Rock-Type Definition and Pore Characterization of Tight Carbonate Rocks Based on Thin Sections and MICP and NMR Experiments

Abstract

Thick tight Ordovician carbonate rocks are present at depths exceeding 5300 m in the Tahe oilfield and their matrix is considered to contain no storage space. An integrated petrographical and petrophysical study was conducted on a set of 25 tight carbonate core samples from Ordovician strata, covering a wide range of lithologies and textures. Six carbonate rock types were characterized by integrating both petrographical and petrophysical data, including thin-section observations and porosity, permeability, mercury injection capillary pressure and nuclear magnetic resonance (NMR) measurements. We found that thick grainstone and limestone with half-filled fractures exhibited good reservoir properties. NMR testing is an invaluable tool for characterizing pore structures in tight carbonate rocks. For example, six rock types can be identified from the NMR T₂ distributions and the changes in pore volume under different pressures (up to 20 MPa) can be calculated. NMR technology can be used to perform rapid and accurate rock-type identification and pore network evaluation in tight carbonate rocks. The results provide an experimental foundation for NMR logging interpretations and advance the understanding of geological and geophysical characteristics of ultra-deep carbonate reservoirs.