The Bakken and Duvernay have proven to be the most sought after formations in North America today. With many companies currently getting high returns from these formations due to precise placement of fracturing intervals, it has demonstrated how mapping out the horizontal section of your well can be a huge advantage. With both formations being high in Uranium, a typical Gamma Ray curve is not sufficient when trying to correctly identify the source formation in comparison to the over and under lying formations.
How does the SGR (Spectral Gamma Ray) Work?
The SGR tool uses an analyzer to split the natural gamma radiation emitted from the formation into energy levels, which are then broken up into the following three isotopes:
Potassium – 1.46 MeV
Uranium – 1.76 MeV
Thorium – 2.62 MeV
These three isotopes from the SGR tool, if summed together using API units, are equal to the conventional Gamma Ray tool output value.
Formations such as the Bakken and Duvernay have high Uranium salt deposits, that falsely identifies it as clay shale, rather than it’s Dolomite/shale composition. Many times during the drilling operation the wells toe up or toe down veers into the adjacent formation, the MWD Gamma Ray, and conventional Gamma Ray tool would not see a great variance in values between the clay shales and Dolomite formation due to this highly radioactive Uranium.
4 Things the SGR is used for:
With the increased vertical resolution of the SGR tool in comparison to a Gamma Ray tool and the ability to quantify incident Gamma Ray energies into Thorium, Potassium and Uranium, the SGR can aid in the interpretation of:
Depositional Environments
Clay Typing
Identifying Fractures
Differentiating radioactive carbonates from shales
What 2 things affect the quality of the SGR data?
Mud system – Certain additives to the muds such as KCL, Barite and Bentonite will either emit or absorb external gamma rays
Logging Speed – The SGR tool is statistical, therefore the slow it is logged the better the quality will be
With the high uranium content formations being evaluated, the requirement to provide the most out of every well drilled, it has been proven how affective the SGR tool works.
