The organic matter (OM) in shales is of two types, primary and secondary. Kerogen constitutes the primary OM; these are insoluble in organic solvents and comprise most of the OM present. As shales thermally mature over time during burial, the kerogen become progressively altered/transformed into secondary OM, particularly solid bitumen and pyrobitumen. This secondary OM typically occupies minute interstices between mineral grains. It is also associated with and promotes the formation of pores, at nano- to macro-scales, within the primary OM, providing space for storing hydrocarbons as they are generated during thermal maturation. That generated pore space can also be used to potentially store natural gas and possibly carbon dioxide injected into shale formations. The dimensions (quantity, size, and shape) of OM pores vary with the kerogen types present and are also influenced by thermal maturation degree and the surrounding inorganic minerals in the shale matrix. Imaging and gas-adsorption techniques make it possible to explore the evolution of OM-hosted porosity as thermal maturity advances and its influence on permeability, gas flow and storage. The paleoenvironmental conditions (water depth, oxygen levels, and rate of sedimentation) during shale deposition influences OM preservation. A thorough understanding of all these factors helps in delineating an effective shale gas reservoir for both hydrocarbon production and CO2 sequestration.