A study on fractal characteristics of lacustrine shales of Qingshankou Formation in the Songliao Basin, northeast China using nitrogen adsorption and mercury injection methods

In order to research the fractal characteristics of lacustrine shale, we conducted low-pressure N-2 adsorption and mercury injection measurements on 35 shale samples of Cretaceous Qingshankou Formation in the Songliao Basin, northeast China. The fractal dimensions (D-1, D-2, and D-3) were obtained from N-2 adsorption (P/P-o < 0.5 and P/P-o > 0.5) and mercury injection data, respectively. Accordingly, we analyzed the relationships between fractal dimensions and petmphysical and geochemical parameters. The results show that the fractal dimensions (D-1, D-2, and D-3) are in the ranges of 2.0174-2.5705, 2.5589-2.8425, and 2.1461-2.9728, respectively. There exists a positive relationship between D-1 and D-2, suggesting that micmpores and mesopores have similar surface characteristics. The poor correlations between D-3 and D-1 or D-2 may be related to the different principles of low-pressure N-2 adsorption and mercury injection. The fractal dimensions (D-1 and D-2) correlate positively with pore volumes and surface areas, which indicates that the fractal dimensions can well reflect the pore structure and surface characteristics. The fractal dimensions (D-1 and D-2) decreased with the increasing TOC content, which may be aroused by the low abundance of pores in organic matters. The fractal dimensions (D-1 and D-2) decreased firstly and then increased with the increasing maturity, which is aroused by liquid hydrocarbons generation and cracking. And the fractal dimension D-3 decreased during maturation, which may be induced by compaction. The fractal dimensions (D-1 and D-2) decreased with the increasing Rock-Eval pyrolysis parameters S-1 and S-1/TOC, which suggests that the fractal dimensions can be indicators of oil content of shales.