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Carbonate Buildup Reservoirs, Lower Mississippian Lodgepole Formation, Dickinson Field, North Dakota, USA
An estimated 14–16 million cubic metres (90–100 million barrels) of oil occur stratigraphically trapped in a cluster of Missippian carbonate buildups in the lower Lodgepole Formation. Over 1.3 million cubic metres (seven million barrels) of oil have been produced in the 4 years since discovery of the first buildup as new buildups were discovered and their production was established. This large volume was produced despite periodic operator-restricted production while awaiting unitization. Currently 8 pressure discrete pools have been defined in a cluster of buildups occupying approximately 7,000 acres. The largest pool complex is over 6.5 kilometres long and about 2.4 kilometres wide (4 miles by 1.5 miles) whereas the smallest are of the order of 2.6 square kilometres (1 square mile). The cluster of mounds describe a sub-circular to arcuate geometry on the southern limb of a northwestward-plunging anticline. Mounds are associated with fault zones but the association is unclear. Mound thickness ranges from 70 to 97 metres (231 to 318 feet) and their sides dip steeply up to 45°. Oil of 45° API gravity is produced primarily from fractures, vugs and interparticle porosity from oil columns, over 38 metres (125 feet) thick in some mounds.
Buildups are composed predominantly of mixtures of bryozoan, ostracode and crinoid grainstones and packstones, radiaxial calcite-cemented rudstones, bryozoan cementstones and a variety of styles of stromatactis structures. The bryozoan cementstones are interpreted to be preserved growth centers and one form of stromatactis whereas sheet-like stromatactis are more common on the flanks and occur at an apparent hiatus in mound growth. The paucity of mud and the abundance of multiple stages of coarse, early marine cements suggests these buildups are not “true” Waulsortian mounds, but rather more like lithoherms. Proximal beds surrounding the mound are thin to medium interbeds of mudstones, wackestones and packstones with similar skeletal components dominated by crinoids, bryozoans and a few corals separated by argillaceous and organic-rich shale partings. More distal surrounding beds of typical basinal Lodgepole are dark brown-black mudstones and wackestones separated by organic- and argillaceous-rich bed partings. The lower non-buildup Lodgepole beds are darker black, organic-rich with associated chert and pyrite, all of which diminish upwards into a medium black-grey mudstone and wackestone. It is the non-porous, low permeability surrounding beds that form the seal for the mound trap. Porosity in the buildup averages 4–5% with a few scattered two to four foot intervals of 10–15% porosity. Geometric average permeability measured from full diameter core is about 0.5 md with a range of less than 0.1 md to several Darcy’s in some directions.
Oil produced from the buildups has been analyzed to predominantly correspond to that produced from the black shales of the Devonian/Mississippian Bakken Formation that is locally in contact directly beneath the buildups. The minor admixed oil component is probably sourced from the Lodgepole which generates oil here also
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