Coastal and Deltaic Sedimentation of Upper Cretaceous Eagle Sandstone: Relation to Shallow Gas Accumulations, North-Central Montana

Abstract

Depositional environments of the Upper Cretaceous Eagle Sandstone were studied at outcrops along the Missouri River and its southern tributaries from the town of Virgelle southeastward to the mouth of the Judith River in north-central Montana. In this area, the Eagle is divided into three members--the Virgelle Sandstone Member, 80 to 130 ft (24 to 40 m) thick, and the middle and upper members, which together are as much as 220 ft (67 m) thick. The Eagle Sandstone is underlain by the Telegraph Creek Formation and overlain by the Claggett Shale, both of Late Cretaceous age. The Telegraph Creek accumulated in an offshore-shoreface transition environment and grades upward into the shoreface and foreshore sandstones of the Virgelle Sandstone Member. The basal Virgelle was depo ited along an eastwardly prograding coastal-interdeltaic mainland shoreline. The middle member of the Eagle represents coastal-plain deposition. In the eastern part of the area, the upper part of this member comprises a sheetlike, delta-front sandstone capped by a thin coastal-plain unit. The delta-front sandstone was deposited along a wave-dominated shoreline that prograded over a coastal plain following an overall marine transgression. The upper member lies disconformably on the middle member and is represented by rock types which were deposited in two distinct depositional settings. Interbedded sandstone, siltstone, and shale that exhibit variable bedding types in the northern outcrops probably accumulated in a tidal-flat environment. Progradational cycles of shoreface sandstones are haracteristic of the upper member in the southern outcrop. Chert gravel in the upper member and in the basal part of the Claggett Shale forms a persistent time interval in the northern Rockies and probably represents a lag deposit laid down by the transgressing sea. Natural gas from shallow accumulations in the Eagle Sandstone of the Bearpaw Mountains area is of biogenic origin and was probably generated in the surrounding shales during Late Cretaceous time. Although gravity-induced faults formed after the gas generation provide the final trapping mechanism, the initial control for entrapment was stratigraphic. Most of the early generated gas has remigrated into separate, discrete structural traps where porous reservoirs are developed; some of the gas may have been selectively sealed in original stratigraphic traps. The outcropping depositional units, in particular the reservoir sandstones, can be traced into the subsurface and identified in nearby wells. Thus, an understanding of depositional environments is an important exploration tool for sha low gas accumulations.