The Exshaw and Bakken formations form an elongated belt of multi-layered mudstones, shale, siltstones, and sandstones that emerges from the eastern front of the Rocky Mountains of British Columbia, and then extends into the Western Canadian Sedimentary Basin, including the Alberta Basin and southern Saskatchewan, and then extends partway into the Williston Basin into southwest Manitoba, northwest North Dakota, and northern Montana. The Exshaw and the Bakken are lithologically equivalent formations that range in thicknesses from about 7 to 50 meters. Their sediments were accumulated at about the same time period (from Late Devonian to Early Mississippian, around 360 million years ago) and in very similar depositional environments. These two formations are divided by the Sweetgrass Arch of northern Montana and southeastern Alberta.
The rocks of the Bakken Formation that are located within the North Dakota area of the Williston Basin consist of an organic-rich, lower shale layer, a mostly dolomite-rich sandstone and siltstone middle layer, and a second organic-rich shale layer on top of the sandstone/siltstone middle layer.
The lower shale layer was deposited on the seafloor of a deep-marine (offshore/continental shelf region) environment that had stagnant and anoxic waters (with low oxygen levels and limited circulation). The low oxygen levels allowed the preservation and the accumulation of thick deposits of partially decomposed remains of plankton, algae, and other marine plants and animals that were laid down along with the fine-grained sediments that were eroded from adjacent higher-elevation terrains.
The middle sandstone and siltstone layer was then deposited in shallower depths (nearshore marine region) at a time when sea levels were at first receding, and then later, slowly rising again. These coarser and heavier silts and sands were deposited on top of the fine shale, while the finer shale sediments were carried away by the receding waters further out into the open seas. This middle sandstone-siltstone layer of the Bakken is the usual target layer for the horizontal drilling and multi-stage hydraulic fracturing operations that are being conducted in North Dakota today.
Later as sea levels rose again, the deep-marine environment was re-established and the coarser and heavier sands and silts were deposited on areas closer to the newly established shores, while the fine-grained mudstones and shales were again locally deposited, this time on top of the sands and silt-sized sediments. These fine-grained shales overlying the coarser sandstone-siltstone layer constitute the upper shale unit of the Bakken.
The Exshaw Formation stretches from northeastern British Columbia through western and southern Alberta and southern Saskatchewan. Formed in very similar environments as the Bakken, the Exshaw however is generally 10% thinner compared to the Bakken. The upper (organic-rich) black shale member of the Exshaw (that would correspond to the upper black shale member of the Bakken) is usually poorly developed or altogether missing, but the Exshaw does have the same (or very similar) sandstone-siltstone layer overlying a lower, organic-rich, black shale layer as with the Bakken. Based on currently available core samples, the middle layer of the Exshaw Formation seems to have lower porosities and permeabilities compared to the middle layer of the Bakken Formation; however, tight oil with API gravities measuring in the low 30s has been found in the middle layer of the Exshaw at depths between 1000 to 2000 meters.
Some geologists consider the shale-rich lower portion of the Banff Formation (which directly overlies the Exshaw Formation) in the Western Canadian Sedimentary Basin as also equivalent to the upper shale member of the Bakken Formation in the Williston Basin. The Sappington Member of the Three Forks Formation in north-central Montana also closely matches the Exshaw Formation, and both are considered lithologically equivalent. Also, an Alberta Basin Bakken Petroleum System has been proposed as consisting of the Exshaw and the overlying Banff and Pekisko formations as well as the underlying Big Valley, Stettler, Wabamun, and Crowfoot formations.
Publicly reported proved and probable reserves of the Bakken rocks within the Saskatchewan and Manitoba portions of the Williston Basin amount to about 225 million barrels of oil; the Bakken rocks of the North Dakota and Montana areas of the Williston Basin have an estimated 3.65 billion barrels of recoverable oil.
References:
1. http://oilshalegas.com/exshawshale.html
2. https://www.neb-one.gc.ca/clf-nsi/rnrgynfmtn/nrgyrprt/l/tghtdvlpmntwcsb2011/tghtdvlpmntwcsb2011-eng.html
3. http://www.cspg.org/documents/Conventions/Archives/Annual/2011/137-The_Alberta_Bakken.pdf
4. http://www.cspg.org/documents/Conventions/Archives/Annual/2011/178-Regional_Stratigraphic_Study_of_Exshaw_Bakken_Fm.pdf
5. http://www.geoconvention.com/uploads/2012abstracts/core/321_GC2012_Wabamun_Bakken_equivalent_Exshaw_and_Banff_Fm.pdf
6. http://www.ags.gov.ab.ca/publications/wcsb_atlas/a_ch27/ch_27.html
7. http://www.eprinc.org/pdf/EPRINC-BakkenBoom.pdf
