Oil saturation quantitative evaluation in lacustrine shale: A novel insight from NMR T1-T2 experiments

Oil saturation is important for shale reservior to identify favorable sections and mapping the geological sweet spot. Current oil saturation evaluation methods, including experiments and empirical formulas, are not suitable for shale reservoir because of the complex mineral, fluid components and pore structure characteristics. To establish the shale oil saturation calculation model, X-ray diffraction, one-dimension and two-dimension nuclear magnetic resonance (NMR), and oil-water two-phase displacement experiments were employed on shale samples collected in the upper sub-member of the fourth member of the Eocene Shahejie Formation in the Dongying sag, Jiyang Depression, Bohai Bay Basin. After data analysis, the reason for whether oil is produced in the displacement experiments were explained, distribution characteristics of different shale components in the NMR T1-T2 map were analyzed, and a new shale oil saturation calculation method was proposed using NMR T2 sensitive parameters that reflected the changes of NMR T2 spectrum morphological characteristics with different oil saturation calibrated by NMR T1-T2 map at different displacement stage. The results indicated that the pore structure of shale samples is complex and show strong heterogeneity according to the NMR T2 spectrum, and the distribution of shale pore size is the main factor determining whether there is oil in the volumetric cylinder in the displacement experiment under the premise of the slight difference of wettability. NMR T1-T2 map is an effective way to identify different components (kerogen and solid bitumen, adsorbed oil, free oil, structural and adsorbed water, free water) of shale samples, and usually, kerogen and solid bitumen distributed in the top left of the T1-T2 map with T1>10 ms, T2<0.1 ms. Based on this, T2 threshold for free oil and adsorbed oil are 2 and 0.2 ms, and the corresponding threshold of pore radius are 40 and 4 nm according to the NMR theory. As NMR T2 spectrum sensitive parameters, geometric mean and interval porosity corresponding to the first peak are positively and negatively correlated with oil saturation respectively. With understanding this, oil saturation calculation method is established using the above two parameters and the Root Mean Square Error (RESM) between the measure oil saturation and the calculated results is 5.78%, which reflecting the accuracy and validity of the method. In general, this method allows the shale oil saturation to be accurately calculated and provides a parameter basis for the determination of favorable sections and evaluation of resource of shale oil reservoir. Moreover, it also offers a new idea for the oil saturation predication by NMR logging.