Insights into the pore structure and fractal characteristics of Permian Lucaogou Formation shale from the Jimusar sag, Junggar Basin

Abstract

The Permian Lucaogou Formation in the Jimusar sag, Junggar Basin, northwestern China, is increasingly receiving attention for its shale oil potential, which is still in the exploration stage. In this work, core samples were obtained from two wells in the Jimusar sag, and their mineralogy, pore structure, and fractal characteristics were investigated by XRD (X-ray diffraction) analysis, LPNA (low-pressure N2 adsorption) analysis, FE-SEM (field emission scanning electron microscopy), CTS (casting thin sections), and fractal analysis. The results showed that the major mineral compositions were plagioclase (average 34.82%), quartz (average 17.07%), and dolomite (average 17.66%). Compared with well-known marine shales in China, the shales have higher feldspar (average 44.64%) and carbonate (average 26.03%), and lower quartz (average 17.07%). Based on the FE-SEM and CTS, organic matter, intergranular, interlayer, dissolved, intercrystalline pores, and microfractures were observed. The specific surface area ranged from 10.181 to 26.648 m2/g, and the total pore volume was between 0.03595 and 0.07102 cm3/g. The results of the DFT-PSD curves indicated that shales showed multi-modal distribution characteristics, whereas the results of the BJH-PSDs curves revealed that shales displayed unimodal distribution characteristics with a peak between 20 and 40 nm. Pore sizes of less than 5 nm had an important influence on the specific surface areas, whereas pore sizes ranging from 20 to 40 nm had an important influence on the total pore volume. The fractal dimension values ranged from 2.7564 to 2.8382. The fractal dimension decreased as the average pore size increased and increased as the total pore volume and specific surface area increased.