Stratigraphy and diagenesis of the Mississippian Bakken Shale-Lodgepole Limestone sequence, Williston Basin, North Dakota

Correlation of electric logs, cores and cuttings of the Mississippian Lodgepole Limestone indicates it was deposited by three sequences of prograding clinoform shaped wedges that filled the early Williston Basin. The previously defined subintervals of the Lodgepole Limestone in Canada (Scallion, Virden, Whitewater, and Flossie Lake) are now correlated throughout the U.S. portion of the Williston Basin. From 1993-95, six 300 ft (91 m) tall, mound shaped reefs have been discovered in the lower Lodgepole in North Dakota. Each mound has an average of 18 million bbls of oil in place. Results of this study show that the mounds nucleated and aggraded during the second transgressive systems tract in a deeper water, 500 ft (152 m), toe of slope environment. Therefore, by mapping locations of the toe of slope during the second and third transgressive systems tracts and overlaying that on a thermal maturity map of the underlying Bakken Shale, one can predict where other productive mounds will occur.; Oil production from the underlying Bakken Shale is found along a strip known as the "Bakken Fairway". The fairway is located where thermally mature shales are overlain by non-shaly limestone of the second highstand systems tract, defined as the upper Virden subinterval. Horizontal wells in the fractured Bakken Shale may be draining fractures that extend up from the shale into the clean brittle limestone above.; Burial depth of the Bakken Shale-Lodgepole Limestone contact grades from 2500 ft (762 m) at the edge of the basin to 12000 ft (3657 m) in the center. The general diagenetic sequence in the limestone above the contact, is bioturbation, micritization, fringe cements, framboidal pyrite, nodular concretions, organogenic dolomite, early dog tooth spar, inclusion-rich syntaxial overgrowths, unconformity related dolomite, silicification, neomorphism, dissolution, hematization, iron-rich dolomite, equant cements and late syntaxial overgrowths, saddle dolomite, anhydrite, fluorite, celestite, marcasite, sphalerite, chalcopyrite, millerite, and oil migration.; Oil migration in the shales and mudstones is believed to occur along stylolitic seams that are concentrating kerogens and bitumins. In the Lodgepole Limestone, oil occurs along stylolitic seams that have been dolomitized with a late stage iron-rich dolomite. Vertical fractures in the lower Lodgepole, below 2.2 km, are being cemented with dolomite, calcite and celestite that formed during oil migration.