Movable Oil Saturation Quantitation Evaluation in Shale Oil: Novel Insights from T ₂ Distribution

The quantification of movable shale oil is crucial for the effective exploration and development of shale oil resources. Nuclear magnetic resonance (NMR), a nondestructive and noninvasive technique, has become an indispensable tool for evaluating movable oil saturation. However, the small core sizes, high-frequency instrumentation, costly measurements, and significant losses of light hydrocarbons pose substantial challenges in accurately assessing movable oil. To solve these issues, we developed a novel workflow for determining the movable oil saturation. Our approach begins with the rapid acquisition of NMR data using a vehicle-mounted NMR measurement operating at a 2 MHz magnetic field frequency. Therefore, we can acquire pressure-sealed full-diameter cores, which can provide timely and accurate NMR-based characterization from the original formation. Subsequently, the longitudinal relaxation time-transverse relaxation time (T ₁-T ₂) distribution derived from full-diameter cores can be seen as constraints, we utilize the continuous wavelet transform to analyze the T ₂ distributions from bound fluids (organic matter + clay water), organic pore oil, and inorganic pore water. This process extracts the shape and position parameters of spectral peaks at multiple scales within the wavelet domain. Gaussian distribution functions are constructed at each scale, allowing for the reconstruction of T ₂ distributions for each fluid component. The optimal reconstruction results were selected to determine the movable oil saturation. The results indicate the average absolute error of movable oil saturations between 1D NMR and 2D NMR is 4.78%, which confirms the workflow provides a significant assessment toward movable oil saturation, thereby contributing to enhanced reserve estimation and increased production from unconventional shale oil resources.