Fractal behaviors of NMR saturated and centrifugal T2 spectra in oil shale reservoirs: The Paleogene Funing formation in Subei basin, China

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 (E(1)f(2)) in Subei Basin, China. Nuclear magnetic resonance (NMR) T-2 (transverse relaxation time) spectra were used to gain insight into pore size distribution. Fractal analysis was performed on the NMR T-2 spectra measured at saturated and centrifugal status, respectively, with the aim to reveal the different fractal behaviors of irreducible pore realms (centrifugal T-2 spectrum) and the entire pore systems (saturated T-2 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 T-2 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 T-2 spectra show that fractal dimensions calcu-lated from centrifugal T-2 spectrum are higher. In addition, fractal dimensions for centrifugal T-2 spectrum show negative correlation relationships with BVI and T-2cutoff values. The different fractal behaviors of saturated and centrifugal T-2 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.