Explaining Nuclear Magnetic Resonance T2 shifts in sandstones with anomalous porosity.

Some sandstones present anomalously high porosities after burial, which result from different mechanisms including the early growth of quartz and clays on host quartz grains (Bloch et al., 2002). These rocks typically show a macroporosity and some microporosity well identified by Nuclear Magnetic Resonance (NMR) relaxometry. However, the comparison between NMR and petrophysics measurements show a systematic mismatch in pore-size estimations in most samples expressed as a shift in NMR T2 to lower relaxation times. Here we show the results obtained on three sedimentary units that present anomalous porosity: the clay-coated sandstones from Água Grande (Recôncavo Basin), the microcrystalline quartz-coated sandstones from Fontainebleau Formation (Paris Basin), and the sandstones coated with illite either microcrystalline quartz from Uerê Formation (Solimões Basin). Hematite-coated sandstones from Juruá Formation (also from Solimões Basin) were also studied to characterize the effects of magnetic minerals in the rock matrix on NMR data.