Large accumulations of biogenic gas are known to exist in shallow (<3,000 ft deep) Cretaceous reservoirs of the Northern Great Plains (mainly Montana) and southeastern Alberta and southwestern Saskatchewan, Canada. The 1995 USGS National oil and gas assessment made a mean estimate of 1.15-1.18 x 10 12 m 3 (41-42 trillion cubic ft) of potential additions to reserves of continuous-type (unconventional) gas in Cretaceous shallow biogenic gas plays of northern and central Montana (Rice and Spencer, 1995). About 90 percent of the 1995 gas estimate was in hypothetical plays. The controls on the boundaries of these plays were poorly understood at the time of the assessment in 1995. As a follow on to the 1995 assessment, a multidisciplinary project (Ridgley and others, 1999) was undertaken in order to better understand the controls on the gas accumulation and to better define the play boundaries. The focus of the project is on Cretaceous rocks from the top of the Mowry Shale to the base of the Judith River Formation, and within this geologic time slice the project will define the petroleum system of the shallow biogenic gas. To achieve these goals, we are integrating geologic, structural, hydrologic, and engineering studies with known and new geochemical data on the gas and co-produced water to understand the controls on the spatial distribution of potential gas accumulations and to understand the origin and time of generation and migration of the gas. An additional focus of the project is to determine the controls on the sites of shallow biogenic gas in the large southeastern Alberta gas field in Canada. This gas accumulation is in rocks age-equivalent to those that host the shallow gas in the Northern Great Plains. Over one-half of the 1995 national oil and gas assessment was based primarily on similarity of the potentially productive facies in Montana to the facies that host large shallow biogenic gas resources in southeastern Alberta and southwestern Saskatchewan. We will determine if the Canadian analog is appropriate for the Northern Great Plains shallow gas system. We plan a series of topical reports that will provide the background information required for the next assessment of the shallow biogenic gas system in the Northern Great Plains in late 2000.
