Magnetic matrix effects on NMR relaxation times in sandstones: A case study in Solimões Basin

Abstract

Pore distribution estimates based on Nuclear Magnetic Resonance (NMR) can be strongly affected by the presence of magnetic field gradients between the pore and the grain’s surface. Here we investigate the effect of the magnetic matrix on Uerê and Juruá reservoir sandstones (Solimões Basin, Brazil), where quartz, illite and hematite coatings developed and contributed to the partial preservation of the original porosity. Samples were divided into three groups based on petrographic observations, magnetic data and type of iron-bearing minerals. Group 1 and Group 2 samples consist of subarkoses with variable amounts of coating of microcrystalline quartz. Besides, the mineral matrix in Group 2 samples contains paramagnetic iron-bearing minerals such as Fe-dolomite, siderite and pyrite, besides Fe-illite. Samples from Juruá Formation form the Group 3 and consist of hematite-coated sandstones. Macroporosity for these samples is indicated by a peak above 0.1 s in NMR T2 time distributions, whereas intermediate peaks in the range of 0.1–0.01 s are generally related to the secondary porosity generated by mineral dissolution. Group 2 and Group 3 samples present significant shifts of the highest T2 peaks towards shorter times. These shifts are directly related to the magnetic mineralogy of the rocks (paramagnetic and hematite minerals, respectively for Group 2 and Group 3 samples), with these samples presenting the highest magnetic susceptibilities. T2 displacements in the studied samples are caused mainly by relaxitivity related to the Fe cements and the strongly magnetic hematite coatings. The diffusion coupling caused by coatings does not influence significantly the NMR response. We note that magnetite impurities are not enough to influence the diffusion relaxivity neither the NMR porosity estimates, however the localization of hematite at the pores surfaces of Group 3 samples lead to an underestimation of the porosity.