Experimental study on the existence of nano-scale pores and the evolution of organic matter in organic-rich shale

Abstract

Abstract To explore the characteristics of nanopores in organic-rich shale, the shale of the Upper Yangtze area under different maturity are studied. Scanning electron microscopy (SEM), low temperature nitrogen experiment (LP-N2-GA) and the Frenkel-Halsey-Hill (FHH) fractal model were used. Results show that the intergranular pores and organic pores are most developed, and the pore size is distributed from a few nanometers to several tens of micrometers. The SEM gray scale images make the pore development characteristics of shale be observed more intuitively. Based on the results of low temperature nitrogen adsorption test and FHH fractal model, the results show the fractal dimension of shale is positively correlated with TOC and quartz, but negatively related to clay minerals and feldspar. There is a good relation between maturity and pore volume.With releasing of pressure, the pore evolution gradually returns to the normal compaction trend, and the pore volume of the shale decrease. Graphical abstract: There are many nano-scale pores in the shale, and the pore characteristics determine the characteristics of shale reservoirs. At the initial stage of sedimentary burial, when Ro is less than 0.7%, the pore volume and specific surface area of the initial shale are large. At the medium maturity stage (0.7%<Ro<1.3%), the pore volume and pores caused by mechanical compaction to some extent. As the degree of thermal evolution deepens to the high maturity stage (1.3% <Ro < 2.6%), the shale is generating large amount of gas. The shale belongs to the low-porosity and low-permeability reservoirs, and the generated oil and gas cannot be effectively transported in a short period of time. The overpressure of the reservoir and the inter-mineral pore characteristics caused by the organic pores and organic acids formed by the hydrocarbon generation are more complicated. When shale is in the stage of over-maturation (Ro>2.6%), as the increase of buried depth, the end of hydrocarbon generation process and long-term pressure release, the under-compacting effect gradually disappear. Porosity evolution gradually returns to normal compaction trend in the shale.