Quality-assured evaluation of freshwater-bearing hydrocarbon reservoirs

The petrophysical evaluation of clastic freshwater-bearing hydrocarbon reservoirs cannot be approached using the standard composite Archie equation because these reservoirs show departures from the assumptions that underpin its application. The problem is placed on a sounder technical footing by defining the preconditions for the application of non-Archie saturation models and showing where and how these are satisfied. This exercise assumes that porosity has been evaluated at an earlier stage of the petrophysical workflow. It reveals that there is no fixed salinity level below which reservoirs show freshwater character in an electrical sense: the level is reservoir-specific. Where the non-Archie models do break down, a conditioned pseudo-Archie approach is proposed. This approach demonstrably enhances the estimation of hydrocarbon saturation, typically by up to 20 saturation units, as evidenced through groundtruthing core measurements and comparisons with other log deliverables. These comparisons can, of course, impart additional quality assurance. The approach is illustrated using field examples. On this basis, a protocol is outlined for the optimum acquisition of petrophysical data in clastic freshwater-bearing hydrocarbon reservoirs. Central to this protocol are charts for identifying the most appropriate values of the porosity and saturation exponents. The adoption of this protocol leads to a significant reduction of uncertainty in the volumetric evaluation of these challenging hydrocarbon accumulations.