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SPWLA Distinguished Speaker Series. The petrophysicist contributes to maintaining and prolonging the lifetime of oil and gas fields by ensuring the reservoir’s 3D geomodel is accurately initialised with properties that include porosity, permeability, water saturations and net cut-off.This is very challenging as there is only limited core and electrical log data available at the well locations.

The petrophysicist helps to determine a field’s hydrocarbon in place by modelling the distribution of fluids using a water saturation vs. height (Swh) function.A good Swh function ensures the three independent sources of fluid distribution data are consistent.These being the formation pressure, core and electrical log data.The Swh function must be simple to apply, especially in reservoirs where it is difficult to map permeability or where there appears to be multiple contacts.It must accurately upscale the log and core derived water saturations to the reservoir model cell sizes.

Using capillary pressure theory and the concept of fractals, a practical Swh function has been derived.Several case studies are presented showing the match between this function and well data.The function gives an accurate prediction of water saturations, even in wells where a resistivity log was not run due to well conditions.

Logs and core data from eleven fields, with very different porosity and permeability characteristics, depositional environments and geological age, are compared.This study demonstrates how this Swh function is independent of permeability and litho-facies types.A method is presented to identify swept and by-passed hydrocarbon and to normalise true vertical depths between wells.

The fractal function defines the free water level, the hydrocarbon to water contact, net reservoir and the irreducible water saturation.The function provides a simple way to quality control electrical log and core data and justifies using core plug sized samples to model water saturations on the reservoir scale.