Nanopores Structure and Multifractal Characterization of Bulk Shale and Isolated Kerogen—An Application in Songliao Basin, China
Zhikai Liang, Zhenxue Jiang, Zhuo Li, Yan Song, Fenglin Gao, Xiaoqing Liu, Shiqi Xiang
Abstract
Understanding the anisotropy-controlling factors of shale within organic matter can provide novel insight into the transport process and gas storage in typical shale. A series of techniques, such as low-temperature CO2/N2 adsorption (LP CO2/N2), scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), and reflected light microscopy (RLM), has been used to characterize the pore structure characteristics and the heterogeneity of the pore size distribution. In this study, LP gas adsorption experiments of the Shahezi Formation in the Changling Fault Depression, Songliao Basin, China were conducted to calculate the multifractal parameter Δα (the multifractal spectral width) and H (Hurst exponent) and investigate the pore structure and heterogeneity characteristic. The results demonstrate that the pore volume (PV) and surface area (SA) of the kerogen samples have a range of 0.045–0.131 cm3/g and 18.921–135.707 m2/g which are 1.180–3.066 fold and 1.431–5.583 fold greater than those of the volume and surface area of the corresponding bulk shale samples, respectively. The Δα value of the bulk shale samples are similar to the corresponding isolated kerogen samples in the micropores, whereas the value of the isolated kerogen samples of Δα are higher than that of the bulk shale samples in the meso-macropores. The PV and SA have similar influences on the multifractal parameters, suggesting that the multifractal parameters are closely related to the pore structure parameters and significant for evaluating the development of pores in shale. The clay and quartz contents have an obvious impact on the heterogeneity of the nanopore structure in the full pore size of the Shahezi shale. Other factors, such as the TOC content and Ro, play an inconspicuous role in the pore structure–heterogeneity connection. On the basis of SEM, EDS, and RLM imaging, OM pores show significant heterogeneities among different kerogen particles. According to the pore geometrical morphology and genetic types, the main controlling factors of the heterogeneity kerogen pores can be summarized as the remaining original kerogen pores and differences in hydrocarbon generation potential.