Organic and Inorganic Porosity, and Controls of Hydrocarbon Storage in Shales

Porosity is a primary characterization parameter for organic-rich shales, as gas/oil exists within the pore spaces within these formations. Careful analysis and quantification of the various attributes of the complete range of pore sizes dimensions, shapes and connectivity is required. The results of such analysis applied to a suite of samples from a specific province are typically quite revealing about the gas storage potential of the shales and how that evolves with advancing thermal maturity. While different techniques exist to assess porous structures in shales, low-pressure gas adsorption (LPGA) techniques using N2 and CO2 are generally the most informative and reliable tools for assessing the complex nano- to macropore distributions in shale reservoirs. In this chapter we discuss the different facets of using LPGA techniques for assessing these reservoirs. N2 is widely for assessing predominantly the mesoporous and part of macroporous components of the pore-size distributions. Whereas, CO2 is used to better access the microporous components of the pore-size distributions in shales. At low experimental temperatures, N2 is unable to fully access the fine complex network of mesoporous and microporous components, particularly which are present within organic-matter. Comparative analysis of the pore properties of coals and shales (organic-rich and organic-lean) confirms that the porosity of organic-rich horizons tends to be underestimated by the low-pressure N2-adsorption method compared to the CO2 adsorption method. Pore surface fractal dimension D1, representing the fractal dimension at lower relative pressures is observed to be consistently lower than the pore structural fractal dimension, D2 for shales occurring in different basins and geological settings/ages. The impact of the particle crush-size of the samples analyzed can influence the pore-size distribution measurements and should be carefully selected to provide consistent shale-porosity interpretations.