Classification of Reservoirs and Fluids in Tight Sandstone Based on the NMR Logging Technique─A Case Study of the Upper Paleozoic in the Ordos Basin

Abstract

The Upper Paleozoic strata in the Ordos Basin are dominated by tight sandstone gas reservoirs characterized by strong heterogeneity and widely distributed aquifers. Laterally, there is no evidently continuous distribution of water and gas, and longitudinally, the gas and aquifers are mostly in isolated intersections without uniform gas–water contact, rendering difficulties for effectively identifying the reservoir and fluid properties by using traditional mud logging technology. To solve these problems mentioned, the reservoirs were classified by integrating the T 2 spectra of cores obtained through nuclear magnetic resonance (NMR) logging, regional geological characteristics, and gas test results. It is indicated in the result that the “three-division” method can reflect the type of storage space, and the structural movement and sedimentary thickness in the basin are primary factors for the formation of reservoirs of various types. The “nine-division” method is closely related to characteristics of “initial tightening and subsequent accumulation” of the basin and the gas test results, which can give insights into natural gas charging status in the tight sandstone samples, thereby effectively identifying fluid properties.