Seismic attribute and petrophysics-assisted interpretation of the Nanushuk and Torok formations on the North Slope, Alaska

Abstract

Exploration of the Brookian sandstone reservoirs in the Nanushuk and Torok formations on the North Slope of Alaska is a hot topic and presents opportunities to the oil and gas community because of their shallow depth, vast extent, and scope of development, etc. The consecutive hydrocarbon discoveries announced by Repsol-Armstrong, Caelus Energy, and ConocoPhillips in 2015, 2016, and 2017 have indicated the presence of the vast recoverable resources on the North Slope in the Nanushuk and Torok reservoirs. In this study, we investigate the detailed geophysical and petrophysical characteristics of these reservoirs. Our goal is to detect dominant geologic features in these formations using a combination of seismic attributes at the regional scale and analyze critical petrophysical and rock physics properties to evaluate formation heterogeneities and identify the reservoir targets by integrating well-log and core data at the well-scale. We show that the Nanushuk Formation is expressed as topset reflections, whereas the Torok and Gamma-Ray Zone (GRZ) formations are expressed as foresets and bottomsets on the seismic reflection data. Using seismic attributes, we mapped the extent of different geomorphological features, including shelf-edges, channels, slides, and basin floor fans, all with significant amplitude anomalies. The shelf-edges continue for 10s-100s of miles along N/NW and EW directions, depending on the areas. The internal characters of these formations delineated by conventional well-logs and advanced petrophysical analysis reveal their vertical heterogeneities and complexities, in terms of reservoir properties. We conclude that the reservoirs are both vertically and laterally heterogeneous, and these are thin-bedded low-resistivity reservoirs. Only a few zones in the parasequences are oil-saturated. We show that a combination of low VP/VS ratio and acoustic impedance can be a useful proxy to detect the hydrocarbon-bearing sand intervals in these formations.