The use of NMR spectroscopy to validate NMR logs from deeply buried reservoir sandstones

A Lower Jurassic deeply buried sandstone oil reservoir offshore Mid Norway was logged with NUMARʹs MRIL-C tool. The NML data have been compared with standard logs as well as laboratory NMR and standard petrophysical core measurements. The two formations studied are in the oil zone, Formation A being characterised by an extensive distribution of pore lining chlorite, while asphalt staining is observed in the pores of cleaned Formation B samples. These characteristics are also recognised on the NMR T1 data at irreducible water saturation with n-decane. Formation A samples is clearly water-wet while samples from Formation B show a mixed-wet behaviour. A good match is found between NMR derived and measured porosities and saturations of the core plugs. The detrimental effect of chlorite on the T2 relaxation is demonstrated by strong reduction of relaxation times for the longest T2 components. Under-estimation of porosity (MPHI) from the NML data is related to the oil saturation of the logged formation. This phenomenon is attributed to a lower hydrogen index in the oil phase and diffusional processes induced by the applied magnetic field gradient. The under-estimation of porosity leads also to a general under-estimation of permeability. The fraction of bound water (BVI) derived from the NML data is lower in the mixed-wet formation. This compensates to some extent for the underestimated porosity in the permeability estimates. It is further suggested that the BVI/MPHI ratio may be used as a wettability indicator.