George W. Shurr1
(1) Geoshurr Resources, L.L.C, Ellsworth, MN
Abstract: Hydrodynamic controls on basin margin gas accumulations in shallow-marine depositional systems
Rocks deposited in shallow-marine systems host important gas reserves on the margins of cratonic basins. Conventional reservoirs in the Northern Great Plains are found in Cretaceous transitional marine, shore face, and shelf sandstones; unconventional reservoirs are in interbedded siltstone and shale deposited on a siliciclastic shelf. In contrast with the more continuous basin center gas accumulations, basin margin gas is mainly biogenic and is closely associated with geologic structures including regional fracture systems. Basin margins are attractive shallow gas targets for astute independents.
Hydrodynamics play an important role in basin margin gas traps. Ground water movement is demonstrated by: 1) tilted gas-water contacts, 2) nonhorizontal planes of constant pressure, 3) production patterns off-set from anticline axes, and 4) diagnostic gas compositions. Basin margin gas accumulations result from regional ground water flow up dip or down dip and from interaction of flow systems with local geologic structures.
The Cretaceous Dakota Sandstone has conventional gas reservoirs with un-dip regional ground water flow on the eastern margin of the Williston Basin. Gas production at the turn of the century in central South Dakota and southwestern North Dakota ended when numerous artesian water wells modified isopotential surfaces to effectively eliminate hydrodynamic traps.
Cretaceous shallow marine shelf rocks have basin margin gas accumulations on several specific geologic structures located on the western margin of the Williston Basin. Hydrodynamic trap components are identified in conventional reservoirs in the Judith River Sandstone in Plevna Field, MT, and the Shannon Sandstone in West Short Pine Hills Field, SD. Unconventional reservoirs have hydrodynamic aspects om the Eagle Sandstone on Cedar Creek Anticline, MT., the Gammon Formation in Little Missouri Field, ND, and the Phillips and Bowdoin Sandstones on Bowdoin Dome, MT.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana