The Bakken at Parshall Field: Inferences from New Data Regarding Hydrocarbon Generation and Migration

Abstract

Summary URTeC 1578764 Detailed organic and inorganic petrography investigations on two wells in the eastern Williston Basin indicate that hydrocarbon generation, migration and trapping may have different controls than previously recognized. Both of these wells, the Long 1-01H well on the eastern side of the Parshall Field and the Harstad et al. 1 well on the western side of the Stanley Field, 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 inorganic petrography in this well indicates two distinct hydrothermal expressions of an igneous event which influenced the thermal history of the stratigraphic succession. One type of hydrothermal mineralization 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 observed in the Bakken comprises quartz and carbonate mineralization which defines microfabrics resulting from hydrothermal fluid flow within permeable sediments. These similar features in both the Harstad and Long wells indicate that the middle Bakken member was a paleo-aquifer for hydrothermal fluids during maturation. The thermal maturity achieved as a result of this hydrothermal activity, VIRF Ro (normal) approximately 0.85%, is also very similar at both locations, suggesting laterally extensive fluid movement with convective heat transfer. Both the upper Bakken shale and overlying Lodgepole Formation in Long 1-01H feature inclusions of trapped oil, which are inferred to have been emplaced at the time of hydrothermal activity. Although the fluid inclusions have not yet been tested, these observations imply that the hydrothermal activity is likely to have influenced Bakken hydrocarbon generation. The uniform maturity of all analyzed units from the Mississippian Lodgepole Formation to the Lower Devonian Prairie Formation in Harstad et al. 1 suggests that hydrothermal fluid movement was unrestricted both vertically and laterally within a 2000ft+ permeable paleo-aquifer interval. Periodic movement along the structural lineaments near the Long and Harstad wells may have provided conduits for the hydrothermal fluids. The regionally extensive F Salt may have acted as a seal by preventing open fault fractures and hydrothermal fluids from moving further upward in the stratigraphic section. Mathematical modeling of the convective heat transfer was beyond the scope of this study. Intense microbrecciation and iron mineralization of the Upper Pennsylvanian to Lower Cretaceous sediments above the F Salt in the Harstad et al. 1 well are postulated to result from high level igneous intrusion, contemporaneous with hydrothermal activity in the mainly Devonian succession below the F Salt. Vitric tuffs in Upper Cretaceous sediments indicate minor magma extrusion at this time, which helps to constrain the timing of the igneous and hydrothermal event. Some organic matter in Lower Cretaceous to Mississippian sediments was chemically altered by igneous volatiles at the onset of the igneous and hydrothermal activity. These altered assemblages provide paleomaturity data indicating Bakken sediments had achieved Ro (normal) 0.70% and were already generating oil in the Parshall Field area prior to the Late Cretaceous igneous and hydrothermal event. Oils now reservoired at Parshall Field are therefore likely to be a mixture of oil generated prior to the Late Cretaceous hydrothermal event with oil generated in conjunction with the hydrothermal event. Some of the Bakken oil in the Long 1-01H well could also have migrated up dip from the west. 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 based on accurate vitrinite reflectance data be considered and incorporated into the appraisal.