Abstract: Synthetic Seismic Sections of Selected Stratigraphic Traps and Aquifers in Southeast Powder River Basin, Wyoming

Robert T. Ryder, Robert C. Anderson, Alfred H. Balch, William J. Head, Myung W. Lee

Detailed stratigraphic sections in the vicinity of the Old Woman anticline indicate that sandstone units in the Minnelusa Formation and Canyon Springs Sandstone Member of the Sundance Formation are potential exploration targets for stratigraphically trapped oil in the southeast Powder River basin. In the Red Bird field on the north plunge of the Old Woman, structure oil is trapped in the 80-ft (24 m)-thick Canyon Springs Sandstone Member where it wedges out updip against an erosional outlier of red siltstone and claystone units of the Spearfish Formation. Oil also is trapped in the porous 50-ft (15 m)-thick first Leo sandstone, an economic unit in the Minneslusa Formation, where it grades laterally into anhydrite, anhydrite-cemented sandstone, and dolomite. A similar type of trapping mechanism in the Minnelusa Formation is responsible for the Pine Lodge oil field 18 mi (29 km) southwest. Synthetic seismic sections based on numerous sonic and density logs from the Old Woman area suggest that the porous sandstone units of the Minnelusa Formation (^phgrv = 2.60 g/cc, V = 11,000 ft or 3,353 m/sec) can be differentiated on seismic records from equivalent nonporous anhydrite-cemented sandstone units (^phgrv = 2.75 g/cc, V = 18,000 ft or 5,486 m/sec). Likewise, the Canyon Springs Member (^phgrv = 2.25 g/cc, V = 9,300 ft or 2,835 m/sec) should be recognizable on seismic records where it pinches out against the nonporous Spearfish Formation (^phgrv = 2.55 g/cc, V = 2,500 ft or 3,810 m/sec).

The seismic method also may be useful in predicting high water-yield wells in the Madison Limestone aquifer. Synthetic seismograms constructed from the sonic logs of several wells indicate that the Madison Limestone with good porosity development (velocity of porous zones = 17,000 to 17,500 ft or 5,182 to 5,334 m/sec) may be distinguishable on seismic records from relatively impervious intervals of Madison Limestone (V = 18,000 to 21,000 ft or 5,486 to 6,400 m/sec).

AAPG Search and Discovery Article #90971©1976 AAPG-SEPM Rocky Mountain Sections 25th Annual Meeting, Billings, Montana