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Comparison of 2-D and 3-D Seismic Data to Image Shallow Pop-Up Structures in the Eagle Sandstone Gas Play, North-Central Montana

Johnson, Eric H. 1 (1) Johnson Geophysical, Billings, MT

 

In north-central Montana, gravity slides on the flanks of the Bearpaw Mountains, riding on detachment surfaces in the Cretaceous Niobrara and Greenhorn formations, produced multiple bands of thrust-faulted structures that ring the mountains, outward for a distance of 50 km. Within each of the structural bands that encircle the mountains, the Cretaceous Eagle Sandstone has been broken and incorporated into a coalesced complex of fault-bounded blocks that range in size from less than 40 acres to more than 300 acres. The faulted blocks are often elevated hundreds of meters above the adjacent strata and appear to have “popped up” through the overlying rock layers. Individual blocks can yield 200 to 1,000 MMCFG from 10 to 50 m of sandstone having 25-percent porosity.

 

2D and 3D seismic data are compared for an area north of the Bearpaw Mountains, where the regional Eagle Sandstone is about 375 m deep and pop-up structures range from 100 to 300 m deep. Imaging the shallow structures is a challenge for both 2D and 3D seismic. 2D seismic lines exhibit better data resolution than corresponding lines from 3D data, due to a closer in-line sampling interval (8 m) and higher fold. 3D data resolution is a practical compromise between sampling interval (25 m) and acquisition cost. However, even with only 1 to 3 fold data above 300 m, 3D seismic excels due to its spatial imaging of complex structures and superior data migration. 3D seismic data can clarify fault orientations and delineate small traps previously overlooked in this complicated structural area. 

 

AAPG Search and Discovery Article #90055©2006 AAPG Rocky Mountain Section Meeting, Billings, Montana