Lithotype characterizations by Nuclear Magnetic Resonance (NMR): A case study on limestone and associated rocks from the eastern Dahomey Basin, Nigeria

Three representative rock types (limestone, sandstone, and shale) and glauconite samples collected from Ewekoro Quarry, eastern Dahomey Basin in Nigeria were characterized using low field 2 MHz and 20 MHz Nuclear Magnetic Resonance (NMR) techniques. NMR T-2 relaxation time decay measurement was conducted on disc samples under partial water-saturation and full water-saturation conditions using CPMG spin-echo routine. The T-2 relaxation decay was converted into T-2 distribution in the time domain to assess and evaluate the pore size distribution of the samples. Good agreement exists between water content from T-2 NMR distributions and water imbibition porosity (WIP) technique. Results show that the most useful characteristics to discriminate the different facies come from full saturation NMR 2 MHz pore size distribution (PSD). Shale facies depict a quasi-unimodal distribution with greater than 90% contribution from clay bound water component (T-2s) coupled to capillary bound water component (T-2i) centred on 2 ms. The other fades with well connected pore structure show either bimodal or trimodal T-2 distribution composed of the similar clay bound water component centred on 0.3 ms and quasi-capillary bound water component centred on 10 ms. But their difference depends on the movable water T-2 component (T-2l) that does not exist in the glauconite facies (bimodal distribution) while it exists in both the sandstone and limestone facies. The basic difference between the limestone and sandstone facies is related to the longer T-2 coupling: T-2i and T-2l populations are coupled in sandstone generating a single population which convolves both populations (bimodal distribution). Limestone with a trimodal distribution attests to the fact that carbonate rocks have more complex pore system than siliclastic rocks. The degree of pore connectivity is highest in sandstone, followed by limestone and least in glauconite. Therefore a basic/quick NMR log run on samples along a geological formation can provide precise lithofacies characterization with quantitative information on pore size, structure and distributions. (C) 2017 Elsevier Ltd. All rights reserved.