Print this pageRIDGLEY, 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