/g to 0.031 cm3/g and from 0.015 cma/g to 0.054 cm3/g, respectively. The total surface area of the Niutitang Formation shales varies from 0.47 mE/g to 2.93 m2/g, as compared to a higher value of 3.62M3.93 m2/g for the Longmaxi Formation shales. The specific surface area of the initial shales is mainly due to nanopores which are less than 10nm in diameter, but the nanopores which are larger than 10 nm in diameter become the dominant contributors in the pyrolyzed shales. The specific surface area of the pyrolysed Niutitang Formation and Longmaxi Formation shales displays a positive correlation with TOC, but it shows a poor correlation with the contents of clay and brittle minerals, suggesting that changes in specific surface area are mainly controlled by organic nanopores.