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The Mississippian Limestone of northern Oklahoma and southern Kansas is comprised of four basic depositional environments. These are the Mississippi ‘Chat’, Chester, Meramec and Osage. The Mississippi ‘Chat’ is the uppermost member at the unconformity between the Pennsylvanian and Mississippian aged rocks and is considered to be a very good hydrocarbon reservoir. These ‘Chat’ reservoirs are very wide spread, vary in gross thickness from a few feet to 80′ thick and are heterogeneous in nature. Net porosity thicknesses greater than 5% range from a few feet to greater than 40 feet. This is caused in part through uplift, alteration, erosion and deposition of the original limestone commonly referred to as Mississippi Lime. The ‘Chat’ is comprised of varying amounts of weathered chert, limestone and dolomite. Porosities range from 3 to 35%. The resistivities are generally low due to the amount of weathered chert and appear to be wet on logs by conventional petrophysical analysis. These reservoirs are in fact not wet but do produce some water associated with the production of hydrocarbons. Water rates vary between wells depending on the actual mineral composition of the formation being completed. The Mississippi Lime underlies the ‘Chat’ and is comprised of the Mississippi Chester, Manning, Meramec and Osage. It is deep water to shallow marine limestone sequence with interbedded dolomite facies enhancing porosity and permeability. This zone is also productive in the prospect area in addition to the Chat. Porosities range from 5 to 15% with water saturations ranging from 25 to 60%. Net porosity thickness varies greater than 5% range from 10 to 100 with an average of 30 to 50. Due to the heterogenic porosity development within the Mississippian formations it is advantageous to drill horizontal wells to interconnect a larger percentage of porosity with one well bore. Fracture treatment of the formation during completion in addition to the horizontal placement of the well bore will add significant conduits for production of hydrocarbons adding significant recoverable reserves. These reserves would not be recovered with a conventional vertical well.
The Bakken petroleum system is part of a larger depositional system laid down in the Williston Basin during the Phanerozoic period with sediments up to 16,000 feet thick. The Bakken system covers parts of North Dakota and Montana in addition to parts of Saskatchewan and Manitoba, Canada and includes the Bakken, Lower Lodgepole and Upper Three Forks Formations. The Bakken Formation is comprised of three distinct members, the upper and lower Bakken’s organic rich shale layers, and the middle Bakken member which is primarily sandstone and siltstone. The middle Bakken is the primary reservoir rock, together with the Lower Lodgepole and Upper Three Forks, although all the Bakken Fomation members have reservoir potential. The upper and lower Bakken Formation members are also source rock. Porosity and permeability are, generally, low over the most of Bakken, The estimated average porosity and Permeability in the Bakken at 5% and 0.04 millidarcy, although value varies widely over the play. Porosity is sometimes enhanced by natural fracture system in the Bakken. The Bakken shale oil field may hold between 10-12 billion barrels of recoverable oil. This could also be changed, as the whole North Dakota could still be sitting on oil. There has also been natural gas released from the shale fracture in the Bakken Formation. Although the Bakken resource is immense, exploiting its full potential will require development or refinement of a number of technologies including microseismic fracture monitoring, extended reach and rotary steerable drilling, enhance multi-stage fracturing and water issue.