The Three Forks Formation is becoming increasingly important in terms of hydrocarbon productivity in the Williston Basin, North Dakota. This growth has been attributed not only to the availability of new drilling and completion techniques, but also to the availability of new geological information. Core data has allowed the understanding of depositional environment, facies distribution, and other aspects that impact the reservoir quality.
The description carried out on 6 cores and analysis of characteristics and evidence in additional 23 wells, indicates that the Three Forks Formation was deposited in a very shallow and extensive epeiric platform. The arid climate dominant during the late Devonian, in combination with changes in sea level, provided the right setting for the development of an evaporitic platform and deposition of storm/tidally dominated lithologies, in the sedimentary record. All these conditions together, pose a unique setting for the sediment deposition and impact the reservoir quality, due to the unusual hydraulic conditions in this kind of environment.
One of the most challenging aspects in the definition of reservoir quality in carbonate tight reservoirs is the characterization of porosity and permeability due to the complexity of these types of rocks. A methodology that has been proven to be accurate for the definition of flow units in conventional reservoirs, using core data analysis, was carried out in these tight rocks, in order to establish a relationship between facies distribution, flow units and well log response. This definition was done using the SML methodology (Stratigraphic Modified Lorentz Plots). This methodology was introduced by Gunter et al. (1997) and illustrates in a very simple way the variability of reservoir storage capacity and flow capacity by the interpretation of slope changes on the plot.
In North Dakota, a series of wells (20), with core data analysis available were plotted under this methodology in order to obtain an image of the flow units slope's trends and distribution. As a preliminary result, all the wells show in general a very accurate response between facies, SML plots and hydrocarbons saturation. Some assumptions about reservoir quality in intervals not tested can be done due to the behavior in the slopes. The slopes obtained in the Upper Three Forks productive interval (around 45 degrees) are present in other intervals within Three Forks Formation as with the Middle and Lower Three Forks. Additional cross plots based on core data like porosity vs. permeability and irreducible oil saturation vs. irreducible water saturation plots have showed being a good approach in the definition of reservoir quality in this unconventional reservoir type.