The "Bakken Pool" is an administrative term that includes the Bakken formation and the Three Forks Sanish formation.
The NDIC website shows the accumulated totals of oil production from the "Bakken" and the "Bakken/TFS." I assume the Bakken is limited to the Bakken formation in the "Bakken Pool"; and, the "Bakken/TFS" is limited to the TFS formation in the "Bakken Pool."
Geo News, July, 2010: the Bakken potential.
The Original Post
I have posted this several times: there is confusion over "Bakken" vs "Three Forks Sanish." This is the best answer I have seen: the NDIC and the USGS consider the oil coming from the "Bakken" and the "Three Forks Sanish" to be coming from the "Bakken Pool." That's why on the daily activity report, the pool is always listed as "Bakken" when the well targets the Bakken formation or the TFS formation. [Update, October 14, 2010: click here to see an exception. Is this new, evolving, or did I miss similar wells earlier?]
And that's the key: "formation." The Bakken formation is composed of the Upper, Middle, and Lower Bakken members/units, all separate strata. Likewise, the Upper TFS and the Lower TFS are separate strata.
Formations, members/units, and strata are all geologic terms.
On the other hand, "Bakken Pool" is an administrative term. The USGS and the NDIC prefer to lump these five (5) formations together as one pool (see below -- it makes sense).
The five formations can be very, very thin in some areas, and it is very possible a) the geologist is not 100% sure which formation the tip of the drill is in; b) a long lateral may pierce more than one formation; and/or, c) the administrative paperwork to date may not always be "perfect," and it might be difficult, expensive, and even unnecessary to sort out exactly what formation the lateral is in, especially when the formations may communicate in some localities.
The NDIC defines the Bakken Pool as 50 feet above the upper Bakken limit and 50 feet below the lower Bakken formation line. One needs to see the "completion report" to see which formation(s) was (were) targeted. Most of the reports linked at my site do not include the completion report, but the report that is provided after the well comes off the confidential list.
The stratigraphic nomenclature varies, for example:
Periods: Mississippian and Devonian
Groups: Three Forks and Saskatchewan
Formations: Bakken, Big Valley, Torquay (the Three Forks group)
Formation: Birdbear (the Saskatchewan group)
Periods: Mississippian and Devonian
Groups: "Bakken Pool" and Jefferson
Formations: Bakken and Three Forks (the "Bakken Pool" group)
Formation: Birdbear (the only formation in the Jefferson group)
Of course, the next question that arises is this: at the NDIC website, cumulative oil production is broken down by "formation" on one of the pages. The NDIC has broken out two "formations" as 1) the Bakken; and 2) the Bakken/TFS. It appears for North Dakota this is logical: as noted above, the "Bakken Pool" group is composed of the Bakken formation and the Three Forks Formation. But it is confusing that they label it as the "Bakken/TFS." They don't do that with other formations, such as the Red River which is part of the Madison Group.
Here's another recent discussion regarding this issue; the discussion begins well down into the thread once you get there.
I have recently posted that EOG may be planning to drill three wells from one pad (in the Parshall), the wells targeting, respectively, the Middle Bakken, the Upper Three Forks Sanish, and the Lower Three Forks Sanish.
Interestingly enough, the 1990 boom in North Dakota targeted the Upper Bakken.
So, there you have it. Within the "Bakken," it sounds like there could be five formations: Upper, Middle, and Lower Bakken; and, the Upper and Lower Three Forks Sanish (more correctly referred to as unit 1- 6).
Note: one of the pioneers in the Bakken, Marathon Oil (MRO), considers the "Bakken" to be composed of five distinct units: the Lodgepole; the upper, middle and lower Bakken; and, the Three Forks (Sanish).
On another note, the Spearfish/Madison Pool lies directly above the Bakken Pool. The Spearfish/Madison Pool is much thicker, extending from the shallower Spearfish, all the way down to the Madison, which lies directly above the Bakken formations. Back in 2006, Julie LeFevre published an article which included data to suggest that the Madison Pool does not "leak" oil into the underlying Bakken.
According to a research paper by Roy Neset, Jr., Department of Geosciences, Fall, 2009, the source rocks for the middle Bakken member are the upper Bakken shale and the lower Bakken Shale; the source rock for the Three Forks Sanish member is the lower Bakken shale. Neset provides data to suggest that the lower Lodgepole Formation, which overlies the upper Bakken shale source rock, could be an economically viable oil-producing zone. An abbreviated synopsis of that research can be found here.