Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo Research paper Sensitive parameters of NMR T 2 spectrum and their application to pore structure characterization and evaluation in logging profile: A case study from Chang 7 in the Yanchang Formation, Heshui area, Ordos Basin, NW China Jianping Yan a,b,* , Xu He b , Shaolong Zhang b , Chunzhen Feng c , Jun Wang d , Qinhong Hu e,** , Jingong Cai f , Min Wang d a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Chengdu 610500, China b School of Geoscience and Technology, Southwest Petroleum University, Chengdu 610500, China c Changqing Division of PetroChina Logging Company, Xi’an 718500, China d Institute of Exploration and Development, Shengli Oil Field, SINOPEC, Dongying 257015, China e Department of Earth and Environmental Sciences, The University of Texas at Arlington, Arlington, TX 76019, USA f State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China ARTICLE INFO Keywords: Nuclear magnetic resonance Sensitive parameters Pore structure Tight sandstone ABSTRACT The evaluation of the pore structure of tight sandstone reservoirs has a significant influence on the effective exploration and development of tight sandstone oil. Laboratory measurements including scanning electron microscopy (SEM), mercury-injection capillary pressure (MICP), gas adsorption, nuclear magnetic resonance (NMR), and Nano-CT can provide detailed pore structure data. However, only NMR results can be used to evaluate the pore structure in a logging profile due to NMR logging. In this study, the relationships between NMR T 2 distribution, pore structure, and pore size of tight sandstone in Chang 7 of the Yanchang Formation, in the Heshui area of the Ordos Basin, are analyzed based on NMR principles. The characteristics of the NMR T 2 distribution of different rock samples are analyzed; the sensitive parameters of the NMR T 2 spectrum are pro- posed. These parameters are then used to classify pore structure types in tight sandstone reservoirs and divide the active layers in the logging profile of the study area. The results indicate that different pore size structures have different distributions, and that the NMR T 2 spectrum can highlight the difference in the pore structure type and pore size distribution using sensitive parameters such as T 2 P 2 (the value of T 2 corresponding to the last peak of the bimodal NMR T 2 spectrum) and T DM (the mean value of T 2 relaxation time obtained by weighted average method). As the pore structure of the rock samples in the study area worsened, their porosity and permeability worsened, T 2 P 2 and T DM decreased, and the displacement pressure and NMR irreducible water saturation in- creased. The results of classifying the pore structure types obtained in the logging profile are helpful in evalu- ating the effectiveness of the reservoir and in the broad application of NMR logging in terms of understanding pore structure. 1. Introduction The exploration and development of tight sandstone oil, an un- conventional oil resource, has attracted considerable attention in China with the decreasing production of conventional oil and gas (Jia and Chi, 2004; Zou et al., 2012; Zhao et al., 2015; Jia et al., 2017). The pore structure of rocks refers to the geometry, size, and dis- tribution of pores and throats as well as the relationships among them (Fu et al., 2015). However, low-permeability tight sandstone reservoirs usually have complex pore structures. Under laboratory conditions, various experimental methods could be used to characterize pore structures, including scanning electron microscopy (SEM), mercury injection capillary pressure (MICP), low-pressure N 2 adsorption, small- angle neutron scattering, and nuclear magnetic resonance (NMR). Many quantitative pore structure parameters, such as displacement pressure (P d ), average pore throat radius, specific surface, and T 2 geometric https://doi.org/10.1016/j.marpetgeo.2019.08.025 Received 22 June 2019; Received in revised form 14 August 2019; Accepted 14 August 2019 * Corresponding author. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (Southwest Petroleum University), Chengdu 610500, China. ** Corresponding author. E-mail addresses: yanjp_tj@163.com (J. Yan), maxhu@uta.edu (Q. Hu). Marine and Petroleum Geology 111 (2020) 230–239 Available online 20 August 2019 0264-8172/ © 2019 Elsevier Ltd. All rights reserved. T