Of particular interest in unconventional reservoir characterization is an NMR log of total porosity and macro-pore hydrocarbon saturation, where both quantities are independent of a mineralogy model. A log of the macro-pore hydrocarbon saturation has a direct impact on calculating hydrocarbon reserves. It helps identify sweet spots in the reservoir to optimize horizontal-well placement for hydraulic fracturing and production. It also helps avoid water production which would negatively affect the economics of the well. However, NMR logs in unconventional shale are challenging due to potential overlapping signal in the 1-dimensional (1-D) 𝑇𝑇2 domain between micropore water and bound hydrocarbon (i.e. bitumen), and, macro-pore water and hydrocarbons. In response to this challenge, NMR core-analysis in unconventional organic-shale has proven that 2-dimensional (2-D) 𝑇𝑇1 − 𝑇𝑇2 correlation maps and the 𝑇𝑇1/𝑇𝑇2 ratio can be a powerful technique for fluid typing and saturation. One limitation is that these techniques often just compare fully hydrocarbon-saturated with fully brine-saturated cores to calibrate a set of cutoffs in 𝑇𝑇1, 𝑇𝑇2, and/or 𝑇𝑇1/𝑇𝑇2 ratio. These cutoffs are then blindly applied to 𝑇𝑇1 − 𝑇𝑇2 maps from logs or cores of unknown saturation to determine the macro-pore hydrocarbon saturation in the unconventional organic shale. An example from the unconventional Point-Pleasant formation is shown where the traditional 𝑇𝑇1 − 𝑇𝑇2 cutoff technique to determine macro-pore hydrocarbon saturation breaks down, which is remedied by measuring 𝑇𝑇1 − 𝑇𝑇2 maps on mixed hydrocarbon-water saturated cores. The results show that instead of using cutoffs, the log-mean 𝑇𝑇1 , log-mean 𝑇𝑇2 , and log-mean 𝑇𝑇1/𝑇𝑇2 ratio correlate strongly against macro-pore hydrocarbon saturation of the mixed-saturated cores. In particular, for the Point-Pleasant organic-shale formation, the log-mean 𝑇𝑇1 is much more sensitive to macro-pore hydrocarbon saturation than the log-mean 𝑇𝑇2 or log-mean 𝑇𝑇1/𝑇𝑇2 ratio. The calibration of macro-pore hydrocarbon saturation from log-mean 𝑇𝑇1 is found to be different above and below a para-sequence boundary (nonconformity) in the organic-shale interval, the results of which can be used to interpret NMR logs. Details of the time-efficient technique used to obtain the mixed hydrocarbon-water saturated cores are shown.
