Assessments of shaly and low permeability hydrocarbon reservoir sandstones are always problematic. Traditional log-based techniques provide rock porosity but at best can only estimate permeability. Recent advances in the nuclear magnetic resonance (NMR) log technology provide additional benefits through a more direct permeability estimation based on computed volumes of movable and immovable fluids. However, the presence of clay, in reservoir rocks proves to be problematic not only for the traditional log measurements but also for NMR log interpretation. Many shaly sandstones that have good porosity appear to have highly variable permeability. In the presence of clays the most commonly used T_2 cutoff values seem to eventually yield inaccurate permeability estimates. This paper presents results of a study that focuses on NMR measurements on shaly sand samples taken from various oilfields in Australia. The study suggests that the T_2 cutoff values for the samples appear to be significantly different from the 'standard' values, in order for NMR-derived irreducible water to match core-derived irreducible water. This is also shown by the samples' resulting NMR-derived permeability estimates that are in more satisfactory agreement with the directly measured permeability values. The overall results underline the need to study T_2 cutoff values more closely for specific reservoir rocks before their practical uses in the field.
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