Date of Award

Spring 2017

Degree Type


Degree Name

Master of Science (MS)


Geological Engineering

Committee Chair

Larry Smith

First Advisor

Chris Gammons

Second Advisor

Curtis Link


The Bow Island Member of the Thermopolis Formation in Montana is a low-pressure gas producer, which formed due to the first (Kiowa-Skull Creek) transgressive–regressive cycles of the Cretaceous Western Interior Seaway.

The Bow Island Member and its correlative units (Viking and Muddy Formations) have been extensively exploited for oil and gas throughout Canada and Wyoming, but remains relatively unmapped in Montana. The area of stratigraphic study is located at section 15, T36N, R5E Liberty County, Montana. This area located on Dafoe Ranch exhibits a moderately exposed stratigraphic section of Cretaceous Thermopolis Formation. The Bow Island Member lies between the Skull Creek Shale Member and the Shell Creek Shale Member of the Thermopolis Formation. The measured stratigraphic section of the Bow Island is approximately 270 feet. The section shows a progradational parasequence set of five coarsening upward sequences deposited in an offshore to uppershoreface depositional environment. Very fine to coarse-grained sandstone sequences are separated by mudstone flooding surfaces. The sequence boundary between a highstand systems tract and a transgressive systems tract is marked by a sandstone containing black chert pebbles. The previously recognized bed of chert-pebble lag gravel marks a position of erosion and the Albian eustatic drop in sea level. The unconformity marked by the middle Bow Island top is interpreted as second order and the parasequences are interpreted as fourth to fifth order.

Well-log data from exploratory and producing wells are used to conduct subsurface and sequence stratigraphic mapping of the Bow Island Formation. Correlation of 814 wells located near East Butte and the surrounding area into Liberty and Hill counties are used to map surfaces recognized in outcrop to the subsurface. Four informal units correlated are: Bow Island top, middle Bow Island, lower Bow Island, Skull Creek Shale. Four structure contour maps, four isochore maps and two 3D visualizations are shown to illustrate the Bow Island in a 35-township area in north central Montana. Most of the controls for gas production in the area are structural, a new subsurface domal structure is detailed in the 3D visualizations. Stratigraphic traps within the Bow Island could not be determined from log rasters.

The PANalytical TerraSpec Halo Mineral Identifier was used to identify minerals within samples obtained from the exposed beds of the sediments in the Bow Island Member to determine the cause for decreased production in wells drilled in the area. Illites and micas are the predominant minerals throughout the column. A combination of processes (particle plugging, clay swelling, change in pH) lead to decreased permeability within the member.


A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geoscience: Geology Option