Depositional environments and stratigraphic architecture of the Late Cretaceous Milk River and Eagle formations, southern Alberta and north-central Montana: relationships to shallow biogenic gas

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ABSTRACT The Milk River and Eagle formations in southern Alberta and north-central Montana form the core of the first Upper Cretaceous clastic wedge in this part of the Western Interior Foreland Basin. The wedge is a host for shallow biogenic gas in both Canada and the USA. Facies analysis in outcrop and cores shows two overall marine progradational successions with markedly differing depositional environments. The lower, Virgelle succession, is dominated by swaley and hummocky cross-stratification and represents a storm-dominated shoreface deposit. The upper, Upper Eagle succession, is dominated by a high mudstone content and has tidal indicators. Importantly, no abundant swaley or hummocky cross-stratifications can be found within the Upper Eagle succession, indicating deposition under dominantly fair-weather conditions in a restricted, probably deltaic environment. Down-dip correlation with 2760 wireline logs reveals that each succession consists of a series of three progradational allomembers separated by minor transgressions. The three Virgelle allomembers step consecutively farther basinward and were terminated by a regional transgression. The three Upper Eagle allomembers also step seaward in a shingled manner and were partly drowned before being terminated by the regional Pakowki/Claggett transgression. Production data indicates that only the Upper Eagle A, B and, when present, C allomembers are productive in the Bearpaw Mountains area. All Virgelle allomembers commonly test wet, probably due to a large amount of inter-connectivity among the coastal sandstones of the allomembers, which enabled the migration of hydrocarbons up structural dip. Unconventional reservoirs such as the Alderson Member of the southern Alberta Milk River Gas Pool are time equivalent to the Upper Eagle allomembers. Because deltaic sedimentation with little or no storm activity prevailed during Upper Eagle time, the Alderson fine-grained rocks could be the result of a plume of sediment of deltaic origin (pro-delta) being deposited on the shelf during prevailing fair-weather conditions. End_Page 155------------------------