Fractal behaviors of NMR saturated and centrifugal T2 spectra in oil shale reservoirs: the Paleogene Funing Formation in Subei Basin, China

Abstract Fluorescence thin section and scanning electron microscopy (SEM) analysis were performed to delineate pore spaces and pore network assemblages of oil shales of Member 2 of Paleogene Funing Formation (E1f2) in Subei Basin, China. Nuclear magnetic resonance (NMR) T2 (transverse relaxation time) spectra were used to gain insight into pore size distribution. Fractal analysis was performed on the NMR T2 spectra measured at saturated and centrifugal status, respectively, with the aim to reveal the different fractal behaviors of irreducible pore realms (centrifugal T2 spectrum) and the entire pore systems (saturated T2 spectrum). Relationships between NMR parameters and fractal dimensions were revealed. The results show that the pore spaces consist of 1) large pore realms including interparticle pores, and microfractures, 2) small pore realms including intragranular dissolution pores, intercrystal micropores and organic matter pores. The T2 spectra are mostly bimodal and left-skewed due to abundance in small pore realms. The right peak or tail distribution is associated with large pore realms. Fractal analysis performed on saturated and centrifugal T2 spectra show that fractal dimensions calculated from centrifugal T2 spectrum are higher. In addition, fractal dimensions for centrifugal T2 spectrum show negative correlation relationships with BVI and T2cutoff values. The different fractal behaviors of saturated and centrifugal T2 spectra reflect the complex pore assemblages in oil shales. The small pore realms determine the microscopic complexity, while large pore realms control the macroscopic reservoir quality of oil shales. However large pore realms (interparticle pores and micro-fractures) are not self-similar with the small pore realms, and can’t be described by fractal dimension. Fractal analysis gives insights in the heterogeneous assemblage of pore systems and helps describe complexity of pore structure.