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

Abstract In order to research the fractal characteristics of lacustrine shale, we conducted low-pressure N2 adsorption and mercury injection measurements on 35 shale samples of Cretaceous Qingshankou Formation in the Songliao Basin, northeast China. The fractal dimensions (D1, D2, and D3) were obtained from N2 adsorption (P/Po   0.5) and mercury injection data, respectively. Accordingly, we analyzed the relationships between fractal dimensions and petrophysical and geochemical parameters. The results show that the fractal dimensions (D1, D2, and D3) 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 D1 and D2, suggesting that micropores and mesopores have similar surface characteristics. The poor correlations between D3 and D1 or D2 may be related to the different principles of low-pressure N2 adsorption and mercury injection. The fractal dimensions (D1 and D2) 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 (D1 and D2) decreased with the increasing TOC content, which may be aroused by the low abundance of pores in organic matters. The fractal dimensions (D1 and D2) decreased firstly and then increased with the increasing maturity, which is aroused by liquid hydrocarbons generation and cracking. And the fractal dimension D3 decreased during maturation, which may be induced by compaction. The fractal dimensions (D1 and D2) decreased with the increasing Rock-Eval pyrolysis parameters S1 and S1/TOC, which suggests that the fractal dimensions can be indicators of oil content of shales.