Experimental studies on nanopore evolution in organic matter-rich shales

Abstract

The evolution of nanopores when highly-matured or over-matured shales encounter secondary hydrocarbons was characterised in two groups of Paleozoic marine shale samples. The samples were subjected to hydrous pyrolysis to investigate the evolution of organic-hosted pores and mineral-hosted pores. The samples were collected from the Upper Permian Dalong Formation (sample JG) and the Lower Silurian Longmaxi Formation (sample CN) in the Sichuan Basin, Southwest China. X-ray diffraction, low-pressure CO2 and N2 isotherm adsorption, and scanning electron microscopy (FE-SEM) were utilized to determine the mineralogical compositions and the nanoporosity evolution characteristics of the samples. FE-SEM observations showed that the inorganic pores and organic pores in the JG samples increased as the pyrolysis temperature increased from 400 °C to 525 °C, while the pores in the CN sample did not change significantly. The maximum N2 and CO2 adsorption capacities of the JG shale sample peaked at the pyrolysis temperature of 450 °C. Meanwhile, the maximum N2 adsorption capacity of the CN shale sample reached its maximum at the pyrolysis temperature of 500 °C. The maximum CO2 adsorption of the CN samples generally decreased with increasing thermal simulation temperature, reaching a minimum at 525 °C. The pore volumes of the original and pyrolyzed samples of JG and CN were calculated based on the N2 and CO2 adsorption data. The experimental results are summarized as follows. (1) The pore volumes of the shale samples with different maturities all increased in the pyrolysis simulation, but pore volume increases observed in the matured sample were higher than those in the over-matured sample. (2) Laboratory pyrolysis experiments can increase organic matter (OM) micropores when the gases are generated from the kerogen/bitumen/oil. (3) As maturity increases further, OM micropores become connected together by the formation of new micropores, forming mesopores or even macropores. (4) In the processes of hydrocarbon generation, the kerogen type and carbonate mineral recrystallization or dissolution also have a great influence on the evolution of pores.