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Very High Resolution Aeromagnetic Evidence for Structure, Faults and New Oil and Gas Targets - Sweetgrass Arch, Montana

Pearson, William C.1, Richard F. Inden2
1 Pearson Technologies Inc, Lakewood, CO
2 LSSI, Inc, Denver, CO

Structurally and stratigraphically entrapped oil and gas fields in the Sweetgrass Arch Province of Montana are studied with a very high resolution aeromagnetic survey. A century of drilling and production has discovered several oil and gas fields including the giant Cutbank field in Toole, Pondera and Glacier Counties. Commercial hydrocarbons have been discovered in Mississippian carbonates and in Jurassic and Cretaceous sands. Well control and field studies confirm that structures and faults are instrumental in controlling reservoir entrapment.

This proprietary 22,000 line mile high resolution aeromagnetic survey was flown with a 250 meter by 1000 meter flight line spacing. Flight altitude was 300 feet above ground. Surveying utilized GPS navigation, digital diurnal monitoring, high sensitivity Cesium vapor magnetometer and video ground recording. Careful deculturing, profile analysis and grid filtering produced impressive images of residual structural highs, basement faults, intrasedimentary faults and regional wrench faults. Color SUNMAG/AUTOFAULT and gray shade images highlight the structure and fault trends at selected “pseudo depth slices”. Detailed 2-D Werner and 3-D Euler depth estimation provide a separate set of fault picks at different depths in the section.

By relating Sweetgrass Arch oil and gas production to the magnetic fault and structural patterns, this exploration approach is extended to new plays in the shallow biogenic gas (and coalbed methane?) plays of the Upper Cretaceous Judith River, Eagle, Niobrara and the Tertiary Fort Union sections. The magnetic reconnaissance exploration approach is shown to be a good non-invasive exploration tool for prospecting in the environmentally sensitive Montana disturbed belt to the west of the survey.