Fractal characteristics of pore structure of continental shale in the process of thermal evolution

Fractal dimension can characterize quantitatively the complexity of pore structure of a reservoir and provides ideas for evaluation of shale reservoir. With the shale Chang 7 of Yanchang Formation in Ordos basin as the object, field emission scanning electronic microscope was used to observe the pore characteristics of thermal evolution stages, through the low temperature nitrogen adsorption experiment, the fractal characters of shale pore structure of different evolutionary stages were studied. FHH model was applied to calculate the pore fractal dimension. The relationships between the fractal dimensions and composition, the pore structure parameters were investigated. The results show it had not reached gas window for remaining a low thermal evolution stage for the primary sample, immature source rocks had not begun to generate a lot of hydrocarbons. With thermal maturity increasing, pores began to form gradually in the organic matter, especially, interlayer nanopores generated for the decomposition of organic matter within the clay. There existed mainly ink bottle-like pores and a few of parallel tabular pores, peak pore diameter was in 2~4 nm and 40~50 nm. With increase of maturity, the relative quantity of pores in the two above ranges of pore diameter increased, fractal dimension increased successively, the fractal dimension was 2.592~2.717. The fractal dimensions were negatively correlated with TOC, but show no obvious relationship with the quartz and clay mineral content, the proportion of micropores and mesopores increased with thermal maturity, the average diameters decreased, pore surface was more complicated, the specific surface and fractal dimensions increased. The relationship between total pore volume, micropore volume, mesopore volume and fractal dimensions were positively correlated, while macropore volume and fractal dimensions were poorly correlated. The fractal dimension could be used to evaluate the complexity of reservoir, it provides support for shale reservoir evaluation and adsorption.