Abstract The Upper Cretaceous Milk River Formation in southeastern Alberta and southwestern Saskatchewan has produced more than 2 tcf of dry (>99% methane) microbial gas ( δ 13 C PDB –65 to –71‰) that was internally sourced. Production is from underpressured fine-grained sandstone and siltstone reservoirs, whereas the gas was generated in interbedded organic-bearing mudstones with low organic carbon contents (0.5–1.50%). The formation experienced a shallow burial history (maximum burial, <1.3 km [<0.8 mi]) and cool formation temperatures (<50°C [<122°F]). Petrologic and isotopic studies suggest that methanogenesis began shortly after deposition and continued for at least 20 to 25 m.y. Mercury injection capillary pressure data from the Milk River Formation and the overlying Upper Cretaceous Pakowki Formation, which contains numerous regionally extensive bentonitic claystones, reveal a strong lithologic control on pore apertures and calculated permeabilities. Pore apertures and calculated permeabilities in Milk River mudstones range from 0.0255 to 0.169 μm and less than 0.002 to 0.414 md, respectively, and claystones from the overlying Pakowki Formation have pore apertures from 0.011 to 0.0338 μm and calculated permeabilities of 0.0017 to 0.0065 md. The small pore apertures and low permeabilities indicate that claystones and mudstones served as seals for microbial Milk River gas, thereby permitting gas to accumulate in economic quantities and be preserved for millions of years. Based on the timing of gas generation, the gas system of the Milk River Formation can be considered an ancient microbial gas system, which is one of several ways it differs from that of the Devonian Antrim Shale, Michigan Basin, where microbial gas generation is a geologically young (Pleistocene and younger) phenomenon. The difference in timing of gas generation between the Milk River and Antrim systems implies that gases in the two formations represent end members of a spectrum of microbial gas accumulations in fine-grained rocks, with the Milk River Formation being an excellent example on which to base a paradigm for an ancient microbial gas system.