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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. Introduction During progressive burial a trend of porosity-loss with depth is observed on sandstone units due to the increase in mechanical compaction and quartz cementation. However, the porosity can be preserved in special conditions, for example, when fluid overpressure occurs in rocks with low thermal maturity, due to early hydrocarbon emplacement or through clay or quartz grain-coating (Bloch et al., 2002). Anomalously high porosity caused by coatings of microcrystalline quartz and clay in sandstones have been studied by many authors in different depositional settings (Pittman et al., 1992; Mc Kinley et al., 2003; Worden et al., 2012; Ajdukiewicz and Larese, 2012; French & Worden, 2013). However, their microporosity structure has not been studied, including the micropore families associated with coatings. The NMR is a non-destructive tool well suited for the study of these pores, since it provides information about pore size distribution and can be performed in well log exploration, i.e. in-situ. In the present study we characterized the pore space of samples from Fontainebleau and Uerê Formation, which show anomalous porosity due to the presence of microcrystalline quartz coatings, and Água Grande Formation, which shows chlorite coatings. The complete dataset was published in Jacomo et al. (2018, 2019).

Explaining Nuclear Magnetic Resonance T2 shifts in sandstones with anomalous porosity