--> Integrated petrophysical characterization of the Nanushuk Formation on the North Slope, Alaska

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Integrated petrophysical characterization of the Nanushuk Formation on the North Slope, Alaska


The Nanushuk Formation is one of the emerging plays on the North Slope, Alaska. In this study, conventional and advanced well-logs, Modular Formation Dynamics Tester (MDT), and routine core data were integrated to characterize the Nanushuk Formation. In terms of advanced petrophysical logs, Pulsed Neutron Spectroscopy (PNS), Nuclear Magnetic Resonance (NMR), and sonic scanner tools from dry and hydrocarbon-show wells were used. In places, where the PNS logs were not available, joint inversion of conventional well-logs was performed to generate a series of depth-by-depth statistical multi-mineral solutions to identify hydrocarbon-rich sandstone layers. General well-log correlation shows several cleaning-upward sequences punctuated by a sharp increase in gamma-ray response. This pattern is indicative of progradation and flooding surfaces at the top of the parasequence set. This formation is dominantly composed of sandstone, siltstone, and mudstone, with subordinate coal layers. For refined petrophysical characterization, well-logs were processed to remove the borehole effects. Then, the clay volume was estimated from either PNS log or multi-mineral solution calibrated to core-based mineralogy data to remove its effect from porosity and fluid saturation estimates. Conventional porosity logs calibrated to core data show the presence of sandstone with porosity values in the range of 9-30% in the Nanushuk Formation. This is a low-resistivity pay (~8-10 ohm-m) at most places due to the presence of conductive clay layers. Hence, traditional Archie’s equation cannot be applied to estimate fluid saturations correctly; otherwise, reservoir estimates would be wrong. However, there are exceptions at a local scale, where the sandstones exhibit high resistivity, which may be due to a decrease in clay content and the presence of permafrost. The water saturation based on the Simandoux method, which removes the clay effect, matches well with core data. NMR-log-based long T2 (relaxation time), indicative of oil, was validated with MDT data and core photographs showing the oil-stained Nanushuk Formation. Each parasequence is composed of several petrophysical zones, with varying mineral proportions, porosity, permeability, and hydrocarbon saturation. Only a few zones in some of the Nanushuk parasequences are oil-saturated.