Use of Resistivity and Density Borehole Image Logs to Identify and Distribute Facies in the Pikka Unit - A Case Study from the Nanushuk Formation, North Slope, Alaska

Abstract

Over the past decade, the North Slope of Alaska has yielded several major hydrocarbon discoveries in deltaic topsets of the Brookian Nanushuk Formation. Together the Nanushuk topsets and genetically related foreset and bottomset beds of the Torok Formation comprise part of a giant clinothem system that prograded across the Colville Foreland Basin during the lower Cretaceous (Aptian through Cenomanian). The discovered Nanushuk topset play contains stratigraphically trapped hydrocarbons within multiple fairways trending roughly north to south along the basin’s extent. The Nanushuk topset play was first discovered in the Pikka Unit by Repsol and partners during the 2013 winter drilling campaign. The Pikka Unit is located at the eastern edge of the Nanushuk-Torok clinothem system and underlies the modern-day Colville River. Here, the Nanushuk Formation comprises shelf-edge deltaic and shoreface deposits, characterized by intercalations of fine-grained litharenites and silty mudstones. The layered character of the formation is readily recognized in electric logs due to density and resistivity contrasts between those main lithologies. Following the initial Pikka discovery, 14 appraisal wells were drilled in the unit, including 2 horizontal and 2 high angle wells. An extensive and diverse borehole image dataset was acquired and includes wireline high resolution oil-based mud resistivity and logging-while-drilling azimuthal density images. In addition, more than 1000 feet of continuous core was collected in three wells (Qugruk-8, Pikka B, Pikka B ST1). Borehole images were then used to orientate the high resolution CT scan images of the cores, which afterwards were integrated with the image log analysis. This study presents a case on how the integration of core sedimentology and detailed borehole image log analysis were used to guide and predict the facies distribution across the Pikka unit.