The assessment of geological uncertainties in the petrophysical characterisation of reservoirs should be a common and standard approach, but a unique robust methodology is not yet available "off-the-shelf". Moreover, it is not granted that a unique approach may prove equally robust in all environments.

It is therefore mandatory to develop a specific, sound methodology for the correct evaluation of the petrophysical uncertainty according to the reservoir characteristics, in order to propagate the correct results through the following reservoir modelling processes.

A fit-for-purpose approach was developed in the scope of the non-conventional petrophysical interpretation [7] of a thin-layered reservoir, in order to evaluate the uncertainty associated to the most critical petrophysical properties, i.e. porosity and water saturation.

The results proved that the approach was both robust and flexible enough to be applied to a specific interpretation process, in a complex geological and petrophysical environment. The quantitative evaluation of the uncertainty associated to the petrophysical properties provided a significant improvement in the knowledge of the true uncertainty finally fed into the reservoir model and risk analysis.


This paper describes the fit-for-purpose approach developed to evaluate the petrophysical uncertainty in a critical formation, characterised by the presence of thin and very thin sand/shale alternations, where a conventional methodology could not provide robust estimates of the petrophysical properties and their uncertainty and, consequently, of HOIP.

The workflow includes three main steps:

  1. First, the definition of the high-resolution petrophysical model of the reservoir, by means of e-tlacTM, the proprietary methodology developed by Eni to provide the quantitative characterisation of the petrophysical properties and the evaluation of Net Pay and NTG in thin-layered reservoirs

  2. Then, the analysis of the uncertainty. The specific environment required a fit-for-purpose adaptation of a step-by-step approach, that was eventually integrated into the e-tlacTM methodology

  3. Finally, an accurate sensitivity analysis of the results (including porosity, water saturation and NTG) was done to provide consistent estimates to the risk analysis process

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