Nuclear Magnetic Resonance and Pore Coupling in Clay-Coated Sandstones With Anomalous Porosity Preservation, Água Grande Formation, Recôncavo Basin, Brazil

Continuous chlorite coats are known to preserve the porosity in deeply buried sandstones by forming physical barriers to quartz early overgrowth. Sandstones from Água Grande Formation, Recôncavo Basin, Brazil, present anomalous porosity due to development of chlorite coating and is therefore suited for studying the nuclear magnetic response to this effect. Samples from this unit were classi¿ ed into three groups according to their texture, composition and abundance of chlorite coatings: Group 1 with low amounts of coating, Group 2 with high amounts of coating and a nonreservoir sample (Group 3). Group 1 samples show wide NMR T2 distribution, while Group 2 samples present a bimodal T2 distribution. Nonreservoir Group 3 samples showed only a T2 peak in much shorter T2 times. To interpret the NMR results, transmitted light optics, scanning electron microscopy, porosity and permeability measurements, micro-CT, X-ray diffraction, magnetic susceptibility and hysteresis were used. We conclude that the longest T2 (> 0.1 s) peak of reservoir samples (Groups 1 and 2) is due to intergranular macropores, the intermediate peak is due to feldspar or clay intraclasts dissolution and the shortest peak (~0.01 s) is due to the chlorite coating itself, with minor contribution from secondary microporosity. The microporosity is predominant in the nonreservoir sample and relates to the clay-bound water. The shift to shorter times of longer T2 peaks in samples with higher contents of chlorite-bearing sandstones is likely related to diffusive coupling.