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Assessment of the Mowry Shale and Niobrara Formation as Unconventional Hydrocarbon Systems, Powder River Basin, Montana and Wyoming

Lawrence Anna and Troy Cook
US Geological Survey, Lakewood, CO

A recent U.S. Geological Survey oil and gas assessment of the Powder River basin identified the Mowry Shale and Niobrara Formation as the primary hydrocarbon sources for Cretaceous conventional and unconventional reservoirs in the basin. The Mowry Shale has a 2-3 percent total organic content consisting of Type II and Type III kerogen. The Niobrara has an average of 3 percent TOC of Type II kerogen. Burial history modeling indicated that hydrocarbon generation for both formations started at about 0.60 percent Ro at a depth of about 8,000 ft. At maximum depths, Ro for the Mowry is about 1.2 to1.3 percent and about 0.80 percent for the Niobrara.

The Mowry and Niobrara continuous reservoirs were assessed using a cell based methodology that utilized production data. The size of each cell was based on geologic controls and potential drainage area of analog fields. Current and historical production data were used to determined the estimated ultimate recovery distribution for untested cells. Only production data from unconventional fractured shale reservoirs with vertical wells were used. The minimum, median, and maximum total recovery per cell for untested cells is 0.002, 0.25, and 0.35 MMBO, respectively, for the Mowry. For the Niobrara, the minimum, median, and maximum total recovery per cell for untested cells is 0.002, 0.028, and 0.5 MMBO.

Sweet spots were identified by lineaments and faults, which are believed to be areas of greater delivery potential than non-sweet spots; an upper limit of 8,000 ft was defined by over pressuring caused by hydrocarbon generation.

Mean estimate of technically recoverable undiscovered continuous resources for the Mowry is 198 MMBO, 198 BCF (billion cubic feet of gas), and 11.9 MMBNGL (million barrels of natural gas liquid), and for the Niobrara is 227 MMBO, 227 BCFG, and 13.6 MMBNGL.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado