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Quantitative geophysical interpretation of the Nanushuk and Torok formations on the North Slope, Alaska

Abstract

Exploration of the Brookian Nanushuk and Torok formations on the North Slope of Alaska is a hot topic now. The recent discoveries, as well as the USGS (2017) estimates of mean undiscovered and technically recoverable 8.7 billion Bbl of oil and 25 Tcf of natural gas in these formations, indicate the presence of the vast Previous HithydrocarbonNext Hit resources. The goal of this study is to detect dominant geologic features and identify the reservoirs using a combination of several post-stack and angle-stack (near, mid, and far) 3D seismic volumes, and petrophysical data. On the post-stack seismic data, the Nanushuk Formation is expressed as topset reflections, whereas the Torok and Gamma-Ray Zone (GRZ) formations are expressed as foresets and bottomsets. In this study, we used Sobel-filter similarity, coherent energy, reflector convergence, and spectral decomposition attributes on the identified seismic horizons. Using the corendered seismic attributes, we identified the presence of channels, shelf-edges, canyons, mass-transport-deposits, large slide blocks, and basin floor fans, all with amplitude anomalies. However, an assertion that these amplitude anomalies could be Previous HithydrocarbonNext Hit-bearing (or Previous HitDirectNext Hit Previous HitHydrocarbonNext Hit Previous HitIndicatorsNext Hit, DHI) would be wrong. The identification of DHIs in these formations is inconclusive based on post-stack seismic data at several places. Some dry holes look good on post-stack amplitude anomalies. Amplitude anomalies can be caused by sand-shale laminations, tuning thickness, cemented horizons, and chaotic slumping, etc. in this geologic context. In general, fluid, especially if oil-bearing sandstone, can be observed on far-angle stack seismic data and Amplitude Variation with Offset gradient volume. We used P-wave and S-wave velocity information from dipole sonic logs and core data from Previous HithydrocarbonNext Hit-show and dry wells. Based on the relations between P-impedance, S-impedance, and velocities, the wells with Previous HithydrocarbonTop shows have a combination of lower VP/VS ratio and acoustic impedance than dry wells. This pattern also depends on the burial depth and clay volume. Further analysis of the difference in amplitudes between the near-angle stack and far-angle stack data and acoustic impedance inversion results proved the difference in acoustic signatures near a discovery well versus a dry well.