Subsurface Mapping and Preliminary Stratigraphic Analysis of the Lower Cretaceous Bow Island Formation, North-Central Montana

The Bow Island Formation is an Albian age sequence of shales to sandstones deposited in southern Alberta and north-central Montana during the Early Cretaceous transgressions and regressions of the Western Interior Seaway that flooded the interior of North America. It is a shallow, low pressure gas producer in Montana, produces oil further north around Calgary, and is equivalent with the Muddy Formation in Wyoming and Colorado. Extensive mapping using both well logs and dozens of cores has been completed throughout Alberta.
(Pederson et al., 2002). Mapping and stratigraphic correlations in Montana are very incomplete, consisting of two published cross sections, very little sedimentology, and no subsurface mapping (Porter et al., 1998; Peterson, 1966).

Structurally, the area is controlled by the Kevin/Sunburst dome along the Sweetgrass Arch. This anticlinal structure gently dips towards the north and has undergone multiple periods of movement. Northeast of this dome are the Sweetgrass Hills, three Eocene igneous intrusions and dikes thatintrude and uplift the formation.

Production occurs from several zones of sandstone that cap coarsening upward cycles and are located at the top or flanks of the dome. Also, gas can be found along zones of ravine infill related to the late Albian lowstand unconformity that can be seen in well logs throughout Montana, and from other stratigraphic truncations related to third-order parasequences. Currently there are 1066 wells producing 13,000 MCF/D throughout Montana from the Bow Island Field (Montana Board of Oil and Gas data). The majority of these fields were discovered as a secondary target while drilling toward deeper horizons. Despite these great number of wells, the work has not been published to tie these producing intervals together over a large enough area to propose an exploration model for the Bow Island.

This work provides the first publically available detailed mapping of the Bow Island throughout north central Montana and also touches on the structures in the mapping area that are controlling production in the numerous gas fields.

Pederson et al., 2002, Sequence Stratigraphic Architecture: Albian Bow Island Formation, Bulletin of Canadian Petroleum Geology v 50 p 441-477.

Porter et al., 1998, Post-Mannville/Kootenai Lower Cretaceous Rocks, North-Central Montana: MBMG Sp Pap 13.

Peterson, 1966, Sedimentary History of the Sweetgrass Arch: 17th Billings Geological Society guidebook p 112-132.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California