Estimating vertical and horizontal resistivity of the overburden and the reservoir for the Alvheim – Boa field

Abstract

Towed streamer EM data was acquired in October 2012 over the Alvheim – Boa Field located in the Norwegian sector of the North Sea. It is a challenging target consisting of a medium sized oil & gas field exhibiting an average transverse resistance located at 2,100 m below mudline. A depth model was defined for both the overburden and the reservoir interval based on an available well log, and the data was inverted as a series of 1D inversions for all common mid-points along two survey lines to form 2D resistivity sections. Both the vertical and horizontal resistivities were inverted for by minimizing the difference in the frequency responses between forward modeled data and the acquired towed streamer EM field data. Hence, the inversions were done with only a ten layer depth model as background information plus one value for the under-burden. The reservoir interval itself displays high anisotropy as expected since the reservoir is a turbidite. It consists of high resistivity hydrocarbon-charged sands inter-bedded with low resistivity shales giving rise to an effective anisotropy ratio of around 5, whereas the proximal overburden layer exhibits an anisotropy of 2.6. When the anisotropy can be evaluated, the net-to-gross can be estimated facilitating a much improved quantitative estimate of the hydrocarbons in place. Further, when an anisotropic reservoir is located in proximity to or directly on top of basement it can be detected by means of the anisotropy alone. The basement is likely to be isotropic or even display inverse anisotropy due to vertical fractures being more abundant, wider and hence also better conducting than tight horizontal fractures.