--> --> Stratigraphy and Sedimentology of Productive Lower Carboniferous Reservoirs in the Bakken Petroleum System, western Montana and Alberta

AAPG Rocky Mountain Section Meeting

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Stratigraphy and Sedimentology of Productive Lower Carboniferous Reservoirs in the Bakken Petroleum System, western Montana and Alberta

Abstract

Lower Carboniferous strata in the Bakken Petroleum System of the northwestern United States and southern Canada comprise the Banff Formation (part), upper Sappington Shale, and Cottonwood Canyon Member of the Lodgepole Formation, that are correlative with the Upper Bakken Shale to the east. The western facies record a major episode of basin reorganization and erosion related to the Antler Orogeny. They overlie an erosional lag that locally rests on the Jefferson/Duperow and extends as far east as Elm Coulee Field. They are capped by an erosional surface at the base of the Lodgepole/ Banff limestones that has stripped them from local paleohighs. This unconformity bound sequence is interpreted as its own transgressive systems tract that passes into a correlative conformity in the Williston and Elbow Basins, where the upper Bakken shale has historically been seen as having sharp, but conformable contacts. Rocky Mountain Section – AAPG: 2019 Annual Meeting 48 Internally, the western facies consist of two members; a lower black shale identical to the Upper Bakken Shale and an overlying sandstone that is not present in the Williston Basin. The sandstone is a very fine-grained, medium bedded sandstone with black silt and mudstone stringers that has been thoroughly bioturbated in the Zoophychos Ichnofacies at an intermediate subtidal depth. The sandstone is very carbonaceous containing flakes and stems of organic matter, making it difficult to identify on gamma ray logs having relative gamma API values ranging from 60 – 140 GR units in the productive reservoir. A western source for the sand is postulated, perhaps from an uplifted forebulge extensively colonized by terrestrial plants. In southern Alberta, this sandstone reaches thicknesses of 45+ feet and produces in the Ferguson field, down dip of the Kevin-Sunburst Dome. Here it is referred to as the Banff Sand or the “Alberta Bakken” emphasizing that it is not correlative to the older Middle Bakken or Exshaw. The Ferguson and related sand bodies are erosional remnants of what was once a much thicker depositional system that has been extensively eroded beneath the basal Lodgepole/Banff unconformity. Trapping occurs by truncation of the reservoir along the flank of a paleohigh in an up-dip position on the northern flank of the Dome.