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Reservoir Pore Structure Classification of Carbonate Rocks Based on Lithology Analysis and NMR Experiments


An integrated petrographical and petrophysical study was carried out on a set of 20 carbonate core samples in Ordovician reservoirs with depth more than 5500 m, covering a wide range of lithologies and textures. In this study various carbonate rock-types have been characterized, by integrating both sediment-petrological and petrophysical data, including thin sections, porosity, permeability, low-field Nuclear Magnetic Resonance (NMR) and Mercury-injection capillary pressure (MICP). Based on the petrographical and petrophysical analysis, 6 groups of rock types were identified, and each of them characterized by a unique NMR signature: (1) Grainstone, (2) Packstone, (3) Wackstone, (4) Mudstone, (5) limestone with full-filled fractures and (6) limestone with half-filled fractures. NMR T2 distributions were linked to pore body size and T2 logarithmic (T2lm) was calculated. It is apparent that packstone, wackstone and mudstone of the carbonate reservoirs in this study, yield smaller pore body sizes (T2lm < 20 ms), as well as narrower pore throats (average radius < 150 nm) and lower permeability values (typically below 0.1 mD). The grainstone samples yield bimodal T2 distributions, with a first peak related to the cement matrix pores and a second peak related to intraparticle pores. The T2 distributions of limestone with fractures reflecting larger pore sizes (T2lm > 90 ms) and higher permeabilities (up to 10 mD). Additionally, each rock type's NMR characters were tested under different pressure, and their sensitive responses were also analyzed, especially for the second peak of grainstone and limestone with fractures. For all samples, permeability was inferred from NMR spectra using Schlumberger Doll Research (SDR) model. The study aims to develop an NMR-based approach to characterize various carbonate rock-types, calibrated by geological and petrophysical analysis. The results allow an in depth understanding of the NMR signal of each carbonate rock-type, and can be used as a guide to interpret NMR logging data.