Parshall Field: Inferences From New Data Regarding Bakken Hydrocarbon Generation and Migration

Newman, Jane; Edman, Janell; LeFever, Julie; Howe, James

Detailed organic petrography investigations were done on two wells in the eastern Williston Basin. These two wells are the Long 1-01H well on the eastern side of the Parshall Field and the Harstad et al. 1 on the western side of the Stanley Field. Both of these wells are located near southwest-northeast trending structural lineaments. The Harstad well reached total depth at 13,246 feet in the Ordovician Red River Formation, and organic petrography in this well indicates two distinct types of hydrothermal alteration. One type of hydrothermal alteration occurs in the mainly Devonian strata below the Mississippian F Salt, including the Bakken, and the other type occurs in younger strata above the F Salt. Hydrothermal alteration is also observed in the Bakken in the Long well, and it is this hydrothermal alteration that is the focus of this presentation. Periodic movement along the lineaments near these two wells may have provided conduits for the hydrothermal fluids.

The upper Bakken shale in the Long 1-01H is pervasively mineralized at a range of scales, whereas the lower Bakken shale is significantly less modified. Similar results are observed in the Harstad well. Quartz and carbonate mineralization define microfabrics resulting from hydrothermal fluid flow indicating the middle Bakken was an aquifer for hydrothermal fluids during maturation. The Lodgepole Formation exhibits similar features. The mineralization in the upper Bakken shale and Lodgepole Formation features inclusions of trapped oil, which are inferred to have been emplaced at the time of hydrothermal activity. This observation implies the hydrothermal activity is likely to have influenced Bakken hydrocarbon generation.

The hydrothermal activity also appears to have affected vitrinite reflectance measurements in the Bakken. Organic petrography in the Long well shows there is actually more orthohydrous normal vitrinite present in the low TOC middle Bakken than in the more organic rich upper and lower Bakken. The %Ro in the middle Bakken is 0.84, which indicates the Baken in the Long well is likely at peak oil generation. Other authors have noted localized higher maturity areas within shale resource plays and have used these localized maturity variations to help locate "sweet spots" for exploration. Consequently, when evaluating resource play acreage, it is recommended that evidence for hydrothermal activity and localized maturity variations be considered and incorporated into the appraisal.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013