Title: Lower Cretaceous Greybull [“Dakota”] Valley-fill Deposits, Northern Bighorn Basin, Montana
Speakers: Ed Coalson and Bryan DeVault, Vecta Oil & Gas, Ltd.
Location: Denver Marriott City Center
Date: April 7th, 2010
Abstract: Published in The Outcrop, April 2010
Oil and gas are produced from lower Cretaceous sandstones of the Greybull formation in numerous structural closures in the Bighorn Basin of Wyoming and Montana. In the Montana portion of the basin, the Greybull is composed of fluvial and estuarine deposits laid down in regionally extensive valley systems. The bulk of the valley-fill deposits are clean, white, quartzose sandstones with good reservoir quality. The sandstones form flow units that are laterally continuous on the scale of miles. They offer the possibility of forming large-scale stratigraphic traps. If these sandstone trends could be shown to have the possibility of containing large quantities of oil and gas, a significant new stratigraphic play might ensue.
Shale beds overlying the Greybull contain mature, effective source rocks for oil. These source rocks are evidenced by TOC measurements exceeding 3%, and probably charged the Greybull reservoirs at Elk Basin, Mosser Dome and several other oil and gas pools. In particular, Mosser Dome appears to be a paleostratigraphic trap that has been partially breached and reduced to the present small structural accumulation. The orignal stratigraphic trap may have contained as much as 200 MMBO to 1.0 BBO, almost all of which has been lost to Graybull outcrops lying southeast of Mosser Dome.
Since Don Stone published on the Greybull (1988), these valley fills have been recognized as good targets for exploration via conventional (P-wave) seismic. P-wave data reveal to locations of this valley-fill deposits with reasonable accuracy, but do not reveal the presence or absence of reservoir-quality sandstones. However recent developments in methods of shear (S-wave) seismic offer the likelihood of “seeing” reservoir sandstones directly; S-wave seismic offers a superior development tool.