Study on fluid mobility in sandwich-type shale oil reservoir using two-dimensional nuclear magnetic resonance approaches

Regarding sandwich-type shale oil reservoir characterized by diversified lithologies and complex fluid mobility, we combined one- and two-dimensional (1D and 2D) nuclear magnetic resonance (NMR) core analysis techniques to quantitatively study the movable oil and fluid saturations of rock samples with different lithologies. First, we used 2D T-1-T-2 NMR to analyze the spectrum curve of the dolomitic mudstone and siltstone samples from a sandwich-type shale oil reservoir at initial and oil-saturated states. We evaluated the occurrence of movable oil saturations in source and reservoir rocks. Then, we analyzed samples of the siltstone serving as the reservoir rock using a centrifuge combined with 1D NMR approach to evaluate the characteristics of movable fluid saturations inside the reservoir. The results showed that the samples of dolomitic mudstone serving as the source rock in the sandwich-type shale oil reservoir in the Lucaogou Formation in Jimsar had the occurrence of kerogen and bitumen signals, the development of small- and medium-scale organic matter pores, and an average movable oil saturation of 15.6%. The siltstone serving as the reservoir rock had an average movable oil saturation of 53.8%, and its movable fluid saturation was positively correlated with porosity and permeability. For samples whose porosity was greater than 12% and permeability was greater than 0.1 mD, their movable fluid saturations usually were higher than 40%. Therefore, siltstone was the preferable diagenetic facies for the development of high-quality reservoirs.