Saturations in the Sto formation were extremely high at the Wisting well: greater than 90%, leading to extremely high measured resistivity. This coupled with the shallow depth of the Sto formation makes this interval, in principle, an ideal target for CSEM methods. However, when assessing the ability of CSEM data to resolve sub-surface resistivity structure, both the resistivity of the target, and importantly the resistivity of the background structure must be considered. Background resistivity in the Northern Barents is notoriously high even in the shallow sedimentary structure (often in excess of 20 Ω.m), with a high degree of electrical anisotropy. In this environment it is important to understand not only whether CSEM data are sensitive to a given structure but also whether a CSEM inversion process is likely to be able to recover a given resistivity structure from survey data - the so-called recoverability problem as discussed by MacGregor and Tomlinson in 2014.

• play
  Step 1: Recoverability
  This recoverability question is illustrated in the figure. The upper panels show the vertical resistivity of an anisotropic model constructed to represent the background observed in the survey area. A high resistivity zone is embedded just beneath the top Realgrunnen to represent a hydrocarbon charged target. The bottom panels show the vertical resistivity derived from 2D anisotropic inversion of noise contaminated synthetic data derived from these models. Whereas a reservoir with a transverse resistance (the product of resistivity and thickness) of 6000 Ω.m² can be easily recovered by the inversion, a 3000 Ωm² target is considerably more challenging and considered to be at the limit of what is likely to be recoverable.


• play
  Step 2: Recoverability limits
  The meaning of this recoverability limit in terms of hydrocarbon saturation is shown in the figure, which illustrates the variation of transverse resistance with hydrocarbon saturation, calculated from Archie’s law calibrated at the nearby Wisting well, and assuming a 20m thick hydrocarbon charged sand. Three different porosities are considered covering the range likely to be encountered in the area. The results suggest that for a hydrocarbon accumulation to be clearly imaged by the CSEM data, the saturation must be 60% or greater.