Characterization of light hydrocarbon reservoirs by gradient-NMR well logging

New methods for acquiring and processing gradient NMR well log data enable signals from gas, light oil, and water to be unambiguously separated and, in many cases, quantified. These methods exploit the combined effects of T1-based and diffusion-based contrast on log response. T1 contrast, which separates the water and light hydrocarbon (oil or gas) signals, is measured by subtracting spin-echo decays measured at different, appropriately chosen wait times. Gas and oil signals are then separated based on the large contrast in the diffusion-induced T2 relaxation times for gas vs. liquid. Practical application of these principles is illustrated with new log examples that also highlight the advantages of NMR over traditional logging methods for detecting and typing light hydrocarbons, especially in mineralogically complex rocks.