Insights into the Bakken: Overpressure, Sweet Spots, and Trap
Charles Bartberger, Eryn Bergin, and Katie Kocman
Published geochemical data on Williston Basin source rocks and oils suggest that prodigious volumes of oil generated by (and presumably expelled from) upper and lower Bakken organic-rich shales should be confined to reservoir rocks of the Bakken Petroleum System, primarily middle-Bakken dolomitic siltstones between the upper and lower shales. With poor reservoir properties that are thought to inhibit updip out-of-basin migration of oil, the middle-Bakken reservoir seemingly should be oil-saturated and overpressured (from significant pressure generated in conjunction with the huge volume of oil) throughout the thermally mature deep basin. Contrary to this model, ubiquitous basin-wide high oil saturations and pressures within the middle Bakken are not observed. Sweet spots where the middle Bakken is characterized by elevated reservoir pressure, low calculated water saturations, small amounts of water produced relative to oil, and exceptionally high recoveries of oil, have limited geographic extent. Such localized sweet spots, including Elm Coulee and Parshall Fields, are believed to be influenced by localized traps that prevent updip out-of-basin migration of oil in the middle Bakken. At Elm Coulee Field on the southwestern edge of the deep-basin Bakken oil accumulation, facies changes within, and local truncation of, the middle Bakken provide the trap. At Parshall and associated fields at the updip edge of commercial Bakken production on the eastern margin of the Bakken accumulation, the trap is a pore-throat trap (associated with eastward deterioration of reservoir properties) that cuts across thermal-maturity trends. Progressive updip decrease in middle Bakken oil recovery on the eastern margin is accompanied by increasingly higher water saturations calculated from wireline logs and progressive increase in percent water produced until sufficiently far updip, the middle Bakken produces only water and no oil. Elsewhere along the periphery of the deep-basin region where Bakken shales reached thermal maturity (on the northeast, north, and probably west), somewhat better middle-Bakken reservoir properties provide a leaky seal that retards, but still allows, updip migration of Bakken oil and dissipation of elevated pressure from the deep basin. Migrated Bakken oil is trapped in middle-Bakken reservoirs in southern Canada at Roncott, Viewfield, Rocanville, Antler, Daly, and Sinclair Fields, far updip outside the thermally mature deep basin.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012