Mineralogy and microstructure are key variables defining the physical properties of a rock. Mudstones show inherently heterogeneous matrix pore-size distributions. They can show organic and inorganic pores and the transport mechanism through pores is different, and, therefore, it is necessary to describe their organic and inorganic porosity. This work uses Field Emission Gun Scanning Electron Microscopy to characterize the Galembo Member mudstones, Cretaceous La Luna Formation, Middle Magdalena Valley Basin, Colombia. There are several pore types in mudstones, including interparticle pores due to flocculation of clay minerals, organoporosity due to burial and thermal maturity of organic matter, intraparticle pores from organisms, intraparticle pores within mineral grains, and microchannels and microfractures, including dissolution, fillings and interlayer fractures. The existence of interconnected pores in such complex fracture-pore system provides effective pathways for primary gas migration and it also provides a storage space for the residual petroleum in mudstones, which is important for the primary migration and storage for mudstone gas resources. The pore connectivity is high and increases towards the top of the Galembo Member. © 2016. Acad. Colomb. Cienc. Ex. Fis. Nat. All rights reserved.