Porosity evolution of the organic-rich shale with thermal maturity increasing

To study porosity evolution of organic-rich shale, five immature shale samples collected from outcrops were analyzed by pyrolyzing them in a thermal simulation experiment. Organic-rich shale samples with different thermal maturities were obtained in the lab and their maturities referred to immature, early bitumen-formation, late bitumen-formation, late oil-formation and oil-cracking stages, respectively. Nano-pores in these five shale samples were then measured on a specific surface area analyzer. A series of nitrogen adsorption isotherms were measured, which showed that nano-pores increase with thermal maturity increasing and so does the porosity in the organic-rich shale, which is obviously different from the porosity evolution in matrix. The porosity changes little at the oil window, but drastically increases in the gas window, which is consistent with calculated value based on the mass balance theory. Therefore, it can be concluded that increase in porosity of the organic-rich shale results dominantly from the organic matter conversion of oil to gas.