RIDGLEY, J. L., S. M. CONDON, L. O. ANNA, T. C. HESTER, E. L. ROWAN, TROY COOK, and P. G. LILLIS
U.S. Geological Survey, Denver, CO

ABSTRACT: Reexamination of Cretaceous Shallow Biogenic Gas Resources, Northern Great Plains, Montana -- A Multidisciplinary Approach

In 1995, the U.S. Geological Survey assessed 40 tcf of undiscovered, continuous (unconventional) gas, in the Cretaceous, shallow biogenic gas play of northern Montana. Much of this resource was in hypothetical plays and was based on analogy to a similar gas play in Alberta and Saskatchewan, Canada. As a follow on to the 1995 USGS assessment, a multidisciplinary approach was implemented to evaluate controls on the continuous, shallow, Cretaceous biogenic gas accumulation in Montana, to better define the play boundary, and to reassess resource numbers. The multidisciplinary approach involves digital integration of geology, hydrology, gas and produced water geochemistry and isotopes, production histories and other engineering parameters, well log characterization, and characteristics of nonproducing wells in both the US and Canada. Focus of the study is on the Upper Cretaceous interval from the base of the Judith River Formation to the top of the Mowry Shale.

Integration of geology (facies) and hydrology (flow units) with gas and water geochemistry has led to reevaluation of the timing/origin of gas generation, timing of gas migration, and timing of subsequent removal and redistribution of the gas under shallow ground water conditions. Analysis of gas chemistry for multiple formations within the interval of focus indicate 1) excessive amounts (for biogenic gas) of ethane, propane, and higher hydrocarbons in more than 20 percent of gases analyzed, suggesting a mixture of biogenic and thermogenic gas; and 2) modification by shallow groundwater, as indicated by gases that become more nitrogen-rich updip in the recharge zone. New gas and water sampling are underway to help explain these observations and their impact on future assessments.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana