Accurate estimation of deterministic recoverable reserves is a formidable task for engineers and geoscientists. It is even more challenging if the reservoir is naturally fractured and exhibits dual porosity behaviour. The major problem is estimating how much of the hydrocarbon, stored in the matrix porosity system, will be recovered via the fracture network. The interconnectivity between matrix and fracture, affectionately termed the X factor, is the important parameter in the exercise. Unfortunately, it is very difficult to constrain. New research into this topic has just begun at La Sapienza, Rome University, and its primary objective is to ascertain whether the deterministic X factor can be constrained sufficiently to be used with confidence when estimating reserves in dual porosity systems. The ultimate goal is to develop a robust deterministic volumetric model of dual porosity systems for use by engineers and geo-scientists involved in screening exploration prospects and commercial opportunities. However, to achieve this goal, the model must provide accurate estimates of original oil-in-place, both in the matrix as well as in the fracture system. The added knowledge of how the matrix and fracture system interact is fundamental in determining appropriate production strategies. Future research will attempt to link in bottom up approaches (numerical simulation of fracture networks) and top down approaches (regional strain intensity mapping) by employing the calibration field data sets currently being set up.