Carbonate rocks are well known for their complex petrophysical behavior where, in contrast to siliciclastic rocks, different parameters, including porosity and permeability, usually are not directly related. This behavior is the result of thorough reorganization of porosity during diagenesis, and it turns prediction of reservoir quality of carbonate rocks into a challenge. The study presented here deals with the problem of utilizing NMR techniques in prediction of petrophysical properties in carbonates.We employ a visual porosity classification as a priori knowledge for better interpreting NMR data for prediction purposes. This allows for choice of suitable T 2 cutoff values to differentiate movable from bound fluids adapted for the specific carbonate rock, thus resulting in better interpretation of NMR data. The approach of using a genetic pore type classification for adapting the conventional method for T 2 cutoff determination, which originally was developed for siliciclastic rocks, is promising. Similarly, for permeability determination on the basis of NMR measurements, the classification of carbonate rocks based on porosity types also shows potential. The approach implemented here has the promise to provide a basis of standardized interpretation of NMR data from carbonate rocks.

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Accurate estimates of porosity and fluid saturations are critical for the proper evaluation of a gas reservoir. These properties are determined by combining data from a dual wait-time (DTW) nuclear magnetic resonance (NMR) log and a density log. The novel deterministic method additionally yields several in-situ gas properties. The density and dual wait-time (DDTW) technique is applicable to reservoirs where the pore-filling fluid consists of one liquid phase and one gas phase. The low proton density of the gas phase causes a reduction in the NMR signal strength resulting in underestimation of the apparent porosity. The amount of polarization for different wait-times depends on the specific spin-lattice relaxation time of each fluid and may cause additional NMR porosity underestimation. The density log, however, delivers a porosity that is overestimated because of the presence of a gas phase. Published correlations for gas properties are used to establish the new deterministic method. DDTW primarily yields the total porosity f, and the flushed zone gas saturation, Sg, xo. Other derived properties are the in-situ gas density, rg, as well as the two NMR-related properties hydrogen index, HIg, and spin-lattice relaxation time, T1g. Two field examples illustrate the method and an error propagation study supports its stability.