Campos Basin Anisotropic Turbidities Formation Evaluation: Challenges And Proposed Solutions

The evaluation of some deep water turbiditic reservoirs offshore Brazil has always been a challenge. In the first discovery vertical wells, the real hydrocarbon saturation was often masked by the presence of laminations as the traditional wireline induction logs, mostly sensitive to the horizontal resistivity component, did not manage to capture the real hydrocarbon saturation. The anisotropy existence was confirmed early by electrical images and cores in a vertical well. Using resolution enhancing methods and simultaneous laterolog and induction analysis a better quantification of the fractions of sand and shale and their respective saturations started to become possible. This analysis was confirmed using multi axis induction tools which provided an alternative way of extracting horizontal and vertical resistivity components. With the advent of Ha/Hz wells as primary mean of reservoir development, both new challenges and opportunities became available. On one side the analysts had to deal with spurious polarization effects which made LWD raw logs unsuitable for a direct use for saturation computation, but on the other side the same measurements, designed to be responsive to the axis transversal to the tool, as a consequence of the well geometry change, became much more sensitive to the vertical resistivity component, historically difficult to be measured. In this paper independent technologies and interpretation approaches are discussed and compared showing field cases in vertical and Ha/Hz wells. The results obtained were consistent and comparable proving the need of evaluating interlaminated reservoirs using non-conventional methods making use of Rh, Rv and Vclay to derive Rsand, R clay(h & v) and relative saturations.