Fractal characteristics of the micropore throats in the shale oil reservoirs of the Chang 7 member of the Yanchang Formation, Ordos Basin

With the development of the global shale oil and gas revolution, shale oil has became an important replacement field to increase oil and gas reserves and production. The Chang 7 member of the Yanchang Formation in the Ordos Basin is an important shale oil exploration series in China. To study the micropore-throat structure characteristics of the Chang 7 member, we have conducted nuclear magnetic resonance (NMR) and high-pressure mercury intrusion (HPMI) experiments to analyze the pore-throat structure features of the Chang 7 reservoir, and we have considered fractal theory to study the fractal characteristics. The NMR results indicate that the T-2 spectral morphology of the Chang 7 reservoir can be characterized by three main patterns encompassing early and late peaks with different amplitudes: the type 1 reservoir contains mostly small pores and few large pores, and the porosities of the small and large pores range from 4.16% to 9.04% and 0.70% to 2.40%, respectively; the type 2 reservoir contains similar amounts of small and large pores; and the type 3 reservoir contains few small pores and mostly large pores, while the porosities of the small and large pores range from 1.81% to 2.74% and 3.32% to 5.64%, respectively. The pore-throat structure parameters are obviously affected by the pore size distribution, which in turn influences the reservoir seepage characteristics of the reservoir. The micropore-throat structure of the reservoir exhibits obvious piecewise fractal characteristics and mainly includes dichotomous and trilateral fractals. The type 1 reservoirs are dominated by dichotomous fractals, and these two fractal types are equally distributed in the type 2 and 3 reservoirs. The fractal dimension of the pore throats of different scales exhibits a negative correlation with the corresponding porosity, but no correlation is observed with the permeability, indicating that the size of the reservoir determined by pore throats imposes a strong controlling effect on their fractal characteristics.