Origin and significance of natural gases of Montana

About two years ago, I was assigned to a project to evaluate the hydrocarbon potential of the Cretaceous system of the northern Rocky Mountains; namely, in Montana, North Dakota, and South Dakota. I had no problem identifying potential traps; both structural and stratigraphic, but recognized the fact that most of the production from the Cretaceous in Montana is natural gas. More important, this production is from relatively shallow depths, often less than 2,000 feet. I realized that not only stratigraphy and structure, but also the origin, migration, and accumulation of hydrocarbons must be fully understood for effective exploration of new resources in the northern Rocky Mountains and elsewhere.