Magnetic resonance for downhole complex-lithology earth formation evaluation

Many petroleum-bearing reservoirs are known as complex-lithology, complex-mineralogy earth formations where the existing nuclear magnetic resonance (NMR) analysis models require modification and extension to work properly. This paper discusses the effects of mineralogy variation in complex-lithology formation rocks that will cause NMR response variations. In particular, the existence of iron-rich authigenic clays and the nonquartz grains in siliciclastics affect the surface relaxivity and the internal field inhomogeneity. Using a simplified pore-lining clay model, we estimated that the magnitude of the internal gradient in siliciclastics is comparable with or greater than that of the instrument-generated field gradient common to the currently used NMR logging tools. To account for these effects for reservoir fluid identification and quantification from NMR data, an inversion model is created, which includes the mean susceptibility of the grain minerals, the correlation between pore size and the relaxation time of the wetting-phase fluid in the pores and the geometric restriction to the fluid molecular diffusion.