Understanding porosity and its development is essential when characterizing source-rock reservoir quality. Using a field-emission scanning electron microscope (FESEM) on argon-ion polished shale samples, images show that most of the porosity occurs within organic matter (OM) and that the pore geometry and characteristics vary with increasing thermal maturity. Comparison of the OM in shales from numerous plays reveals that the OM is heterogeneous. High magnification FESEM images (>30,000X) show the OM is composed of spherical to cylindrical units, 7–45 nm in diameter dispersed in an organic matrix, where the porosity has developed within this arrangement. These subunits and pore structures are evident in rocks from low to high thermal maturity (e.g., %Ro equivalent 0.6 to >3). The common presence of OM with this texture in open pores and between clay platelets in larger, open, stress-protected pore bodies supports an interpretation that the OM was formed within intercrystalline space as a " colloidal-like" system. Observations suggest that with the onset of thermal maturation and degradation of the OM, molecular self-assembly took place through noncovalent interactions to form a colloidal-like system. Large pores and high organic porosity do not occur in all OM within a sample and OM with no significant visible porosity does occur in high-stress locations and in thin sheets between mineral grains. The presence of minerals within the OM may have an effect on visible organic porosity size and distribution.
Porosity occurring within organic matter is well documented in gas shales but also occurs in the less thermally mature oil shales. Understanding fluid flow through this porous media, particularly at nanometer scales, requires integration of molecule size, pore throat size, molecule-pore wall interaction, and the relation of these properties to the transport of molecular components through a porous network. Perhaps most importantly, the analysis leads to understanding gas- and oil-shale production, stimulation, and flow properties. Examining organic porosity, we can hypothesize that formation of the connected pore system increases with thermal maturation. As immature kerogen undergoes thermal maturation, hydrogen bonds are initially broken resulting in the early formation of oil, then wet gas, and finally of pure methane.