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Subsurface Characterization of the Bakken/Three Forks Unconventional Tight Oil Reservoirs, Williston Basin, North Dakota Through the Use of High Resolution Mud Gas Mass Spectrometry

Abstract

We present the findings of a multi-well study integrating high resolution mud gas mass spectrometry with core, cuttings, completion and production data. The study uses the integrated methodology to refine geologic targeting based on facies interpretation (lateral well placement via geo-steering) while developing a better subsurface understanding of the Bakken/Three Forks reservoirs. Routine mud gas detection while drilling distinguishes only lighter hydrocarbon compounds (i.e. C1 thru C5) and face interpretation challenges due to variable gas component extraction efficiency, recycling effects and gas fractionation through a mini-separator. These complications prevent true quantitative analysis from traditional mud gas logging and render qualitative interpretations difficult. High-resolution mud gas mass spectrometry detects a wider range of hydrocarbons (e.g. C1-C10) and non-hydrocarbon gases (e.g. He, H, CO2), which can provide improved understanding of subsurface parameters including oil/water saturations, porosity, existence of natural fractures or fault zones and reservoir compartmentalization. Reservoir rock cuttings collected while drilling were analyzed using a traditional “source rock” pyrolysis technique to identify zones within the lateral well which contain free hydrocarbons. This, along with X-ray diffraction analysis, completion fall-off pressure and production data provide an independent means of validating the signatures seen in the mud gas data. The integration of cuttings, core and advanced mug gas analysis has helped better characterize the subsurface and provides a template for future lateral drilling with the ability to customize hydraulic facture completions to more effectively and efficiently produce hydrocarbons in the Williston Basin.