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The Need of a Paradigm for Ancient Microbial Gas Accumulations – the Upper Cretaceous Milk River Formation, Alberta and Saskatchewan

Neil S. Fishman1, Paul G. Lillis1, Donald L. Hall2, and Debra K. Higley1
1 U.S. Geological Survey, Denver, CO
2 Fluid Inclusion Technologies, Inc, Broken Arrow, OK

The Upper Cretaceous Milk River Formation, deposited within the Western Interior Seaway, has produced >3 tcf of dry (>99% CH4), internally sourced (based on cogenetic methane/water isotopic data and nitrogen content of produced gas), microbial gas (δ13CPDB -65 to -71‰) from a large (>25,000 km2) area in southeastern Alberta and southwestern Saskatchewan. Production is from underpressured fine-grained sandstone, siltstone, and mudstone with variable porosity (<1 to 26%) and matrix permeability (<1md to 200md). Therefore, the Milk River appears similar to some self-sourced, “shale” gas reservoirs. However, a low organic carbon content (0.5-2%), shallow burial depth (<1.5 km), cool formation temperatures (<50°C from basin reconstruction), limited natural fracturing, and prolonged (~20 m.y.) duration of microbial gas generation that commenced shortly after deposition (from petrologic, isotopic, and fluid inclusion studies) are characteristics that distinguish the Milk River from other self-sourced reservoirs, including the organic-rich, fractured Mississippian Barnett Shale.

Self-sourced gas reservoirs, as “unconventional” reservoirs, represent a paradigm shift from “conventional” gas reservoirs, and study of the Milk River demonstrates that key characteristics of unconventional reservoirs vary markedly. The broad areal extent of Milk River production, coupled with known production of microbial gas in other rocks deposited within the Western Interior Seaway, not only demonstrates the importance of these resources but also points to the need for a separate model that addresses microbial gas accumulations and helps target locations where similar accumulations might exist. The Milk River seems an excellent example upon which to base a paradigm for ancient microbial gas systems.