ANCIENT MICROBIAL GAS IN THE MILK RIVER FORMATION, ALBERTA AND SASKATCHEWAN: HOW HAS IT STAYED AROUND FOR SO LONG?

The Upper Cretaceous Milk River Formation in southeastern Alberta and southwestern Saskatchewan has produced more than 3 tcf of dry (>99% methane) microbial gas (δ¹³C_PDB -65 to -71‰) that was internally sourced (from cogenetic methane/water isotopic data). Milk River production is from underpressured fine-grained sandstone and siltstone reservoirs; gas was generated in interbedded organic-bearing mudstones. Thus, the Milk River appears similar to some self-sourced, “shale” gas reservoirs. However, low organic carbon content of mudstones (0.5-2%), shallow burial depth (<1.5 km), cool maximum temperatures (<50^oC), limited natural fracturing, and prolonged (~20 m.y.) generation that commenced shortly after deposition (based on petrologic and isotopic studies) are characteristics that distinguish the Milk River from other self-sourced systems (e.g.. the organic-rich Mississippian Barnett Shale).

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. For fine-grained sandstone reservoirs in the Milk River, pore apertures range from 0.3 to 10 µm and permeabilities range from 0.3 to 121 mD. In contrast, pore apertures and calculated permeabilities in interbedded mudstones range from 0.02 to 0.17 µm and 0.002 to 0.4 mD, respectively. For the Pakowki, claystones have pore apertures from 0.01 to 0.07 µm and calculated permeabilities of 0.0017 to 0.017 mD. The small pore apertures and low permeabilites of Milk River mudstones, as well as claystones in the Pakowki, indicate that microbial gas in the Milk River was "sealed" by both the intraformational mudstones and the overlying claystones. It is therefore not surprising that gas has accumulated in the Milk River and has been preserved for millions of years in sufficient quantities for exploitation.