By Kathleen Neset, President of Neset Consulting Service
An interesting question – but gas from all Bakken and Three Forks wells is not the same. Similarly, wells throughout the Williston Basin that target and produce from the Bakken Oil System produce gas of different components and make up. Why is this important?
Let’s first talk about the Bakken Oil System. The Bakken Oil System is comprised of both the Bakken and Three Forks formations. To break this down a little further, the Bakken formation is made up of the three main rock subunits: Upper Bakken Shale, Middle Bakken, and Lower Bakken Shale. The formation immediately beneath the Bakken is the Three Forks formation. Although the Three Forks is a separate formation – it has some similar rock characteristics as the Bakken and more specifically, the Middle Bakken. The Three Forks is thinly laminated sequence of dolomites and shale with some varying amounts of siltstones, sandstones, clays, and other rock types present. This rock has characteristics like the Middle Bakken, making both rock units the reservoir or storage part of the formations. The Upper Bakken Shale and the Lower Bakken Shale are primarily the source of the hydrocarbons.
As the layers of shale in the Bakken formation are the source of the oil, the layers of Middle Bakken and underlying Three Forks formation are the storage units. Therefore, both formations are filled with hydrocarbons from the Bakken Shale layers. This unique sequence of source rock and reservoir rock is one aspect that makes the Bakken and Three Forks so special.
Now, if the shale layers are the source of the oil and gas throughout the Williston Basin, why would the composition of the associated gas differ? This is due to several reasons: depositional environment of the area, vertical depth of the particular rock units, temperature and pressure that the particular area is subject to, alterations to the rock unit over time, and several other factors.
Wells that produce oil typically produce oil, gas and water in varying amounts.
As stated, wells of the Bakken Oil System produce from the Bakken and Three Forks storage units but the oil is sourced from the Bakken Shales. That means that the oil from the shales is forced downward into the underlying Three Forks rock units due to the natural formation pressures of the Bakken. As we continue to explore various areas of the Williston Basin, we are finding variations in the composition of the produced gas.
Gas from the Bakken Oil System is unique in that it has a high concentration of natural gas liquids. These are the components of the gas known as ethane, propane, butane, and natural gasoline. These components are very valuable commodities and add additional economic value to the oil and gas produced. By removing these NGLs and marketing them separately value is added to the produced resource.
Variations occur in the gas produced throughout the Williston Basin and Bakken-Three Forks production area. Some of this variation is due to the environment present 370 million years ago when the Three Forks formation was being deposited. Some of the region was deep water and ocean environment, some was nearer to a shoreline environment, some was quiet water environment, and other variations occurred. These depositional environments along with variations in climate, organic makeup of the deposited material, and other factors all contribute to the chemical makeup of the oil and gas we are now producing present day.
The variations in Bakken and Three Forks produced gas also plays a part in the composition of the crude oil produced. These natural variations are seen throughout the basin and are part of what makes the Bakken Oil System – crude oil and produced gas, a high quality, high gravity grade of oil. This high quality hydrocarbon product of crude oil and gas adds value to the product while maintaining properties of stability similar to crude oil produced from other formations in the Williston Basin. Continued work to develop the infrastructure for North Dakota’s valuable gas production market will work to strengthen this great resource that we are working to produce. With continued production and development, the infrastructure to get the natural gas and NGLs to market will only get better. This is another valuable resource that is helping to ensure North Dakota’s and America’s Energy Independence.
Kathleen Neset is owner of Neset Consulting Service, which provides wellsite geology and geo-steering services throughout the Williston Basin. For more information, contact Neset Consulting Service at 701-664-1492 or firstname.lastname@example.org.
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