Duration of Microbial Gas Generation in the Upper Cretaceous Milk River Formation, Southeastern Alberta and Southwestern Saskatchewan

Fishman, Neil S.¹, Donald L. Hall², Elizabeth L. Rowan^3
1 U.S. Geological Survey, Denver, CO
² Fluid Inclusion Technologies, Inc, Broken Arrow, OK
³ U.S. Geological Survey, Reston, VA

Petrologic and fluid inclusion studies have been integrated with burial history data to evaluate the duration of methanogenesis in the Upper Cretaceous Milk River Formation (~85-81Ma), a prolific producing unit (>5 TCF), southeastern Alberta and southwestern Saskatchewan. Gas characteristics (methane ¹³C_PDB –65 to –71‰, dry gas >99% methane) point to a microbial origin.

Methane-bearing inclusions are present in various generations of authigenic carbonates. Concretions, which contain intergrown siderite, dolomite, and calcite, formed early, as evidenced by unbroken, hollow microfossils and undeformed labile grains. Methanic inclusions (presumed primary) in concretionary minerals point to coeval methanogenesis and concretion formation during early diagenesis. Calcite also completely cements some thin sandstones, which have varying (34-22%, n=26) amounts of intergranular volume (IGV). Because the IGV of calcite-cemented sandstones provide a good estimate of the degree of mechanical compaction at the time of cementation, sandstones with high IGV’s were cemented relatively early, whereas those with lower IGV’s were cemented later. The lower IGV’s are those expected for sandstones that remained uncemented through burial of ~1.5 km, the maximum depth of burial experienced by the Milk River. Maximum burial was reached ~20 m.y. after deposition. Importantly, primary methane-bearing inclusions are present in calcite cement, regardless of its age. 

The presence of primary methanic inclusions in carbonates of varying age implies that methanogenesis continued throughout the timing of carbonate precipitation. Thus, methanogenesis took place during at least the first 20 m.y. after deposition, and might have also continued while the Milk River remained at maximum burial.