Study on the pore structure and fractal characteristics on shale rock and isolated organic matter in lacustrine shales from the Changling fault depression in the Songliao Basin, China
The stock of shale gas in the Shahezi shale reservoir in Changling fault depression, Songliao basin is believed to be worth exploring. To conduct an in-depth study on the pore structure and fractal characterization of organic matter (OM) can help better understand the pore system of shale reservoir, which has implications for the exploration of lacustrine shale. In order to demonstrate the nanoscale pore structure and irregularity of the isolated OM, we collected a large number of samples and then conducted a series of laboratory experiments, such as the XRD, SEM, CO2, and N2 adsorption experiments conducted to determine the pore structure parameters and reveal their heterogeneity according to FHH theory. As suggested by the experimental results, the pore volume of the isolated OM ranges between 0.034 and 0.056 cm3/g, which is approximately 0.90-3.06 times that of bulk shale samples. As for the fractal dimensions D1 (2.594 on average) and D2 (2.657 on average) of bulk shale, they are larger as compared to isolated OM, indicating that inorganic minerals can make a significant difference to the heterogeneity of shale pores. The fractal dimensions (D1 and D2) of bulk shales show a close correlation with the parameters of pore structure, while there is no significant correlation observed between the dimensions of isolated OM and its parameters. In addition, thermal maturity and solid bitumen have only limited impact on the OM pore structure of isolated OM samples. Then, we conducted a further research to reveal that the insoluble OM macerals derived from terrestrial higher plants can be used to explain the difference in pore structure and heterogeneity between isolated OM samples. Therefore, we arrived at the conclusion that the composition of macerals depends on the exact pore structure and fractal characteristics of isolated OM samples with similarity in thermal maturity