Print this pageJennie L. Ridgley1, C.F. Gilboy2, D.H. McNeil3, S.M. Condon1, J.D. Obradovich1
(1) U.S. Geological Survey, Denver, CO
(2) Saskatchewan Energy and Mines, Saskatchewan
(3) Geological Survey of Canada, N/A
ABSTRACT: Stratigraphic and Structural Influences on Sites of Shallow Biogenic Gas Accumulation in the Upper Cretaceous Belle Fourche and Second White Specks/Greenhorn Formations of Southern Alberta, Southwestern Saskatchewan, and Northern Montana
Biogenic gas is produced from different sandstone units in the Upper Cretaceous Belle Fourche Formation in southeastern Alberta and southwestern Saskatchewan, Canada, and in the Bowdoin dome, Bears Paw Mountains, and Sweetgrass Hills areas, Montana. Limited shallow biogenic gas production also occurs in bioclastic limestone and calcarenite in the Second White Specks Formation in Canada, and the Greenhorn Formation in Montana. Areas of current biogenic gas production in the Belle Fourche partly reflect historic practices of step-out drilling and do not completely reflect the true geographic/stratigraphic distribution of potential reservoirs. A principal control on the spatial distribution of productive gas reservoirs in these formations is structure. During the Late Cenomanian, reservoir sandstone facies were eroded sequentially from south to north in the upper and middle parts of the Belle Fourche throughout the gas-producing area; loss of these facies is most pronounced in southwestern Saskatchewan. The erosion of these rocks was probably caused by differential movement on fault-bounded blocks in response to thrust loading and subsequent relaxation of tectonic forces in the Disturbed Belt west and southwest of Great Falls and Cut Bank, Montana, coupled with uplift in central Saskatchewan. Movement on the blocks occurred prior to the final stage of the Greenhorn transgression; strata assigned to the Second White Specks and Greenhorn formations rest unconformably on various units of the Belle Fourche. Foraminiferal biostratigraphic data and new ages on key bentonite marker beds have been instrumental in determining the extent of the unconformity.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado