The Williston Basin is a sedimentary and structural basin encompassing 52,000 square miles in North Dakota. The basin itself contains rocks from every geologic period from the Cambrian to the Tertiary. However, I am going to zone in on the late Devonian to early Mississippian aged units, or in and around the Bakken formation if you will. Organic-rich shales in the Bakken have been recognized as excellent petroleum source rocks and are believed to have supplied the large amounts of oil found in reservoirs above and below the Bakken.
During the Middle and Upper Devonian Epochs the Williston basin was a part of the larger western Canada basin of deposition. This was a time that was characterized by predominantly carbonate deposition with a thick evaporite in the lower part (Prairie Formation) and cyclical carbonate with some thin clastic and evaporite beds in the upper part (Duperow, Nisku, Three Forks). Deposition was continuous or nearly continuous into the Mississippian, but the center of the Madison depositional basin was nearly coincident with the present Williston basin. Mississippian deposition began with predominantly carbonate deposition, and evaporites increasing in the upper part.
The Three Forks Formation (Devonian) is roughly 250 feet thick and composed of shales, siltstones, dolostones, and sandstones. The Three Forks is a potential target for operators. Lying at the top of the formation is the Sanish Sand at the contact between the upper Three Forks and the lower Bakken Shale. Core analysis indicates the Sanish Sand is more Bakken related than Three Forks. The very fine-grained sandstone is highly burrowed and contains porosity and permeability similar to the Middle Bakken section. I’ll compare properties a little more when I discuss the Middle Bakken.
I’ve included a map showing Sanish Sand completed wells in order to get an idea of a spatial trend. Just an example of how Drilling Info can aid in various kinds of research.
Here is a map showing regional fractures in western North Dakota. The open tension fracturing along the Antelope field drape fold exposes the Sanish Sand for conventional and unconventional drilling methods.
The Lower Bakken Shale is very similar to the Upper Bakken with the main difference being thickness. The lower member is roughly 50 feet thick while the upper member is around 25 feet thick. Both are dark brown to black, fissile, non-calcareous organic rich shale. Both are pyritic and often thinly laminated. Average TOC is around 10% and even possibly as high as 20% in some areas. The rock is fairly fossiliferous containing brachiopods, fish teeth, bones, and conodonts. The fossil record indicates partially restricted marine conditions, evident by a neatly stratified water column. In other words, a quiet, deep, no turn-over, oxygen starved environment. This offshore setting is a key ingredient to an excellent source rock.
This brings me to the Middle Bakken, aka the “sweet spot.” This area is roughly 8 – 14 feet thick and around 75% oil saturation. However, middle member maximum thickness reaches 140 feet. From the Three Forks to the Lodgepole, only the Sanish Sand of the Upper Three Forks and the Middle Bakken siltstone have measurable matrix reservoir properties. The Middle Bakken being a little more porous and having a porosity value of around 8 – 10%. The Sanish has around 5-6%. Permeability in the Middle Bakken is around 0.05 md compared to the Sanish which is a little less than 0.1 md.
The depositional environment has a lot to do with permeability and porosity of the rock. The Middle Bakken was deposited during higher energy conditions. There must have been either regional uplift or a time of sea level regression in order to create this shallow water carbonate scenario. Basically, imagine low sea level during Three Forks deposition, high level during Lower Bakken, low level during Middle Bakken, high level during Upper Bakken, and then once again low level during Lodgepole deposition.
These rock properties and lithologies make the Middle Bakken a good place to frac. Plus, by having source rocks above and below, along with natural fractures, this allows for some of the best producing wells in the play. By looking back at the Regional Fractures map you can see the mass of wells in Mountrail County. Once again, using Drilling Info, I wanted to show some production values for Bakken completed wells in Mountrail County.
The Lodgepole Formation will wrap up this sequence. Also, sometimes referred to as the “False Bakken.” The Lodgepole is close to 900 feet thick deposited under normal marine conditions. The formation consists of limestones and dolomites. The Lodgepole is a major producing reservoir in Manitoba. However, the lower North Dakota Lodgepole or roughly lowest 100 feet of the formation would be best suited for any economically viable amount of oil and gas.
I look forward to more in depth look at production in respect to these reservoirs and plan to release a blog in the near future covering that. A good place to find more info on Williston Basin geology is the Geology Folder of Drilling Info DNA.
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