CSEM-driven exploration over the Nordland Ridge

Abstract

Exploration efforts in the Nordland Ridge over the past two decades have resulted in the Norne, Urd, Alve, Falk, and Linerle discoveries. Besides these discoveries, several wells have failed expectations. Exploration efforts in this area are challenging because of the large variation in the rock properties of potential Triassic to early Jurassic traps that could be filled with viscous (heavy) oil. Application of controlled-source electromagnetic (CSEM) technology to derive the electrical resistivity distribution of the subsurface was thought by some oil companies to be the key that could unlock the exploration potential in the area. However, two dry wells in 2006 (6608/11-5 “Valkyrie”) and 2011 (6608/11-7s “Phoenix”) drew negative attention to CSEM within the exploration community because CSEM data were available in both cases prior to drilling. The wells targeted Jurassic Åre sands very close to the existing Linerle discovery with heavy oil in reservoir-quality Åre sandstones. The first well encountered less than 5 m of hydrocarbon traces in the Åre sands, while the second well was dry with a poorer-than-expected reservoir at target depth. This work examines the quality and reliability of the most recently acquired 3D CSEM survey. Previous interpretations are discussed in light of the 3D data and the latest drilling results. The CSEM-derived distribution of resistivity in the subsurface is in good agreement with all well log data proving that CSEM provides reliable information that can be used for interpretation and decision making. A strong and localized resistivity anomaly remains untested, although the last two wells drilled very close to it. Two interpretations were evaluated to explain the measured resistivity. The first inferred the presence of a localized sabkha evaporite in the late Carnian, while the second suggested the presence of a hydrocarbon-filled reservoir in erosional products near the base Cretaceous unconformity.