Characterization of pore size distributions of shale oil reservoirs: A case study from Dongying sag, Bohai Bay basin, China

Shale oil reservoirs characterized by complex pore structures typically contain a wide pore size range from nanometers to millimeters. The pore size distribution (PSD) is one of the most critical parameters affecting fluid storage and transport properties in shale oil reservoirs. Quantitatively and accurately determining PSD is of great importance for evaluating the shale oil occurrence state and movable fluid volume. In this study, four major techniques, including nitrogen adsorption (NA), scanning electron microscopy (SEM), mercury injection capillary pressure (MICP) and nuclear magnetic resonance (NMR), were adopted to determine pore sizes in ten shale oil reservoir samples from Dongying sag, Bohai Bay basin, China. NMR T-2 spectra were converted to PSDs by using a new subsection calibration method based on the NA and SEM measurements; using the linear and power exponent conversion models, these two models are compared. The results show that the subsection calibration is an effective method for shale NMR T-2 spectrum conversion. Shale oil reservoir T-2 spectra can be converted to full-scale PSDs by combining NA micropore (< 100 nm) PSDs and SEM mesopore (100 nm-1000 nun) PSDs using linear and power exponent models. The power exponent model is more useful for determining the approximate initial transverse surface relaxivity (rho(2,0)) values for the shale samples. However, if the calibration PSDs are not sufficient, the average conversion coefficient (51.295, in this study) of the linear model is recommended to determine an approximate NMR PSD. The results also indicate that there is a wide range of pore sizes from micropores to macropores (> 1000 nm) in shale oil reservoirs; however, they are mainly connected by small-scale throats. Therefore, the comprehensive characterization of shale oil reservoir pore structures may be efficiently and accurately determined by a combination of NMR and MICP.